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Old 02-08-2012, 15:32   #1
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Join Date: Apr 2008
Location: SF Bay
Boat: Catalina C-42
Posts: 168
New Reefer

I just replaced a 15 year old Nova Cool 12v DC refrigeration system because the aluminum roll-bond box evaporator failed due to corrosion. The corrosion caused a refrigerant loss.

The new system consists of a Danfoss BD35 complessor & condensor unit, and a 40 inch by 10 inch flat roll bond evaporator bent in to an "L" shape the evaporator is oriented vertically in the reefer box. The system is controlled by a Carel electronic thermistat. The BD35 is set to run at 2500 rpm. No problem cooling the 6.1 cubic foot box.

My question is in regards to the setpoint and differential for the Carel thermistat. The thermistat is sensing evaporator plate temperature. I currently have the setpoint at 16 degrees F with the differential at 17 degrees F. So the compressor turns on when the evaporator reaches 33 degrees and shuts off when the evap temp reaches 16 degrees. Box temps are about 42 degrees at the top of the box and 36 degrees at center and vertical midpoint of the box (a little too cool). If I raise the differential above 17 I get icicles forming off the evaporator because it defrosts. Gut feel says I shouldn't let the evaporator defrost.

If I raise the setpoint and reduce the differential, I get increased cycling of the compressor.

Am I on the right track here? The system runs 15 minutes on, and about 25 minutes off as presently set. Ambient daytime temps are 75 to 80 degrees F during the day.
rodney_k is offline   Reply With Quote
Old 02-08-2012, 21:11   #2

Join Date: Aug 2003
Boat: Dragonfly 1000 trimaran
Posts: 6,879
Re: New Reefer

Stick a little computer fan ( 2 inch dia or so ) in the box pointing straight down.
Hook it up to the fan output on the electronics and it will only run when the compressor runs
It will even out your box temps within 2-3 degrees while drawing nearly nothing.
I insulated the bottom of our evap (freezer) with a thin layer of Aerogel.
The box is apartment sized, 43 degrees F. at the top, 40 F. at the bottom, freezer compartment averages 10 degrees F. while using approx 30 aH / day in the PNW with 75 degree days.
(BD35 with keel cooler)
senormechanico is offline   Reply With Quote
Old 03-08-2012, 09:56   #3
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Join Date: Apr 2008
Location: SF Bay
Boat: Catalina C-42
Posts: 168
Re: New Reefer

Thank you Senor. Not too worried about the stratification in the box. More curious about which way to move setpoint and differential to reduce run time.

I do have a 3" fan which I will wire up as you suggest this weekend.
rodney_k is offline   Reply With Quote

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