Thermostatic Expansion Valve (Txv) - Carrier 48TC 17 Series Service And Maintenance Instructions

THERMOSTATIC EXPANSION
VALVE (TXV)
All 48TC's have a factory installed nonadjustable
thermostatic expansion valve (TXV). The TXV will be a
TXV Operation
The TXV is a metering device that is used in air
conditioning and heat pump systems to adjust to changing
load conditions by maintaining a preset superheat
temperature at the outlet of the evaporator coil.
The volume of refrigerant metered through the valve seat
is dependent upon the following:
1. Superheat temperature is sensed by the cap tube sens-
ing bulb on suction the tube at the outlet of evapora-
tor 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,
exerting pressure on the underside of the diaphragm.
Therefore, the bulb pressure equals the evaporator
pressure (at the outlet of the coil) plus the spring pres-
sure. If the evaporator load increases, the temperature
increases at the bulb, which increases the pressure on
the topside of the diaphragm, pushing the carrier
away from the seat, opening the valve and increasing
the flow of refrigerant. The increased refrigerant flow
causes increased leaving evaporator pressure which
is transferred through the equalizer tube to the under-
side of the diaphragm. This causes the pin carrier
spring pressure to close the TXV valve. The refriger-
ant flow is effectively stabilized to the load demand
with a negligible change in superheat.
DIAPHRAGM
PUSHRODS
FEEDER TUBES
INLET
OUTLET
NEEDLE
VALVE
SPRING
DISTRIBUTOR
Fig. 20 − Thermostatic Expansion Valve (TXV) Operation
bi-flow, bleed port expansion valve with an external
equalizer. TXVs are specifically designed to operate with
PuronR or R-22 refrigerant, use only factory authorized
TXVs. See Fig. 20.
CAPILLARY TUBE
COIL
BULB
EXTERNAL EQUALIZER TUBE
Replacing TXV
CAUTION
PERSONAL INJURY HAZARD
Failure to follow this CAUTION can result in injury
to personnel and damage to components.
Always wear approved safety glasses, work gloves,
and other recommended
Equipment (PPE) when working with refrigerants.
1. Disconnect all AC power to the unit. Use approved
lockout/tagout procedures.
2. Using the gauge set approved for use with Puron (R−
410A) refrigerant, recover all refrigerant from the
system.
3. Remove the TXV support clamp.
4. Disconnect the liquid line at the TXV inlet.
5. Remove the liquid line connection at the TXV inlet.
6. Remove the equalizer tube from the suction line of
the coil. Use a tubing cutter to cut the brazed equaliz-
er line approximately 2 inches (50 mm) above the
suction tube.
7. Remove the bulb from the vapor tube above the evap-
orator coil header outlet.
8. Install the new TXV; avoid damage to the tubing or
the valve when attaching the TXV to the distributor.
Protect the TXV against over−temperature conditions
by using wet rags and directing the torch flame tip
away from the TXV body. Connect the liquid line to
the TXV inlet by repeating the above process.
9. Attach the equalizer tube to the suction line. If the
replacement TXV has a flare nut on its equalizer line,
use a tubing cutter to remove the mechanical flare nut
from the equalizer. Then use a coupling to braze the
17
Personal Protective