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lana
07-12-2005, 01:30 PM
Hi there,
I am working on a freezing tunnel with double-stage Bitzer compressor. One compressor is connected to three evaporators in the same room. The problem I have is three TEVs with MOP are acting differently i.e., there are three different superheat values and three different evaporating temperatures. There is one common suction line to the compressor.
My question is : What is the best and simplest way to bring the three evaporating temperatures to the same value?
And what is happening when three different pressures mix in the suction line?
I appreciate any experience you may have on this issue.
Cheers:)

US Iceman
07-12-2005, 02:59 PM
lana,

Are the evaporators defrosting at different times? Do the evaporators have back-pressure regulating valves? How do the evaporators defrost?

If the evaporators are connected to a common suction line, then obviously the suction pressure is relatively constant. I am assuming the suction pressure is at the desired pressure?

If this is the case, the something is happening to the system upstream of the evaporators. TXV's, liquid line piping, flash gas, no subcooling,or ... something?

Can you provide a piping and layout sketch to help you?

US Iceman
07-12-2005, 03:04 PM
And what is happening when three different pressures mix in the suction line?

Well... the three pressures mix to some equilibrium condition at the lowest pressure. The lowest pressure will be the suction pressure where the last evaporator connects to the suction line.

If there are back-pressure regulators, the higher "controlled" pressure simply expands down to the lower pressure.

chillin out
07-12-2005, 08:49 PM
My question is : What is the best and simplest way to bring the three evaporating temperatures to the same value?
Answer

Do the evaporators have back-pressure regulating valves?
As US Iceman says, all you have to do is fit these and this will control evap pressure for evaps.

The only problem you might have is that when the 2 lower temp evaps are cut off on stat but the 3rd want cooling, you might short cycle the comp depending what control you have on it.


Chillin:) :)

lana
07-12-2005, 09:26 PM
lana,

Are the evaporators defrosting at different times? Do the evaporators have back-pressure regulating valves? How do the evaporators defrost?

If the evaporators are connected to a common suction line, then obviously the suction pressure is relatively constant. I am assuming the suction pressure is at the desired pressure?

If this is the case, the something is happening to the system upstream of the evaporators. TXV's, liquid line piping, flash gas, no subcooling,or ... something?

Can you provide a piping and layout sketch to help you?
Thanks for your reply.
The defrost is electrical and there is no back-pressure regulating valve. The design suction pressure is 0 Psig (-42°C for R22) but now we have 5 Psig. All three TEVs have MOP -20°C. There are sight glasses before each TEV and all are full of liquid. The subcooling is high because there is a plate-heat exchanger as the sub-cooler.
One TEV opens and closes regularly and the outlet temperature for that evap. moves from -28 to -32°C. But other two evaporator's outlet temps stay at -20°C and only go down when the TEV adjusting screw is closed. Then the temp goes to say -28°C and stays at that value. But as I said one of them is opening and closing. The superheat for that evaporator is 0°C and the other two is +20°C!!!
What I think we should try is to adjust each TEV for say 6°C superheat and then see what will happen.
Thanks for any ideas.
Cheers.:)

Dan
10-12-2005, 02:18 AM
Lana, take a close look at the suction piping and TEV bulb locations.


But as I said one of them is opening and closing. The superheat for that evaporator is 0°C and the other two is +20°C!!!

If the first evaporator is operating at a very low superheat, then is it possible that the downstream expansion valves are sensing this low temperature and throttling?

US Iceman
10-12-2005, 04:25 AM
lana,

A piping sketch would really help. My instinct says the problem is piping related. Marc O'Brien asked this also.

Since you have a common suction line, the evaporator pressures should be relatively equal (depending on the TXV MOP operation of each valve).

If the piping is not exactly correct, liquid from one evaporator can run into the outlet of another coil, causing the TXV to close (liquid present at the TXV bulb). This same problem can also be caused by a single suction riser connecting all three evaporators, especially when the coils are stacked on top of each other.

Can you prepare a piping sketch and scan it into a PDF?

US Iceman
11-12-2005, 02:07 AM
Oil too, the evap nearest the compressor (downstream evaps) can collect oil from upstream evaps. When they're stacked then usually the bottom evap.

That too. I forgot about the oil when I wrote this. Thanks, Marc.

The forum has seen some interesting threads in the last few days that come down to system piping & installation techniques.

Most of the problems are dependent on the installation (if the equipment has been correctly selected)...

lana
12-12-2005, 08:47 AM
Hi everybody,
Thanks for your contribution.

I attached a piping system for you. Please have a look and see if you can help.
Cheers:)

Renato RR
12-12-2005, 11:25 AM
The equallizer of all 3 TEV sense that the pressure after evaporator is low and actuate the closing of TEV acording to spring create pressure + equallizer pressur.The superheat on first evaporator must be increased from 0 to 6 C and then see what hapends.The TEV with MOP have vapor in thermostatic element and if sensing bulb becomes wormer then TEV the migration of vapor is shure.Try to vorm TEV with lighter.
The rest 2 evaporators must reduce superheat from 20 to 6 C.

Renato

lana
12-12-2005, 01:42 PM
The TEV with MOP have vapor in thermostatic element and if sensing bulb becomes wormer then TEV the migration of vapor is shure.Try to vorm TEV with lighter.

There are heating elements on top of the TEVs.
Cheers:)

US Iceman
13-12-2005, 01:20 AM
Hi lana,

Thanks for the piping diagram. Do you also have the TXV model numbers and manufacturer?

In your diagram you said the suction line (after all three evaporators) went to the ceiling. How much height difference is this?

Then the suction line goes down to the compressor.

If I am understanding your comments properly, the suction line has a trap built-in. The height of the trap appears to be the evaporator height plus some additional elevation, before the suction line goes down to the compressor.

Are the individual suction lines coming from the evaporators free draining as you show it, or, are they on the same elevation?

Maybe liquid from one evaporator is flowing backwards into the other suction lines (or building up to the bulb location), causing those TXV's to close down and cause the higher superheat readings you are seeing.

An isometric drawing would show the relevant elevation changes between the components, but based on your diagram and explanations, my comments above are my best guess right now.

TXiceman
13-12-2005, 04:16 AM
You need to have evaporator pressure regulators on the two higher temp evaporators and let the compressor runat the lowest suction pressure.

If you are having trouble with your TXV control, is th eTXV sized considering the high degree of liquid subcooling you are running. A TXV operating with 90 dF liquid is completely different from a valve running with liquid subcooler to 40 or 50 dF liquid. The valve is oversized when you subcool so much.

Another issue is the all of the suctions are properly trapped before the return to the suction maim and should alsl be reverse trapped at the main, to prevent liquid and/or oil from running back on one evaporator due to location of being inactive.

Ken

lana
13-12-2005, 07:26 AM
Hi ,

Dear USIceman , the suction line after the trap goes up 2m and then goes horizontally and then down to the compressor. The TEVs are Danfoss TEX2-1.2 with 04 oriffice (Range B) MOP-20°C.
As TXiceman mentioned this TEV gives more capacity with high degree of subcooling. Now I am changing the oriffice and see what happens.
I tried to adjust the individual TEV for 6°C superheat and situation got a little bit better but still I have high suction pressure. I will let you know the results.
Thanks guys for your input.
Cheers:)

lana
11-01-2006, 08:34 AM
Hi everybody,
Happy new year.

I have changed the suction piping in the system and also the TEV nozzel. Everything became OK and the room temperature reached -38°C. Thanks for your help guys.

Cheers.:)

Dan
12-01-2006, 01:45 AM
Lana, please tell us how you changed the piping, and what your theory is regarding your problems.:)

lana
13-01-2006, 10:52 AM
Lana, please tell us how you changed the piping, and what your theory is regarding your problems.:)

Hi dan,

The three evaporator outlet pipes were connected to the common suction pipe incorrectly. Therefore, when adjusting one TEV it would effect the other two. What I did is , I changed the connection of three suction pipes from the evaporators to the common suction pipe by adding three inverted traps at the top of the common suction pipe. Secondly, I changed the TEV nozzel to a smaller one because with the sub-cooler opperating there is a large amount of sub-cooling available therefore, a smaller nozzle will give the same capacity according to the Danfoss catalougue. There were heating elements on the top of three TEVs with MOP which stopped the charge migration. So everything is fine now and I got -38°C in the room without any problem.
Cheers:)