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

[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

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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 .

[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.