Variable speed fridge compressors

[Brian_UK]

Apologies for my lack of technical knowledge but this question arises from one asked on alt.hvac regarding the Danfoss 12/24volt compressors BD35 running on R134a.

These comps use capillary lines for off cycle equalization and the compressor can be speed changed in 500rpm steps from 2000rpm to 3500rpm.

My knowledge of speed/pressure square laws provide me with some thoughts on what might happen to the suction and discharge pressures but my understanding of the effects on refrigerants is incomplete.

If the machine is set up to provide a -15°C evaporating pressure at 2000rpm but we needed a quick pull down would the pressures changes cause much effect on the evaporator temperature or will the increased gas flow counteract these?

The scenario I am thinking about is using one of these comps for a vehicle based cold box which could be 'fast' cooled before use.

[Brian_UK]

Sorry just realised where I posted the above but couldn't quite see it as Commercial or Industrial Fridge.

[Brian_UK]

Thanks Marc, I now have some more data/knowledge to digest, I'll just have to get the notepaper and pencil (plus calculator) on the go.

[Dan]

Hmmm. Thought I posted a replay way back when. I agree with Marc's thoughts. Although I disagree that suction "has to drop with higher speeds and the same evaporator."

Looking at it from a different perspective in hopes of addressing your question regarding "quick" pulldown, I have a comment or two. Of course, speeding up a compressor will dramatically increase the rate of pulldown in a high load situation. It won't necessarily lower the suction pressure to what you might consider design, however simply because of the high mass flow related to greatly increased refrigeration effect. But the pressure will be lower than if you didn't increase the speed of the compressor.

The inefficiencies of a speeded up compressor during pulldown are more than offset by the uncomfortable efficiency of a motor sluggishly trying to overcome a high suction pressure and hot space temperatures. Further, the increased efficiency of a compressor operating more slowly during low load conditions perhaps is what variable speed design focuses on as the greater good.

Think of it this way, lets say you controlled the speed of the compressor by attempting to maintain a certain suction pressure. The compressor would certainly find its entire speed range in use during the cycle between pulldown and satisfaction. And would most likely see the highest suction pressures when its top speed is insufficient for the hottest load that it sees. But basically you would be operating at a single suction pressure. Wonder how a condenser should be sized... methinks toward the high end of the load range. Maybe with a fan attached to a shaft from the compressor?

I am not clear if you are considering a cap tube system as the sole metering device. I would advise against it if you want a quick pull-down. But even still, some of the dynamics are in place. You will increase your head pressure which will tend to feed more refrigerant through the cap tube, but I don't see it working out to much advantage for quick pull downs. Although I can see it working okay for unloading purposes okay. Home fridge duty and that sort.

[zolar1]

Perhaps the variable speed compressor is designed so that system draws lower current on startup, then gets a fast pull down (temporarily), then stabilizes at the 2000rpm rating for the designed refrigeration effect.

Kind of like 'priming' the system with refrigerant for faster cooling.

Interesting theory though.

[Brian_UK]

Originally posted by zolar1
Perhaps the variable speed compressor is designed so that system draws lower current on startup, then gets a fast pull down (temporarily), then stabilizes at the 2000rpm rating for the designed refrigeration effect.

Actually Zolar, these compressors are designed for caravan/boat/RV usage (amongst others) and you are correct in saying that the current draw is important.

It allows the OEM to set a maximum battery drain and sell that fridge that way.