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Thermal Expansion Valves

Emerson Climate Technologies

Balanced Port TXV Operation

In conventional TXVs, as the pressure drop across

the TXV port changes due to changes in head pressure

or suction pressure, the operating superheat of the TXV

will vary.

Depending on the operating conditions under which

the superheat was originally set, this “unbalance” can

sometimes result in compressor ooding or evaporator

starvation. A unique design concept called “Balanced

Port” cancels the effect of this pressure unbalance, per-

mitting the TXV to operate at a fairly constant superheat

over a wide range of operating conditions.

There are 2 fundamental Balanced Port designs:

Double Ported Design (Figure 11a) – In this design,

there are 2 paths for the refrigerant to ow. One path

creates a force that tends to push the pin in the “open”

direction; whereas the other path creates a force push-

ing the pin in the “closed” position. These paths are de-

signed in such a way that the forces generated in each

path are equal to one another, resulting in a “balanced”

design.

Single Ported Design (Figure 11b) – In this design, the

valve pin has a shoulder added that is on the inlet side

of the valve. The high pressure times the area of the

shoulder results in an upward (closing) force. The pres-

sure differential across the pin results in a “downward”

force. By designing the shoulder carefully, the downward

force is negated or “balanced”.

Any refrigeration system which experiences changes

in operating pressures because of varying ambient, gas

defrost, heat reclaim, or swings in evaporator load will

benet from using a balanced port TXV.

M.O.P.

Maximum Operating Pressure (sometimes referred

to as Motor Overload Protection) is the ability of a TXV

to close down, starve, or shut off if the suction pressure

should approach a dangerously high predetermined limit

condition. These conditions could overheat a suction

cooled compressor or load the crankcase with too dense

a vapor pressure. With the TXV in a closed condition

the compressor has a chance to pull the suction back

down to satisfactory operating conditions. Once below

the MOP, the TXV will re-open and feed normally or until

there is an overload again.

Power Element Charges

There are several basic types of charges in use

today. Most common are the: liquid charge; gas charge;

liquid cross-charge; gas cross-charge; and the adsorp-

tion charge.

Liquid Charges

The power element contains the same refrigerant

as the system in which the TXV is used. When manu-

factured, it is put into the remote bulb in a liquid state.

Volume is controlled so that within the design tempera-

ture range some liquid always remains in the bulb.

Therefore, the power element pressure is always the

saturation pressure corresponding to the temperature of

the remote bulb.

OUT

BALANCED CAGE

ASSEMBLY

CONVENTIONAL

CAGE ASSEMBLY

Fig. 11

Fig. 11a

Fig. 11b

1 2 ... 8 9 10 11 12 13 14 15 16 17 18 ... 71 72

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