Capacity control for multi-stage compressors

In a refrigeration means under microprocessor control for cooling a zone including a closed circuit serially including compressor means, condenser means, economizer means connected to said compressor means, expansion means and evaporator means, a method for operating the refrigeration means during pulldown at high ambient temperature where the compressor means has three banks, a crankcase, a suction inlet connected to the evaporator, means for controlling mass flow to the three banks, and a discharge connected to the condenser comprising the steps of:

• supplying gas from the suction inlet to the first and second banks;
• supplying gas from the crankcase to a third bank of the three banks;
• delivering compressed gas from the third bank to the discharge;
• selectively connecting the first and second banks to either the crankcase or the discharge whereby when said first and second banks are connected to the crankcase they act as a first stage and the third bank acts as a second stage and when the first and second banks are connected to discharge they act as a single stage and the third bank acts as a single stage in parallel with the first and second banks;
• sensing at least one of ambient and condenser entering air temperature;
• sensing zone temperature;
• sensing zone set point;
• comparing zone temperature and zone set point and if the sensed ambient or condenser entering air temperature is on the order of 100.degree. F., or above, the sensed zone temperature exceeds the zone set point by 5.degree. F., or more, and the first, second and third banks are in single stage operation, performing the serial steps of:
• reducing the capacity of said first and second banks to reduce crankcase pressure;
• switching over from single stage to two-stage operation;
• enabling the economizer whereby capacity is increased and the pulldown is speeded up.