View Full Version : Hot Gas Defrost for Tray Tunnel Freezer Evaporator

27-09-2017, 12:10 PM
Dear Guys,

Need your help. We are planning to install a tray tunnel freezer with 380kW refrigeration load (2 evaporators). We are looking for option to install a stand alone ammonia refrigeration system which is a packaged chiller operating at -40/+38 degC. This package chiller composed of screw compressor, LP Vessel, Ammonia Pump and condensing unit.

My issue is if the stand alone refrigeration unit enough to supply hot gas defrost for the tray tunnel freezer evaporators. Any thought? Thanks

27-09-2017, 05:20 PM
what kind system do you have .
is it flooded system ,
or it is pump system NH3

For flooded system in tunnel freezer hot water defrost is better than hot gas

27-09-2017, 09:34 PM
As mbc say's hot water better, as usually have a cleaning cycle anyway & this is carried out then.
System would not be able to generate enough hot gas if defrosting either 1 of the 2 evaporators.
An artificial load or heat sink is probably the only other way in some cases to generate hot gas for defrost.
CO2 systems use hot glycol circuit in evaporator, not sure if it has been used for this purpose as usually more expense.

27-09-2017, 11:43 PM
Hi MBC/Ranger1,

Yes indeed, the system uses ammonia pump to send liquid going to the evaporator. Are you saying that hot gas from the discharge is not enough to defrost the evaporators? Thanks You

27-09-2017, 11:54 PM
Yes because not enough can be generated for good defrost.
Compressor probably unloaded with nothing to do, so no gas generated.

27-09-2017, 11:56 PM
Thanks guys,

probably a best solution is hot water defrost or manual(air) defrost.

Do you have any files/media for evaporators using hot water defrost. Thanks

28-09-2017, 04:12 PM
What type of Tray Tunnel Freezer you are purchasing is it one stage or two stage temperatures operation and type of operation, for batch freezing or continuous operation. The system you planning to install as per your notes is liquid recirculation [NH3 pumps and re-circulators] . Hot gas defrost will work , is good for low and medium temperatures.
Industry standard recommendations for hot gas defrost: 1/3 of the evaporators in a system should be defrosted at one time. Example in your case :if your total tunnel evaporator capacity is 108 tons (380 kW) [380 kw / 3.517 kw/tr], then you need three evaporators each sized for 36 tons (126 kW) of capacity that should be defrosted at once.

For continuous operation you can use sequential defrost , each evaporator in the group of evaporators is defrosted in turn while the remaining evaporators in the group continue to operate in the refrigeration mode , say two evaporators running while one is on defrost .
Air defrost system [batch processes only] or sequential defrost. Water defrost is not recommended for sequential defrost systems or continuous operation, also need to watch the water temperature and defrost procedure.
I recommend you to consult the factory for clarification


Camille Zabbal

29-09-2017, 11:34 AM
Hi Camille,

The tray tunnel freezer composed of 2 190kW evaporator. And the refrigeration attached is a packaged chiller operated at -40/+38 degC. The issue is the hot gas capacity during defrosting as Ranger said.

02-10-2017, 08:08 PM
Actually, I don't see how this should not be possible....

depending on the level of automation.

if a defrost cycle is started, where all discharge gas is diverted from the condenser to one of the evaporators, and the hotgas return is lead back into the suction drum, this will create a state where there is more refrigerant being lead back to the suction side of the compressor than there is refrigerant being moved away. from experience, the compressor will try to decrease capacity, before the suction pressure rises and the compressor finds itself needing to increase capacity to deal with the sudden influx of gas :)

A hotgas defrost can be made on a system like this :)

02-10-2017, 09:29 PM
I think we would have to know layout of tunnel & it's exact operation, product, before considering it.
-40 is hard to defrost anything at the best of times, as big unit's, lot's of hot gas.

03-10-2017, 07:57 PM
Renger 1
l think best way for his defrost in that system as I wrote is hot water .

03-10-2017, 09:12 PM
Renger 1
l think best way for his defrost in that system as I wrote is hot water .

mbc, I agree with you but still stand by what I wrote, as we know very little.
Convince Tycho, not me!

03-10-2017, 10:17 PM
Hi all,

many years ago we installed one similar system ... two tunnels running in parallel ... outcome of defrosting was not good ... it was not enough hot gas only from one tunnel to defrost the second one ... it was good if in time of defrosting was working some other deep freezing room or some other high temperature room ... operators needed quite a lot of time to learn how to manage defrosting ... kind of game ... new experience for all of us ...

With tray tunnel I am not sure what about tunnel capacity during defrosting of one evaporator(sequential defrosting) ... installation of third evaporator is possible, but sequential defrosting process cannot be cheap .... also installation of complete required equipment would be at higher price ....

I believe it is batch freezing and in my opinion the best will be ... finish with freezing of one batch - discharge the batch - fast defrosting with water - cleaning and drying of tunnel and start with second batch ...

Of course I can be wrong, because we do not know anything about facility ....

Best regards, Josip