cooling load- defrost

hellos
what is the guideline what allowance has to be made for the defrost on the cooling load calculation
electrical elements as well as hot gas defrost

For your set cooling period, look at the number of defrosts needed, the length of time each defrost lasts and the power of the heating elements. Then use the heat input from all of those defrosts as a heat load in your cooling load calculation.
The hot gas defrost wont be as easy to calculate, youd need to work out the amount of heat transfered from the hot gas.

I think you would overestimate the heat load from defrosting if you used the input power of all the heating elements. A substantial amount (I would say most) of that heat energy is used to melt ice, which has no effect on local temperature.

If you knew the mass of the evaporator and materials of construction (easy enough to figure out), and had a decent idea of its temperature at the end of a defrost cycle, you could figure the refrigeration load required to cool the evaporator back down after defrost. This would get you in the ball park.

MrFreez717

Hi All,

Once the heat load estimate is concluded (product, infiltration, holding load, fans, people, etc). I would adjust the the "refrigeration run time" in a 24 hour cycle to accommodate for the defrost load. In a cold room / freezer room I would potentially work off 21 x run hours. This will increase the duty required.

See attachment of heat loss during defrost of a blower coil for a project I did for a cold room. I would not normally do it and had to scratch my head but the client wanted it. The exercise proved interesting as the result was that not much heat was rejected into the room as one would imagine.

It is important to set up the defrost sequence. The snap freeze time with cooling on but fans off after defrost will remove residual heat in the steel after defrost and after the drip time.