Controlling variable speed compressor in system with expansion valve

I have a mobile AC unit that we use for cooling a survey instrument. It has a masterflux variable speed compressor and a Danfoss TUAE expansion valve.

I would like to automatically control the compressor speed to keep a steady output air temp.

But as I understand it the expansion valve is already controlling the coolant flow into the evaporator to keep the suction temperature constant. The bulb is mounted on the exit of the evaporator coils.

If I slow the compressor isn't the valve simply going to open up and put more coolant through the evaporator. I guess that at some point the valve is completely open and the evaporator temperature is controlled by the compressor rpms,, but that doesn't sound like a good solution.



I am looking for comments.

If you think about it the cooling duty is being controlled by the expansion valve.

If you have a specific temperature that you require then perhaps using a portable cooler is not the best option.

what do the ductless systems with variable speed compressors do? Do they control both the expansion valve and the compressor electronically?

robert

Compressor slows down, low side pressure rises, valve starts closing. The catch is that then the TXV essentially becomes oversized and might not be able to regulate properly.

Quote:

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Originally Posted by rebible

what do the ductless systems with variable speed compressors do? Do they control both the expansion valve and the compressor electronically?

robert

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Short answer is yes, the electronic expansion valve regulates in response to the demand on the evaporator and the compressor follows the EEV.