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Thermal Expansion Valves

Emerson Climate Technologies

= 45.4 PSIG

= 35 PSIG

= 10.4 PSIG

35 PSIG = 40°F

35 PSIG = 50°F

TXV with internal

equalizer on evaporator with

no pressure drop.

Fig. 1

is charged with the same refrigerant as that in the sys-

tem.

The power assembly pressure (P1), which corre-

sponds to the saturation pressure of the refrigerant gas

temperature leaving the evaporator, moves the TXV pin

in the opening direction.

Opposed to this opening force on the underneath

side of the diaphragm and acting in the closing direc-

tion are two forces: the force exerted by the evaporator

pressure (P2) and that exerted by the superheat spring

(P3). In the rst condition, the TXV will assume a stable

control position when these three forces are in balance

(P1 = P2 + P3). See gure 1A.

Thermal Expansion Valves

The most commonly used device for controlling the

ow of liquid refrigerant into the evaporator is the ther-

mostatic expansion valve (TXV). Also known as thermal

expansion valves, TXVs are precision devices designed

to regulate refrigerant liquid ow into the evaporator in

exact proportion to evaporation of refrigerant liquid in

the evaporator.

Refrigerant gas leaving the evaporator can be regu-

lated since the TXV responds to the temperature of the

refrigerant gas leaving the evaporator and the pres-

sure in the evaporator. This controlled ow prevents the

return of refrigerant liquid to the compressor. The TXV

controls the ow of refrigerant by maintaining a pre-de-

termined superheat.

An orice in the TXV meters the ow into the evapo-

rator. Flow is modulated as required by a needle type

plunger and seat, which varies the orice opening. The

needle is controlled by a diaphragm subject to three

forces:

1. The power element and remote bulb pressure (P1)

2. The evaporator pressure (P2)

3. The superheat spring equivalent pressure (P3)

These forces are shown in Figure 1.

Internal Equalizer

Three conditions are present in the operation of a

TXV:

1. The balanced forces

2. An increase in superheat

3. A decrease in superheat

The remote bulb and power element make up a

closed system (power assembly), and in the following

discussion, it’s assumed that the power assembly

If the temperature of the refrigerant gas at the

evaporator outlet (remote bulb location) rises above the

saturation temperature corresponding to the evaporator

pressure as it becomes superheated (P1 greater than

P2 + P3), the TXV pin moves in an opening direction.

When the temperature of the refrigerant gas leaving

the evaporator decreases, the pressure in the remote

bulb and power assembly also decreases and the com-

bined evaporator and spring pressure cause the TXV pin

to move in a closing direction (P1 less than P2 + P3).

For example, when the evaporator is operating with

R-134a at a temperature of 40°F or a pressure of 35

psig and the refrigerant gas leaving the evaporator at

the remote bulb location is 45°F a condition of 10°F

superheat exists. Since the remote bulb and power as-

sembly are charged with the same refrigerant as that

used in the system R-134a, its pressure (P1) will follow

its saturation pressure-temperature characteristics. With

the liquid in the remote bulb at 45°F, the pressure inside

the remote bulb and power assembly will be 40 psig

acting in an opening direction. Beneath the diaphragm

and acting in a closing direction are the evaporator pres-

sure (P2) of 35 psig and the spring pressure (P3) for a

10°F superheat setting of 5 psig (35 psi + 5 psi = 40 psi)

making a total of 40 psig. The TXV is balanced, 40 psig

above and 40 psig below the diaphragm.

The following sections describe the operation and ap-

plication of single-outlet TXVs in two general categories:

internally equalized and externally equalized.

1 2 3 4 5 6 7 8 9 10 11 ... 71 72

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