View Full Version : hot gas flow for hot gas defrosting

22-10-2012, 04:19 PM
As per ASHRAE Refrieration 1/3 of the evaporators should be undertaken for defrosting at a time for hot gas defrosting. i.e. hot gas generated by 2/3 of the refrigeration plant is required for effective defrosting of the evaporator. Therefore, in case there are only two evaporators in the system and one has to provide hot gas defrosting, the hot gas generated by 1/2 the refrigeration system may not be sufficient. I would like to know, if hot gas defrosting is provided in such system, what will be effectiveness of the defrsoting? Will the defrosting period will be longer? if longer, how to estimate the time required for defrosting. Could any one guide in this case?

22-10-2012, 11:53 PM
Trial and error, are they in separate rooms ?

23-10-2012, 05:08 AM
I totally agree with NH34life, there are so many variables envolved that you have not included in your question.

23-10-2012, 07:44 AM
OK I will try to more specific. There are 2 evaporators of 130 kW capacities working on Ammonia refrigerant with evap temp of -40 deg C. The refrigeration system is with two stage reciprocating compressors & evaporative condensers. Both the evaporators are to be defrosted by hot gas. During defrosting of one evaporator the other evaporator will be operating normal. As per ASHRAE hot gas requirement for defrosting of one evaporator there should be other evaporators of double the capacity running normally. However in this case we have only other evaporator of equal capacity running. Naturally the hot gas available is 50% only. What way this will affect the effectiveness of defrosting? Will the defrosting time will double?

25-10-2012, 12:22 AM
This is not question of efficiency. Assume that you operate evaporator 1 and defrost evaporator 2. When defrost initiated, a lot of hot gas will go into evap.2. Much more than gas produced by evap.1. Condensing pressure will go down. At certain point amount of vapor evaporated in evap.1 will equalize with amount of vapor produced by evap.1. Assume that this pressure is 8 bars or 120 psig. One evaporator will defrost at this pressure another will not. It depends.

12-02-2013, 08:04 PM
It boils down to you do what you have to do. I have a blast freezer with 2 units, 60 tons and 20 tons. A storage freezer with 3 12 ton units and a cooler with a 12 ton unit. I can not defrost one of the blast freezer units and run the other since any air movement in the room causes unsatisfactory defrosting. So, when that 60 ton coil needs to defrost I have perhaps a total of 48 tons with which to do it. Force on all three storage freezer units and jump out the stat so they all call for refrigeration.

It gets better. If the defrosting unit steals too much of the hot gas I run low/out of liquid for the liquid injection cooling on the screws.

Then... the powers that be decide that both coils in the blast freezer are to be defrosted simultaneously to maximize production time. Now it's 80 tons in defrost with around 48 tons supplying the gas.

It works. Sometimes lose the plant on screw oil temperature when the pilot receiver for LIC runs dry but, for the most part, it works.

20-02-2013, 02:56 AM
I think that the issue is more complex than that. You are talking about nominal load of the evaporator, and normally you select the evaporator for pick load or for mean load. But what happen when the freezer is not receiving goods. The load is small and you will have less heat to defrost the second coil.Observe, at the moment you defrost the coil and use hot gas to, you are introducing to the system a negative heat It is like another condenser but at a lower atmosferic temperature.
So you have to 1) make a balanace of heat getting in and getting out from system at low load 2) you have to control the pressure in the receiver. Maybe you have to install VFD or shut off condenser.
Regards Gwapa

20-02-2013, 08:22 PM
Would you think of heating glycol and pumping this over or through the evaporators, have seen it on a GEA ammonia system, one defrost per day though and evaporators adapted for use.