Hot Gas Defrost

[CJG21]

Can anyone tell me where I could find a schematic of a hot gas defrost system?

Is a hot gas system an efficient one or would you be safer installing an ordinary electric defrost system on a coldroom?

Any help is much appreciated, I;m trying to complete an assignment on them and dont have any experience on the system. :confused:

[GXMPLX]

Can anyone tell me where I could find a schematic of a hot gas defrost system?

What type of systems are you thinking of? There are many schematics!


Is a hot gas system an efficient one or would you be safer installing an ordinary electric defrost system on a coldroom?

It depends, if you can produce superheated vapour WHILE you use some of it to defrost the answer would be yes. If you have to produce it only for defrost it will depend on the size of compressor and the amount of ice you produce.

But there are some situations you better use electric, like when the evaporator is not designed for gas defrost.



Any help is much appreciated, I;m trying to complete an assignment on them and dont have any experience on the system. :confused:

Well this is not the first time someone posts here his homework, so we try not to always give correct answers!

[Tycho]

Can anyone tell me where I could find a schematic of a hot gas defrost system?

Here is a schematic of how it would look on a manually operated hotgas system on the evap side.

http://www.pbase.com/image/99657966.jpg

On this type, the operator would have to close the suction valve and then slowly open the hotgas valve.
the SCH valve on the liquid line would close, and when the pressure inside the freezer rises above 4.5 barg the OFV valve opens and allow the liquid to drain out of the evap and back to the liquid return line (of wet suction if you want).

this is the one I had in hand, but if you have further questions, don't hesitate to ask. worst case scenario is that you learn something :)



Is a hot gas system an efficient one or would you be safer installing an ordinary electric defrost system on a coldroom?


The hotgas is always there, so it is always readily available.

However, you have to take into mind the cost of the extra valves and pipe work that will be needed for making a hotgas system, since at minimum, you will need two extra valves.

compared to

an off the shelf evap with electrical defrost already mounted.

also take into concideration how often will the evaps need defrosting?
is it a storage room with traffic in and out once a month or many times each day?
what is stored in the room, and is it allready frozen when going into the room, will it give of moisture?

A storeroom with in and out once a month will not need many defrosts. and hotgas could be a solution.

a room with lots of traffic will need frequent defrosts and electrical would be better

also, on ammonia plants, I dare say you have to have hotgas defrost available along with electrical defrost. This, so that during normal operation the system uses electrical defrost, but once a month or depending on if oil is building up in the evap you should be able to flush the oil back to the suction accumulator so it can be drained properly (and by properly I mean you shouldnt have to waggle on a ladder 6 meters up in a room holding -25 C to drain oil from the evap :))

the reason for having electrical also is that the ice buildup on the evap may require a defrost every 3-6 hours and I just don't have the heart to use hotgas on a system like that


Any help is much appreciated, I;m trying to complete an assignment on them and dont have any experience on the system. :confused:

[powell]

Colin,

Here's the link to Heatcrafts IOM manual. Pages 19-20 cover HGD.

http://www.heatcraftrpd.com/resources/wiringdiagrams/H-IM-64L.pdf

[Peter_1]

I suggest not to use hotgas defrosting in a non-industrial single evaporator application.
Too expensive in valves and more prone to failure.
Tycho, we inject the condensed liquid back in the main liquid line and not in a liquid return line. We then insert of course some flash gas in the liquid line at the end of the defrost but this isn't for long.