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

[US Iceman]

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.

[Refrigerologist]

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.

[US Iceman]

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]

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!

[SteinarN]

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

[Refrigerologist]

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!

[US Iceman]

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. You did not strike me as the bashful type.

[philfridge]

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.

[NH3LVR]

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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.

[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.

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.

[philfridge]

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 .

[Refrigerologist]

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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 .

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]

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.

[philfridge]

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 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.

[WINJA]

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