Thermostatic Expansion Valve (Txv) - Carrier 48HC D17 Series Service And Maintenance Instructions

THERMOSTATIC EXPANSION VALVE
(TXV)
All 48/50HC's have factory installed nonadjustable
thermostatic expansion valves (TXV). The TXVs are
INLET
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. 23.
The volume of refrigerant metered through the valve seat
is dependent 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,
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
pressure. 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 underside of the diaphragm. This causes
the pin carrier spring pressure to close the TXV valve.
The refrigerant flow is effectively stabilized to the
load demand with a negligible change in superheat.
DIAPHRAGM
PUSHRODS
FEEDER TUBES
OUTLET
NEEDLE
VALVE
SPRING
DISTRIBUTOR
Fig. 21 - - Thermostatic Expansion Valve (TXV) Operation
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. 18.
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 unit. Use approved
lockout/tagout procedures.
2. Using gauge set approved for use with Puron
(R- - 410A) refrigerant, recover all refrigerant from the
system..
3. Remove TXV support clamp.
4. Disconnect the liquid line at the TXV inlet.
5. Remove the liquid line connection at the TXV inlet.
6. Remove equalizer tube from suction line of coil. Use
tubing cutter to cut
approximately 2- - inches (50 mm) above the suction
tube.
7. Remove bulb from vapor tube above the evaporator
coil header outlet.
8. Install the new TXV avoiding damage to the tubing or
the valve when attaching the TXV to the distributor.
Protect the TXV against over- - temperature conditions
y 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,
18
Personal Protective
brazed equalizer line