Valve "Hunting"

[Abe]

My "book" wont arrive for a while so I have another question.....

TXV hunting........How does one ascertain when a valve is doing this, a recent installation the back pressure kept fluctuating between 30 Psi and 25 Psi, which I can gather was the valve shutting down and opening up again. I could also tell from the compressor noise as the load increased and decreased. Was the valve hunting? Or should the back pressure have remained stable throughout its operation with a minimal fluctuation.
System was running on R404a, 1.5hp hermetic LBP compressor, chilling down to +1 C.(cabinet ytemp)
Evaporating temperature around -28C.
Danfoss valve with a number 1 orifice, or capsule....
Valve bulb was fixed tight to the side of the return pipe on the evaporator.

[Gary]

Yes. This is hunting. It is generally caused by refrigerant feeding too fast for the load, which could mean oversized orifice, light load, or excessive pressure drop across the orifice.

[Brian_UK]

Originally posted by Aiyub
...a recent installation .....System was running on R502

How recent was this installation that you were using CFC R502 and does your customer know?

Or this this a military usage that we shouldn't really know about ?

[Abe]

I meant 404a.........slip of the tongue...or keyboard I guess

[Brian_UK]

That's OK then, I'll let you off this time.

[Jack Lester]

Brian was correct in asking how long ago the system was installed. It is posible that the components are mismached. I would check the bulb placement and superheat first though
Jack

[Brian_UK]

Mind you, thinking sideways (as I do) why do we have an R404a system running at 1°C ? This seems a bit of overkill to me, wouldn't something like R134a have been a better choice?

[Jack Lester]

Does seem odd. +1c cabinet and a -28c evap. Yikes!! Rather large TD.

[dan wong]

I have a very similiar problem, Back pressure goes up and down. (intermittenly)

3/4hp, medium temp hermatic, R409a. Ambient temp 80F. Cap tube. This system is used to chill water in loster tank between 40F-45F. System has been in use 5-7 years - no problem.

aprroximately, Three to four time a week, system will not maintaine temp. Loster tank temp goes to approx 55F - 60F. Our gage show BP varie from 10psig to 35psig (slowly). Hi side varie from 175psig to 290psig. We've replaced liquid line filter dryer, replace cap tube, deep vacuum, recharged with fresh R409A........no help.

At this point, we beleived its probablly a bad compressor, However, I am reluctant to replace the compressor, because most of the time it work fine and no other solid evident to indicate a bad compressor.

Any Idea why my cap tube system acts like its "hunting" ?
Any suggestion what the problem is?

[Prof Sporlan]

Any Idea why my cap tube system acts like its "hunting" ?

A cap tube, of course, will not in itself cause a hunting problem. It is simply a fixed restriction, and is not capable of being "fooled"' by varying superheat conditions at the outlet of the evaporator. But a cap tube could "hunt" when applied to an evaporator coil poorly design for the application.

The Prof would suspect you have a bottom fed evaporator coil. Since the system chills water, one might suspect a serpentine coil design in which the evaporator circuit starts from the top of the chiller barrel, runs immediately to the bottom of the barrel, and then spirals its way back up.

With this type of coil design, if refrigerant velocities in the coil aren't sufficiently high, the refrigerant will trap in the bottom portion of the coil. When sufficient liquid is trapped in the coil, a slug of refrigerant will form which may pass thru the remaining portion of the coil and onto the compressor. This could explain why a cap tube system "hunts".

[dan wong]

professor, you are absolutely correct. The Eavaporator is a long piece of clear plastic tubing, coil up in a sepentine style lay at bottom of this loster tank.

Since this is how the system was design and worked for many years. Hard to say a " design" problem. What can I do to correct this problem?

[Prof Sporlan]

Since this is how the system was design and worked for many years. Hard to say a "design" problem. What can I do to correct this problem?

There are four ways to increase the refrigerant velocity in the coil:

1. Increase the load on the coil. You may be able to accomplish this task by lengthening the coil. Keep in mind that adding coil surface will also raise your suction pressure and temperature.

2. Increase the refrigerant liquid temperature entering the cap tube. This technique is obviously more useful for systems employing mechanical subcoolers which can be turned on or off. With your system, subcooling probably cannot be affected significantly.

3. Replace the cap tube with one having greater flow capacity. One may be able to shorten the existing cap tube to effect this change. Obviously, this approach will prove to be a bad idea if the system currently operates with relatively low suction superheat.

4. And perhaps the obvious: replace the coil with tubing having a smaller ID.

[dan wong]

Thank again. Those are very good solutions.

Initially I like to try the following because its easier,
1) They have a water filter / airerator pump. would it help if I take water from pump pass it by the Evaporator coil first, then up to the water filter.?
2) The soft Plastic evaporator coil tied in a bundle can be un-bundle, move around, to absorbe more heat from the water. The only place I can not move is where the ***** enter and exist the evaporator-that is fix.

what do you think.? Of course if above don't work, I am going to do all those you mentioned.

Thank again.

[Prof Sporlan]

Initially I like to try the following because its easier,
1) They have a water filter / airerator pump. would it help if I take water from pump pass it by the Evaporator coil first, then up to the water filter.?

It is not clear how this would increase load on the evaporator. Would the water be warmer coming directly from the pump? Or would the flow rate improve? If so, load would be increased on the evaporator.

2) The soft Plastic evaporator coil tied in a bundle can be un-bundle, move around, to absorbe more heat from the water.

The Prof likes this approach to increase load on the evaporator, and he suspects you will have better luck here.

[dan wong]

Originally posted by Prof Sporlan
It is not clear how this would increase load on the evaporator. Would the water be warmer coming directly from the pump? Or would the flow rate improve? If so, load would be increased on the evaporator.

Because the evaporator is coiled up in a bundle, water surounding the evaporator is very cold (concentrated) comparing to the rest of the water, by moving some of the warmer water throught the evaporator area, I beleive the evaporator will see it as "heat load". ( I could be wrong)

In any case, You have correctly identify the problem and possible solutions. I will try them all. I am sure one of this solution will work.

Once again, THANK YOU, Your advice is greatly appreciated.

[wellcold]

thanks

[farley]

Aiyub

For the record I dont know verymuch about Garys TEV (Electric) Valves but the hunting would be more evident and quicker on them I believe than on your stated TXV (Mechanicaly. by spring and bellows. operated). A nother factor is load has Gary said. But too much load will cause the valve to stay open. too little will close it down. It will tend to "hunt" when operating and a load is added.

[Nigel]

Originally posted by Jack Lester
Does seem odd. +1c cabinet and a -28c evap. Yikes!! Rather large TD.

Large TD Jack?, emormous comes to mind (lol)

[Gary]

For the record I dont know very much about Garys TEV (Electric) Valves but the hunting would be more evident and quicker on them I believe than on your stated TXV (Mechanicaly. by spring and bellows. operated). Another factor is load as Gary said. But too much load will cause the valve to stay open. too little will close it down. It will tend to "hunt" when operating and a load is added.

TXV = TEV = thermostatic expansion valve
EXV = EEV = electronic expansion valve

A valve will tend to hunt when the load is very low.

As Jack and Nigel have pointed out (and I missed it ), the difference between box temp and evap temp is enormous on the system in question and TXV/TEV hunting is the least of it's problems. It seems more likely to have a hunting restriction, rather than a hunting valve.

[Abe]

Ok,

I know I have had "eyebrows raised" about this large or enormous TD.

The explanation is simple, Im an experimenter, ( have I spelt that right? ) anyway, sometimes, I do things a little differently, mind you, just sometimes.

On this occasion I used a R404 compressor capable of -18 c operation on a chiller system with the thermostat cutting in out at at around minus 2c

I know, I should have used R134A with a High Back Pressure compressor, but I didnt, I wanted to see how it works out

The system is functioning ok, I havent heard of any probs.........yet

I shall have to wait and see if compressor fails early...........or perhaps some fridge visionary can tell me what to expect.......

[Gary]

Let's start over. With R404A, 25-30PSI does not correspond to -28C evap temp. This does not compute. Did you check or adjust the TEV superheat?