Trane RTHD

RTHD-SVX01D-EN 83
Operating Principles Mechanical
Lubricant Recovery
Despite the high efficiency of the oil separators, a small percentage of oil will
get past them, move through the condenser, and eventually end up in the
evaporator. This oil must be recovered and returned to the oil sump. The
function of active oil return is accomplished by a pressure-actuated pump
referred to as the “gas pump.”
The gas pump, mounted just beneath the evaporator, is a cylinder with four
ports controlled by two solenoids. The pump serves to return accumulating
oil in the evaporator to the compressor at regular time intervals. As the refrig-
erant-oil mixture enters the gas pump from the bottom of the evaporator, a fill
solenoid opens to allow refrigerant vapor to be vented into the top of the
evaporator, and is then closed. A second solenoid then opens to allow refrig-
erant at condenser pressure to enter the gas pump. Simultaneously, a check
valve prevents reverse flow back into the evaporator. A liquid refrigerant and
oil mixture is displaced from the gas pump cylinder and is directed through a
filter to the compressor. The oil then combines with oil injected into the com-
pressor and returns to the oil sump via the oil separators.
Oil Cooler
The oil cooler is a brazed plate heat exchanger located near the oil filter. It is
designed to transfer approximately one ton of heat from the oil to the suction
side of the system. Subcooled liquid is the cooling source.
The oil cooler is required on units running at high condensing or low suction
temperatures. The high discharge temperatures in these applications
increase oil temperatures above the recommended limits for adequate lubri-
cation and reduce the viscosity of the oil.