domestic refrigeration question 2

[martinakenic]

Hi,

I have one question about „how to work domestic fridge“. I know that compressors in domestic fridge work in cycle, but what happen when compressors off.
The pressures and temparatures in condenser and evaporator after some time are equal and liquid from condenser goes throught capilary tube in evaporator - all liquid without changing phase ? (am I right?). And compressor start when this pressure and temparature are the same (that the reason why we can use very cheap compressors).
But i dont understand, when we start compressors all liquid in evaporator will be sucked by compressors and compressors will be flooded or not? and how compressors get in to working conditions? In domestic fridge we dont have some magnetic valve which will close when compressors off or some acomulators after evaporators...
Also when we by a new fridge this fridge was in the warehause for a long time and the liquid have enough time to go everywhere even to compressor and how compressors get in to working conditions?

Thanks

Kind Regards

 

[itsmoked]

It's a good question but you're not realizing that there won't be any liquid back to the compressor because the small amount of refrigerant let thru the cap-tube will be vaporized by the time it has gone thru the captube->evaporator->condenser and made it back to the compressor inlet. As for sitting in the warehouse the installed charge is engineered to be mostly vapor everywhere in the system at room-temperature. 70F verses 34F.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[martinakenic]

Thanks for quick answer, and one more to clarify:
1. when compressor go to off and pressure and temparatures in condenser start to equal with temparatures and pressures in evaporator, liquid when pass through capilary tube will be vapur or will be liquid? and when it comes in evaporator will be start to evaporate (but fridge does not work compressors is in off cycle)?
2. If I understand second answer, when I buy fridge, and I put a fridge in a kitchen and stay a fridge about two hour before start , all refrigerant fluid in system will be vapur in normal kitchen conditions, but if I put a fridge on cold condition outside and put in kitthen and immediately turn on, compressors will be in trouble?
Kind Regards

 

[itsmoked]

Because of the cap-tube the refrigeration continues even after the compressor is off. The remaining liquid expands thru the cap-tube typically becoming a wet gas as it leaves the tube and going completely to gas in the evaporator or in the heat of the mechanical space in the refer. Eventually the refrigeration comes to a halt when all liquid is vaporized. The system then waits for the thermostat to power on the compressor again.

The compressor must be off for a long enough period for most of the liquid and hence pressure on the compressor/condenser side of the cap-tube to equalize via the always open cap-tube or the compressor can not mechanically start with the size motor usually provided. That is why when a power failure occurs while the thermostat is trying to run the compressor and then the power comes back on in less than 5~10 minutes the compressor fails to start - the room lights dim and a large CLICK can be heard from the refer. That's because the motor was unable to get the compressor piston over top-dead-center and the motor over-current thermal switch trips opening the compressor motor circuit. In a minute or 3 it will cool enough to try it again. Eventually the piston makes it over top-dead-center and the compressor gets running. The time period allowed more gas to get thru the cap-tube towards equalization.

2) Domestics are pretty robust as they don't want highly problematic returns occurring. You do not have to worry about it unless you're talking about it being subzero outside like 20F below or something. In that case I'd suggest you leave the refer sit for 2 hours and the door open in the warmed house. Though likely they would even shrug that off. The worst that can happen is slugging. A domestic could probably start with liquid in the cylinder and just stall exactly as described above. The failed start would've heated the compressor enough to vaporize any liquid in the cylinder enough to allow a start on the next cycle attempt.

What a refer cannot survive is typically the disaster of laying them over on their sides. This causes the oil in the compressor to go places it shouldn't and worse that oil carries debris sitting otherwise safely in the crankcase up into the cap-tube. That can fatally plug it! If you simply have to lay it on it's side for transport then it should be stood up in place for 24 hours before powering it. That allows the oil and debris to gravity back out of the cap-tube before being mechanically rammed into the cap-tube under pressure.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

The main point that martinakenic seems to be missing is that the total volume of the refrigeration loop is constant but there is much more volume in the low pressure side of the refrigeration loop than in the high side. So only when the compressor is running, is refrigerant liquified by squeezing it into the low-volume, high-pressure side of the loop. When the compressor stops the loop is filled with a medium-pressure, high density vapor. This is why it is important to not over-fill or under-fill a refrigeration loop. If the high pressure condenser becomes filled with liquid, the hot gas cannot get condensed and the compressor will become overloaded.

Also, all compressors have some sort if gas/liquid separator prior to the compressor intake. In the case of hermetically sealed compressors this is the case of the compressor itself. Oil and any liquid refrigerant fall to the bottom of the case, while the compressor intake is at the top.