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Negative Suction Pressure Of Propane Refrigeration Compressor

Started by Guest_Giman_*, Jan 14 2013 12:04 AM
propane refrigeration

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#1 Guest_Giman_*

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Posted 14 January 2013 - 12:04 AM

Hi Guys

I'm working on a C3 refrigeration loop design.
Since process side of gas chiller shall be cooled upto -35 degC, propane is selected as a chilling medium.
Saturated liquid propane @ 40 degC is letdown to near atmospheric pressure, so that temperature is dropped to about -40 degC. This mixed-phased low pressure propane will be used as a chilling medium to acheive -35 degC of process side cooling.
I guess everyone here well understands above typical description of propane refrigeration chiller.
Now, here is my question.
Why propane refrigerant does not operate below atmospheric pressure?
If propane is letdown to negative pressure, it can be acheived even lower temperature than -40 degC.

In GPSA section 14 fig 14-3, the figure shows compressor suction pressure is 98kPa(a), that is negative pressure.
If it is ok with negative pressure at compressor suction, why propane chiller does not operate at lower temperature than -40 degC?

I'm just curious.

#2 latexman

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Posted 14 January 2013 - 08:12 AM

Propane is not operated at negative suction pressure, because IF there was a leak, oxygen (air) will leak in; it will accumulate until it reaches the MOC (minimum oxygen concentration - to sustain combustion), and then IF the process exceeds the autoignition temperature or WHEN there is an internal spark (mechanical failure) - BOOM!

Ingress of air and moisture will also decrease the efficiency of the refrigeration.

#3 Hamid_r_2003

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Posted 14 January 2013 - 02:54 PM

in addition to my friend statement i believe that there is no need to lower temperature. Now you have 5 C temperature approach which is enough as shell&tube exchanger design requirement. moreover most of compressors and pump in plants operate with negative suction pressure with no trouble.

#4 Guest_Giman_*

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Posted 14 January 2013 - 09:26 PM

Thanks for replies.

Negative compressor suction pressure can cause air ingress to refrigeration loop.

Either i dont think 5 degC of min. approach cannot be achieved by kettle type S&T exchanger.
I just got a curiosity about why typical C3 refrigeration has a limitaion of -40 degC cooling while i'm working on C3 refrigeration design.

How much possibility of air ingress does it have when compressor suction is operating @ negative pressure?
Is it related to compressor seal condition or vibration?
What about vacuum distillation unit? Does it also have possibility of air ingress?

So many questions, I'm a inquisitive boy. Thanks.
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