Precision Direct Phase Change

I'm looking at building a direct phase change system that can hold temperatures with an accuracy of +- 1C from -10C to 110C. It needs to hold 100W at -10C. I'm planning on using a 1/4hp compressor running 404a with a heater cartridge in the evap and using an OMEGA PID controller. Attachment 10352Attachment 10352

The Evap is a copper block roughtly 2"x2"x1" with fins. The outer piece is molded plastic and there will be spray foam between the plastic and the evap.

I'm also planning on using a hot-gas bypass valve

How difficult would this be?

I'll say :)

With an evaporator temperature of -10°C, a 1/4 HP compressor will have a capacity of 1250 btu/hr. 100w is equivalent to 341.3 btu/hr, the compressor would be grossly oversized. Using the hot gas bypass would limit the evaporator temp. from dropping below the specified -10°C, but could overheat the compressor if using a capillary tube for the metering device. A TXV would be more stable, but properly sizing the compressor would be a better choice. Using the heater controller and PID controller would be an alternative to the hot gas method, either or, only one would be required. Again, I would look at matching the compressor closer to the heat load produced.