Condenser Check Valve and Elevation - Reason, necessity

[limerick14]

I am working on a standard propane refrigeration system consisting of a screw compressor, horizontal 2 pass condenser, receiver and a chiller. The condenser is not pressure or temperature-controlled and floats with the ambient. There is only a manual louvre control.
My question is about the check valve typically installed downstream of the condenser. It seems common practice to install a check valve after the condenser. The reason for that is to prevent hot gas from entering the receiver through the hot gas condenser bypass going backwards into the condenser. For the longest time, we got told to overcome the cracking pressure of the check valve, a specific elevation upstream of the check valve must be maintained. The common practice is 5-6 ft. This is supposed to provide sufficient head to crack the check open and keep it open.
So we now build a unit on which we cannot maintain even one-foot elevation. We debated what would happen. There is a different opinion and non of them seems consistent or coherent to me. It seems everyone knows something from someone, but nobody understands the whole picture, including me.
Some say the liquid propane will pile back up into the condenser. This will, in return, reduce condensing area, which will increase discharge pressure, which will push the liquid out. Then the cycle starts again. They call it surging.
Others say the compressor will always provide a driving force to crack the check valve open, and no liquid will pile up.
Anyway, I am interested in anyone's experience in that matter. Thanks to all in advance.

[Segei]

It would be good to have some drawings. As far as I know typical refrigeration plant has check valves on discharge of each compressor to prevent back flow when compressor stop. It's not clear for me why you need check valve between condenser and receiver.

[RANGER1]

Page 8 Danfoss NRD & KVR seems more set up for that application, but many ways to do things.

https://assets.danfoss.com/documents...8en-000101.pdf

[Peter_1]

It would be good to have some drawings. As far as I know typical refrigeration plant has check valves on discharge of each compressor to prevent back flow when compressor stop. It's not clear for me why you need check valve between condenser and receiver.

I had same remark, I don't see the need for a check valve ...and also he bypass is not clear for me ...indeed ...a schematic would clarify more

[limerick14]

Ok, I sketched the system up. It’s really a standard industrial refrigeration system. Nothing special about it. I may explain the Hot Gas Condenser Bypass (HGBP).
The HGBP only operates on startup. Imagine it's cold outside. Let’s say -20C. The receiver will be very cold and the pressure in the receiver will be very low. Hence there may not be enough pressure to drive the liquid propane to the chiller. On startup hot gas coming straight from discharge is introduced into the receiver to raise the pressure and provide this driving force. Without the HGBP it would impossible to start the system.
The check valve downstream of the condenser prevents the hot gas from flowing back into the condenser. It is this check valve and the pipe from the condenser to the receiver what I want to discuss, not the compressor discharge check valve.
So the common industry practise is to provide 5-6 feet elevation between the condenser and the check valve. The reason is the check valve has a cracking pressure of about 1PSI. Hence a static liquid head on the check valve is required to crack it open.
What I am interested in is knowing if anyone has experience in operating a similar unit but with less than 5’ or no elevation at all.

[limerick14]

Ok, I sketched the system up. It’s really a standard industrial refrigeration system. Nothing special about it. I may explain the Hot Gas Condenser Bypass (HGBP).
The HGBP only operates on startup. Imagine its cold outside. Let’s say -20C. The receiver will be very cold and the pressure in the receiver will be very low. Hence there may not be enough pressure to drive the liquid propane to the chiller. On startup hot gas coming straight from discharge is introduced into the receiver to raise the pressure and provide this driving force. Without the HGBP it would impossible to start the system.
The check valve downstream of the condenser prevents the hot gas from flowing back into the condenser. It is this check valve and the pipe from the condenser to the receiver what I want to discuss, not the compressor discharge check valve.
So the common industry practice is to provide 5-6 feet elevation between the condenser and the check valve. The reason is the check valve has a cracking pressure of about 1PSI. Hence a static liquid head on the check valve is required to crack it open.
What I am interested in to know if anyone has experience in operating a similar unit but with less than 5’ or nor elevation at all.

*UPDATE* I cant upload a picture here. Wont let me.

[Segei]

You can replace check valve by solenoid.

[limerick14]

Segei, This is an industrial application, not a refrigerator. There is 100GPM liquid propane going through these lines. I don't want to replace the check. Don't get me wrong I don't want to discuss changes to my process. I try to find someone who has experience with running a similar system with no or little elevation between condenser and receiver. I am interested in how a system would run under these conditions. I understand I am asking a very specific question, which not many people have experience with. I really don't want to discuss what eventually will happen, or what could happen. I did this already extensively with many people. I need someone who knows what I am talking about and has seen it happen. I need firsthand experience, not speculations. Sorry did not want to sound rude or so.

[RANGER1]

Limerick14,
The link I posted explains commercial way of doing it on smaller scale.
Danfoss KVR & KVD, read it to see what is done, do not dismiss.

https://assets.danfoss.com/documents...8en-000133.pdf

In your case, but without proven in field experience it will work if you close bypass once running.
1psi pressure drop between condenser/liquid receiver, then liquid will flow no problem.
If bypass stays fully open liquid will bank up if depending on bypass line size & capacity.

There is probably one specific person on this forum who will more than likely know as worked on these types of plants.

Alternative is a Process engineering or Industrial refrigeration company, or website, where you will get the information in a complex way.

[Segei]

Ranger1.
I think that mentioned plant is different then Danfoss design. Typically in industrial refrigeration liquid refrigerant drain to the receiver by gravity. For that condenser and receiver should have equalizing line. I assume that this plant has air condenser. To open check valve they need 5-6 feet of liquid head. If liquid head is not sufficient, liquid will collect in condenser until check valve open. However, condenser can be 90% full of liquid and in summer time head pressure can be problem. We saw it many times in evaporative condensers when liquid traps were not designed properly and part of the condenser was flooded.

[limerick14]

[QUOTE=Segei;331833]Ranger1.

Thanks, I see you understand what I am asking. We don't equalize pressures as this is not necessary on a single-section condenser. Anyway, you are right the consequence of too little elevation is likely backing up liquid in the condenser. This is what most people say will happen or has happened in the past. Then the explanation goes like this. When the condenser fills with liquid, the area decreases, the discharge pressure increases and the propane will be pushed out. Then the cycle starts again. This is what some people call surging. What I am not necessarily are convinced about it is the fact the liquid will back up into the condenser only because the liquid head is needed to push the check open. The compressor always provides a driving force and pushes open whatever check is in the flow. The argument against this is typical that the vapour in the condenser collapses into liquid and therefore the pressure decreases. This can't be right because the receiver is at the same pressure the condenser is, minus small line losses. There is no such thing as no pressure because vapour collapses. Anyway, my question is; why would the compressor not push open the check before the liquid reaches the condenser?

[mbc]

hi
if your are going to use check valve you should put equlizing pipe between condenser and reciver with REG valve to control flow rate .
this pipe provent reciver from damaging when it is full of liquid
and temperature in the ambient is changing and we have expanding liquid.
we keep liquid in condenser in winter time our during defrosting time . liquid reduce our exchanging area in condenser and it give us more pressure in condenser when we need .

if you do not want to use equlizing pipe
the valium of reciver should be big from your all refrigerat in the system.

[knighty]

email the schematic to me and I'll upload it here for you?

[RANGER1]

Ranger1.
I think that mentioned plant is different then Danfoss design. Typically in industrial refrigeration liquid refrigerant drain to the receiver by gravity. For that condenser and receiver should have equalizing line. I assume that this plant has air condenser. To open check valve they need 5-6 feet of liquid head. If liquid head is not sufficient, liquid will collect in condenser until check valve open. However, condenser can be 90% full of liquid and in summer time head pressure can be problem. We saw it many times in evaporative condensers when liquid traps were not designed properly and part of the condenser was flooded.

Segei, was just trying to show that it is common.
The difference is no NRD differential valve in Danfoss talk.
Even if bypass open on start up, condenser is going to condense liquid to some extent until things get going.
The Danfoss KVR would be the same as a ICS with CVP to my understanding.
Have done it with 2 air cooled condensers in parallel, no balance line, need more refrigerant for it to have condenser filled & still have liquid receiver level.
In this case hot gas, or balance line would have to be regulated, or adjusted after start up.
If check valve in horizontal spring could be lighter so does not have to overcome gravity or it can be piped that way, just a suggestion.

AI251086497566en-000808.pdf (danfoss.com)

[RANGER1]

[QUOTE=limerick14;331834]

Ranger1.

Thanks, I see you understand what I am asking. We don't equalize pressures as this is not necessary on a single-section condenser. Anyway, you are right the consequence of too little elevation is likely backing up liquid in the condenser. This is what most people say will happen or has happened in the past. Then the explanation goes like this. When the condenser fills with liquid, the area decreases, the discharge pressure increases and the propane will be pushed out. Then the cycle starts again. This is what some people call surging. What I am not necessarily are convinced about it is the fact the liquid will back up into the condenser only because the liquid head is needed to push the check open. The compressor always provides a driving force and pushes open whatever check is in the flow. The argument against this is typical that the vapour in the condenser collapses into liquid and therefore the pressure decreases. This can't be right because the receiver is at the same pressure the condenser is, minus small line losses. There is no such thing as no pressure because vapour collapses. Anyway, my question is; why would the compressor not push open the check before the liquid reaches the condenser?

I think if low flow rate check valve could surge until compressor increases capacity.
Could you fit check valve with horizontal flow with lighter spring to counteract lower head.
Do you control hot gas bypass somehow, or is it manual?

[limerick14]

[QUOTE=RANGER1;331838][QUOTE=limerick14;331834]

I wish I could attach something to this thread but it won't let me.
About the check. Yes, sometimes the spring is removed or weakened by heating it up with a blowtorch.
The HGBP is just a regulator. It works like this. The backpressure control valve (BPCV) on the compressor is let's say set at 150 PSIG. This means until 150PSIG the BPCV is closed. This is because we need at least 150PSIG to get the compressor oil moving through the system. When the Compressor starts, this is typically achieved within seconds. Once we hit 150PSIG the valve goes quickly to full open. However, the volume downstream of the BPCV is much larger and requires time to be pressurized. As we also need to get the suction side of the compressor going we need to get propane to go to the chiller but without pressure, it can't go. Hence the HGBP opens. The HGBP regulator or control valve is set around 25 PSI lower than BPCV set point is. This makes gas go on startup straight into the receiver but closes the HGBP once 150PSIG is achieved.

[frank]

I wish I could attach something to this thread but it won't let me.

You need 14 posts before you can upload attachments

[RANGER1]

Limerick14,
If you wanted control of liquid drain, a valve like this can be used, you then know exactly where valve is, no pressure drop, ICS with ICM motorized control valve.

https://assets.danfoss.com/documents...7en-000501.pdf

Other option is Hansen or Parker gas powered valve type
https://static1.squarespace.com/stat...2272/C457b.pdf

In process plant a Fisher or similar control valve could be used.

I know this is not your question, but many ways to do things.

Depending on type of plant, always difficult to change things, or modify plant from original design.

Obviously check valve is the simplest & cheapest thing to do.