View Full Version : Defrost calculations

04-04-2007, 07:00 PM
Hey all. I am still working on the refrigerated warehouse design for my school project, that i was asking before.


Now I am trying to figure out the defrost system for the evaporators, and honestly have no clue where to start or anything. I was thinking of using hot gas defrost or cycle reversal for the system. Below are the temperatures that I have calculated for the freezer coming in and out of the evaporators.

Any help I can get would be a plus, from what equations to use and what I need to calculate to jsut where I can find the right place to get this information.

Temp air in: -35 F
Temp air out: -40 F
Temp NH3 in and out: -41 F



04-04-2007, 08:36 PM
Hi bnuncc07,

If your system uses ammonia then you should go for hot gas defrost.
I hope ammoina guys will help you on that.

Go to link below and find a paper named Defrosting. It explains a lot and it is free:p .


15-12-2007, 07:40 PM
it is part of the design and selection that the thickness of frost formed is stated.

from coil surface and thickness, u get the kW to melt the frost.

next, you decide the method to defrost.

3] water
4] hot gas

after that, you have to satify yourself on the final air temp before defrost cycle stops. that will give u good indications on the process.

hope this helps even though it is bit late.

16-12-2007, 12:13 PM
Because you're posting this question in the NH3 section, hotgas is the only right way to do this.
Cycle reversal is for split AC's and small refrigeration applications.

All rooms that goes below 2C (difficult to read isn't it if you're used to work with SI units) must have a hotgas defrost.

You then need evaporators that can handle hotgas defrost (additional coil in the drain pan)

17-12-2007, 03:54 AM
yes, sometimes, if the client has heat regen facilities, you may want to channel hot water for defrosting mechanism.

energy saving as the water can be recycled & does not reduce the overall coolign performance.

well, all possibilities have to be explored, right?

17-12-2007, 05:01 AM
Hi Ben
There may be some energy savings in using water defrost but there are a lot of problems with water defrost systems. The holes in the distribution trays can block, no ice removeal of return bends as the trays are only over the coil block, drains can block and flood the room and increased defrost times.
I agree with Peter that hot gas is the best option and it would be my prefered choice.