AEC ACOA Series Air-Cooled Central Chillers - Manual (Page 38)

5-6 Capacity Control
The standard AEC/Application Engineering ACOA chiller capacity control operates as follows:
• As the chiller load initially drops, the suction pressure of the compressor(s) start(s)
dropping proportionately, thus balancing minor load variations.
• Variation of unit capacity in response to system load requirements is controlled by an
operating thermostat which monitors the return water temperature.
• Single compressor packages are controlled by cylinder unloading or hot gas bypass. Up to
20 tons (60,480 Kcal/hr) nominal size cylinder unloading is achieved by blocking the
suction gas from entering the cylinders on one bank.
Hot gas bypass operates by imposing an artificial load on the evaporator. Discharge gas
from the compressor is introduced to the liquid-vapor mixture of refrigerant downstream of
the expansion valve. The discharge gas is cooled by the liquid refrigerant present in the
turbulence of the evaporator so that the final temperature does not rise. Hot gas bypass
does not offer any energy savings; it does allow the cooling capacity of the equipment to
vary precisely with the load requirements.
• On multiple compressor units, a combination of cylinder unloading and compressor staging
controls capacity.
Note: See Figure 19 on the following page to determine the capacity control scheme for your
specific unit.
ACOA Series Air-Cooled Central Chillers
Page 37