Carrier WeatherMaster Puron 48HC D17, Service And Maintenance Instructions, Page: 15 / 100

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Fig. 19 — Thermostatic Expansion Valve (TXV)
Operation
TXV Operation
The TXV is a metering device that is used in air conditioning
and heat pump systems to adjust to the changing load condi-
tions by maintaining a preset superheat temperature at the out-
let of the evaporator coil. See Fig. 20.
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.
Fig. 20 — TXV Sensor Bulb Location
OUTLET
INLET
PUSHRODS
DIAPHRAGM
NEEDLE
VALVE
SPRING
DISTRIBUTOR
FEEDER TUBES
CAPILLARY TUBE
COIL
BULB
EXTERNAL EQUALIZER TUBE
TXV SENSOR
BULB
CLAMP
CIRCUIT 1
CIRCUIT 2
TXV
(CIRCUIT 2)
TXV
(CIRCUIT 1)
TXV SENSOR
BULBS
TXV
SENSOR
BULBS