expansion valve

[mrahman01]

hi all friends, i am new and first time posting this. we are doing ammonia based refrigeration systems and manufacturing package type chillers to chill glycol water using plate type heat exchanger with phase separation vessel at top. We have installed about 25 such chillers. we are facing temperature problem with some of the chillers. we are not installing expansion valve in liquid line from separaion vessel to PHE. Should we install an expansion before PHE?

[PaulZ]

Hi mrahman01
You don't need an expansion valve before the PHE it's best to have as little restriction in this line as possible. I presume when you say temperature problems you are referring to the temp drop through the PHE. If so I would suspect there is oil in the PHE. It takes very little oil to affect the performance of PHE's.
Let us know how you go.
Paul

[mrahman01]

Thanks for reply. Usually we do not install expansion valve at this position and it is free fall easy flow of NH3 from separation vessel to PHE.

When i say temperature problem, it means the desire temperature of glycol water. I remember three cases where we face such problem. At one plant where the system was design to acheive 2 deg C chilled water temp, at - 5 deg C evaporation temp. We never able to achieve that and got 5-6 deg C after all the inspection and setting of PM valve etc. Then we added more plates in the phe, but cannot get better result. Only minor difference. After studying different drawings at intenet we found that we should do an experiment by installing an expansion valve as usually there is an expansion valve before evaporator. it was 100 mm and we got the result. Now I am designing two package chillers and I was making these expansion valves as part the design. Only thought behind this is the pre-evaporation before entering the PHE and then full evaporation in PHE to get max. cooling effect.

[PaulZ]

Hi mrahman01
We have installed many PHE's for glycol/water, alcohol/water and water cooling and have never had an expansion valve fitted.
These PHE's were flooded heat exchangers and rely on natural convection through the PHE. Generally we would get 4-5 oC drop across the PHE with a liquid temp of between 0 and -20 depending on the concentration of the brine solution and the application.
I think we may be talking about different designs.
Where we have installed an expansion valve before a PHE was on a pumped liquid system.
If you are not getting the required temp drop across the PHE the pump flow rate could be too high.
Is the PHE piped in parallel flow or counter flow because this can also affect the performance of the PHE.
Paul

[mrahman01]

It is parallel flow design PHE and flow is by gravity. NH3 at one side and glycolwater at other side