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

[Refrigerologist]

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!

[US Iceman]

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.

[US Iceman]

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!

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.

[Refrigerologist]

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?

[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.

[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.

[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

[NH3LVR]

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.

[US Iceman]

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.

[NH3LVR]

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.