Defrost Without Pump Down?

[man from trane]

I worked on this Hussmann condensing unit that serves a 10X16 WIFF. R-22, electric defrost. It takes about 3 minutes to pump down after closing the llsv. Sometimes the Demand Cooling trips after it shuts down on the LP switch at the end of the pump down. I'm not sure why. I know the comp runs hot due to missing insulation on the suction line, but after the compressor shuts off the D/C should lose power so it can't trip.

Anyway, I noticed that when it goes into defrost the timer kills the compressor and llsv both, with no pump down. Looking at the wiring diagram, it is supposed to be that way. Shouldn't it pump down first and then go into defrost? Where does all the liquid in the evap go as the coil warms up? :confused:

[Boise500]

WI freezers don't have to pump down to defrost. The liquid in the evap will turn to gas. Some manufacturers put in a solenoid just for cheap insurance in case the tev doesn't seat on shut down.

[man from trane]

That answers my question, thanks!

It only took 13 months for a reply :)

[powell]

That answers my question, thanks!

It only took 13 months for a reply :)


Wow, how did we miss this post?:oops:


I include pump down on jobs, cooler or freezer. It helps prevents refrigerant migration to the compressor.

[man from trane]

Wow, how did we miss this post?:oops:


I include pump down on jobs, cooler or freezer. It helps prevents refrigerant migration to the compressor.

That sounds like a good idea. Maybe the lack of pumpdown is why the compressor has a knock and doesn't pull down.

[FridgeMagnet]

"WI freezers don't have to pump down to defrost. The liquid in the evap will turn to gas. Some manufacturers put in a solenoid just for cheap insurance in case the tev doesn't seat on shut down. "

I hate to differ but the tx valve will not seat in a defrost. As we know its only function is to control superheat and so when its sensing bulb sees the increased suction temperature during defrost, it will open to try to maintain its setpoint. if it is a MOP valve (Maximum Operating pressure) it will throttle within its range and control the amount of refrigerant in the evaporator, and will not overfeed the evaporator. But if its not, it will open wide and after the defrost terminates there is a huge potential for it to overfeed and slug liquid when the compressor starts. (thus the reason for pump-down) If there is no CRO valve to keep the suction pressure at a minimum, the compressor may trip out on low oil pressure or high head.

I do agree with powell, I would definitely add pumpdown after defrost for the preceding reason.

In regards to the demand cooling, there are only three reasons it can trip
1. High condesing (above 310 F, it trys to keep the discharge temp between 280-290 F)
2. High return Gas temp (above 65 F) superheat needs to be between 6-8 F and suction line insulated to compressor to maintain.
3. Abnormally low suction Pressure

These three will directly affect your discharge temp and then trip the unit

Your right trane man, that it should be offline when compressor is off, check to see if the current sensing relay is working properly.

Depending on what your system is doing, pressures and temperatures, there are some other things to look for.

An excellent Copeland bulletin on demand cooling is AE4-1287 R4, google it and you will see everything u need to know. (i have quoted from it in my post)

[powell]

Here's a link to Copelands AE bulletins.;)

http://www.hvacrinfo.com/ae_index.htm

[man from trane]

Thanks for the info! I always wondered why some freezers use the ZP pressure limiting power elements on the TXV.

[craig1]

Thanks for the info! I always wondered why some freezers use the ZP pressure limiting power elements on the TXV.

also prevents overloading of compressor on initial pull down and after defrost.

[r404a]

"WI freezers don't have to pump down to defrost. The liquid in the evap will turn to gas. Some manufacturers put in a solenoid just for cheap insurance in case the tev doesn't seat on shut down. "

I hate to differ but the tx valve will not seat in a defrost. As we know its only function is to control superheat and so when its sensing bulb sees the increased suction temperature during defrost, it will open to try to maintain its setpoint. if it is a MOP valve (Maximum Operating pressure) it will throttle within its range and control the amount of refrigerant in the evaporator, and will not overfeed the evaporator. But if its not, it will open wide and after the defrost terminates there is a huge potential for it to overfeed and slug liquid when the compressor starts. (thus the reason for pump-down) If there is no CRO valve to keep the suction pressure at a minimum, the compressor may trip out on low oil pressure or high head.

I do agree with powell, I would definitely add pumpdown after defrost for the preceding reason.

In regards to the demand cooling, there are only three reasons it can trip
1. High condesing (above 310 F, it trys to keep the discharge temp between 280-290 F)
2. High return Gas temp (above 65 F) superheat needs to be between 6-8 F and suction line insulated to compressor to maintain.
3. Abnormally low suction Pressure

These three will directly affect your discharge temp and then trip the unit

Your right trane man, that it should be offline when compressor is off, check to see if the current sensing relay is working properly.

Depending on what your system is doing, pressures and temperatures, there are some other things to look for.

An excellent Copeland bulletin on demand cooling is AE4-1287 R4, google it and you will see everything u need to know. (i have quoted from it in my post)




It would also see a high suction pressure. TXVs act upon 3 forces. Two try to shut the valve: Suction pressure and spring pressure. One force acts to open the valve: bulb pressure.

concerning the OP, some manufactures and techs seem to believe that refrigerant GAS in the coil is a good thing upon the application of electric heat as the gas may evenly spread said heat throughout the entire coil and get a better defrost.

r404a

[FridgeMagnet]

"It would also see a high suction pressure. TXVs act upon 3 forces. Two try to shut the valve: Suction pressure and spring pressure. One force acts to open the valve: bulb pressure.

concerning the OP, some manufactures and techs seem to believe that refrigerant GAS in the coil is a good thing upon the application of electric heat as the gas may evenly spread said heat throughout the entire coil and get a better defrost."



To clarify, i have no problem with gas in the coil during defrost and can see the potential benefit, but then you would surely have to have a CRO valve at the compressors inlet (and i think it well to have an mop valve as well). I prefer Pumpdown as it also prevents liquid migration during the off cycle.

In regards to the txv, there are surely 3 forces (actually 4 if u include the pressure drop accross it) and it will see a high suction pressure. But as that pressure and temperature in the evaporator rise, so will the bulb pressure. It all adds up to its set point, and though it may open a bit, or close a bit, i jus don't believe it will seat. But surely by the time our compressor starts, it will be looking to fill the evap. (:o)-E-B

[man from trane]

Some TXV bulbs are covered in thick ice, so I don't think the valves will be opening in that case.

[VTP99]

but then you would surely have to have a CRO valve at the compressors inlet (and i think it well to have an mop valve as well).

I didn't know you could use both.

[FridgeMagnet]

Tx bulbs should be insulated, but after a defrost any ice should be gone.