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

[Chunk]

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.

[Brian_UK]

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.

[US Iceman]

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

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

[US Iceman]

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.

[NH3LVR]

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]

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.

[US Iceman]

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.

[nevgee]

[quote=US Iceman;96656]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. . . Therefore, if someone tells you they have the valve set for 15°F (8.3°K) 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?

[iceburg]

[quote=nevgee;110646]

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. . . Therefore, if someone tells you they have the valve set for 15°F (8.3°K) 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?

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]

[quote=iceburg;111642]

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

Robbers dont do that over here

[US Iceman]

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:



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

[Chunk]

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.

[Refrigerologist]

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!