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  1. #1
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    TXV's To Adjust or Not to Adjust - That is ....


    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.



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    Re: TXV's To Adjust or Not to Adjust - That is ....

    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.
    If it ain't broke, it doesn't have enough features yet.


    Damo

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by Refrigerologist View Post
    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.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    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/Lit...20010/10-9.pdf
    Last edited by powell; 11-03-2008 at 01:33 AM.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by chunky munkey View Post
    I have to because my commissioning engineers dont have time to, due to quick turnaround on jobs.
    ......
    Now thats a frightening statement.

    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.
    Brian - Torquay, Devon, UK
    I have to stop saying "how stupid can you be?" to my co-workers.
    They're starting to take it as a challenge...

    BASIC MAINTENANCE. If it doesn't move and it should then use WD40. If it moves but it shouldn't then use Duct Tape.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by powell
    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.

    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!
    If all else fails, ask for help.


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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by taz24
    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.

    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.
    If all else fails, ask for help.


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    Re: TXV's To Adjust or Not to Adjust - That is ....

    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.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by US Iceman View Post
    I agree with taz. Except for the part about ouch them.

    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)


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

    taz.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    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.
    If all else fails, ask for help.


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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by Brian_UK View Post
    Now thats a frightening statement.

    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.
    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.
    If it ain't broke, it doesn't have enough features yet.


    Damo

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    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!
    Last edited by Refrigerologist; 11-03-2008 at 09:07 PM.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    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'?
    Last edited by Refrigerologist; 11-03-2008 at 09:06 PM. Reason: Grammar

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    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.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    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.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by SteinarN
    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.
    If all else fails, ask for help.


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    Re: TXV's To Adjust or Not to Adjust - That is ....

    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.
    If all else fails, ask for help.


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    Re: TXV's To Adjust or Not to Adjust - That is ....

    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.
    If all else fails, ask for help.


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    Re: TXV's To Adjust or Not to Adjust - That is ....

    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.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    [quote=Refrigerologist;96601]Many engineers' in this industry say "you should never adjust a TXV superheat setting as they are factory set".

    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

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by US Iceman View Post
    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!
    Last edited by Refrigerologist; 10-04-2008 at 08:00 PM.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    [quote=philfridge;100895]
    Quote Originally Posted by Refrigerologist View Post
    Many engineers' in this industry say "you should never adjust a TXV superheat setting as they are factory set".

    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
    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 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!
    Last edited by Refrigerologist; 10-04-2008 at 07:59 PM.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by Refrigerologist
    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.

    Quote Originally Posted by Refrigerologist
    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.
    If all else fails, ask for help.


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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by Refrigerologist
    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!

    Quote Originally Posted by Refrigerologist
    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!

    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.
    If all else fails, ask for help.


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    Thumbs down Re: TXV's To Adjust or Not to Adjust - That is ....

    [quote=Refrigerologist;101113]
    Quote Originally Posted by philfridge View Post

    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 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 . As you mentioned Danfoss valves these are preset at 2 1/2 turns out and this does not need any adjustment in my experience

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by US Iceman View Post
    Wiredrawing is just the term I have heard used for years to describe this.
    Do not forget we are the oldtimers now
    Wiredrawing is the correct term and in common use in steam equipment.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by NH3LVR
    Do not forget we are the oldtimers now
    Speak for yourself!! Just because I have thinning grey hair does not mean I'm getting old.
    If all else fails, ask for help.


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    Re: TXV's To Adjust or Not to Adjust - That is ....

    What makes you kids think you're old?

  29. #29
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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by US Iceman View Post
    Speak for yourself!! Just because I have thinning grey hair does not mean I'm getting old.
    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.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    [QUOTE=philfridge;101154]
    Quote Originally Posted by Refrigerologist View Post
    Like i said you do not need to adjust the valve if you know what you are doing with orifice selection . As you mentioned Danfoss valves these are preset at 2 1/2 turns out and this does not need any adjustment in my experience

    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 ) 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 .
    Last edited by 750 Valve; 11-04-2008 at 01:09 PM.
    ...and she said "give it to me you big fridgie"

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    [quote=750 Valve;101205]
    Quote Originally Posted by philfridge View Post


    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 ) 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 .
    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 me I will also add that I too find Danfoss valves to be the best and most consistent.
    Last edited by Refrigerologist; 11-04-2008 at 04:05 PM.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by US Iceman View Post
    That's where I have a problem with adjusting stems. Just because the valve has one doesn't mean it should be turned!



    I disagree!

    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?
    Last edited by Refrigerologist; 11-04-2008 at 04:58 PM. Reason: Can't spell the word no , I mean know

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    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?). 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.
    If all else fails, ask for help.


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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by US Iceman View Post
    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?). 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.
    Last edited by Refrigerologist; 11-04-2008 at 05:08 PM.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by Refrigerologist
    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.
    If all else fails, ask for help.


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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by US Iceman View Post
    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. Excellent, 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!
    Last edited by Refrigerologist; 11-04-2008 at 06:30 PM.
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    Re: TXV's To Adjust or Not to Adjust - That is ....

    I have been looking for a lively discussion for a while.
    Keep up the good work!

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by SteinarN View Post
    I have been looking for a lively discussion for a while.
    Keep up the good work!
    Hi Steiner,

    I don't need any encouragment This one will run and run. Longer than Les Miserables!
    My tools. Screw driver, Hammer & a Condom:
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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by Refrigerologist
    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.

    Quote Originally Posted by Refrigerologist
    .... 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.


    Quote Originally Posted by SteinarN
    I have been looking for a lively discussion for a while.
    Do not be afraid to join in the fun. You did not strike me as the bashful type.
    If all else fails, ask for help.


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    Exclamation Re: TXV's To Adjust or Not to Adjust - That is ....

    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 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.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    e
    Quote Originally Posted by philfridge View Post
    I have to disagree 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.
    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.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by philfridge View Post
    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 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. 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.
    Last edited by Refrigerologist; 12-04-2008 at 07:16 PM.
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    Re: TXV's To Adjust or Not to Adjust - That is ....

    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.
    Last edited by 750 Valve; 13-04-2008 at 09:02 AM.
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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by 750 Valve View Post
    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 trouble
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    Re: TXV's To Adjust or Not to Adjust - That is ....

    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.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by Refrigerologist
    I just wish I hadn't started this thread, I knew it would be trouble
    Call it a community service.

    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!
    If all else fails, ask for help.


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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by NH3LVR
    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.
    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).
    If all else fails, ask for help.


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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by US Iceman View Post
    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.

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    Re: TXV's To Adjust or Not to Adjust - That is ....

    Quote Originally Posted by NH3LVR
    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).
    If all else fails, ask for help.


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    Lightbulb Re: TXV's To Adjust or Not to Adjust - That is ....

    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. 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 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 . The compressor is not oversized but will just not have to do much work .

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