Hello,
We are trying to build a system that we could use to vary the AC from 3,000 BTU/hr all the way up to 50,000 BTU/hr. This is for a equipment test setup. We were thinking of using a vfd to drive the motor/compressor, but we have many concerns. We want to avoid using a reciprocating compressor (just preference.) It is also must run on R134a.

We have been talking to Bitzer regarding their screw compressor (VSK-3161-15Y) but they said not to run it below 20 Hz and the capacity of the unit is linear with frequency. They were concerned with oil pressure loss. This isn't going to work since we need to be able to go 3,000 BTU/hr.

Then we thought about running the refrigerant through two sets of condensers and evaporators to control our capacity of one unit and place the other evaporator outside of where we need it. This adds too many variables and makes it difficult to control the capacity of the half we are using.

We were thinking of using a scroll compressor due to their durability. Has anyone ever ran a scroll compressor at very low rpm for extend periods? Are there any issues? Is there any possible system set-ups we haven't thought of yet.

We also must run R134a, most scroll compressor's literature is done with R22 or R404a. Is there a correction factor to approximate the capacity of a compressor rated with R22 or R404a if we would use R134a? Is it even safe to run a compressor with a different refrigerant then designed?

Thank you for taking time to read and respond to this. This project has only lead to more questions than answers.