Two stage DX cooling coil

[reftec021702]

Dear fellows:

Some field inspectors like to instructs us on installing solenoid valves at the liquid inlets of two-stage DX cooling coils. Could anyone tell me the reason for this? Are they really required or just optional?

Thank you!!!

 

[chilled]

A pumpdown system maintains the intergrity of the compressor oil and reduces, if not eliminates compressor oil foaming and improper compressor lubrication on start-up.

Solenoids are definately not an option, but can be the difference between compressor(s) lasting 2 years or 20 years.

Thus, the controller energizes a solenoid and the compressor is controlled by the low pressure control. (Plus the additional safeties)

 

[TXiceman]

You do need the solenoids to pupm down the coil so that when you restart, you are insured that the coil is dry on the tube side and lessens the chance of liquid slugging at start up.

Ken

Ken
KE5DFR

 

[imok2]

On pump down the cut-in pressure of the low-pressure control should be set at a high enough pressure to ensure that the system will not short-cycle if residual pressure remains in the low side of the system once it’s pumped down.

Compressor short-cycling can be devastating to motor windings and starting controls because of overheating. However, the cut-in pressure has to be low enough to ensure that the system will cut in once the liquid-line solenoid is energized by the thermostat to start the next on cycle. These pressures are dependent on refrigerant type and the box temperature desired.

 

[DrRTU]

LLSVs are installed for three reasons 1. Pump down sequence – see previous posts on pump down use and operation. 2. Charge migration control – during the off sequence, refrigerant will move to the coldest part of the system. The LLSV minimizes the refrigerant moving and potentially moving to the suction of the compressor. The combination of crankcase heaters and a LLSV keeps most systems out of problems.
3. Evaporator capacity control – you can decrease the evaporator surface area by isolating a coil / TXV. When a compressor is unloaded or common compressor(s) is turned off, you need to match the evaporator area to the compressor capacity. The LLSV will remove a section of an active coil / TXV. Most TXV are only good to 50% of the nominal tonnage.

You should review the manufactures installation instructions details on the application of when a LLSV is required. I would not want a local inspector telling me he wants a pump down on systems that does not require one. Most codes require the system to be installed as per the manufactures instructions.