Author Topic: DanFoss Compressor system Performance  (Read 5694 times)

Richard

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DanFoss Compressor system Performance
« on: May 15, 2012, 07:25:17 pm »
New installed BD50 Performance.

OK, we've just moved aboard today - it is HOT. About 35degC.
The major hassle was moving the contents of our home fridge.
Our approach was to put everything into large tupperware containers and use these as dividers in the fridge - (which is deep like a chest freezer).
The system is a Frigoboat with a Danfoss 50 compressor and the largest evaporator plate they sell.
The fridge is now PACKED.
It has been running for the past 4 hours - there's frost on the plate and it's cold in there... BUT the thermostat hasn't switched it off yet.
The compressor is hot enough to fry an egg on, as is one of the copper tubes, so I switched the "Smart Speed Control" to "Manual" and have the compressor set at a low speed now just in case.
It's pulling about 4A at the moment.

Questions:

* How hot is the compressor supposed to get?

* I mounted the temperature sensor for the digital thermostat near the TOP of the fridge box, on the opposite side of the evaporator plate, reasoning that it is the furthest place away from the cooling source and the warmest place, so if the temp sensor is at the right temp, then the rest of the fridge will be even cooler. Although having just done some googling, it appears that a lot of people put their sensors on the BOTTOM of their fridge box... ? Which is correct?

* Having thought about it - the tupperware around the food is probably acting like a HUGE chunk of insulation and is restricting cool air to move around (and get to the temp sensor too?). Should I put a whole pile of wholes in all our tupperware to facilitate the cold air getting in and around?


ANSWER:

Performance and energy efficiency of your unit will depend on proper thermostat temperature control of evaporator, in box thermo efficiency and compressor cooling.
Another point to remember is at 3500 rpm maximum capacity of BD50 compressor will only deliver its maximum 500 Btu when evaporator temperature is at plus 10 degrees F (-12 C). Btu output is greatly reduced at lower evaporator temperatures. To put 500 Btu of energy into perspective think of the amount of energy produced by flame of a candle per hour. Lowering temperature of a box full of warm food may take 24 hours before all product and box temperature is stabilized at desired temperature. Add insulation of plastic storage containers may add a few minutes to temperature pull down.

In order to normalize temperature of all product to be cooled in a refrigerator there needs to be air movement around product. Air cooled by evaporator will naturally tumble down and warm air will rise circulating heat back to evaporator. Upright refrigerators always have open shelves permitting air movement. Top loading boxes need provisions for air movement like side and bottom stand off rails.

Yes these compressors do run hot at times because of condenser’s cooling medium’s temperature and the amount of work they are attempting to do. Overheated compressor burnout is a common failure on most hermetically sealed electrical compressors but not so with these Danfoss BD compressors. As compressor temperature rises so will amperage and pressure increase overload signaling control module to stop compressor before it is damaged.  Cooling of this BD compressor is accomplished by return temperature of refrigerant and supplemental air cooling from condenser fan.. Returning suction gas flow is controlled by evaporator’s low pressure. On Frigoboat’s new water cooled units they have added a fin type heat conductor to outside of compressor but it is too small dissipate much of compressor’s heat.

In your system if the thermostat’s temperature sense tube is not attached to evaporator it will reduce refrigerant flow by controlling box temperature rather than evaporator temperature and may be overpowering evaporator by not cycling compressor when desired evaporator temperature is reached. Most all small heat absorption by conduction evaporator systems like yours will use thermostats that control evaporator temperature instead of box temperature to achieve energy efficiency and best performance. If heat transfer at evaporator were equal to or greater then capacity of compressor then a box temperature sensing thermostat could be used.
My recommendations are:
•   Don’t worry about compressor heat or insulation affect of Tupper wear at this time. Install thermostat’s sense tube where Frigoboat has designed it to be attached. 
•   If you have one of the nonstandard electronic thermostats not intended for refrigerators set it for an evaporator temp of +10 F (-12C) and set differential off cycle for 8 (13C) to 10 (12C) temperature rise before compressor restarts.
•   After box temperature is stable with thermostat set to your desired box temperature set compressor speed. If your unit is equipped with an automatic speed control, place it in automatic. The best compressor speed will be when compressor runs at its lowest speed to maintain desired box temperature yet runs slightly less than 50% of the time.


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AB cycle times Email Question

Is there a "standard" cycle time for an AB refrig? I don’t know if mine is cycling too much or not enough.

ANSWER:
The answer is, NO. Best guess answer to question is if compressor runs less than fifty percent of the time running at its slowest speed and box is maintaining desired temperature it is efficient. Older BD2.5 and BD3 compressors run at fixed speed cycling depends on box’s hourly heat load. If they cycle to often 6 to nine minutes of compressor running energy is lost with each unnecessary cycle.

There are many factors that affect the number of cycle times per hour that change Adler Barbour's compressor cycling times such as:

Super and sub cooling of high pressure gas is either too much in cold weather or poor condenser cooling in warm weather.
Incorrect refrigerant charge.
Ambient temperature change.
Poor insulation.
Moisture in insulation.
Thermostat setting.
If unit has a BD50 compressor it will be less than 10 years old its compressor speed can be set too high or too low wasting energy.

To assure yourself that this AB system is operating correctly check to see that thermostat is installed correctly per AB instructions. Thermostat’s temperature sense tube must not touch evaporator anywhere except where it is designed to be clamped. After unit has operated for at least one day with thermostat set at mid range there must be frost covering at least 90% of evaporator’s surface area and no frost on return line outside refrigerated area on line returning to compressor. Make sure condenser airflow is adequate and warm heated air is not allowed to pass through condenser a second time.

If you are satisfied with refrigerated box’s maintained temperature and compressor cycles twice an hour running less than 50% of the time you are getting great performance.
« Last Edit: May 25, 2012, 02:23:41 pm by Keith Kollmann »