PDA

View Full Version : Hot gas/Cool gas defrost





JONAH
02-04-2002, 10:30 PM
Hi guys,
i have another query youll be able to shed light on.
I have a low temp pack that utillizes both hot(direct discharge) gas and what i term as cool(off top of reciever)gas as defrost.
The system was set up for cool gas defrost,but over the months ive been monitoring pack for ive found excessive frosting of low side stubs, accums,drier cores etc.
Even been causing oilpress tripping on one or two comps where excessive frosting has been occuring.
S/HEAT ETC OK!EVEN DRAINED ONE BLOWER TO SEE IF I HAD EXCESSIVE OIL RESIDING IN THERE!
I changed defrost from cool to hot gas and for the last 6months stubs fine no nuisance trippin on oil even oil back in resevoir,can you tell me my advantages and disadvantages to hot gas defrost vs cool gas

thanx again jonah

its good to be able to share problems and queries with other engineers because i tend to find i end up knowing more about job than my employer and also a lot of tech HELP desks!thanxl

Dan
03-04-2002, 03:39 AM
Hi Jonah. Some questions for you:

Is the frost both on the liquid line coming out of the receiver and the suction branches going to the compressors?

Is the frost constant or only during or after a defrost?

Is the Coolgas utilizing a liquid line differential valve to maintain the differential? Does the hot gas utilize the same control scheme?

The merits claimed for Coolgas defrost are that there is less temperature shock to the piping and evaporators as they switch from refrigeration to defrost compared to hot gas defrost. 95 deg F gas as compared to 180 degree gas, for example.

Since most of the defrost is latent... done at saturated conditions, many feel that the saturated gas (cool gas) provides an equivalent defrost without the extreme expansion of piping that a hot gas defrost incurs.

Your problem could be too low a differential setting during the cool gas defrost. It sounds like you are getting incomplete defrosts with the cool gas. The frosting of the suction components could be indicative of all the condensed liquid in the evaporator and suction line roaring back to the compressor when the refrigeration restarts.

Also, an incomplete defrost will lead to operating with an iced up coil which could cause liquid overfeed throughout the refrigeration cycle... again washing the oil from the crankcase.

Having a receiver full of liquid could cause a cool gas defrost to actually send liquid out to the defrosting coil instead of saturated vapor - another possibility.

I fail to see why the liquid line drier would frost up in any scenario, however.

Andy
03-04-2002, 09:37 PM
Hi, would the drier cores refered to be drier shells fitted to the suction lines as filters?
Regards. Andy.

JONAH
03-04-2002, 10:02 PM
thanx for input chaps.
yes andy i actually meant sl drier shells complete with cores.
sorry dan when i said driers i meantlow side driers not liquid line
the latter of your thoughts might be ontrack m8
about 8 months ago we had a discharge pipe crack and loose complete charge.the engineer who recharged system asked me at time how far up liquid level indicator is sufficient,
in the past ive found everything has been ok with half reciever charge aprox 230kg,
when i went to site to check operation liquid level much higher with all stubs calling.
im wondering if the extra charge is causing liquid to be sent rather than sat vapor.
im on site there tomorrow so ill have a good check.

thanx again jonah

Dan
04-04-2002, 04:13 AM
That would interest me. Also, I would like to understand more about your defrost pressure differential schemes. Somebody once shared an observation with me that I never really mentally attended to: During hot gas defrost, a receiver loses liquid.. but during a Koolgas defrost the receiver gains liquid. Perhaps this would be a good puzzle for all of us to ponder over.:)

Dan

Dan
05-04-2002, 03:32 AM
Along my lines of thinking as well, Marc. But would a hot gas defrost be any different? Let's say we use a discharge pressure differential valve to gain the differential. The liquid pressure feeding the branches is less affected? Thus the still refrigerating cases tend to starve less (than a liquid suppression scheme) and the receiver level drops as a result of all the added condensing surface added during defrost?

It sort of works for me.... so the issue in that case would be more a description of possibly different receiver levels depending on whether we use a liquid depression scheme as opposed to a discharge elevation scheme rather than Koolgas versus hot gas?

Gary
05-04-2002, 06:39 AM
I'm picturing the drop in receiver pressure pulling liquid from both the inlet and the outlet, partially draining the condenser liquid seal and the liquid line.

Gary
05-04-2002, 01:59 PM
I have never worked on this type of system, so this is purely speculative, but I'm thinking the liquid line pressure supressor acts as a dam, building the liquid level upstream (in the receiver).

Thinking it would help if I had a clue...LOL

Dan
06-04-2002, 04:53 AM
To add to Gary's thinking, when the receiver is used simply as a surge tank, Koolgas defrost will tend to reduce the pressure and fill the tank unless receiver pressurizing valves are in place.

I think that surge receivers with hot gas discharge blocking valves suffer the same tendency too, now that I recall some old problems with Hill and Tyler racks.... hmmm.. but they also employed floating head pressure schemes and flash gas was a problem wherein employment of various receiver pressuring schemes attempted to increase the subcooling by pushing the refrigerant out of the tank. Lot's of dynamics and points of view to take.

Anyway, back to Jonah's original question, I think it is possible that you could have a 60% running receiver level and draw enough refrigerant into the tank to frustrate the defrost by sending liquid out of the receiver before the defrost is complete. And then, Katie, bar the door.

Sawdust
10-04-2002, 12:15 PM
Hi guys, just a few comments here, it appears to be a very small system which makes me wonder if the pipe sizing for the defrost gas is correct and if it is sufficient to keep the compressor running enough time to maintain a pressure in the receiver. I know it is the enthalpy we are looking at to accomplish the defrost but the superheat in the hot gas must be giving a fair benifit. I have had experience where if the defrost is left on for too long that you actually start condensing the gas in the evaporator and get a liquid build up there causing slugging on start up. In South Africa we prefer the hot gas route as it seems to be more effective and quicker.

Dale