What is pull down load and how it is calculated?

It is similar to product load. It is basically heat that need to be removed from a refrigerated product to reduce its temperature to desired level.

Load is calculated using this relation:

Q=W*C*(T2-T1) 

W: Weight of Product

C: Specific Heat

T1: Entering Temperature

T2: Space Temperature

 

If you need more details refer to Principles of Refrigeration : Roy J. Dossat

Pages146,153 and154

When the air conditioning system is shut off at the end of the operating day, some heat still may be stored by the building this heat is known as pull-down load

 

If you have a high suction pressure, the compressor will develop some very high pressures internally before the rotor port open to the discharge. A variable Vi compressor can take care of this to a point. The manufacturers provide a maximum suction pressure for each compressor based on compressor Vi. This is a sustained or lengthy run condition to pull a large system down.

 

Pull down conditions, consider evap temp at least10c° higher and select motor, provide crank-case PR regulator or MOP valve. But do not forget to size condenser to match pull down load.

 

Example, if your design needs -35 c°ࣴࣴࣴࣴevap, take pull down suction temp at -25 c°at least.