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Friobernal
04-11-2002, 08:52 PM
Hi all,

I´ve heard that there are differents types of hot gas defrosting. I´ve been looking for in the net drawings or explanations to know the different circuit, but i´ve found nothing. Can anybody explain it to me?

p.d. If somebody puts jpg. it´d be great!!!

Regards

Volnei
06-11-2002, 11:18 AM
Dear Friobernal,

You may clarify your doubts consulting ASHRAE Handbook Refrigeration or IIAR Ammonia Refrigeration Piping Handbook.
Both of them have a good explanation about hot gas defrost.

Best Regards,

Volnei

Andy
06-11-2002, 07:17 PM
Hi, Friobernal:)
what kind of hot gas defrost were you thinking of. Hot gas on a pumped ammonia system, or a simple system such as reverse cycle?
Give us an indication and I may be able to sourse a drawing.'
Regards. Andy.

Friobernal
06-11-2002, 09:47 PM
Hi

I just have an old drawing from a Searle brochure with 2 types of refrigerant defrost systems:

1. Counter flow refrigerant defrost
Defrost gas inlet to the drip tray, later getting into the coil, and getting out through distributor (I think that in this case there must be a great pressure drop, Isn´t it?

2. Parallel flow refrigerant
Defrost gas inlet to the drip tray, later getting into the coil through a collector, but not through a distributor. I don´t know if this collector is only for defrosting or it is also used to colect the liquid, after the distributor, for evaporating???:confused:

I´m a little bit confused

Regards

Miguel

Andy
06-11-2002, 11:26 PM
Hi, Friobernal:)
I have seen these drawings in the past and from memory the system works like this.

System 1/
The hot gas is introduced through the coil in the tray and the into the main refrigeration coil block. At this stsge the suction line has been isolated from the compressor by either a three port diverting valve or a solinoide. The condensed refrigerant leaves the cooler via the refrigerant distributor around the expansion valve via a check valve. Refrigerant liquid then enters the main/common liquid linewhich has a pressure differential valve fitted before any of the cooler liquid lines, this valves causes the hot gas to be at a higher pressure than the cooler suply liquid creating hot gas flow.
A system such as this can only be used on a multiple cooler set up as it requires the main plant to run at 66% to 75% to create hot gas for defrosting.

System 2/
This is a stand alone set up ie one cooling load one compressor plant.
The compressor hot gas is diverted from the condenser to the cooler tray and then to the refrigeration coil, but the hot gas enters after the expansion device and the liquid line solinode which has been closed. The condensed liquid returns into the suction where it is trapped in a suction accumulator/re-evaporator. The re-evaporation element is via a coil in the suction accumulator (which in this case may be hot gas not liquid refrigerant fed). After the accumulator a crankcase regulator is fitted this stops the compressor from being overloaded towards the end of defrost and also by default builds up pressure in the evaporator to condensing conditions suitable for ice removal.
After defost the compressor stays off for pressure equalisation, on start up normally the liquid solinoide open allowing hot gas to flow down the liquid line (the liquid is still in the accumulator) this hot gas boils off the liquid where is is compressed and condensed to be suppliedv to the expansion as normal.
Hope this helps.
Regards. Andy.