how do i calculate actual mass flow through a compressor without flowmeters?

[scubasteve4sq]

Actually, I think that energy savings in industrial refrigeration is a science. You can't measure real life capacity without measuring flow.
Rule of thumb. Keep capacity of screw compressors as maximum as possible and run reciprocated compressors as trimming ones. Efficiency of reciprocated compressor doesn't suffer as much as efficiency of screw compressor when it unloads.
Reduce minimum allowable condensing pressure as low as possible(winter operation). Keep optimum wet bulb approach for summer operation of the condensers.
I'm just curious. How did you determine that you have 3-5 psig of air in the system?

that's my assessment as well, without flow meters i think we are pissing in the wind.

as for non condensibles, i just take the temperature of the ammonia exiting the condensers, a vlookup table in excel checks that temp against an ammonia saturation table and derives the corresponding pressure, then that is subtracted from actual head pressure and the difference is air and non condensibles in the towers. Dalton's law of partial pressure.

[Segei]

You've done it right if liquid not subcooled. Most likely is not.
Since 2005 I educate people how to save energy in industrial refrigeration. Have written 100+ newsletters and you can read them on website www.skenergy.ca (articles). Optimization of the refrigeration plant operation is the most cost effective way to save energy.
Let's review VFD for one screw compressor. Compressor VFD save energy. How much? VFD recover losses related to part load operation. These losses depends of slide valve position and compression ration. Compression ratio of your compressors is 3-4. This is very low. It means that VFD will recover some losses but not a lot. VFD itself use 3-3.5% of energy. It means that at 100% load compressor with VFD will use 103% of energy. At capacity 85% efficiencies of compressors with and without VFD will be equal. At 70% load compressor with VFD will be 3% more efficient than compressor without VFD. It means that if compressor operate between 70% and 100% average efficiency of compressors with and without VFD will be equal.

[scubasteve4sq]

You've done it right if liquid not subcooled. Most likely is not.
Since 2005 I educate people how to save energy in industrial refrigeration. Have written 100+ newsletters and you can read them on website www.skenergy.ca (articles). Optimization of the refrigeration plant operation is the most cost effective way to save energy.
Let's review VFD for one screw compressor. Compressor VFD save energy. How much? VFD recover losses related to part load operation. These losses depends of slide valve position and compression ration. Compression ratio of your compressors is 3-4. This is very low. It means that VFD will recover some losses but not a lot. VFD itself use 3-3.5% of energy. It means that at 100% load compressor with VFD will use 103% of energy. At capacity 85% efficiencies of compressors with and without VFD will be equal. At 70% load compressor with VFD will be 3% more efficient than compressor without VFD. It means that if compressor operate between 70% and 100% average efficiency of compressors with and without VFD will be equal.

i agree with everything you have said here, but keep in mind, i can run all the recip compressors at 100% base load(most efficient) and trim with a screw compressor at 100% slide valve. its more beneficial that just one compressor.

thanks for the links to your articles, i am enjoying reading them, overall i feel we are in pretty good shape, as we do most of the things you suggest.

[Segei]

I gave you that example to show that most likely little or no benefits of VFDs for your screw compressors.
To save energy in industrial refrigeration, people should know optimum set points and should know how to achieve these set points. I don't know where is your plant located in USA but most likely optimum condensing pressure for winter operation is below 100 psig. You have 150 psig. It means a lot of room for improvement. Actually, lowering condensing pressure very often gives people majority(over 50%) of the energy savings.