11 replies to this topic

[vds002]

I was wondering if anyone at this forum has ever came across a heat exchanger which uses refrigerant as a cooling medium.
I had a few question about how to control the cooling.
1. Do I need to measure the refrigerant return temperature and control the refrigerant inlet valve?
2. Do I need to measure the heat exchanger process side outlet temperature and control the refrigerant inlet valve?
3. Do I need to cascade the refrigerant inlet flow with temperature measurement.
4. Do I need to cascade the refrigerant inlet pressure with temperate measurement.

[fatimah]

hi vds002

i did design before for HE with refrigerant as the cooling medium. well, for two cooling & heating mediums; you may require

a- the flow rate
b- temp inlet & outlet
c- inlet pressure & pressure drop

you adjust the inlet valve to control the flow rate. refrigerant outlet temp is controlled by the inlet temp of the heating medium. in my opinion, controlling the valve will affect the flow rate and nothing to do with temp control. See if anybody here master in process control can elaborate further.

Thanks.

[Art Montemayor]

VDS:

I have written numerous posts on this subject, complete with detailed schematic flow diagrams as well as heat & mass balances. At present I am not free to take the time to do that all over again, so I suggest that you employ our Forum SEARCH machine to find that information in our Forums. The attachments that I have posted for download are always in an Excel Workbook.

When you discuss refrigeration systems, please be specific. There are various type of refrigeration systems and we are merely wasting our time if we can’t guess what system you mean or have. If you are not experienced or know nothing of refrigeration systems, then tell us. We cannot train you in refrigeration; we can only help you by discussing your specific problem. We cannot solve education problems – for that you have universities and thermodynamic courses where the subject is covered.

To answer your specific questions:

• No, you do not measure the refrigerant return temperature. You “control” the refrigerant inlet valve (otherwise known as the refrigerant expansion valve) by stroking or adjusting it to suit the refrigerant’s liquid level in the evaporator. Here, I am assuming that you are dealing with a submerged evaporator and not a “direct expansion” type of system.
• No, you do not “need to measure the heat exchanger process side outlet temperature and control the refrigerant inlet valve”. See #1, above.
• I do not understand what is meant by “need to cascade the refrigerant inlet flow with temperature measurement”. Please supply a schematic flow diagram of what you propose or what you are confronting.
• The response is the same as for #3.

We can address any and all specific engineering questions if you supply all the required basic data and a complete description of your application. By compete description, this includes an engineering sketch or schematic flow diagram.

I hope this helps and await your response.

[vds002]

Art,

I appreciate you taking time to response with such a long reply. Even I dont like spoon feeding so I did search the forum before posting the question. My question is not related to process design or equipment design. I am trying to get some inputs on process control. The heat exchanger has refrigerant on the coolant side trying to cool chlorosilane on the process side. Refrigerant in -45 C out -40 C. Process in -15 C out -20 C.

I am trying to get an idea of what should be my measured variable and what should be my controlled variable. Also will simple feedback look work or do i need cascade control.

Appreciate any help....
-vds

[riven]

Control your refrigerant flow rate depending on process temperature.

The only thing that measuring your return line temperature will tell you is how well your chiller is coping and if you can increase efficiency i.e. installing a higher COP machine and ambient heat losses.

[Art Montemayor]

VDS:

I'm trying to help you, but you are not reading through on my post OR you don't understand what refrigeration terms I am using. That is why I am stressing that you MUST understand the basics of refrigeration. A refrigerant (ammonia, freon, ethylene, etc. etc.) cools by being converted into a cold, saturated LIQUID + GAS mixture. It is primarily the liquid that does the cooling by giving up its latent heat. Read my response in detail. Riven is not correct in that the refrigerant flow rate is controlled by the process temperature - unless we are talking about an intermediate cooling medium. A refrigerant (ammonia, freon, ethylene, etc. etc.) is controlled normally as I described: by the evaporator's liquid level.

Are we discussing a refrigeration system product or are you talking about a "generic" refrigerant - such as cooling water or supercooled glycol solution? Please be specific with your data and information because this thread will become a lengthy one that doesn't accomplish what you need or gives you help you seek. You have not answered my questions - what type of process are you talking about. Please furnish a detailed schematic flow diagram. I don't believe that is too much to ask for.

[vds002]

Its a shell and tube partial condensor...
The heat exchanger has refrigerant on the coolant side trying to cool chlorosilane on the process side. Refrigerant in -45 C out -40 C. Process in -15 C out -20 C...
The refrigerant is freon 410A .............
The refrigerant liquid is coming from header and expansion occurs at the TCV-60701 control valve located on the inlet of the heat exchanger..
The plant has refrigeration system (entire system) that supplies liquid refrigerant to different shell and tube heat exchangers....
I am controlling the TCV opening based on process outlet temperature...
do i need to use the refrigerant pressure in cascade loop...

i have attached the figure...let me know if u cannot open it....

Attached Files

•  REFRIGERATION QUESTION.JPG   58.51KB   64 downloads

Edited by vds002, 03 February 2010 - 06:55 PM.

[riven]

I do not see why any more information is required from the control scheme provided. Pressure may be used as an indication of fouling on the refrigerant side but I do not expect that it is needed as long as common sense with this regard is used.


Art
As I suspected vds002 is not talking about the refrigeration machine but about a refrigeration header supplying a cold service fluid to a heat exchanger. Vds002 is not interested in controlling the refrigeration machine itself; i.e., he was not referring to the exchanger inside the refrigeration loop; something that was not clear initially. We can essentially forget about the refrigeration device. I assume this is what you are referring to?

Vds002
Question: why did you not post the picture in the first place and base your question on that? Not being clear has lead to two correct responses to your initial question. Of course many of us have seen refrigeration exchangers both as described in your drawing and in a refrigeration loop. Always post as much information as you possibly can. I would suggest always posting a P&ID and then explaining the control scheme and the situation around it that you wish to ask questions about. Even when talking about equipment to people as familiar with the equipment as I am, there is always the requirement for visualization/P&ID.

[vds002]

Riven,
Are you saying Flow-Temperature cascade is sufficient or should i just use temperature to control the expansion valve or should I use a pressure-temperature cascade to control the expansion valve

I discussed this issue with one of our engineers and she said I dont need a flow-temperature cascade. A simple temperature feedback loop should be sufficient to control the expansion valve.

I talked to another process engineer and he said I need to measure the refrigerant return pressure and cascade it with temperature to control the expansion valve.

-vds

Edited by vds002, 05 February 2010 - 04:23 PM.

[joerd]

VDS,

Please do the following: read about Vapor compression refrigeration cycles in Perry, Wikipedia, Citizendium, or similar. Then re-read all of Art's responses in this thread.
Look at your control scheme and ask yourself:
- what is the phase of the refrigerant after it exits the exchanger: vapor, liquid, or both. What should it be?
- does your current control scheme guarantee the desired fluid phase?
- how do you control the pressure (and hence the temperature) of the refrigerant?
- what happens if you don't get the desired process temperature? what if it is too high? or too low? How does your control scheme react? Does any of the answers above change during these variations?

Typically, like Art said, the letdown valve is controlled by the level in the exchanger. The exchanger duty can be varied by either changing the level setpoint (submerge more/less tubes) which is rather non-linear and indirect, or by changing the pressure in the exchanger (i.e. the refrigerant temperature) so that it "pinches out."

Oops, I think I just gave myself away. Must be Friday.

[Zauberberg]

Depending on the heat exchanger (refrigerant evaporator) design, there is an optimum level - in terms of % of maximum liquid level - that results in maximum heat transfer rates. You want to maintain that particular level. Higher levels can put refrigerant compressor in danger (= liquid carryover), while lower levels will result in smaller heat transfer rates and therefore smaller duties, because of smaller heat transfer coefficients when heat (cold) is transferred from refrigerant vapors instead of being transferred from pool of boiling refrigerant.

If we are speaking about conventional refrigerant exchanger, it is controlled exactly in the way as Art explained: by the refrigerant liquid level control valve at the inlet of evaporator. That way, you always maintain the refrigerant level inside the exchanger: higher flows of hot fluid will ask for mor refrigerant flow, and vice versa.

If you need to control the outlet temperature of hot process stream, that is done by controlling refrigerant boiling/evaporation pressure - if we are speaking about pure refrigerants.

Best regards

[Art Montemayor]

VDS:

I have taken a little longer to answer your second post because I was put on a very hot issue that had to be resolved quickly on a project I am working on. I worked over the weekend on what I wanted to include in this, I hope, my final response.

Some knowledgeable and experienced people - like Riven, Joerd, & Zauber have already contributed to this thread in a positive manner and perhaps you don't need my input anymore. However, I want to express and point out that what you depict in your basic data reveals that you have a very complex heat transfer operation. Whether this operation is being proposed at the design level or if it is an actual, operating one is an important question here that you have not identified. Please refer to the attached Excel Workbook and to the detailed comments I have included there. I have tried to identify and understand what you are confronting and I believe I have interpreted it correctly - in accordance with the basic data you furnished.

Please bear in mind that what you are facing is a "DX" type of refrigerant application. It is not a typical submerged, pool-boiling evaporator type of design. You have a situation where you are attempting to partially condense a saturated(?) process vapor into a supercooled liquid(?) and a saturated vapor using a 2-phase refrigerant flow that is converted into a superheated refrigerant vapor. This is a very difficult and expensive operation to carry out in one, 2-pass tube side, BEM TEMA type of heat exchanger.

Please read my comments in the workbook to understand the details of why the operation is a difficult one.

I hope this helps you out.

Attached Files

•  Partial Chlorosilane Condenser.xls   367KB   83 downloads