Selection of Controls for Hot gas defrosting

[vchatharam]

Could any one please tell me how to size and select controls for hot gas defrosting for a frozen cold room with gravity feed ammonia refrigeration.

Regards
R Sivakumar

 

[Macbeth]

By controls do you mean?
How long should each step be Such as
15 Min for pump out
20 min for hot gas Cycle
5 Min for vent Cycle
5 Min for fan delay

Or do you mean Use Defrost clock PN. XXX-XX-XXX

Or do you mean does any one know of a good system integrator who specializes in Refrigeration Control Systems.

The size of the controls system depends on the number of units, pipe layout and number of valves per unit. Also type of valves such as Dual pressure reg on suction ect. Give more info and I will be glad to assist.

 

[VEEKRISH]

Sivakumar,

Are you having multiple evaporators ?

The problems you may encounter in hot gas defrost are -

Since no liquid is vaporized in the evaporator during the defrost cycle, the amount of gas available from the compressor will be limited. As defrosting progresses, more liquid remains in the evaporator and less refrigerant is returned to the compressor for re-circulation. As a result the system soon runs out of heat even before defrosting is complete.

Another more serious problem is a possibility of liquid flood back to the compressor. This is more likely at the begining of a defrost cycle or just after it is complete.

The way to over come this would be to provide a re-evaporator

For controls all you need will be good solenoid valves and timers.

Lemme know if you need more input.

Krishnan

 

[chillin]

By controls, I'm guessing you mean control valves, not necessarily control sequence or logic. To properly defrost a freezer evaporator you will need sufficient hot gas, as veekrish has stated. Providing you don't try to defrost more than 1/3 of your total system capacity, you should have enough gas, which is the case with many ammonia refrigeration systems.

You must define "gravity feed". If this is a flooded fan coil, then there are two important design elements required to properly defrost the coil. The first is an offset in the horizontal run of the liquid line at the coil to allow for proper draining of the coil during defrost. The second is the sizing of the flooded surge drum. If defrost condensate is relieved into the drum, the drum should be sized to accomodate this volume of liquid. It should also be large enough to prevent liquid carryover due to high velocities at the end of the defrost cycle.

To properly size these lines and valves, the capacity of the coil, operating conditions (Tevap, Tliq), defrost pressure and vessel size are needed. You can consult 1994 ASHRAE Refrigeration Chapter 3 for more information.

chillin