Thermostatic Expansion Valve (Txv); Txv Operation - Bryant Preferred Series Service And Maintenance Instructions

THERMOSTATIC EXPANSION VALVE (TXV)

All 581J units have factory-installed nonadjustable thermostatic
expansion valves (TXV). The TXVs are bleed port expansion
valves with an external equalizer. TXVs are specifically designed
to operate with Puron ® or R-22 refrigerant. Use only factory au-
thorized TXVs. See Fig. 18.
DIAPHRAGM
PUSHRODS
FEEDER TUBES
INLET
OUTLET
NEEDLE
VALVE
SPRING
DISTRIBUTOR
Fig. 18 — Thermostatic Expansion Valve (TXV) Operation
TXV SENSOR
BULBS
CIRCUIT 2
CAPILLARY TUBE
COIL
BULB
EXTERNAL EQUALIZER TUBE
CLAMP
Fig. 19 — TXV Sensor Bulb Location

TXV Operation

The TXV is a metering device that is used in air conditioning and
heat pump systems to adjust to the changing load conditions by
maintaining a preset superheat temperature at the outlet of the
evaporator coil. See Fig. 19.
The volume of refrigerant metered through the valve seat is de-
pendent upon the following:
1. Superheat temperature is sensed by the cap tube sensing
bulb on suction the tube at the outlet of evaporator coil.
This temperature is converted into pressure by refrigerant
in the bulb pushing downward on the diaphragm which
opens the valve using the push rods.
2. The suction pressure at the outlet of the evaporator coil is
transferred through the external equalizer tube to the
underside of the diaphragm.
3. The needle valve on the pin carrier is spring loaded, exert-
ing pressure on the underside of the diaphragm. Therefore,
the bulb pressure equals the evaporator pressure (at the
outlet of the coil) plus the spring pressure. If the evapora-
tor load increases, the temperature increases at the bulb,
which increases the pressure on the topside of the dia-
phragm, pushing the carrier away from the seat, opening
the valve and increasing the flow of refrigerant. The
increased refrigerant flow causes increased leaving evapo-
rator pressure which is transferred through the equalizer
tube to the underside of the diaphragm. This causes the pin
carrier spring pressure to close the TXV valve. The refrig-
erant flow is effectively stabilized to the load demand with
a negligible change in superheat.
TXV SENSOR
BULB
TXV
(CIRCUIT 1)
15
TXV
SENSOR
BULBS
TXV
(CIRCUIT 2)
CIRCUIT 1