Expansion valve tips

[philfridge]

Found this im sure it will be of use.Although i disagree with the adjustment of the tev
http://odeo.com/audio/15899933/play

[marc5180]

Thanks for sharing Phil, i'll have a read.

[Brian_UK]

Sorry Phil, I got a dead link there.

[philfridge]

Yes seems to have gone dead got another one here. http://www.stara.co.kr/filedata/sporlan/QT11.pdf
And this one http://www.real-world-refrigeration....on-repair.html

[powell]

Found this im sure it will be of use.Although i disagree with the adjustment of the tev
http://odeo.com/audio/15899933/play

phil,

Okay, you must be looking for a debate.

Why do you disagree with the adjustment statement?

[pomdekiwi]

superb site and just at my Amature level many thanks for that one.

Got any more ??

pom

[philfridge]

If only it was that easy. Ever heard of a searching expansion valve? Want to occupy yourself for many hours? Just sit there and watch a valve in the real world do it's thing.
What a mess!!! The superheat goes down to 3 or 4 degrees then slowly rises to around 12 to 15 degrees. Then begins to repeat the cycle.
Over and over and over it goes. It never settles on one value and stays!!!
So, what's you superheat? Who the heck knows!!! Take your pick! Take an average! Flip a coin! Throw darts at a chart!
Want to know the dirty little secret? Who cares!!! Want to know what really important?

Pay attention to the compressor!!! It will tell you what you need to know about the superheat.
Assuming the rest of the system is operating normally, the compressor on a medium temp system should have a moderately cool suction bell around the suction line.
That's it!!!
If the compressor body is sweating or cool up to the heads, superheat too low. If the suction line is warm at the compressor, superheat too high.
The whole idea of superheat is to get just the right amount of refrigerant into the evaporator.
Too much, it floods back and damages the compressor. Too little, you starve the evaporator, losing cooling capacity. Simple, simple, simple, that's my motto. Keep that refrigeration repair simple.

[US Iceman]

Pay attention to the compressor!!! It will tell you what you need to know about the superheat.
...
If the compressor body is sweating or cool up to the heads, superheat too low. If the suction line is warm at the compressor, superheat too high.

Finally. A voice of sanity in the wilderness.

I like the way you think. If the system load changes are relatively stable (and most are if they are designed right) you can watch what happens at the compressor. I mentioned something like this in another thread. Depending on the operating conditions you can quickly determine how well the TXv's are operating by looking at the compressor.

I am beginning to think we should have locks on TXV's to keep them from being adjusted...


This debate will continue ad nauseum because of a simple little thing like superheat.

[star882]

If only it was that easy. Ever heard of a searching expansion valve? Want to occupy yourself for many hours? Just sit there and watch a valve in the real world do it's thing.
What a mess!!! The superheat goes down to 3 or 4 degrees then slowly rises to around 12 to 15 degrees. Then begins to repeat the cycle.
Over and over and over it goes. It never settles on one value and stays!!!
So, what's you superheat? Who the heck knows!!! Take your pick! Take an average! Flip a coin! Throw darts at a chart!
Want to know the dirty little secret? Who cares!!! Want to know what really important?

Pay attention to the compressor!!! It will tell you what you need to know about the superheat.
Assuming the rest of the system is operating normally, the compressor on a medium temp system should have a moderately cool suction bell around the suction line.
That's it!!!
If the compressor body is sweating or cool up to the heads, superheat too low. If the suction line is warm at the compressor, superheat too high.
The whole idea of superheat is to get just the right amount of refrigerant into the evaporator.
Too much, it floods back and damages the compressor. Too little, you starve the evaporator, losing cooling capacity. Simple, simple, simple, that's my motto. Keep that refrigeration repair simple.

If an expansion valve periodically oscillates in superheat, the valve is hunting.

[US Iceman]

Ever heard of a searching expansion valve?

If an expansion valve periodically oscillates in superheat, the valve is hunting.

Why repeat the same thing that philfridge mentioned? All you did was use different words.

[star882]

Why repeat the same thing that philfridge mentioned? All you did was use different words.

In a properly operating system, the valve should not hunt.

[powell]

phil,

I remember those days. The only superheat gauges we had were in a kit with two brass dial gauges, I think made by Marsh.

They would peg out sitting in my hot van. Had to use ice water to re-calibrate them, what a pain.

A Copeland engineer, now gone, use to teach in his class that the compressor belly should feel the same as a babies butt.

[philfridge]

I am beginning to think we should have locks on TXV's to keep them from being adjusted...

Powell, take note.

This is a brilliant idea and should not be frowned upon. This is because the TEVs are factory set and do not need to be messed about with. All Fridge Engineers need to check the whole system thouroughly before blaming the TEV as they seldom never go wrong or need adjustment ! . US iceman you have my vote.

[powell]

Powell, take note.

Take note to what?

I agree with the comments made.

powell

[philfridge]

Take note to what?

I agree with the comments made.

Oh sorry powell,
Glad to see you on my side

[nevgee]

The whole idea of superheat is to get just the right amount of refrigerant into the evaporator.
Too much, it floods back and damages the compressor. Too little, you starve the evaporator, losing cooling capacity. Simple, simple, simple, that's my motto. Keep that refrigeration repair simple.

Think you meant it the other way round ...large superheat starved evaporator. Small superheat risk of flooding back.

[Billy Ray]

i am generally on side with all you guy/galls ref tev's & the inherent problem with the tech man who 'insists' during a service call or maintenance to give the valve a 'tweek', its likes boys continually having a fumble whilst watching bay watch!

what i would add further is that the adjustment screw is there for a purpose!

it should be set during comissioning, to match the valve & orifice to the particular application. Then left.

Billy Ray

[Graham]

In my 20+ years service in refrigeration I have only had to adjust a TEV about 5 times and that was because the guy before me had adjusted the thing. My dad always said "If there is a screw with a spring. Leave it!" although he was refering to a carburetor at the time.

[Billy Ray]

Lets look at this from a different angle,

These are 'made-up' figures for discussion purpose only.

We select a cooler for an application, balanced against a condensing unit etc.

At design conditions the cooler has a cooling capacity of 4.5kW.

We now select a Ex. valve to match.

Lets say a danfoss TE2.

A No.2 orifice provides 3kW
A No.3 provides 4kW
A No. 4 provides 5kW

etc, etc.

Which one do we choose? And do we adjust it?

Or do we look for another manufacturer which valve does 4.5kW & do not adjust?

Billy Ray

[Andy]

Hello Billy Ray

I would choose the No4, but I would check the pressure drop accross the valve for the actual duty at the actual pressure drop.

The No4 will probably need turned in a little when the load settles, dependant on actual pressure drop and subcooling.


Kind Regards Andy D

[nevgee]

We now select a Ex. valve to match.

Lets say a danfoss TE2.

A No.2 orifice provides 3kW
A No.3 provides 4kW
A No. 4 provides 5kW

etc, etc.

Which one do we choose? And do we adjust it?

Or do we look for another manufacturer which valve does 4.5kW & do not adjust?

Billy Ray

Well lets go for the No. 3 as Danfoss TEV’s have a + 25% capacity so this unit will do the job admirably. Thing is though, I think we’re all getting side tracked about duty and evaporator capacity etc. It’s the bulb that determines the temperature and superheat value at the point where the bulb is located!

The TEV is factory set , and will work at that value regardless of where the bulb is located. If you install to the perfect layout with the bulb and pilot line in the perfect place then the TEV should give you the perfect super heat. But we know this never happens and the bulb could be further away from the evaporator than the factory anticipated, thus the evaporator is prone to flood and carry over. Conversely the closer we position the bulb the more likely the evaporator will not be fully utilized and duty of the coil is likely to be reduced.
In this situation surely then you would need to adjust the little screw. Opening it up to increase capacity and reducing it to restrict liquid flow. Iether way to maximise the capacity of the evaporator.