3 stage cascade, need 3 compressors with 10:1 ratios. (R14 compressor)

[mytekcontrols]

I just looked at the polycold energy requirements. For the 1100 it requires 18 kW for 1.8 kW of cooling. I'm pretty sure I could get better energy efficiency out of a 3 stage cascade, right?

I don't think so. An autocascade only uses one compressor, and no isolated intermediary HXC's like on a cascade system. To me it's kinda like comparing an autotransformer (single winding with taps) to a standard transformer having two isolated windings. There are inefficiencies when trying to cross the threshold, so in a step-up or step-down application, the autotransformer has better efficiency, but you do loose the isolation. Pretty much the same thing applies in a auotcascade as compared to a cascade. The cascade gives you isolation, which makes it easier to figure out the refrigerant charge, but you sacrifice efficiency in the process. An autocascade is very tricky to develop a charge for, but once you have, you reap the rewards in higher energy efficiency.

But we must also factor in how cold do you really want to get? Because overshooting your target by a large degree, probably means that you are not optimized for the particular application. And not being optimized also eats up additional energy. So for instance a Polycold normally runs as cold as -140 to -145C under light loads. If in your case you really want -120C at a moderate load, a different choice of refrigerants may yield better efficiency by giving up the ability to run colder. This would be true for either a cascade or autocascade.

So perhaps in your final application a more specific charge might be in order (i.e. reduce the amount of Argon).

I'm glad you and Pat made the connection, and they had something to suit your needs. I still occasionally do contract work for them, and will most likely be involved with testing whatever unit you guys decide to go with.

One last thing...
18 kw would be at maximum rated heat load for a PFC-1100ST, with less heat load, you'll draw less energy. Also doing a good job of matching your needs to a given Polycold will help keep the energy requirements down. The 1100's use a 10 hp compressor and the next lower model uses a 6.5 hp compressor, with the hp ratings getting incrementally smaller as the model number also gets smaller.