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TRG-TRC011-EN 45
period three
Capacity Control
notes
This example purge system consists of a purge tank, a small refrigeration
system, a pump-out system, and controls. The purge’s refrigeration system
includes: a small compressor, an air-cooled condensing coil, an expansion
valve, and an evaporator coil located inside of the purge tank.
When the chiller is operating, air migrates to the absorber, the area of the chiller
operating at the lowest pressure. In this example purge system, an eductor
system moves the air from the absorber to the condenser. Because the purge
evaporator operates at a lower temperature and pressure than the chiller
condenser, a mixture of refrigerant vapor and air is drawn from the chiller
condenser into the purge tank. Inside the purge tank, the refrigerant condenses
on the cold tubes of the evaporator coil, collects in the bottom of the purge
tank, and returns to the chiller condenser as a liquid.
The air does not condense, but instead accumulates in the top portion of the
purge tank. Eventually, enough air accumulates to cover a large portion of the
purge evaporator coil. The air insulates this coil, impeding heat transfer and
reducing the temperature of the refrigerant inside the purge evaporator coil.
When the purge refrigerant temperature drops below the setpoint, a controller
signals the need for a pump-out sequence. The controller opens the isolation
valves, allowing the air to be pumped out of the purge by a vacuum pump.
When the purge refrigerant temperature rises again, the controller closes the
isolation valves.
The purge controls can be used to track and record how often pump-out occurs.
Excessive purging activity may indicate an air leak or depletion of the corrosion
inhibitor. The results can be decreased capacity, increased risk of internal
corrosion, and possible crystallization. Leaks can be detected early by
comparing pump-out activity over the last 24 hours to the 30-day average.
refrigerant vapor from
refrigerant vapor from
chiller condenser
chiller condenser
liquid refrigerant
liquid refrigerant
returning to
returning to
chiller condenser
chiller condenser
purge evaporator
purge evaporator
coil
coil
purge
purge
tank
tank
vacuum
vacuum
pump
pump
isolation valve
isolation valve
Figure 53