Author Topic: Refrigeration current draw  (Read 7369 times)

Rmax

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Refrigeration current draw
« on: September 19, 2012, 08:52:36 pm »
I have a 2001 Polar Mate installed by the PO of my boat.  I bought the boat in 2007 and since I have owned it, according to my battery monitor, it has always drawn about 8.25 amps after the first couple of minutes it runs.  Initially it draws a bit over 10 amps.  It is water cooled and the pump draws about 1.2 amps.  Based on the literature it came with, it should only draw a little over 5 amps with the pump running.  I have read on your site that the high amperage draw indicates that the compressor is nearing its end, however, since it has been this way for at least six years, that wouldn't seem to be the case.  I have replaced the wires between the battery and the fridge so there is less than a 3% voltage drop so I know it is getting adequate voltage.  Any idea what may be causing the higher than normal amperage?

Richard

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Re: Refrigeration current draw
« Reply #1 on: September 20, 2012, 10:43:06 am »

Grunert’s 12 volt Frost Mate condensing units were sold with two different manufacturers compressors Danfoss or Spanish/Mexican. Danfoss fan cooled BD2.5 or BD3 compressors when correctly serviced with pure refrigerant will draw 5 amps after evaporator temperature is stabilized. Adding a 1.2 amp water pump to compressor and fan load would then not exceed 6.2 amps. For a Danfoss compressor system to operate at 8.25 amps it would indicate poor condenser cooling or too much refrigerant in system. It is also possible the water pump is drawing 3 amps instead of 1.2 amps. If compressor were causing high amperage electronic control module would prevent it from running.

Too much refrigerant and contaminated refrigerant can be eliminated as a cause of high amperage by frost pattern on evaporator and lines. Correct amount of pure clean Freon 12 in BD2.5 or 134a refrigerant in BD3 compressor is indicated by frost covering 90% of evaporators surface area, and no frost outside refrigerator box on line returning toward compressor.

Sufficient condenser cooling can be verified by line temperature of no more than 110 degrees F coming out of condenser and filter on its way to evaporator.

Water pump can be eliminated as high amp cause by turning it off and reading compressor and fan amperage of only 5 amps if evaporator is somewhat cool.

Rmax

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Re: Refrigeration current draw
« Reply #2 on: September 22, 2012, 09:34:02 pm »
Richard,

Thanks for getting back to me.

I checked out the things you suggested and found the following:
-the water pump is drawing 1.19v.
-the evaporator seems to have frost on at least 90% of its surface and I did not see any frost on the tube coming out of the box.
-the temperature at the tube coming out of the top left of the compressor and going to (or coming from-not sure which) the ice box is 85.  The temp at the tube coming out of the middle right of the compressor going to the cooling fins and fan got up to 115, although about 1/2" from the compressor the temp of the tube was also about 85.  The hottest spot (120) was the compressor itself near the tube going to the cooling fins.

The water temp was somewhere in the 70s and there seems to be plenty of water coming out the discharge through hull.

The amperage draw was only 7.25, one amp less than before.  I don't know if this is because the water temp was cooler (probably in the low 80s last time I ran it) or because I replaced the power supply wires with heavier gauge ones (went from 10 to 8awg) so I would have less voltage drop.

Based on the above, do you think the most likely cause of the high current draw is inadequate cooling of the compressor?  I would probably know from the temp readings if I understood them, but I don't know exactly where you were talking about when you said the temp should be no more than 110.

BTW-I guess I have the BD3 compressor since it uses 134A refrigerant, although for some reason I thought it was a BD35 or 50.  A picture of the compressor is attached.

Thanks for your help with this.

Richard

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Re: Refrigeration current draw
« Reply #3 on: September 23, 2012, 10:52:41 am »
Pump current is not the problem.

Frost line indicates refrigerant charge is correct.

•   High pressure line coming out of compressor below center line and going to air cooled fined condenser will be very hot 120 to 135 degrees F.
•   Line coming out of fan cooled condenser on your unit will inter seawater condenser will only be cooled slightly unless fan is running.
•   There are two large line connectors on left side of picture, the one at bottom not seen in picture is refrigerant gas after process heat has been removed going to refrigerator’s evaporator plate. In order for the refrigeration process to be efficient the lower high pressure line to evaporator must be cooled to less than 115 degrees. Based on water temp 70s degree and good water flow I would think high pressure line temperature would be under 100 degrees and closer to seawater temp.
•   After refrigerant dues its work in evaporator the line returning to top of compressor will be cool say 65 to 80 degrees.

High line temperature and amperage could be caused by sea growth inside condenser, has condenser been cleaned recently?

The electronic control module in picture confirms you have a variable speed BD35 or BD50 compressor. These compressors when cooled by pumping water are generally less efficient. For these compressors to realize energy efficiency compressor must run at slower speed, this means they run longer and increased pump running time offsets any energy saving.

You need to confirm what compressor speed is set at by determining what size resistor was installed in thermostat wire. When compressor runs at slow speed compressor and pump runs. When compressor runs to fast it overpowers evaporator and cycles off to many times wasting daily energy. Considering your reported amperage reading it sounds like your unit’s compressor is a BD50 running at maybe maximum speed of 3500 rpm.

To determine speed resistor size disconnect thermostat wires C and T from control module and with thermostat anywhere in the on position use an ohmmeter to read resistance of wiring. Most companies insert resistor in wire near module. Zero resistance indicates no resistor and speed is at minimum 2000 rpm , a 277 ohm resistor produces 2500 rpm, 693 ohms 3000 rpm and 1500 rpm 3500 rpm.   

Rmax

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Re: Refrigeration current draw
« Reply #4 on: September 26, 2012, 05:14:43 pm »
Richard,

You diagnosed the problem.  There was a 1470 ohm resistor in one of the thermostat wires.  When I removed it the amperage with the water pump running dropped from 8.26 down to 4.75.

Was the compressor most likely at max speed so it would cool the box down more quickly when first turned on, or is there another reason?  Other than the compressor running a bit longer, is there anything I should look out for that may indicate I need to reinstall the resistor?

Thanks again for your help.  I would like to send you something for helping me fix this problem but I did not see a mailing address on your website.  Is there an address I can send it to?

Robert

Richard

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Re: Refrigeration current draw
« Reply #5 on: September 27, 2012, 11:15:13 am »
The correct variable compressor speed needed to performance balance a system can only be determined after refrigeration system is installed. Adler Barbour when they sell a system or just the evaporator include a thermostat with speed resistor sized to marry capacity of compressor to evaporator’s capacity, but this still dues not consider box size, amount of insulation, or climate conditions.
Remembering then that lower compressor speeds increase compressor’s Coefficient Of Performance. The trick is to find a compressor speed anywhere in between 2000 and 3500 rpm that uses the least daily amp-hrs and still maintains desired sustained box temperature.

Yes, removing what may be an oversized speed resistor cut running amperage in half it also extended compressor running time. Now the question is have you increased or reduced daily amp-hrs consumed while maintaining desired box temperature? A general rule is if compressor runs less than 50% of the time with no resistor you are obtaining the best speed performance available from that compressor. Boats equipped with an amp hour meter can help you fine tune what resistor is best for system application. If your present setup is running more than 50% of the time you may want to increase speed to 2500 rpm by adding a 277 ohm+ or – 25 ohm resistor. The water cooling pump will waste one amp daily for each hour compressor runs over that of a correctly installed air cooled unit.


I have been helping boaters with refrigeration problems for 30 years thanks for the offer but my advice has always been free.
 

Rmax

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Re: Refrigeration current draw
« Reply #6 on: September 27, 2012, 12:58:29 pm »
I do have an amp hour meter so I will try the different speeds and figure out which uses the least daily amperage.

If my unit is designed for water cooling, can I turn off the water pump if I provide adequate ventilation, or if it has been designed for water cooling will air cooling be inadequate?

Richard

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Re: Refrigeration current draw
« Reply #7 on: September 27, 2012, 04:12:35 pm »
Danfoss application instruction specifications for BD compressors indicates they recommend air cooled condensers only.  I was advised by one boater with a Frost Mate who comes south each winter that he turns off water pump in Florida’s warm seawater to reduce daily amp-hrs used. For fan air cooled condenser to be more energy efficient than pumped water condenser air interring it must be no warmer than cabin’s ambient air. This means after warm air leaves condenser it must not pass through condenser again. With your amp-hour meter you can determine for yourself the energy savings of low compressor speed while fan cooling without water pump running. 

Rmax

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Re: Refrigeration current draw
« Reply #8 on: September 27, 2012, 06:39:30 pm »
Richard,

Thanks again for all of your advice.

I am going to buy your book on 12v refrigeration and read it before I ask any more questions it may answer.

Regards,
Rob