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Refrigerologist
10-03-2008, 09:10 PM
Many engineers' in this industry say "you should never adjust a TXV superheat setting as they are factory set".

I would welcome comments on this statement.

Chunk
10-03-2008, 09:15 PM
I have to because my commissioning engineers dont have time to, due to quick turnaround on jobs.

It just means i have more time to play around when they dont.

I work on supermarket packs so its not too critical what they are set to as long as the cases work.:eek:

taz24
10-03-2008, 09:56 PM
Many engineers' in this industry say "you should never adjust a TXV superheat setting as they are factory set".

I would welcome comments on this statement.


They are pre set to about 6k superheat.
For 95% of all aplications you do not need to ouch them.
Only on rare occasions do you need to adjust.

If the valve and orrifice are sized correctly then there is no need to adjust, from experience the problems occur when there is a miss match in valve, evap and system design.

taz.

powell
10-03-2008, 11:57 PM
Refrigerologist.

The statement "never adjust a TXV factory superheat setting" reads in my mind as NOT checking superheat. Just install, charge and move on.

The operating superheat setting for a refrigeration system depends on the temperature difference between the refrigerant and the medium being cooled.

A newly installed, properly sized TXV's SH should be checked and then fined tuned if needed.

Here's a link to the Parker/Sporlan (Sparker) website for more detailed info on adjusting superheat.

http://www.parker.com/literature/Literature%20Files/Parker.com/Literature/Sporlan/Sporlan%20pdf%20files/Sporlan%20pdf%20010/10-9.pdf

Brian_UK
11-03-2008, 01:01 AM
I have to because my commissioning engineers dont have time to, due to quick turnaround on jobs.
......Now thats a frightening statement.:eek:

If the commissioning staff are not allowed sufficient time to perform their task properly then they have not commissioned the job, merely turned it on.:(

This is either due to the customer not wanting to pay for a proper job or the installing company trying to save a buck.:mad:

US Iceman
11-03-2008, 04:21 AM
The operating superheat setting for a refrigeration system depends on the temperature difference between the refrigerant and the medium being cooled.


Aha! Now we are getting down to the real basics.:cool:

You cannot get more evaporator superheat than you have temperature difference between the entering air temperature and the saturated evaporating temperature. Probably a few degrees less to be more accurate!

This is hardly ever considered and I suspect we will get a lot of questions about this.

If you only have 10F (5.5K) of temperature difference, the maximum evaporator superheat you can get is 10F (5.5K) in theory. 8 to 9F (4.4 to 5.0K) is probably the best you can get.

Therefore, if someone tells you they have the valve set for 15F (8.3K) of superheat, ask them to prove it!;)

US Iceman
11-03-2008, 04:33 AM
They are pre set to about 6k superheat.
For 95% of all applications you do not need to touch them.
Only on rare occasions do you need to adjust.

If the valve and orifice are sized correctly then there is no need to adjust, from experience the problems occur when there is a miss match in valve, evap and system design.


I agree with taz. Except for the part about ouch them.:rolleyes:

If the valves are properly selected for the installed application, then by all means check the superheat. I think you will find they are pretty darn accurate with the factory setting.

I think I may have started this by saying don't adjust the TXV's. I will stand by that comment.

In practice what I mean is don't try adjusting the valve on a service call. It either works or does not. If it has failed then change it.

Too many people adjust these to get some desired effect such as higher or lower suction pressure, trying to get rid of bubbles in a site glass, or heaven knows what else.

When you truly do have to adjust a TXV it is usually because someone else was there adjusting it before you.:(

NH3LVR
11-03-2008, 06:07 AM
I DO adjust expansion valves. ONCE.
If I am called to a plant with a over or underfeed condition, I will adjust expansion valves. ONCE.
If they do not maintain their setting-change them.
Mostly an NH3 guy, I find the factory seting on TX valves is usually not correct.
But if have to adjust them more than once, there is a problem.

taz24
11-03-2008, 12:17 PM
I agree with taz. Except for the part about ouch them.:rolleyes:

When you truly do have to adjust a TXV it is usually because someone else was there adjusting it before you.:(


The Ouch feeling when working with TEV's (TXV):eek:


Thats when you lose your temper with them and want to beat the c**p out of them with a spanner:D.

taz.

US Iceman
11-03-2008, 04:45 PM
I just remembered something due to NH3LVR's comments on ammonia TVX's. How often do you rebuild or replace ammonia TXV's?

For people who work on ammonia systems this is the result of a very common issue called wire-drawing. Ammonia TXV's are very prone to this and the result is the pin and seat wear away. Eventually the valve orifice is worn out and can't control the liquid flow for superheat control anymore.

Adjusting an NH3 TXV with this problem is a waste of time.;)

Chunk
11-03-2008, 08:36 PM
Now thats a frightening statement.:eek:

If the commissioning staff are not allowed sufficient time to perform their task properly then they have not commissioned the job, merely turned it on.:(

This is either due to the customer not wanting to pay for a proper job or the installing company trying to save a buck.:mad:

Hi Brian.

Just to give you a picture of what goes on.

We do a shutdown when the store shuts at say 10pm then case fitters come in and rip everything out.By 6am new cases in.Pack goes on vac for a day,gas is put in and cases turned on.Then we can setup controllers and get cases down to temp.

Within 2 hours of cases being turned on the store is loading cases whether they are working or not,this is where the problem occurrs.

One commissioning engineer per site having to set all case and pack controllers running comms and getting it all connected to monitoring system and only given 3 or 4 days to do this.

All i can say is it is not like the old days when stores actually shut for a week so all work can be done efficiently,but this is how these supermarkets want to play nowadays.

Refrigerologist
11-03-2008, 08:52 PM
My main point would be: If expansion valves are not to be adjusted, why then does the manufacturer bother to fit an adjustment screw?

Each installation is different. The liquid line pressure will vary from system to system even when operating at the same condensing and evaporating conditions. An evpaporator installed 20metres above the condensing unit will not have the same valve inlet liquid pressure with an evaporator installed 20 metres below the condensing unit. The same type of valve may have been installed. But the capacity of each valve will vary slightly due to the variance in diferrential pressure alone. This doesn't take into account a systems that use the same valve and orifice but the evaporating temperure for one is -36C and the other 5 C.
I agree that in an ideal world, after the commissioning engineer has checked and set the superheat, there should be no need to fiddle with it. Unfortunatley, I have met & worked with quite a few 'engineers' (trained monkeys could have been as useful), who have no idea what superheat is, let alone how to check it!

Refrigerologist
11-03-2008, 09:04 PM
Iceman makes the point of wire draw, I have no idea what that means. Of course if an HFC, HCFC or CFC plant were operating short of refrigerant for a long period of time this would wear the orifce seat and valve needle due to the increase in friction. Again it would be pointless adjusting the valve, although you might get away with a new orifice.

Is this the same as 'wire draw'?

abet_meneses
08-04-2008, 03:08 PM
whether to adjust or not the txv is an option doing the job.in my experience,txv of the orifice type often need to adjust,while fixed capacity txv seldom need adjustment.

SteinarN
08-04-2008, 04:04 PM
I mostly install Danfoss TEV. I find them to have a good factory setting suitable for most normal conditions. It is however absolutely necessary to decrease the superheat when installing a system with a low TD evaporator.

I once conected a new blast chiller to a sentraliced system operating at -8c. Coudn't get the temperature in the chiller below +3c. Found the TEV was adjusted to 10k static superheat. As the temperature in the chiller aproached 10k above evaporation temperature, the TEV closed completely. Adjusted the TEV to a few k static superheat. Afterwards I got the temperature down to -2c or -3c. No problem with liquid entering the compressor as the system has a suction gas/liquid line heat exchanger.

US Iceman
08-04-2008, 04:36 PM
It is however absolutely necessary to decrease the superheat when installing a system with a low TD evaporator.


That is a valid point and one not often considered. I forgot about this when posting my comments earlier (although I have mentioned this before in another thread somewhere). If the static superheat setting of the expansion avlve is less than the temperature difference of the evaporator you can have problems with loss of capacity or higher discharge temperatures.

You cannot get more superheat than you have temperature difference. This is a practical limit to recognize in my opinion.

US Iceman
08-04-2008, 04:37 PM
Refrigerologist,

Wire drawing is where the pin carrier and needle are eroded due to flash gas in the liquid feeding the TXV. This happens over a period of time on ammonia valves which requires them to be rebuilt occasionally.

If you have seen a piece of wood that has termites or worms feeding on it, the pin and needle look something like this, only rougher.

US Iceman
08-04-2008, 04:44 PM
One last comment while I'm on this subject for now. When I suggest the general rule of not adjusting expansion valves it is based on preventing other problems.

All too often people start to adjust the TXV's (or replace them) as a general service practice when performing analysis of refrigeration systems.

This happens in much the same way as adding refrigerant to the system when cooling is not sufficient.:(

What I'm trying to suggest is that people need to increase their skill levels and understanding of how things work.

The problem with adjustment stems is that people like to turn them when something is not correct.:o

Gary
08-04-2008, 09:57 PM
An expansion valve should be adjusted if it needs adjusting. The problem is that most service techs can't tell if it needs adjusting or not, so they end up causing problems instead of curing problems. When in doubt, leave it alone. TXV's almost never need adjustment.

philfridge
08-04-2008, 09:58 PM
[quote=Refrigerologist;96601]Many engineers' in this industry say "you should never adjust a TXV superheat setting as they are factory set".

:rolleyes: This is correct and if you select the right size orifice no adjustment is required . Its all about knowledge and experience , however not everyone knows how to select the correct size orifice they then start messing with the t.e.v ?? why o why just leave them valves alone and get some training :D:D

Refrigerologist
10-04-2008, 07:54 PM
Refrigerologist,

Wire drawing is where the pin carrier and needle are eroded due to flash gas in the liquid feeding the TXV. This happens over a period of time on ammonia valves which requires them to be rebuilt occasionally.

If you have seen a piece of wood that has termites or worms feeding on it, the pin and needle look something like this, only rougher.

Thnaks Iceman,:) so it is the same as an HFC or CFC system, flash gas in the liquid line, or shortage of gas causes the problem. I have never heard it given that name before! I always called it needle and/or orifice seat wear!

Refrigerologist
10-04-2008, 07:57 PM
[quote=Refrigerologist;96601]Many engineers' in this industry say "you should never adjust a TXV superheat setting as they are factory set".

:rolleyes: This is correct and if you select the right size orifice no adjustment is required . Its all about knowledge and experience , however not everyone knows how to select the correct size orifice they then start messing with the t.e.v ?? why o why just leave them valves alone and get some training :D:D

If you read some of my previous posts you will see that I completely disagree. Valve superheat should always be checked and adjusted as per individual system requirements. As I have said, if we are not to adjust the valve then the manufacturer would not bother to fit an adjustment screw http://www.refrigeration-engineer.com/forums/images/icons/icon10.gifhttp://www.refrigeration-engineer.com/forums/images/icons/icon10.gifhttp://www.refrigeration-engineer.com/forums/images/icons/icon10.gifhttp://www.refrigeration-engineer.com/forums/images/icons/icon10.gif I would suggest you look at a selection catalogue for, say Danfoss valves, it will show that an orifice will provide different capacities at differrent conditions and the valve cannot be factory set to cover all of the possible conditions that the valve will need to cover!

US Iceman
10-04-2008, 08:31 PM
Thanks Iceman,:) so it is the same as an HFC or CFC system, flash gas in the liquid line, or shortage of gas causes the problem.


The situation is more readily noticed on ammonia TXV's because the volume of flash gas created is usually much higher than what you would see on HFC's or CFC's. It doesn't take very much flash gas to to start the wear patterns. My general recommendations are: if you don't have subcooling on a high pressure ammonia liquid line you ought to be thinking about rebuilding the TXV's every couple of years.



I have never heard it given that name before! I always called it needle and/or orifice seat wear!

Wiredrawing is just the term I have heard used for years to describe this. Probably from the old timers who tried to teach me.:D

US Iceman
10-04-2008, 08:37 PM
As I have said, if we are not to adjust the valve then the manufacturer would not bother to fit an adjustment screw


That's where I have a problem with adjusting stems. Just because the valve has one doesn't mean it should be turned!:rolleyes:



I would suggest you look at a selection catalogue ...it will show that an orifice will provide different capacities at different conditions and the valve cannot be factory set to cover all of the possible conditions that the valve will need to cover!


I disagree!:eek:

The valve capacity changes because of the orifice size OR the pressure differential across the valve. The adjusting stem is to adjust the superheat for specific conditions. It is not used to adjust the valve capacity directly.

philfridge
11-04-2008, 12:41 AM
[quote=philfridge;100895]

If you read some of my previous posts you will see that I completely disagree. Valve superheat should always be checked and adjusted as per individual system requirements. As I have said, if we are not to adjust the valve then the manufacturer would not bother to fit an adjustment screw http://www.refrigeration-engineer.com/forums/images/icons/icon10.gifhttp://www.refrigeration-engineer.com/forums/images/icons/icon10.gifhttp://www.refrigeration-engineer.com/forums/images/icons/icon10.gifhttp://www.refrigeration-engineer.com/forums/images/icons/icon10.gif I would suggest you look at a selection catalogue for, say Danfoss valves, coit will show that an orifice will provide different capacities at differrent conditions and the valve cannot be factory set to cover all of the possible conditions that the valve will need to cover!
:(:( Like i said you do not need to adjust the valve if you know what you are doing with orifice selection :D:D . As you mentioned Danfoss valves these are preset at 2 1/2 turns out and this does not need any adjustment in my experience :p

NH3LVR
11-04-2008, 02:53 AM
Wiredrawing is just the term I have heard used for years to describe this. :D

Do not forget we are the oldtimers now;)
Wiredrawing is the correct term and in common use in steam equipment.

US Iceman
11-04-2008, 03:28 AM
Do not forget we are the oldtimers now;)


Speak for yourself!!:p Just because I have thinning grey hair does not mean I'm getting old.:rolleyes:

Gary
11-04-2008, 04:35 AM
What makes you kids think you're old?

NH3LVR
11-04-2008, 05:51 AM
Speak for yourself!!:p Just because I have thinning grey hair does not mean I'm getting old.:rolleyes:
First off my hair is not getting thinner, but it is getting grayer.
I recommend Grecian Formula.
On the other hand, I can still pull the 16 hour shifts when required. And watch the twenty somethings complain they are tired.
Excuse me, it is after 8:30. I belong in bed.

750 Valve
11-04-2008, 01:06 PM
[quote=Refrigerologist;101113]
:(:( Like i said you do not need to adjust the valve if you know what you are doing with orifice selection :D:D . As you mentioned Danfoss valves these are preset at 2 1/2 turns out and this does not need any adjustment in my experience :p


I completely disagree, comparing a danfoss tuae valve which has 9 selectable orifice sizes to say a sporlan balanced port valve (SBF series) which has only 5 - leaves a lot of room for mismatching evap load to valve capacity with the sporlan valve. If you believe EVERY single valve will keep 6K superheat then maybe you need to set up some more evaps... distributor and coil design have a big influence on how a valve behaves as well - this is why every valve should be evaluated for its performance and adjusted if necessary to keep a stable superheat.

I commission supermarket cases nearly every day (or night :D) and have been doing so everyday for nearly 8 years straight now and I cannot tell you how many of the same cases with the same evaps, distributors and txv's I come across... if you think they all behave the same as far as superheat control you have another thing coming. I use an Einstein or RMCC in a suitcase and setup probes to log and graph their performance, there are differences in their factory setting to say the least, Alco valves are the worst as far as differing behaviours for the same given conditions, then sporlan and I'd have to say Danfoss are probably the best as far as consistency goes.

I'd love to just start them up and skip the whole txv logging and adjusting but unfortunately the supermarkets over here have consulting engineers that will randomly check superheat in each store on handover - if its not between 4 to 6k you will be made to do them all again :eek:.

Refrigerologist
11-04-2008, 03:41 PM
[quote=philfridge;101154]


I completely disagree, comparing a danfoss tuae valve which has 9 selectable orifice sizes to say a sporlan balanced port valve (SBF series) which has only 5 - leaves a lot of room for mismatching evap load to valve capacity with the sporlan valve. If you believe EVERY single valve will keep 6K superheat then maybe you need to set up some more evaps... distributor and coil design have a big influence on how a valve behaves as well - this is why every valve should be evaluated for its performance and adjusted if necessary to keep a stable superheat.

I commission supermarket cases nearly every day (or night :D) and have been doing so everyday for nearly 8 years straight now and I cannot tell you how many of the same cases with the same evaps, distributors and txv's I come across... if you think they all behave the same as far as superheat control you have another thing coming. I use an Einstein or RMCC in a suitcase and setup probes to log and graph their performance, there are differences in their factory setting to say the least, Alco valves are the worst as far as differing behaviours for the same given conditions, then sporlan and I'd have to say Danfoss are probably the best as far as consistency goes.

I'd love to just start them up and skip the whole txv logging and adjusting but unfortunately the supermarkets over here have consulting engineers that will randomly check superheat in each store on handover - if its not between 4 to 6k you will be made to do them all again :eek:.

The quote at the head of your post is not one of mine. I believe valve superheat should be checked and adjsuted, if it is required, for every system on commissioning. In other words you will get no argument from mehttp://www.refrigeration-engineer.com/forums/images/icons/icon10.gif I will also add that I too find Danfoss valves to be the best and most consistent.

Refrigerologist
11-04-2008, 03:45 PM
That's where I have a problem with adjusting stems. Just because the valve has one doesn't mean it should be turned!:rolleyes:



I disagree!:eek:

The valve capacity changes because of the orifice size OR the pressure differential across the valve. The adjusting stem is to adjust the superheat for specific conditions. It is not used to adjust the valve capacity directly.

I was not suggesting trying to adjust the valve capacity at all. I was merely suggesting that the a valve may have a wide range of uses, ie deep freeze or high temp and therefore some adjustment from factory set point would be necessary. I believe I covered the aspect of specific conditions, such as pressure differential across the valve in an earlier post. Also does anyone here believe that the manufacturer tests every single valve it produces, or is it more likely that it does a batch test of perhaps 3 or 4 out every thousand that are produced?

I stand by the statement if a valve was not meant to be adjusted then it would not have an adjustment screw. You might as well argue that all thermistor sensors are factory tested and therefore they never need calibrating in the field. But we all know this is boll**ks! Electronic controls usually have a calibration setting.

I commissioned 4 systems last week. Identical systems, almost identical horizontal pipe runs, just a couple of metres difference. Different resulting superheats. 2 were correct at around 6 to 7K, 2 were at at around 9K. Same gauges, same temperature probe, same day, difference in outdoor ambient 1K over the day, same indoor conditions. So why the difference? Could it be that the factory setting is not always as stated? The 2 that were operating at 9k of superheat would certainly have worked reasonbly well, but would they have been as energy efficient as the other 2?

US Iceman
11-04-2008, 04:34 PM
OK, since we are all getting along so well I want to add some additional comments.

Let's say you set the superheat correctly at the evaporator. That's the evaporator superheat, right?

Now let's say the suction line is very long and the insulation is less than perfect (that never happens right?:rolleyes:). Now as the superheated gas flows back to the compressor with the evaporator superheat in the vapor, the vapor continues to absorb additional heat from the ambient.

At the suction service valve we can measure the superheat at this point. This is the suction superheat. The higher this gas temperature becomes also decreases the gas density and lowers the mass flow pumped by the compressor. Therefore the compressor capacity decreases. Compressors just pump gas volume. They don't remove kW or BTU's.

Then the vapor enters the motor on a semi-hermetic compressor. This also adds heat to the refrigerant vapor.

Consequently, the higher suction temperatures increase the discharge temperature of the vapor.

Now for the big question....

Why don't we set the superheat on the evaporator to a slightly lower setting to reduce the suction line superheat to improve the compressor performance and reduce the motor and discharge temperatures?

Lower motor and gas temperatures would increase the life of the compressor and improve the oil stability. The compressor runs cooler, so it will last longer.

A slightly lower evaporator superheat means you are using more of the evaporator surface for latent heat transfer, so you are picking up more heat with the same coil surface.;)

Refrigerologist
11-04-2008, 05:05 PM
OK, since we are all getting along so well I want to add some additional comments.

Let's say you set the superheat correctly at the evaporator. That's the evaporator superheat, right?

Now let's say the suction line is very long and the insulation is less than perfect (that never happens right?:rolleyes:). Now as the superheated gas flows back to the compressor with the evaporator superheat in the vapor, the vapor continues to absorb additional heat from the ambient.

At the suction service valve we can measure the superheat at this point. This is the suction superheat. The higher this gas temperature becomes also decreases the gas density and lowers the mass flow pumped by the compressor. Therefore the compressor capacity decreases. Compressors just pump gas volume. They don't remove kW or BTU's.

Then the vapor enters the motor on a semi-hermetic compressor. This also adds heat to the refrigerant vapor.

Consequently, the higher suction temperatures increase the discharge temperature of the vapor.

Now for the big question....

Why don't we set the superheat on the evaporator to a slightly lower setting to reduce the suction line superheat to improve the compressor performance and reduce the motor and discharge temperatures?

Lower motor and gas temperatures would increase the life of the compressor and improve the oil stability. The compressor runs cooler, so it will last longer.

A slightly lower evaporator superheat means you are using more of the evaporator surface for latent heat transfer, so you are picking up more heat with the same coil surface.;)

I would tend to agree with the point you are making, as long as it is useful cooling that is taking place in the evaporator. The problem is that we are using mechanical valves, these can be reasonbly slow to respond to changing evaporator pipe outlet temperatures, as there is a delay from more refrigerant entering the evaporator and the pipe temperature reducing and thus causing the valve to modulate. We of course tend to err on the side of caution and do not usually set superheat below 5K.

What happens if the evaporator fans stop working? The orifice is effectively oversized and so the valve will hunt and may cause premature failure of the compressor due to liquid hammer.

However, with supermarkets using electronic TXV's, which tend to respond more quickly, and with the usual long pipe runs then it is beneficial to set the superheat a bit tighter.

US Iceman
11-04-2008, 05:34 PM
What happens if the evaporator fans stop working? The orifice is effectively oversized and so the valve will hunt and may cause premature failure of the compressor due to liquid hammer.


OK, that's reasonable. However, I think the larger problem with instability is the system operation changing rapidly. Fan cycling controls for head pressure control are notorious for this.

The older style unbalanced port TXV's were/are susceptible to this and hunt easier than a balanced port valve.

If the system operation is stable the loss of single small wattage motor is less important than a change in the pressures. But, that is just my opinion.

Refrigerologist
11-04-2008, 06:27 PM
OK, that's reasonable. However, I think the larger problem with instability is the system operation changing rapidly. Fan cycling controls for head pressure control are notorious for this.

The older style unbalanced port TXV's were/are susceptible to this and hunt easier than a balanced port valve.

If the system operation is stable the loss of single small wattage motor is less important than a change in the pressures. But, that is just my opinion.

Hi Iceman, I agree with what your saying, especially where small low wattage evap fans are concerned. I was thinking more of the larger many hp motors operating large air handlers etc. Of course, there should be an airflow switch installed to prevent the compressor from operating if there is an air flow failure!

I totally agree about on/off type fan control on condensers, even large multiple fan condensers need a lttle thought about which fans to cycle as this greatly affects subcooling if the wrong fans are cycled. But that is a debate for another thread.

The biggest problem is not TXV adjustment it is the lack of understanding of superheat, subcooling and valve selection that are our biggest problems. In otherwords adequate training.

As an aside I downloaded, from a link provided by you, (on another thread) a guide to Danfoss TXV's. I printed it and gave it to my nipper for bed time reading. http://www.refrigeration-engineer.com/forums/images/icons/icon14.gifExcellent, he can see now that what I have been telling him is actually correct as it comes directly form the manufacturer. And I will confess, it gave me a much more structured view of how to adjust the valve to prevent hunting. I have always understood what it is, I have always manged to stop the valve from hunting, but the method stated is so simple and quick!

SteinarN
11-04-2008, 06:30 PM
I have been looking for a lively discussion for a while.
Keep up the good work! ;)

Refrigerologist
11-04-2008, 06:44 PM
I have been looking for a lively discussion for a while.
Keep up the good work! ;)

Hi Steiner,

I don't need any encouragment:D This one will run and run. Longer than Les Miserables!

US Iceman
11-04-2008, 07:44 PM
I was thinking more of the larger many hp motors operating large air handlers etc.


Now those are a problem. Just as bad if not worse is when TXV's are used on VAV (variable air volume) systems. I saw one of these years ago and the compressors had already been changed about 3 times before I was called to look it.



.... the lack of understanding of superheat, subcooling and valve selection that are our biggest problems. In other words adequate training.


No argument from me. This is why I said don't adjust the TXV's in the first place. People adjust them for the wrong reasons most of the time. I was hoping someone might think twice before they adjusted a TXV and would spend some time thinking about what they were doing.




I have been looking for a lively discussion for a while.


Do not be afraid to join in the fun.:D You did not strike me as the bashful type.;)

philfridge
11-04-2008, 08:57 PM
What happens if the evaporator fans stop working? The orifice is effectively oversized and so the valve will hunt and may cause premature failure of the compressor due to liquid hammer.

I have to disagree :D The orifice is not oversized it is the same size and the t.e.v will close down via the bulb sensor, and if it has not been ADJUSTED incorrectly :) liquid should not reach the compressor as the valve is shut down .System will cut out on LP control. No damage done.

NH3LVR
12-04-2008, 02:48 AM
e

I have to disagree :D The orifice is not oversized it is the same size and the t.e.v will close down via the bulb sensor, and if it has not been ADJUSTED incorrectly :) liquid should not reach the compressor as the valve is shut down .System will cut out on LP control. No damage done.

Here I sit on my new deck with my trusty laptop, on the first night of the year nice enough to enjoy a glass of homebrew Ale.
I might as well jump into the fray.:D
The first thing I look for in a plant with TX valves is a Evap with the fans off, or no water flowing etc.
Tx Valves do not shut off tightly enough to completely stop the flow of refrigerant in many cases. This is very often the case with NH3 valves after a couple years.
Yes the Compressor does shut down on Low Pressure. But the unevaporated liquid continues to flow at a small rate.

Refrigerologist
12-04-2008, 06:59 PM
What happens if the evaporator fans stop working? The orifice is effectively oversized and so the valve will hunt and may cause premature failure of the compressor due to liquid hammer.

I have to disagree :D The orifice is not oversized it is the same size and the t.e.v will close down via the bulb sensor, and if it has not been ADJUSTED incorrectly :) liquid should not reach the compressor as the valve is shut down .System will cut out on LP control. No damage done.

I would suggest that some systems that I work on would never shut down on LP due to the fans stopping. Have look at the cut out pressures for a Daikin condensing unit, some of these are many inches of vacuum.

I would also argue the point as the gear I was commissioning and talked about in an earlier post did just this. The fire alarms were being tested whilst I was commissioning. The main fans shut down, but I had the BMS control for unit operation linked so that I could work on my kit undisturbed by on off temp control.

The result, liquid floodback to compressor, foaming oil raised noise. I shut down the kit. And had to wait for the fire alrm test to be completed. The LP switch is a factory inbuilt 1.5bar set switch. The suction pressure with the main fan operating was 4.3bar with the fans off it did not drop below 1.8bar. I dare say it would have dropped further as the coil iced over, but by that time the compressor would have been covered in frost as it was already pumping liquid.

With the fans switched off the valve orifice must become oversized as it is sized for the refrigerant mass flow rate with the fans operating, not with the fans switched off:D. With the fans off the coil becomes a static cooler and cannot possibly boil off the refrigerant that is being delivered by the now oversized compressor. Yes if we are lucky the compressor may stop via LP but not in all instances.

What I would say is that in larger systems it is usual to incorporate an evaporator fan fail switch that also shut the compressor off in the event of loss of airflow. This definitley stops the problem.

750 Valve
13-04-2008, 08:59 AM
philfridge, refrigerologist i apologise for the misquote - I don't know what happened... I just hit the quote button like I normally would, lots of the quotes in this thread appear to be messed up

Anyway i completely agree with US Iceman in regards to total suction superheat and mass flow rates, one of our customers - a large supermarket chain in aus specifies a maximum suction superheat (as do the comp mfg's) but fits stub probes and really likes to see around 10K at the stubs entering the suction header, which normally lands the evap superheat around the 4 to 6K mark, and yes this total superheat is affected by line sizing and insulation quality, and I guess through experience 10K provides a decent mass flow rate (when compared to say 20 - 30K). It can make setting stores up a little tedious when they are so perdantic (spelling?) about stub temps, it requires a little tweaking to keep them satisfied with both evap and total superheats but I guess it keeps us commissioners on our toes a bit.

Refrigerologist
13-04-2008, 01:30 PM
philfridge, refrigerologist i apologise for the misquote - I don't know what happened... I just hit the quote button like I normally would, lots of the quotes in this thread appear to be messed up

Anyway i completely agree with US Iceman in regards to total suction superheat and mass flow rates, one of our customers - a large supermarket chain in aus specifies a maximum suction superheat (as do the comp mfg's) but fits stub probes and really likes to see around 10K at the stubs entering the suction header, which normally lands the evap superheat around the 4 to 6K mark, and yes this total superheat is affected by line sizing and insulation quality, and I guess through experience 10K provides a decent mass flow rate (when compared to say 20 - 30K). It can make setting stores up a little tedious when they are so perdantic (spelling?) about stub temps, it requires a little tweaking to keep them satisfied with both evap and total superheats but I guess it keeps us commissioners on our toes a bit.

No argument from me here! I just wish I hadn't started this thread, I knew it would be troublehttp://www.refrigeration-engineer.com/forums/images/icons/icon10.gif

NH3LVR
13-04-2008, 06:33 PM
If the fans are not runing and the valve closes, what difference does it make which orfice size is installed? Closed is closed. If the coil is flooding the valve should respond unless the piping arrangment and bulb location permit flow through the bottom of the suction pipe without cooling the bulb.
There will be little or no superheat to open the valve.:confused:

US Iceman
13-04-2008, 07:32 PM
I just wish I hadn't started this thread, I knew it would be trouble


Call it a community service.:D

It might seem like you opened Pandoras box, but I don't think so. These are the types of discussions that once in print help to support learning for others.

The only down side in setting the TXV's to control the total superheat is that now the compressor bodies may sweat or have frost on them depnding on the application. Then someone thinks the compressor is flooding back and resets the TXV's so the compressors run HOT AGAIN!:o

US Iceman
13-04-2008, 07:35 PM
If the fans are not runing and the valve closes, what difference does it make which orfice size is installed? Closed is closed. If the coil is flooding the valve should respond unless the piping arrangment and bulb location permit flow through the bottom of the suction pipe without cooling the bulb.
There will be little or no superheat to open the valve.:confused:


Or unless the power element lost it's charge. Then it could potenitally seep liquid since the valve is only responding to pressure changes alone (inlet and outlet).;)

NH3LVR
13-04-2008, 07:45 PM
Or unless the power element lost it's charge. Then it could potenitally seep liquid since the valve is only responding to pressure changes alone (inlet and outlet).;)

I need to think about that awhile. The few cases I have seen the TX valve shut down when the head failed.

US Iceman
13-04-2008, 08:01 PM
I need to think about that awhile. The few cases I have seen the TX valve shut down when the head failed.

This is a little bit of a stretch I know. But as you have stated already, if the fans shut down the TXV should close also.

This is the only way I can think of where the valve might tend to leak liquid sometimes (although I don't think it would not be a great amount by any means).

philfridge
13-04-2008, 10:06 PM
With the fans switched off the valve orifice must become oversized as it is sized for the refrigerant mass flow rate with the fans operating, not with the fans switched off:D. With the fans off the coil becomes a static cooler and cannot possibly boil off the refrigerant that is being delivered by the now oversized compressor. Yes if we are lucky the compressor may stop via LP but not in all instances.

The fact is though the expansion valve bulb sensor clamped on the suction line will sense the freezing line :eek: and shut the valve down stopping almost any liquid flooding back to the compressor. So the evaporator will not need to boil off the refrigerant :D . The compressor is not oversized but will just not have to do much work :rolleyes: .

bill1983
14-04-2008, 12:21 AM
i have enjoyed reading this discussion so far but would like to ask what should the procedure be when, in these times of energy saving, consulting engineers are requesting that operating condensing pressures be reduced from 14.5 bar to 11 bar. if the performance of the valve is in part based on the pressure drop across it, then surely a 3 to 3.5 bar decrease in pressure drop would naturally change the valve/orifice combination's capacity. this surely would therefore force a change of orifice or superheat setting. we do not however receive instruction on this with the comment "the change is not large enough to warrant replacement or adjustment". i have not yet made the time to measure any change, but the 25% change in pressure drop woluld surely have a significant effect on the system operation. does anybody have experience of this?:confused:

FEISTY
14-04-2008, 02:58 AM
Hav to say one of the best discussions in a long time. Spent 7 yrs at a counter now 14 years field with 7 owning my company. Same rule stands... don't mess with a valve unless you have a spare on hand. Totally agree that some techs like to think they know how to read and set properly. A couple of nightmare situations and a " good " tech learns not to touch what he can't understand. I am learning everyday MY LIMITS and look for the problem before I apply the solution. Again....great discussion!!! Thanks.
'

US Iceman
14-04-2008, 04:43 AM
...consulting engineers are requesting that operating condensing pressures be reduced from 14.5 bar to 11 bar.


This is easy to do in the winter time with air-cooled condensers since the condensing pressure set point can be set to lower pressures. You have to remember in this situation the liquid coming off of the condenser has a lot of subcooling or is much colder than normal (in summer).

The colder liquid tends to increase the valve capacity, while the lower differential pressure causes the valve capacity to be reduced.

Therefore, what you loose in one condition the other helps to increase. You have to look at the manufacturers ratings and correction factors to see where indeed this balances out in the this condition.

On the other hand, trying to lower the condensing pressure in the summer is a limit of the condensing heat transfer surface available. If the condenser was sized to reject the full load heat at 14.5 bar, then that is where it will want to operate.

sugino_m
14-04-2008, 09:56 AM
Many engineers' in this industry say "you should never adjust a TXV superheat setting as they are factory set".

I would welcome comments on this statement.
Personally, I will agree with this statement, many TXV is already pre-set from the factory. :);)

But certain case, only by "professional" in refrigeration, you may do some adjustment, which you can find in instruction letter come along with TXV packaging, mentioned to turn clock-wise or anticlockwise vice versa. But straightly recommend only for "professional", which they may spend whole day or even couple days for "tune" and "adjust" TXV into the right proper system they want it to be.:eek:

Hope this is usefull.:D

Regards,

750 Valve
14-04-2008, 01:57 PM
Hav to say one of the best discussions in a long time. Spent 7 yrs at a counter now 14 years field with 7 owning my company. Same rule stands... don't mess with a valve unless you have a spare on hand. Totally agree that some techs like to think they know how to read and set properly. A couple of nightmare situations and a " good " tech learns not to touch what he can't understand. I am learning everyday MY LIMITS and look for the problem before I apply the solution. Again....great discussion!!! Thanks.
'


You are right - don't touch what you don't understand, but if you can't adjust a txv to maximise its operation and just change them whenever they don't do what you want, then... well.... um... I'm lost for words.
How would you go getting a butcher's coldplate to flood fully and not smash valveplates? They don't always do what you want out of the box and no amount of changing parts will help you. Sometimes you need to use the old noggin (aka grey matter) and spend the time to watch the evap operation.

Most danfoss valves require a PD of between 10 and 12 bar across the valve before capacity is dramatically reduced and as mentioned before the subcooled liquid temps decrease the percentage of flash gas and thereby allow capacity to be increased.

nike123
14-04-2008, 02:37 PM
I just want to add here, that most of the tools we have to measure superheat, are not calibrated, low accuracy and repeatability, and measurement's (pressure) are often not taken at evaporator. That fact of life also makes few more arguments in favor to not adjust TEV unless you are 100% sure that it is needed and that your measured data are of any good.

US Iceman
14-04-2008, 03:34 PM
I just want to add here, that most of the tools we have to measure superheat, are not calibrated, low accuracy and repeatability, and measurement's (pressure) are often not taken at evaporator. That fact of life also makes few more arguments in favor to not adjust TEV unless you are 100% sure that it is needed and that your measured data are of any good.


Some very good points! I might add some comments about the pressure/temperature relationship of the refrigerants also.

At low pressures the equivalent saturation temperature can significantly change due a small pressure loss (or bad data from gauge). While at higher pressures the effect of pressure loss ( or again bad data) present much less of an issue.

If you will forgive the IP units... A difference of 1 psi in the gauge reading can amount to as much as 5 or 6 degrees temperature change at low temperature (depending on the refrigerant). At high temperatures, a pressure difference of 1 psi may only be equivalent to 1 degree or less.

So yes, the accuracy of the gauges and data used is very important. On a low temperature display case that might be set for 6-8 degrees (evaporator superheat in Fahrenheit scale) an error of 3 or 4 degrees due to bad data could cause the compressors to run hot or flood.:o

philfridge
16-04-2008, 10:14 PM
But straightly recommend only for "professional", which they may spend whole day or even couple days for "tune" and "adjust" TXV into the right proper system they want it to be.:eek:

Hope this is usefull.:D

Regards,

:) Yer sounds like my kinda job spending a day or two to adjust a t.e.v . Whats the pay like :D

WINJA
17-04-2008, 09:02 AM
Now for the big question....

Why don't we set the superheat on the evaporator to a slightly lower setting to reduce the suction line superheat to improve the compressor performance and reduce the motor and discharge temperatures?

Lower motor and gas temperatures would increase the life of the compressor and improve the oil stability. The compressor runs cooler, so it will last longer.

A slightly lower evaporator superheat means you are using more of the evaporator surface for latent heat transfer, so you are picking up more heat with the same coil surface.;)
carrier did something like this on EVX equipt chillers , the sensor above the oil pump on the o6e compressor was the suction temp sensor , so super heat was calculated after the windings

Refrigerologist
19-04-2008, 12:07 AM
.

The fact is though the expansion valve bulb sensor clamped on the suction line will sense the freezing line :eek: and shut the valve down stopping almost any liquid flooding back to the compressor. So the evaporator will not need to boil off the refrigerant :D . The compressor is not oversized but will just not have to do much work :rolleyes: .

So on a large capacity system then, with the correct valve orifice installed, but an incorrectly set superheat operating at 3K superheat, the valve would fully close to prevent liquid entering the compressor? What if it is a large packaged unit with a very short suction line and a loss of duty over the evaporator? After the valve has closed there will still be a large amount of liquid in the evaporator and with no duty to heat the refrigerant, liquid will enter the compressor for a short period.

I agree in principal with what you say, but with the precursor that the TXV should be set up by the commissioning engineer and only be left at the factory set adjustment if it has been tested and found to be acceptable, or, as required by the system designer/manufacturer.

If the valves are always correct then maybe I shouldn't bother to check the superheat if I have a reasonable head pressure and 5k of subcooling at the valve inlet.

I think it is too easy for all of us, (including me), to generalise about how each system we encounter would operate in such circumstances. There are too many variables from system to system. I don't think any of us here sit on fence. Some, like me feel each valve should be checked and adjusted, others feel they should be left alone. A case for agreeing to disagree, but it is an interesting debate:)

US Iceman
19-04-2008, 02:46 AM
Let's compromise and say the TXV's should only be adjusted when you have a very good reason and understand what you are doing. Otherwise leave them alone.:D

philfridge
19-04-2008, 12:51 PM
Some, like me feel each valve should be checked and adjusted, others feel they should be left alone. A case for agreeing to disagree, but it is an interesting debate:)


This has to be the last comment really :rolleyes: on this debate as I think all reasons for and against the adjustment of t.e.v s have been made. But everyone has their own beliefs on this subject and must decide for themslves whenever if or why the valve should be adjusted. I personally do not interfere with the adjustment screw but i select different orifice sizes.:) A most interesting debate.

Refrigerologist
19-04-2008, 10:00 PM
Thanks guys. A very good debate. No winners or losers thoughhttp://www.refrigeration-engineer.com/forums/images/icons/icon10.gif

US Iceman
20-04-2008, 02:18 AM
Are you throwing in the towel?:p

This was a good discussion and now it's recorded for posterity for one and all.

Refrigerologist
20-04-2008, 01:14 PM
Are you throwing in the towel?:p

This was a good discussion and now it's recorded for posterity for one and all.

Yep! Towel well and truely thrown in. I couldn't take any more beatings from you guyshttp://www.refrigeration-engineer.com/forums/images/icons/icon12.gif

jure
21-04-2008, 09:50 PM
:DHello guys
I am new here
But I think superheat should be checked (and adjusted if needed)on commissioning because you can never be shore what they did in the factory

US Iceman
16-06-2008, 07:04 PM
Commissioning is about adjusting the system components to achieve the rated capacity and capability of the system. However, there is a logic in this same process. MRcoolingMAGIC, hits it directly. During start-ups the system/room is hot. You have to allow the system to stabilize down to operating conditions reasonably close to what the norm might be.

At that point you can adjust the pressure switches, thermostats, etc. And during this time CHECK the evaporator & suction superheats (but don't adjust them yet!).

If you try to adjust a TXV during pull-down of the load you will be on the job for weeks trying to fix it.!:D

nevgee
21-06-2008, 10:00 AM
[quote=US Iceman;96656]Aha! Now we are getting down to the real basics.:cool:

You cannot get more evaporator superheat than you have temperature difference between the entering air temperature and the saturated evaporating temperature. . . Therefore, if someone tells you they have the valve set for 15F (8.3K) of superheat, ask them to prove it!

Oh that wonderful word "superheat" just how it evokes a discussion.

Maybe we should first all agree to the definition of superheat and what it actually is? Because I'll bet my new digital gauges that we get more than one answer to this question.

Who wants to start?:D

iceburg
30-06-2008, 05:41 AM
[quote=US Iceman;96656]Aha! Now we are getting down to the real basics.:cool:

You cannot get more evaporator superheat than you have temperature difference between the entering air temperature and the saturated evaporating temperature. . . Therefore, if someone tells you they have the valve set for 15F (8.3K) of superheat, ask them to prove it!

Oh that wonderful word "superheat" just how it evokes a discussion.

Maybe we should first all agree to the definition of superheat and what it actually is? Because I'll bet my new digital gauges that we get more than one answer to this question.

Who wants to start?:D


OK I'll bite
To measure evaporator superheat:
1) Record the actual temperature at the TXV bulb.
2) Record the evaporating pressure at the TXV bulb.
3) Convert the evaporating pressure to temperature by using those handy pocket pressure/temperature cards. These cards/charts show the saturation pressure/temperature relationship for those refrigerants.
4) Subtract the temperature you converted on the pressure/temperature card from the actual temperature you recorded at the TXV bulb.
5) The difference is the actual evaporator superheat

as you can see this is a copy and paste, the thing that is not mentioned in this is the swing you get as your TXV opens and closes with demand. It takes time to set a valve properly you have to watch the case and use your lowest temp reading and check your pressure on that swing. If you have electronic SORIT valves your pressure also changes with demand.

To set the valves properly takes time. And to answer the begining of this thread. YES every valve should be checked for proper superheat before a store is turned over.

philfridge
30-06-2008, 09:51 AM
[quote=nevgee;110646]

YES every valve should be checked for proper superheat before a store is turned over.




Robbers dont do that over here :o

US Iceman
13-07-2008, 05:38 AM
The system has to be in a stable condition to adjust the TXV when you have to do it. No capacity changes, no changes in the evaporating pressure, nothing. Any chnage leads to a potentially unstable valve setting.


If the system operation is unstable, then this is what you end up doing:

http://farm2.static.flickr.com/1022/754356724_5397681c4d.jpg

You just wear yourself out and don't get anywhere.:D

iceburg
14-07-2008, 11:46 PM
I totally agree US Iceman, we spend the first week with vacuums, leak checking, gassing up, making sure all the fans are running and there are no flood backs to the compressors ect. then we leave for a week sometimes two these are new store startups so we can do that. When we get back to the store the gas and oil has found its place, that is when we start superheats. We get them as close as possible at that point, sometimes once the cases are loaded some more adjustments are needed but not much or many.

US Iceman
15-07-2008, 12:15 AM
...sometimes once the cases are loaded...


This raises an interesting point.

What effect does the added mass of the product in the cases have? It helps to stabilize the loads thus reducing any erratic swings that may occur. Reducing the swings also helps to stabilize the loads, and hence smooth out the operation of the refrigeration system.

It's the same thing as an empty refrigerator. Open the door on an empty fridge and the temperatures shoot up. Open the door on a full fridge and the temperatures are moderated due to the added mass.

rickst29
14-06-2009, 06:17 AM
....
Here's a link to the Parker/Sporlan (Sparker) website for more detailed info on adjusting superheat....

I'm not sure that a "newby" should be posting in this older thread, but (unfortunately) that link has gone "404". I do see wonderful Threads on this BBoard, which probably provide even better information... but is that Sporlan PDF still available anywhere else? Meanwhile- I have question, and it's exactly this topic: "TXV, stay with factory-provided non-adjustable, or replace it with an Adjustable, sized smaller to match compressor?"

Here's my sad history. I'm in USA, a homeowner with pretty much a new installation. (R410A) It's "pretty much new" due to a tech from a now-fired company trying to fix hunting and inadequate cooling, on a second call-back, by adding more charge, and more charge, and more charge... And it all blew up after he left. (Probably the Evap first, but the Compressor quickly followed.) Now to the point and question: I'm definitely not DIY, and my new tech (new company) seems way more competent. But I think that my configuration DOES call for upgrade of the factory-installed TXV to an adjustable, smaller sized TXV for several reasons:


The Evaporator Coil (from USA company 'Advanced Distributor Products') is oversized relative to compressor (17.5 kW versus 12.25 kW), for efficiency reasons.
altitude 5200 ft (yes, WAY up).
lineset > 50 ft, with about seven 90 degree turns (some "sharp", others "soft").

ADP supplies this Evaporator Coil with an R410a TXV. It's a nonadjustable Emerson, the older Type-A, not the brand new Type-C. (So it's == Alco, IIRC.) Factory labels the part to be nominally capable of anything from 12.25 kW (42,000 BTUH) to the full Evaporator rating of 17.5 kW (60,000 BTUH). My Trane compressor is labeled at the absolute bottom of this range.
So, I'm thinking that the TXV had better be operating nearly closed most of the time.

And I don't like that idea, or the thought of it maybe trying to open up so wide as to generate bubbles in the incoming liquid line (from the less capable compressor). I saw severe hunting before, after the first recharge... although other service errors, including crud left in the lines, may have played a role at that time too.

The only way to tune this one-size-fits-all Emerson is by adding or removing charge. (subcooling OR superheat, not both). And the flowrate "curve" is probably too "steep" anyway.) So I'm inclined to buy a Danfoss TR6 "universal upgrade" kit for him to swap in, before doing the final N2 blow-out and charging. Nominal size 4 tons (14.0 kW). Although some of you say that Danfoss makes 'em a lot "bigger" than the BTUH rating, I think that this would still be a better match for the 12.25 kW compressor than the factory-provided Emerson. (And even if the slope is still too "steep", at least some adjustment can be done via spring tension.) Should I maybe buy the Danfoss kit all the way down at "3 tons" nominal size?

The one thing I won't like to do to the new guy is to make him charge it up, run, and test using the factory provided Emerson-- then find that I'm HATING the performance measurements, make him recover it all, then replace the TXV, then re-do the vacuum test and blow out with N2 (again) and re-vacuum (again), initial charge up (again), stabilize for measurements and charge adjustment (again), make changes, wait for it to stabilize (again) ... not a good way to win friends. So I'm inclined to ask him to take it out, replace it with the Danfoss right at the start of the call. BTW, Trane also discontinued the "factory authorized service" relationship with my original installers, and my "new" guys are Trane's current representatives here.

Thanks for reading all this. :) BTW, it's A/C only (not a Heat pump), and outdoor temps here in Reno do exceed 100F (several times each year). But humidity is extremely low, all I need is the cooling.

nike123
14-06-2009, 06:52 AM
http://www.refrigeration-engineer.com/forums/showpost.php?p=145070&postcount=5

yoelyeve
14-06-2009, 07:03 AM
All my installation's I Check superheat A/C 10-12F medium temp 8-10F low temp 6-8F this is what Copeland requires
also doing so you will make sure that you get the most out of the coil and customer saves on electricity by unit shouting off quicker

desA
14-06-2009, 08:10 AM
^^ Thanks nike - good to see you around.

nike123
14-06-2009, 08:14 AM
^^ Thanks nike - good to see you around.

I am little to busy these days.:)

Gary
14-06-2009, 09:15 AM
All my installation's I Check superheat A/C 10-12F medium temp 8-10F low temp 6-8F this is what Copeland requires


It is important to remember that these numbers are only valid when the refrigerated space is at design temp.

When the space temp is higher, the superheat should be higher... and vice versa.

rickst29
15-06-2009, 06:21 AM
First, let me thank Nike for the link to all those "collected" downloads.

But second, I'd like to thank EVERYONE for the clear and full postings on this subject, in both this Thread and elsewhere on the forum. I'm going to proceed with the Danfoss TR6 "upgrade", at the slightly smaller nominal size of only 36,000 BTUH. (There seems to be a lot of agreement that the "slope" on Danfoss TXV's behaves a bit bigger than their labeled sizes, and there's a good deal of extra "headroom" available for hottest weather too.)

And, as you've all emphasized: No fine-tuning of the valve spring for "best" superheat until AFTER high-side pressures are exactly correct, with fairly conservative subcooling at the TXV inlet, during a lengthy run-in.

4 evr learnin
15-06-2009, 03:45 PM
I too am a Supermarket Fridgie and have been watching this disscussion with much relish ! In any given situation any of your scenarios are correct ! Brand new installations very seldomly does a t.x.v fail when new, however does any one give any consideration to the underpaid possibly underskilled fabricator putting these cabinets together in the factory? The T in T.X.V stands for thermostatic which relates to the crosscharged powerhead Controlling the metering of the valve in question! Surely a potentialy overheated power head would result in incorrect metering due to incorrect control pressures in the sensing bulb? I have seen powerheads where the liquid charge has ended up in the powerhead not the sensing bulb! I have also seen T.X.V's where the control pins are bent/siezed, we do not live in a perfect world, unless your the guy taking the O.E.M part out of the box dont trust it. Super heats should be checked , but adjustment comes down to the installation ! Infact I have noticed a growing trend in cabinet manufacturers putting non-adjustable T.X.V'S in there cabinets ! Gee I wonder why ? Because the average apprentice/junior tradesman are not sufficently trained to competetly understand how to either correctly check/adjust correctly a superheat on an average commercial installation! We are not talking about a commisioning engineer with 10+yrs experience we are talking about the Engineers sent to maintain these systems over nuemerous years! One incorrect adjustment every six months over ten yrs can be pretty detrimental to a markets performance! Training costs money and time 4 years and your Qualified Yeah right :) 10 years minium!

How do you check a superheat on a rack-system with a floating suction pressure ? The answer is you check it the same way as you would on any other refrigeration system, (not gonna get in to basics) however more leeway should be given to your superheat a range of 4-6 KTD above your SST ! Set it forget it walkaway, comeback the next day (unless your lucky enough to have data logging equipment in your kit) However this doesnt take into account cabinets that due to poor design running perfectly acceptable superheats will not actually control the cabinet temperature, What do you do then, change the orifice size up? Perhaps due to a gap in orifice sizing this doubles the capacity of the valve. Not suitable, valve hunting! Perhaps you run a superheat of 1KTD and adjust your defrost perameters to suit ! Lets face facts no one is gonna let you redesign the wheel, that costs money, they naint gonna pay! I have seen market quotes almost halved once the consultants have stripped out all your control gear such as E.P.R'S and sorit valves, decent controls etc.

As for old markets if ya onto makin money for your boss just change it, owner/manager at least knows you've done something, with out you rambling on about heat added after your subcooled refrigerant entering BLAH BLAH BLAH If it worked for 5 years and doesn't now....

Sorry for the long winded post!

sunil4seasons
15-06-2009, 04:54 PM
What makes you kids think you're old?

Hi everybody Is there anything available in the market that can show the quantity of refrigerant charged in a system by just connecting it in to the system? plz. comment.

sunil4seasons
15-06-2009, 05:07 PM
Hi everybody Is there anything available in the market that can show the quantity of refrigerant charged in a system by just connecting it in to the system? plz. comment.

Gary
15-06-2009, 06:10 PM
No, there is no way to tell the amount of refrigerant in a system short of removing the charge and weighing it.

You can however tell if there is too much or too little refrigerant in the system by using temperature and pressure measurements to first confirm sufficient airflow through both coils, then check subcooling and superheat (with the refrigerated space at or near it's design temp).

Gary
15-06-2009, 06:28 PM
To get back to the subject:

When the superheat is low, the least likely cause is the TXV adjustment.

When the superheat is high, the least likely cause is the TXV adjustment.

Once adjusted properly (at commissioning, if needed) the TXV almost never loses it's adjustment.

If the superheat is not what you think it should be, look elsewhere for the problem.

Gary
15-06-2009, 06:38 PM
Here's my sad history. I'm in USA, a homeowner with pretty much a new installation. (R410A) It's "pretty much new" due to a tech from a now-fired company trying to fix hunting and inadequate cooling, on a second call-back, by adding more charge, and more charge, and more charge... And it all blew up after he left. (Probably the Evap first, but the Compressor quickly followed.)

This isn't near enough information for a proper evaluation. But... given this limited information, at this point my guess would be evaporator airflow problems... most likely undersized ducts.

rickst29
18-06-2009, 07:48 AM
This isn't near enough information for a proper evaluation. But... given this limited information, at this point my guess would be evaporator airflow problems... most likely undersized ducts.
Your 'guess' is the first place I'd look, too;). But airflow is excellent, absolute bulb temps are excellent, humidity control and overall cooling are already pretty good. After today's commissioning, I'm willing to guess that it will be VERY close to optimal and it's VERY close to optimal after about one full twist towards more more flow, and maybe some recovery of coolant to reduce excess SC.

But thiswas as "close" as we were willing to go today; outdoor ambient temps were barely 80F today, the real "tuning" needs should be for standard conditions (As close as we want to "try" when ambient temps are so low-- barely 80F today.) Here's what we've got, and what we did:

42,000 BTUH compressor, Trane XL14i.
60,000 Evaporator, Advanced Distributor Products ("ADP")
Note: This is an ARI certified combination at SEER 14.00, although my electric costs always seemed a bit too high, even when new.

Upgrade: Danfoss "TR6" TEV, nominal size 36,000 BTUH.

All 3 components are new. (Lineset and Condenser Coil are unchanged.) We removed the ADP-provided TEV, installed the Danfoss. Also installed a Schrader valve near the evaporator, in front of the TEV, to check pressure/subcooling directly at the TEV (that's where it REALLY matters). As a frivolous enhancement, and because I had a small tube of the stuff lying around after a computer upgrade, I put some high quality thermal-transfer compound around the "contact line" between the Danfoss bulb and the return line, before wrapping it all up with insulation. (Arctic Silver #5. I figure, better thermal contact == better accuracy.)

We did have a concern which prevents "fine tuning" today: Outdoor temps barely reached 80F, and the lineset has almost 25 feet through my attic (over a 3-car garage). When the outdoor temp exceeds 100F, common in July, temps in that space can reach 120+. We might have to insulate the liquid line, even though that gives up advantageous heat loss from the naked copper in more moderate conditions. One thing's for sure: Today, the liquid line was able to give off plenty of heat to ambient, even in the attic.

Vapor line goes through that same attic, opf course, with only 3/8" "pipe noodles" as insulation. That's grossly inadequate and will be upgraded. Now remember, my house is at 5000+ ft, so we used a high altitude R410A pressure versus temp chart.

After installation and charge-up (done properly) we had our first surprise: Line set is almost 40 feet long, one way, at Trane-specified diameters, but upon start up, subcooling was almost 20F at the outdoor coil, and even higher in front of the TXV. But superheat was also excessive (28F, even with my cruddy "noodles" insulation), so we concluded that the first thing to do was adjust the TXV for more flow-- before recovering any "unneeded" refrigerant.

The need for "more flow" didn't surprise us at all-- big Evaporator, slightly undersized model of the Danfoss TR6. But the fact that the big Evaporator AND lengthy line set hadn't called for additional charge, IMMEDIATELY, upon the first start up, was a surprise. We'd expected to see lower pressure on the liquid line, but pressures were high-- at the TEV, and back at the outdoor Service tap too.

After 3/4 turn counterclockwise (more flow, less "excess" superheat), and 15 minutes of running time, superheat was stable at 16F (a bit to high, even for "lazy" commissioning, and far too much for optimum performance-- but going the right direction.) liquid pressure and subcooling fell by only a tiny bit. That's what we expected with R410A, but we chose to at least "ballpark" the superheat before recovering coolant from the factory charge. While he did a bit of coolant recovery (his department, I'm not licensed!) I loosened the TEV spring adjustment by another 1/4 turn.

And that's where we have left it: Superheat is now 13F, subcooling at the outdoor service port is 12F. Greater SC at the TEV, but some of that "excessive" SC at the TEV is partly due to heat loss along the naked line in low attic temperatures. I'll have him come back for final settings on a much hotter afternoon in July, when the attic nearly as hot as it can get (and both lines are insulated against that heat). At that time, we'll adjust for much tighter settings: About 4F subcooling (in front of the TEV), and about 6F superheat (in front of the compressor). Total Pressures are just slightly off from Trane design specs already. Just a few PSI on both sides (increase on Vapor side, decrease on liquid side.)

- - - my summary - - -
Well, it took considerable time and money. (TXV was only $70, but I also had to buy a pound of low temp, high-silver solder-- and in USA, it's almost impossible to obtain the old lead-based stuff. (So I bought J.W. Harris Stay-Brite #8, a non-lead non-cadmium premium product, at nearly $45 shipped.) I handed my installer the almost-full spool, as a gift, at the end of the install. :) But for a fully-equiped pro, the TEV and maybe a couple of new Service Ports are the only hardware items-- hundred bucks or so, unless you go with cheapo Schrader valves (and depend on no-loss adapterd for connecting/disconnecting measuring equipment while pressurized.)

Even with the all the finicky wet towel wrapping, he was done with the TEV in only 45 minutes. (I did provide a spare hand in the process, spraying water to keep the paper towels well soaked.) We did, however, make a major mistake with our install: ADP had mounted the non-adjustable "one size fits all" TEV inside the cabinet; and, without thinking enough, we put the new one exactly where the old one hads been.

WRONG! Each of the 3 adjustments required removal of the case panels, a bit hard to get at, with several minutes of flex-duct taping to seal all BIG gaps surrounding the FOUR (4, WTF ???) drainage connector sockets. (Rectangular cutouts in the panel for pairs of round PVC drainage connectors, huge amounts of leakage before taping.) And of course, removal of the panels requires that the A/C be shut down to avoid blowing indoor air all over the place. We should have mounted the new TEV in a segment of horizontal liquid line OUTSIDE of the evaporator coil case. So the "tuning steps" took almost two hours; run continously, when adjustment made while running, it would have taken barely 45 minutes.

ADP, in contrast, didn't even insulate the bulb of the "one-size-fits-all" TEV. And here's the instructions:


1. Run system for at least 10 minutes to allow pressure to stabilize.
2. Add or recover refrigerant until subcooling matches this table: Min=10F, Nominal=12F, Max=14F.
3. Then, if equipped, adjust the valve until the superheat matches this table: Min 8F, Nominal 10F, Max 12F.


I agree with #1, but for 15 minutes after each change. I totally agree with the idea behind #2, but I label the figures as bogus: ADP didn't provide any kind of service port in front of the TXV, so they seem to be assuming that you're measuring outside. And thus, they provide 6-10F of "extra" SC to make up for heat which MIGHT be gained in hot attics, and pressure which MIGHT be lost in filters/driers, and so on. I'm feeling that a lot of bad things can happen to that pressure, as well as the actual line temp, between the Condenser and the TEV. The subcooling figure which really matters is the one taken at the TEV: If you don't have about 3-4F of protection against bubbles at that point, you're either (a) undercharged; (b) losing a lot of pressure from something which needs to be FIXED or CLEANED UP; or (c) gaining heat along the path.

My theory on this step is: you check the high side pressure and temp at the outdoor Service Point only to confirm that it isn't badly wrong or widely different than the pre-TEV measurements. Big looses of pressure imply restriction, (needing clean up or filter replacement), or perhaps a missed leak. (Shouldn't be there, though, because YOU CHECKED.) Any GAINS in temperature need to be prevented. Actual "tuning" of charge and SC should be based on measurement at the TEV, and more than 4F (excluding undependable temp losses in cool outdor conditions) is making the compressor work too hard, and wasting electricity.

But it's their step 3 where they really fell on their faces. "If equipped, adjust the valve until the superheat matches the table." They don't offer an adjustable TXV, you have to buy one yourself. As I just did. Furthermore, their installation guide says tht it's important to match the size of your compressor, but they sell only two sizes: One for everything up to 3.5 tons, And the other for 3.5 all the way to 5.

Adjustable spring tension alone can't make a TEV respond to varying conditions. You CAN crank the spring up or down for one set of conditions, but if the orrifice size is wrong, the adjustments won't work right-- you'll tend towards flooding in one direction, and tend towards starvation in the other. All the mfgrs make a much greater number of products than ADP offers: one size for 12,000 BTUH, another size for 18,000 BTUH, another size for 24,000 BTUH, and yet another size for 36,000 BTUH! ADP makes it easy to order, with just one product for R410a and the other for R22, but the one size can't really work well for all of these different compressor sizes.

It's already working quite well, and So I'm already delighted with the

rickst29
18-06-2009, 08:17 AM
I somehow did a "submit" by accident while editing that title. Should have been "We're adjusting, love the results so far."

And the end wasn't really finished. It's already working quite well. But we can almost definitely take another 9F from that excess Superheat, after improving insulation, via raising flow. Maybe even more. And the subcooling (excess charge) can maybe take a hit too, although ambient temp loss along the liquid line won't be as good as it was today when outdoor temps go up a lot.

9F in superheat is just HUGE for efficiency. I expect the payback on this $70 device, plus $150 or so for the service call, to be less than 3 years.

The only real problem? I've only got about one turn left towards "more flow" on the 3-ton TR6, and this is not a replaceable-orrifice model. If I hit the end of adjustment, it's a complete swap-out to upgrade to the 4 ton. (But hey, this time we'd install it on the OUTSIDE the case! :D ).

rickst29
18-06-2009, 09:06 AM
Brand new installations very seldomly does a t.x.v fail when new, however does any one give any consideration to the underpaid possibly underskilled fabricator putting these cabinets together in the factory?
Yep, and so did the factory guys! The ADP pre-installed TXV was entirely screw-in: output to distributor, and low-side schrader port, AND input side. They even used a non-adjustable open-sided springsteel clamping piece for the bulb, to prevent some idiot from overtightening a traditional clamp and crushing something expensive.

The only thing the "monkeys" could do wrong, really, was crush or kink the the small tubes (the low-side pressure port, and/or the sensing bulb connector). No solder needed or allowed!

As you proclaimed, it was a non-adjustable: saves installation time to just assume it's good and RUN AWAY to the next commissioning job, and leaving way too much SH present reduces the chance of expensive callbacks for liquid-in-the-return-induced compressor damage.

Who will actually NOTICE that efficiency went to heck from a careless install, when the customer bought it according to a piece of paper which said "EER of 14.25" or "SEER 18.00" and never learned how critical it is for the installer to take time to tune their specific installation to reach that performance level?

Hardly anyone. except for customers like me.:D

BTW, they didn't insulate the sensing bulb, either. Takes time, time is money, and who's ever gonna know that insulation was needed in order to get decent temp measurements?

Hardly anyone. except for customers like me.:D

Gary
18-06-2009, 06:58 PM
Your 'guess' is the first place I'd look, too;). But airflow is excellent, absolute bulb temps are excellent, humidity control and overall cooling are already pretty good. After today's commissioning, I'm willing to guess that it will be VERY close to optimal and it's VERY close to optimal after about one full twist towards more more flow, and maybe some recovery of coolant to reduce excess SC.


With all due respect, "excellent" is an opinion, not a measurement.

Until I see the temperature of the air entering the coil and the temperature of the air leaving the coil, the airflow is still highly suspect... and if the airflow (through both coils) is not right, then the SC and SH will not be right. Airflow is crucial.

Gary
18-06-2009, 07:03 PM
I totally agree with the idea behind #2, but I label the figures as bogus: ADP didn't provide any kind of service port in front of the TXV, so they seem to be assuming that you're measuring outside. And thus, they provide 6-10F of "extra" SC to make up for heat which MIGHT be gained in hot attics, and pressure which MIGHT be lost in filters/driers, and so on. I'm feeling that a lot of bad things can happen to that pressure, as well as the actual line temp, between the Condenser and the TEV. The subcooling figure which really matters is the one taken at the TEV...

WRONG... the SC that really matters is at the outdoor unit. If it is above 15F, liquid backs up into the condenser and absolutely kills the efficiency (and eventually the compressor), especially in hot weather.

Gary
18-06-2009, 07:13 PM
With all due respect, "excellent" is an opinion, not a measurement.

Until I see the temperature of the air entering the coil and the temperature of the air leaving the coil, the airflow is still highly suspect... and if the airflow (through both coils) is not right, then the SC and SH will not be right. Airflow is crucial.

Also, in order to judge proper humidity control, we need the temperature at the thermostat as well as the other two temperatures.

US Iceman
18-06-2009, 09:11 PM
42,000 BTUH compressor, Trane XL14i.
60,000 Evaporator, Advanced Distributor Products ("ADP")
Upgrade: Danfoss "TR6" TEV, nominal size 36,000 BTUH.





Note: This is an ARI certified combination at SEER 14.00....


I'll be the first to admit I do not keep up with residential cooling systems, but from a first blush this is not good refrigeration practice.

The evaporator capacity is rated at 140% of the compressor capacity and the TXV is rated at 86% of the compressor capacity. This would indicate the suction pressure could be too high to do sufficient dehumidifying and the TXV would have to opened up to achieve sufficient flow capacity.

BTW, this should have been a separate thread for your discussion.;)

lowcool
19-06-2009, 05:04 PM
back to original title to adjust or not to adjust.adjust otherwise you would not be able to,why is this screw there ?

steemy
20-06-2009, 11:03 AM
of course TXV need to be set after case start up. Say you have 4 identical cases on a system all with external equalised valves and number 3 orifice and you start them up and wait for the sysytem to settle and come down to temperature. You can physically see some valves need to be set before you have even attached your gauges and thermocouple. Some coils look like they arent doing a thing and some a flooding back. connect gauges and what do you see? too much superheat and too little superheat respectively. answer? set the TX valve.