Evaporator Sizing Question

[jpendoley]

Need some advice,

I'm getting ready to order an ice box conversion kit for an air cooled spillover system. Very well insulated box. The box is 6.8 cubic feet. Considering an AB kit- Is there an advantage to going with a larger evaporator plate? Both the CU 100 and 200 use the same compressor (BD50). The CU 200 can also be water cooled which is potentially useful-the CU100 cannot be water cooled-so I'd like to go with the CU200 but the evaporators are sized for the CU200 are sized up to a 15 cubic foot box. My box is half that size-will I a pay an amp draw penalty for going with the bigger plate or will it even out because the compressor will run less?

[SailingHarmonie]

A larger than needed plate will only be an advantage if you slow down the standard speed of the compressor.

This can be done with appropriate sized resistors in the thermostat circuit, or with an automatic system like Coastal Climate Control's Merlin II circuit board. We have installed these on fridge and freezer, and have been extremely happy with the result.

Normally, a compressor driving a larger evaporator is factory programmed (with the resistors) to run at a high speed because the assumption is that there will be a higher load. Conversely, a compressor mated to a small plate will be set to run slower because it is expected to have a lower heat removal load. Or, some manufacturers just set it to run at a medium speed and assume it will work even if not at its most efficient.

Refrigerator compressors will always run more efficiently at lower speed, albeit with a lower cooling capacity.

[RaymondR]

"A larger than needed plate will only be an advantage if you slow down the standard speed of the compressor."

Do you mean speed up SH

[Bowdrie]

I would get on the phone to Dometic, remembering that the companies always like to overstate the "features and performance" levels based on an optimum scenario.
The compressor is-what-it-is, at any selected speed it will move X amount of refrigerant per unit time no matter what size evaporator it's connected to.
Without going into complicated charts which track flow rate and condensing temp and refrigerant expansion ratio, the rate at which the heat in the box is removed is dependent upon the surface area of the evaporator.
All the fancy words only mean that the heat ingress of the box is removed in a shorter run cycle with an evaporator of greater effective surface area.
Assuming the same desired box temp, the off cycle is determined by the rate of heat ingress to the box, the on cycle by rate of heat removal.

[jpendoley]

Thanks everyone for your explanations. I can see speed control is a useful variable to be able to control.
Jim