Hello!

I'm designing a chiller system for a lab scale test stand and comparing quotes from chiller vendors. I was hoping that the combined wisdom of this community will help me to ensure I'm making the smartest choices about system architecture, etc.

Design Criteria:
We have a ~100 gallon bath of kerosene that I want to keep at -35F +/- 5F. There is a process adding between 2 kW and 12 kW of heat to this bath, so I'm designing a chiller to reject all this heat.

The chiller has to be outdoor-rated and air-cooled, capable of rejecting 15 kW of heat at -50F or colder fluid output in an ambient environment up to 100F. I could flow my kerosene directly through the chiller's evaporator (in which case the chiller would have to be Class 1 Div 2 rated because kerosene is flammable) or I could use an intermediate loop of a heat transfer fluid and a brazed plate heat exchanger between this and my process fluid.

Proposed Vendor Solution:
It seems that most vendors converge on the same solution. A simple single-refrigeration cycle design using a Bitzer 2-stage semi-hermetic reciprocating compressor (model S6J-16.2 (21.9 kW) or S6H-20.2 (26 kW)) with R404a refrigerant. They all include some kind of hot gas bypass, but no other frills that I can tell.

Questions:
  1. How do I know whether they've sized the compressor appropriately and aren't selling me an unrealistically high COP/EER that won't actually work? When I contacted Bitzer to size out a compressor, they recommended an S6G-25.2 (30.1 kW)
  2. One vendor chose a Bitzer screw, and one vendor chose 2x Copeland scroll compressors. I'm going to pass on both of these options and stick with the majority vote for the 2-stage reciprocating, unless you guys recommend otherwise
  3. Thoughts on flowing my kerosene directly through the evaporator versus using an intermediate heat transfer fluid? The former seems more efficient, but the latter gives more options for temperature control (turning on and off coolant flow)
  4. Some say that a mechanical/capillary TXV is totally fine, while others are using an electronically controllable one. What's the tradeoff?
  5. Is R404a the right choice, or should I press for R507a? Am I cutting it a bit close with these refrigerants, or am I fine?
  6. Will the suction side of this thing be operating at a slight vacuum? Is that an issue?
  7. Will hot gas bypass this work at such low temperatures? What should I be looking for in terms of false loading? is it okay to turn the compressor on and off to maintain our temperature? What duty cycle is acceptable?
  8. If I use an intermediate heat transfer fluid, what would you recommend? Is EGW 60/40 good enough? PGW? Some sort of silicone based fluid?
  9. Prices range from $45k to $80k with leadtimes from 7 to 14 weeks. It's difficult to discern what I'm getting from each vendor for more or less money, though. Hopefully this sounds reasonable?
  10. What other concerns should I have or what other questions should I be asking? Especially for such low temperatures so close to low end capability of the refrigerant


Thanks for any and all advice!