Hot gas defrost

[Volnei]

Hi guys,

The basic hot gas defrost that I've been using to defrost the evaporators is to inject hot gas in the suction line and returning it by the liquid line, where there is a check valve instaled that makes the hot gas go back to the suction line downstream the main valve. The pressure defrost, inside the coil is set by a pressure control valve at 86 PSI and the hot gas is picked up directly the discharge line, with about 176F.
I would like to know what's the process that hot gas suffers inside the coil during defrostring.
What will be the hot gas state that leaves from the coil?
May anybody could makes a PXH diagram showing its process?

Thanks for all.

Best Regards,

Volnei

[rpkamath]

86 psig for R 22 corresponds to 50 deg F saturated temp.

To melt frost we need a temp above 32 deg F. The temp should be high enough there is heat flow from the gas to the frost. 50 deg F is sufficiently higher than 32 deg F. (A lower temp say 45 deg F (76 psig) also would have been acceptable).

The frost (ice) is melted using the latent heat of condensation of hot gas. So hot gas first cools itself to 50 deg F; then it condenses to liquid releassing "latent heat of condensation". This heat melts the ice.

The heat transfer is very uniform and very very efficient, as it is using the full surface area of the evaporator.

Having said this let us also understand hot gas defrost is more complicsted than this. Firstly during defrost there should be also contnuous supply of vapour to compr suction, without which there would be no hot gas avilable. This is usually achived by having about three to four evaporators in parellel. When one evapr is on defrost other evaporators produce the vapour.

The other complication is handling of liquid coming out of the evaporator. Commonly this is adressed using an accumulator. It is also possible to use one more evaporator coil, along with a capillary, at evaporator outlet to re-evaporate liquid coming from evaporator. The outlet from evaporator should be sent thru this coil during defrost cycle. (This can be done using two sol valves)

[Andy]

Hi, the system Volnei is speaking of uses a check valve around the TEV this allows the condensed liquid from the hot gas which has been cooled/condensed in the evaporator to enter the main ring main liquid line. A pressure drop valve has been fitted in the main liquid line as it leaves the plant room, this is sett for 20PSIG aproximately, this allow the hot gas and the condenser to be at a higher pressure than the main liquid line, thus creating flow. Mainly the suction lines are all stubs on a system such as this, every evaporator or group of evaporators has it's own suction line to the central plant. At the plant a three way valve is fitted, this allow the evaporator suctions to be either in a refrigerate position as a normal suction or a defrost position where the suction stub is blanked off to the main suction and fed hot gas from the compressor discharge header.
Regards. Andy.