19
3.4 Pump-Down Control
Compressors with suction cut-off unloading have inher-
ently greater internal leak paths than compressors with-
out this capacity control feature installed on them.
Compressors with the electric capacity control have been
successfully applied with continuous or automatic pump-
down control. To avoid compressor short-cycling, the
system designer should avoid continuous pump-down
control when the suction pressure cut-in and cut-out
points are within 30 psi (2.1 bar) of each other. Because
the pressure actuated unloaders have a small built-in
high to low pressure path, short-cycling will occur if
continuous or automatic pump-down control is applied.
Do not use continuous or automatic pump-down
control on any compressor equipped with pres-
sure actuated capacity control. Use single pump-out
or solenoid drop (minimum protection) control.
3.5 Unloader Differential Setting
The unloader differential setting is the suction pressure
difference at which the controller loads and unloads the
particular cylinder bank. A differential setting that is too
low could cause the unloader mechanism to short cycle.
For example, when an 06D or 06E compressor unloads,
the suction pressure rises because the compressor
capacity has decreased by one-third or one-half. If the
differential setting is too close, this rise in suction pres-
sure may be enough to load the compressor back-up.
For this reason, Carlyle recommends a minimum differ-
ential setting of 6 to 10 psi (.4 to .7 bar). The final set-
ting should be evaluated for the final application because
many variables are involved:
• Single or multiple compressor application
• Low or medium temperature application
• Unloading 4 or 6 cylinder (1/2 vs. 1/3 load
reduction)
• Refrigerant 12, 22, 502, 507, 404A, or 134a
Allow 1/3 load at 0 SST/115 SCT.
To Next Page
Loading...
Need help?
General
