pump down

[tonydykes55]

Why does freezers use a LL solenoid / pump down to kill the compressor
via a low pressure switch rather than simply put the t-stat in series with the coil ?
I know there is a logical reason.

[caheiman30]

To keep a solid colmun of liquid to the TXV

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[Dchappa21]

Main reasons would be to prevent liquid migration and prevent flood back on start.

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[Dchappa21]

Most low temp pumps also don't like starting with a high suction pressure.


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[mr_guy]

To keep a solid colmun of liquid to the TXV

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WRONG! it prevents refrigerant from leaking out of the evaporator during the off cycle or defrost

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[2sac]

http://hvac-talk.com/vbb/showthread....ight=pump+back

[pat454]

tanx

[Spide]

many lo temp compressors are murdered due to flood back from the evaporator after defrost.
pump down prior to defrost can also speed up defrost time.

[tonydykes55]

I see. When you say "flood back from the evaporator", are you saying that without a LL solenoid, in the off cycle, as a system equalizes, liquid from the condenser migrates to the evap? And then upon start up, the compressor is somewhat flooded? Does this not happen in every system without a solenoid? somewhat?

[ECtofix]

What was said...AND fewer wires from the W/I box to the roof. Plus - lighter load for compressor to start up. Yes...floodback, especially in low-temp (freezer) units.

[2sac]

I see. When you say "flood back from the evaporator", are you saying that without a LL solenoid, in the off cycle, as a system equalizes, liquid from the condenser migrates to the evap? And then upon start up, the compressor is somewhat flooded? Does this not happen in every system without a solenoid? somewhat?

A freezer, be it a walk in or self contained has additional "safeties" so to speak to help prevent slugging or overloading the compressor. Fan delays, accumulators, pressure limiting powerheads on the txv's, and cpr valves all help lessen the effects.

[rayr]

I see. When you say "flood back from the evaporator", are you saying that without a LL solenoid, in the off cycle, as a system equalizes, liquid from the condenser migrates to the evap? And then upon start up, the compressor is somewhat flooded? Does this not happen in every system without a solenoid? somewhat?

Refrigerant will always migrate to the coldest area of the system.

[2sac]

I see. When you say "flood back from the evaporator", are you saying that without a LL solenoid, in the off cycle, as a system equalizes, liquid from the condenser migrates to the evap? And then upon start up, the compressor is somewhat flooded? Does this not happen in every system without a solenoid? somewhat?

In addition to what rayr said
Migration is to the coldest spot in the system. Without a LL solenoid and proper low ambient controls the compressor may never come on and/or never cool properly in areas where the outdoor ambient is lower than the box temperature. I know of a few R12 coolers in Chicago that worked with no llsv thru many winters. They may have worked just ok because of the pressure to temperature relationship. Won't work with the refrigerants that have come into the picture since then.

[Moonray]

I have several walk in coolers where the pump is either wired in series with the stat or with a LPC that controls temp. When satisfied, they simply cycle the pump. Been like that for years. But the condensers are located inside with little change in temp.
I have not seen any freezers this way though.

[tonydykes55]

I understand. So then, in an typical residential split system for example, would I be correct in thinking that since the indoor ambient is typically 68 -74 degress, for example, there is not as much migration as in a 0 degree freezer - therefore no need for a solenoid/pump down? Did I say that right? And the fact that they
are not low pressure systems? Another question: is the defrost termination for the evap fans - strictly to keep from circulating warm air?

[2sac]

Circulating warm air would be the last reason for the fan delay. The main purpose of the defrost termination/fan delay is to lower the load on the compressor at the end of the defrost cycle. It's also used to allow the moisture that has formed on the coil and hasn't dripped into the drain pan during the defrost cycle to freeze, preventing the moisture from blowing out of the coil and forming water droplets on the ceiling in front of the coil.(water droplets can form in other areas for other reasons) Some controls allow you to set for drip time.

[tonydykes55]

thanks, this has been very educational.

[MicahWes]

I have heard many times that the main purpose of a fan delay is to lower load on compressors. I don't really agree.

During hot pull down, the fans stay off until coil is below freezing, then they come on and the coil is very quickly very warm from the warm box. In essence, it did reduce the load, but only for a short time.

During defrost recovery in walk-in's, the fans running would actually help reduce load on compressors due to cold box air being pulled over coil.

[Moonray]

I think the main reason for a defrost termination would be to eliminate defrost if not needed. A typical defrost schedule is set for 40 mins. 4 times a day. On a busy day when the door is open often allowing in humidity and running constantly trying to keep up. It may need 40 mins. to defrost. Which is automatically controlled by the timer to terminate. But when the door is not being opened, like over night, days off, ect. Once the food and the box is at temp, its not working that hard to keep it there. So it may only take 5 or 10 mins. to defrost. At this point the defrost terminator will terminate defrost and cycle the compressor back into cooling. Most are rated for 55 degrees and mounted on the coil or end plate. Coming out of defrost the coil will still have water on it. As mentioned by 2sac. Some will allow for drip time, controlled by the timer. And some will bring the compressor on right away, which will freeze whatever is left before allowing the fans to run. Most are rated for 20 degrees to activate the fans, also mounted on the coil or end plate. I think the main reason for this is to prevent the box from turning into an igloo. Water would be thrown everywhere without using fan delays.

[rayr]

I have heard many times that the main purpose of a fan delay is to lower load on compressors. I don't really agree.

During hot pull down, the fans stay off until coil is below freezing, then they come on and the coil is very quickly very warm from the warm box. In essence, it did reduce the load, but only for a short time.

During defrost recovery in walk-in's, the fans running would actually help reduce load on compressors due to cold box air being pulled over coil.

Stay away from whoever told you that reasoning. Many systems have a CPR (crank case regulator) on them to keep the load low on the compressor when new start up or coming out of defrost.