Ammonia - Heat Exchanger in Series to Evaporative Condensers

[akh]

Dear all,

Thank you for your time in reading and replying. I am a mechanical engineer and I am confused about heat recovery through ammonia condensation.

Ideally I would like to have a vapour to water heat exchanger where the ammonia will condense and reject its heat to a district heating loop using water as a medium.

However, I would like to know what are the complications of having let's say a 2MW heat exchanger pre-condensing ammonia and rejecting heat to water before entering the evaporative condensers in the form of liquid and gas. The heat exchanger will be connected in series to the evaporative condensers.

The evaporative condensers are in parallel and have a total capacity of 13MW, would it be better to have a liquid/vapour separator before entering the evaporative condensers or is it acceptable to send liquid and vapour to the evaporative condensers.

By all means, at one point a specialist industrial refrigeration engineer will be brought on board, my involvement is only on the district heating side. However, I am bit concerned on the implications to the refrigeration system.

Thanks in advance...

[Segei]

If you are going to have vapor and liquid after heat exchanger, put it in parallel with evaporative condensers. It is bad idea put this heat exchanger in series with evaporative condensers.

[mad fridgie]

If you work for, who I believe. then i have simple question before i can answer yours.

What is your return temperature for the central district heating loop?

[Magoo]

You can de-super heat the discharge gas from ammonia but need to know the rest of district heat system as per MF reply.

[RANGER1]

Have seen it done in parallel & gas in liquid drained straight back to liquid receiver.
Heat exchanger has to mounted above liquid receiver so as to allow for gravity feed.
If in series expect pressure drop which you pay a penalty for in power, so defeats purpose.

Best option is to use screw compressor oil coolers, assuming you have screw compressors with oil coolers of some description.

[akh]

Hi all,

Thank you for your replies, you all touched on the same issues I had in mind.

The original concept is to have it in parallel, but there are times where the heat demand would be low, therefore not all of the ammonia would condense to liquid so I was wondering what would happen then, if ammonia went as liquid/gas into the high pressure receiver since it fails to fully condense during low heat demands.

Furthermore in the parallel configuration, is it possible to modulate the flow of superheated ammonia gas going to the heat exchanger during low heat demand ? Do such valves exist for superheated ammonia ? I couldn't find any.

I personally worked on commercial concepts where cooling towers were used and basically had to deal with water to water heat exchangers and everything being more simplistic and within my comfort zone.

At one point the refrigeration experts will be on board and they will be calling all the shots, I am just dealing with the water side of things.

As for the return temperature, this will be a private loop not connected to other schemes,so we will take any temperature we can get off the ammonia/water heat exchanger.

Looking forward to your replies...

[Segei]

Actually, only liquid goes to high pressure receiver. When heat demand is low, more liquid will come from the condensers and less liquid will come from heat exchanger.

[akh]

Actually, only liquid goes to high pressure receiver. When heat demand is low, more liquid will come from the condensers and less liquid will come from heat exchanger.

Hi Sergei,

Would you then have some modulating valve to the gas header distributing to the evaporative condensers and the heat exchangers or how does it work ? or is there some form or trapping after the evaporative condensers to which the heat exchanger would be connected in order to ensure that only liquid goes to the high pressure receiver ?

Thanks...

[mad fridgie]

Sorry my engineering must be poor, I am sure that a private loop, is not a scale rating in temperature.
What is the return loop temperature. This is the primary design criteria.

[Segei]

Hi Sergei,

Would you then have some modulating valve to the gas header distributing to the evaporative condensers and the heat exchangers or how does it work ? or is there some form or trapping after the evaporative condensers to which the heat exchanger would be connected in order to ensure that only liquid goes to the high pressure receiver ?

Thanks...

Hi, Akh.
No modulating valves. Heat exchanger and evaporative condensers should have liquid traps. It is critically important to choose the right size of these traps to provide free draining of the liquid refrigerant.

[akh]

Sorry my engineering must be poor, I am sure that a private loop, is not a scale rating in temperature.
What is the return loop temperature. This is the primary design criteria.

Sorry, all I meant there is no existing district heating scheme at the moment, so we would be happy with any return temperature imposed on us by the heat exchanger supplier and the industrial refrigeration specialists. So at the moment I don't know, to be honest i am just asking things for my own understanding, however in reality design conditions would be supplied at one point.

But the ammonia condensing temperature is 31 deg Celsius so if we assume a 5 degrees approach I would say 26 maybe, I have no idea what is the LMTD for an ammonia to water heat exchanger.

Does this help in any way ?

[Segei]

Probably, heat recovery will improve energy efficiency of the refrigeration plant by 1-2%. However, lowering condensing temperature to 20 - 25 degC can improve energy efficiency by 10 - 20%. Sometimes condensing temperature can go below 20 degC. Why to focus on minor issues?

[RANGER1]

Probably, heat recovery will improve energy efficiency of the refrigeration plant by 1-2%. However, lowering condensing temperature to 20 - 25 degC can improve energy efficiency by 10 - 20%. Sometimes condensing temperature can go below 20 degC. Why to focus on minor issues?

Totally agree 100%
Stick to basics as Segei suggests.

Would like to know what type of compressors are proposed, or likely to be proposed?