Automatic Defrost System

Dear Friends

I think that we have discuss deeply the ammoniac hot gas defrost system for evaporators

The question is how to start the defrost timeless?

What signals should I set to start the defrosting no by time?

We know that a temperature sensor located in the coil (coldest zone) will end the defrost cycle. But how to start each cycle.
If there are 6 evaporator how will be the secuency?

I am dealing with a room at -28ºC ,pumped ammoniac 4:1 , Evaporating at -33ºC/Tc +35ºC ,Total load 420kw
Appreciate you comments
Regards gwapa

I don't know any good reliable sensor to initiate defrosting. Many PLCs use cumulative time to start it. This is operating time of an evaporator.

Hi guapa,
In some cases for freezers I have seen "frost thickness detectors". When a ice or frost thicness is detected, you a get an input for the beginning of defrosting. Owner was happy with these detectors, but unfortunetly I don't know where you can find more information about them.
I hope this helps,
David

To balancing load of Evaporator with each other in liquid line you have adjusting valves((Trotle valve)) . during defrost time also it adjust our liquid return from evaporator to our system .
I use timer (( 24 h or week timer)) or start –stop with timer (( time will adjust by condition of room using))

Why you are looking at timeless control Whatever you do in that system, it shall always be with problems. The ice thickness method though looks fine on paper, is the most troublesome - atleast our experience is that so.

Dear ,
You can use " Eva Coil Temp - Eva Air Out Temp" as a criterion for defrost . As after sufficient ice formation, this diffenence keeps on increasing.Hence in my view if you properly measure the said temps, then you can conculde.

Thanks
amit saxena

To start defrost you can use Air flow switch.
If ice-up in Evaporator, your Air flow comes down and you can use this case by putting Air flow switch to start your defrost system

mbc
could you be more specific?
How the air flow switch work?
the air should be very dry.
Should it?

Hello smpsmp45
Why you said that? We will have a central computer and we can do enything.
The only problem is the ice detector. I agree with you that it is a thing to think. It should be a solution
regards
Gwapa

Dear amitsaxena
tanks for your advice .Could you go more in details
thanks
Gwapa

air flow switch is similar to water flow switch it goes on and off by air. It has plat you
put front of air it push it and switch goes on .you use this on to start your system also
if you have computer system monitoring your system might you can use electronic flow rate to get you start some of them has port it can connect to computer

One method I have seen that works relatively well is using the liquid solenoid valve ON time. After so many minutes have elapsed the coil should need a defrost. The run-time is accumulative for each cycle.

air flow control F62 AA -9001 from PENN Co.