Pressure Control in two-stage refrigeration system

[Rubje]

I have a system similar to the one in the image above. It's a two stage mechanical refrigeration unit with a flash economizer, using propane as a refrigerant to cool a process stream. The compressor is a screw compressor, so positive displacement.

If I'm understanding this system correctly, the expansion valve at the chiller inlet would only change the vapor-liquid ratio in the chiller and 1st stage compressor section. As long as the compressor is capable of dealing with the amount of vapor that is formed in that particular section, the pressure should stay within normal operating range. Am I understanding correctly?

 

[georgeverghese]

The compressor operates to maintain a constant suction pressure at stage 1 inlet. If suction pressure rises, the suction PIC would operate the slide valve (or VFD whichever is applicable in your case) to increase compression capacity. Compression capacity is high limited to the rated throughput the compressor at this suction pressure. You could get some increase in mass throughput if you increases the suction PIC setpoint, up to some thermal limit of this chiller, and also high limited by the the power available at compressor driver. In this example, the cold end DT at the chiller is 5degC, and it may work just as well at say 4degC DT (??). You can also increase throughput if there is some extra cooling capacity at the stage 2D fin fan condensor, since this will in turn result in less flash gas generation at the economiser flash tank which floats on stage 2S.

 

[Rubje]

"You can also increase throughput if there is some extra cooling capacity at the stage 2D fin fan condensor, since this will in turn result in less flash gas generation at the economiser flash tank which floats on stage 2S."

But the 2nd stage discharge pressure would still be limited by whatever the accumulator temperature since it can't exceed the vapor pressure corresponding to that temperature?

 

[georgeverghese]

Yes, that is correct. If you do have additional cooling capacity at stage 2D condensor, stage 2D temp would drop, and so will stage 2D pressure and condensor pressure.

 

[Rubje]

Thank you for the response.

So hypothetically, what would happen if the outlet pressure from stage 2 is pushed based the vapor pressure? Would the vapor simply condense? Would the outlet temperature from the condenser simply increase to match the new discharge pressure?

I imagine it's hard to fully answer this question without knowing the specific control logic of the system.

 

[georgeverghese]

Its the converse. Stage 2D pressure free floats on the condensor discharge temp. If the cooling capacity of the the condensor increases, condensor temp drops, and stage 2D drops to match the corresponding vapor pressure.

 

[Rubje]

That makes sense. thanks

Do you know where i can find an SOP for an industrial refrigeration process? I found documents online that describe the system but I really want to learn more about the controls and how engineers change the various process variables.