What mechanism activates the capacity control of the compressor?

The capacity control of a compressor is primarily activated by suction pressure. This mechanism is crucial for maintaining the efficiency and performance of the refrigeration or air conditioning system. When the suction pressure drops, it indicates a reduction in the load or demand for cooling. The compressor responds to this change by adjusting its capacity, often through various methods such as unloading cylinders or cycling the compressor on and off.

In contrast to suction pressure, other factors also influence the operation of the system but do not directly activate capacity control in the same primary way. For instance, discharge temperature can provide insights into the system's performance but does not directly manage the compressor's capacity. Oil temperature is important for lubrication and overall compressor health but similarly lacks the direct influence on capacity adjustment. Liquid line pressure can also fluctuate with changing conditions but operates more as a measurement related to the refrigerant's state rather than an active control mechanism for compressor capacity.

Thus, the reliance on suction pressure for capacity control enables the compressor to respond effectively to varying load conditions, optimizing system performance and energy use.