Control system of chiller with propane as cooling medium

[Dendrobium1975]

Dear members,

We operate a Propane Refrigerant Unit to provide chilling requirement for some chillers with various purposes. In those chillers (kettle type), shell side is continuously filled with propane in a certain level while the process fluid flows within the tube.
There are two different control systems applied to those chillers.

1.) For a chiller which is used to cool an MDEA-water mix from 43 deg C to 32 deg C, the control system allows the process fluid outlet temperature to be controlled by adjusting the backpressure of propane (by throttling a control valve in the outlet of the shell side/propane side) while the shell side level is controlled by adjusting the propane supply (by throttling a control valve in the inlet of the shell side/propane side).

2.) For a chiller which is used to cool light hydrocarbon (C1-C6) from 11 deg C to -23/-31 deg C, the only control system applied is the shell side/propane side level control by adjusting the propane supply (by throttling a control valve in the inlet of the shell side/propane side). For this chiller, the process side outlet temperature is not controlled but just monitored by a temperature indicator.

I’m still wondering why different control should be applied for those two chillers. Any helpful answer will be very appreciated. Thanks in advance.

 

[BenThayer]

you might have differences due to process limits and not wanting to freeze the water mixture.

it sounds like the light hydrocarbons stream is being cooled to something close to the saturated temperature of the propane boiling at that compressor's suction temperature.

basically, the approach is limiting the temperature of the light hydrocarbons. so all you need to do is dump make-up into and no further throttling is needed.

 

[dcasto]

The MDEA solution freezes to a solid at -15 to -25C. By keeping the back pressure on the refrigerant, you control the lowest temperature you can cool the MDEA to and not freeze it.

Does the propane vapors from the MDEA chiller go back to an interstage pressure on the refrigerant compressor? That would save a bundle of energy.

 

[Dendrobium1975]

BenThayer and dCasto,

Thanks for the enlightenment!

@ dcasto: It's correct. The propane vapor from the MDEA cooler then goes to an Economizer (a vertical vessel) where the vapor from its top then routed to the Refrigerant Compressor Interstage