Compressor starting

sdumper · 10-17-2009, 09:13 AM

Its been colder here in Atlanta lately and I had trouble starting the second compressor on a cascade i use for personal use.

I took a space heater out and warmed the second and first up a bit and it started right up but this has me a little perplexed because it wasnt that cold...maybe 46f -- 8c

Anyone know if this is normal or a symptom of something else.

My compressors are 11k rotaries...

sdumper · 10-17-2009, 09:51 AM

I think i have my answer....
I have a high charge in my first stage and a large charge in my second. The second has never been a problem during the summer because of the outside temp and my use of a large Parker tank for expansion. However on colder days like today my refrigerant is accumulating in my compressors because they are acting like heat sinks and this is raising my compressor pressure high enough that it wont be able to crank. So when I heat my compressors the refrigerant is moving out of my compressors lowering the pressure enough to start.

"While the system is running, there is enough heat being generated by the compressor running to keep from creating refrigerant migration from occurring. Refrigerant vapor always migrates to the coldest part of the system (the compressor). The refrigerant migrates to the compressor crankcase where it is attracted to the compressor oil. This refrigerant vapor condenses and will return to a liquid in the off cycle. On the next start of the compressor, the oil is in a watery state and washes the bearings out, which leads to locked up, frozen or totally burnt out compressors.[3] The temperature is sensed at the compressor, indoor coil, and outdoors. The sensed temperatures are compared and if the compressor temperature is not a specified amount higher than the lower of the other two sensed temperatures, then the crankcase heater is energized. When the compressor temperature rises to or is a specified amount above the lower of the other two temperatures, the crankcase heater is deenergized.[6]"

So if im right how do I get around this do i need to add crankcase heaters??

MeltedDuron · 10-17-2009, 10:30 AM

Higher rated capaitor maybe due to the high presure and load needed to crank it over. Maybe in the colder month add an electric blanket over the unit when not being used

NidStyles · 10-17-2009, 10:52 AM

Quote:

Originally Posted by sdumper

My compressors are 11k rotaries...

Now I'm not too aware with how cascade's work or anything, but wouldn't you want a larger compressor on the first stage???

sdumper · 10-17-2009, 12:26 PM

Quote:

Originally Posted by NidStyles

Now I'm not too aware with how cascade's work or anything, but wouldn't you want a larger compressor on the first stage???

Nope

sdumper · 10-19-2009, 06:17 PM

Quote:

Originally Posted by sdumper

I think i have my answer....
I have a high charge in my first stage and a large charge in my second. The second has never been a problem during the summer because of the outside temp and my use of a large Parker tank for expansion. However on colder days like today my refrigerant is accumulating in my compressors because they are acting like heat sinks and this is raising my compressor pressure high enough that it wont be able to crank. So when I heat my compressors the refrigerant is moving out of my compressors lowering the pressure enough to start.

"While the system is running, there is enough heat being generated by the compressor running to keep from creating refrigerant migration from occurring. Refrigerant vapor always migrates to the coldest part of the system (the compressor). The refrigerant migrates to the compressor crankcase where it is attracted to the compressor oil. This refrigerant vapor condenses and will return to a liquid in the off cycle. On the next start of the compressor, the oil is in a watery state and washes the bearings out, which leads to locked up, frozen or totally burnt out compressors.[3] The temperature is sensed at the compressor, indoor coil, and outdoors. The sensed temperatures are compared and if the compressor temperature is not a specified amount higher than the lower of the other two sensed temperatures, then the crankcase heater is energized. When the compressor temperature rises to or is a specified amount above the lower of the other two temperatures, the crankcase heater is deenergized.[6]"

So if im right how do I get around this do i need to add crankcase heaters??

Bump...

Chilly you around tonight?

n00b 0f l337 · 10-19-2009, 09:38 PM

Crankcase heaters are for scroll compressors.
I'm not sure whats occuring in your case, however when it's cold in that sense its refering to outside condensing units where it is colder. In your case it's across the entire system, and thus simply you have liquid occuring at the lowest points. This may be the compressor, or it may not be. However are your compressors meant to be using start capacitors? Or is there an option for start capacitors? Some rotarys require this.

barbus · 10-20-2009, 02:36 AM

Actually, crank heaters are not just for scrolls.

I have this problem with some outside condensing units, it's usually caused by liquid migrating to the coldest place, and in this instance it sounds like the comp.
I'd ad a crank heater and see how it goes.

sdumper · 10-20-2009, 06:57 AM

Quote:

Originally Posted by barbus

Actually, crank heaters are not just for scrolls.

I have this problem with some outside condensing units, it's usually caused by liquid migrating to the coldest place, and in this instance it sounds like the comp.
I'd ad a crank heater and see how it goes.

Thanks ill give it a try and also check to see if a start cap is an option.