Kollmann Marine

Boat Refrigeration => Boat Refrigeration => Topic started by: Downunder on September 28, 2012, 04:06:09 am

Title: How do I improve performance of 12 volt fridge.
Post by: Downunder on September 28, 2012, 04:06:09 am
I am e-mailing from Australia and I hope you may be able to offer some advice as to how I may be able to improve the performance of my RV fridge. I understand this is a boaters forum but RV's and boats share some similarities on the subject of refrigeration. I will be as brief yet as comprehensive as I can so that you have something to work with. I hope I have posted this right.

Refrigerator: Waeco Coolmatic HDC 190 Litre 2 door upright. Danfoss BD 50F compressor and 101N0210 controller.
Ventilation: Recently changed to internal venting in lieu of external as supplied from factory. Fridge is raised 30mm from base to allow cool air to be entrained and is drawn over the condenser by 2x 80mm fans which discharge this warmer air into the RV at or near roof level. Distance between the condenser and the insulation at the rear is approximately 20mm and overall the distance from the back of the fridge box is approximately 40 - 50mm. 

I recently installed a Guardian compressor speed controller and thermostat and a Merlin smart speed controller, both sourced from Coastal Cool Aids in the US. I liked the idea of having the ability to control compressor speed if necessary as well as a digital readout of exactly what temperature is in the fridge.

My fridge comes from the manufacturer with no internal fan and I suspect this does not assist in obtaining optimum cooling and lesser compressor run times. I understand from reading some of your recommendations that fans can/do help and would be interested in your advice on my particular circumstance. Generally I feel that the compressor runs for extended periods and that with a few modest improvements efficiency could be greatly improved.

(1) The following is a direct exert from the instructions that came with the Guardian. "The thermostat sensor should ideally be located at mid‐height in the box, and not on a wall that has any portion
      of an evaporator mounted on it. The sensor should be isolated from the box wall with a plastic tie or similar".
     I mounted mine mid height as directed (in unused fridge shelf guide channel) raised off the side panel about 3mm with the sensor near the front just back from the door.
     Given I am new to this device I have left in the Auto mode.

    The location of the manufacturers sensor (currently disabled) is to be found on the rear evaporator itself. Given this I have perhaps located the Guardian sensor in the wrong place, what do you think?

(2) I was advised to establish the air space around the rear condenser as stated above and in the interests of directing airflow over the condenser it makes sense to me.  Is this correct in your opinion?

(3) I am contemplating installing a fan to assist in airflow round the box. The size fan I am looking at is 40x40x10mm and has an rpm of 3500. Can you tell me what you believe to be the optimum size and speed of such a fan and where exactly should I fit it to get the best performance? It seems to me that 3500 rpm is too fast and would be too noisy?

(4) What is your opinion of the ventilation system I have described and are the 2x 80mm fans big enough (or too big) for what I am doing? Not sure what the cfm of them is.

(5) There was no fan installed for the compressor or the controller from the manufacturer, should I install one and how do you suggest I can fix it in place. I cannot see any bracket given it may have been an option?

Bottom line is I am trying like everyone else to get the best performance from the fridge under all conditions, particularly the hotter conditions that we get here in Australia. Any advice you can give me would be greatly appreciated. I can also forward pictures should they be beneficial.

Bill James.
Title: Re: How do I improve performance of 12 volt fridge.
Post by: Richard on September 29, 2012, 01:11:51 pm

My guess is controlling box temperature instead of refrigerator manufacturers intended evaporator temp control is wasting power by overpowering evaporator causing compressor to run longer at a higher speed. I am not able to answer your questions and am not sure why you added Guardian compressor speed controller and thermostat and a Merlin smart speed controller. The secret to improving these cabinet refrigerators is condenser heat removal and a good door seal that prevents air infiltration.

I would guess that WAECO believed they designed this Coolmatic HDC 190 refrigerator correctly. There are fewer variables in cabinet refrigeration designs compared to icebox conversions where changing compressor speed can be beneficial when box heat load is less than refrigeration system output capacity. Research and development testing of cabinet refrigerators like this WAECO 190 can provide an application engineer with a base line of performance data. Data on your unit is at 65 watts (5.1 amps at 12.5 volts)  power input at 68 degrees ambient surrounding air temperature, compressor will run 30% of the time. At 86 degrees ambient temperature compressor will run 45% of the time. WAECO’s Data was based on maintaining a refrigerator box temperature of 41 degrees F. The sum of your daily Input power will help you determine a base line before adding gadgets like a speed controller or additional fans. 
Title: Re: How do I improve performance of 12 volt fridge.
Post by: Downunder on September 30, 2012, 01:59:10 am
Thanks for getting back to me.
I installed the Guardian as it automatically regulates the compressor speed and provides the lowest speed that will do the job instead of the compressor running flat out all the time or at a set rpm by installing a diode in line. It also gives me a digital readout of what the temperature is at all times and by leaving the factory unit installed yet not connected I have a ready to go spare should I need it.

I will relocate the temp sensor closer to the evaporter and install a small fan to circulate air around the fridge to more evenly cool the contents when the fridge is full.

Thanks again.
Bill James
Title: Re: How do I improve performance of 12 volt fridge.
Post by: Richard on October 03, 2012, 10:39:34 am
Relocating temp sensor closer to evaporator will not do anything except give a colder false box temperature reading. Refrigerator temperature readings are normally taken in refrigerator where the products that spoil first like milk are located. Box temperature like yours are controlled most efficiently by cycling compressor Off and On based on evaporator temperature not compressor speed.  My question was really what performance data did you have on this refrigerator before redesigning its control? Lowering compressor speed in cooler ambient conditions will reduce daily amp-hrs consumed but increasing speed is less efficient.   

Keeping the old thermostat installed was a wise decision you my find it will provide  better reliable performance. 

Title: Re: How do I improve performance of 12 volt fridge.
Post by: Terry J Taylor on October 10, 2012, 10:18:22 pm
I am also looking to maximize 12v frige. performance, I live off grid in Baja Mex. I am contemplating using a Danfross BD 80F in a regular 19QF frig/freezer. Could you Richard or anyone else give me the best place to source the Danfross and tell me if my project is feasible.
Thank You
Terry Taylor
Title: Re: How do I improve performance of 12 volt fridge.
Post by: Richard on October 17, 2012, 10:21:40 am
I can not see what you expect to gain by bastardizing your unit with a larger compressor. Do you plan on increasing size of condenser, and evaporator to handle 30% more Btu capacity?  Are you saying the present compressor is running at max speed 24 hours per day?  If your compressor runs less than 50% of the time and has good condenser heat disposal increasing compressor size is not going to reduce daily energy consumed.  I would recommend increasing the DC alternative energy power grid with wind and solar equipment. Where you are located for each cubic ft of refrigerator you need over 100 watts of energy and over 200 watts for each cu ft freezer per day.