in a positive displacement compressor having a plurality of banks , operation can be multi - staged or single staged . single stage operation can be of a single bank or plural banks in parallel . switchover between modes of operation is under the control of a microprocessor responsive to sensed inputs . during pulldown under ambient or condenser entering air temperatures of 100 ° f ., or more , suction modulation is used to limit the suction pressure thereby permitting a switchover to two - stage operation and the use of an economizer .