ASHRAE Journal - February 2020 - 42

ASHRAE - CELEBRATING 125 YEARS

gland might
create friction/
hysteresis in the
valve, requiring
design measures
to negate frictional loss in the
TXV.

FIGURE 7 Internal versus external.

FIGURE 6 Early TXV.

External Pressure
Equalization Line

Universal Versus Anti-Hunt Bulb Charges
While there are several types of bulb charges, two common charges used in air-conditioning systems are universal charge and anti-hunt charge (Figure 9).
With a universal charge, the bulb is filled with a liquid
cross charge. Whenever the bulb senses an increase in
suction line temperature, the liquid expands, increasing the pressure in the enclosed volume, and pushes
the diaphragm down, thereby opening the valve and
allowing more liquid refrigerant into the evaporator.
Unfortunately, vaporization is a dynamic process, which
can produce sporadic superheat at the evaporator outlet. Think of liquid refrigerant changing to vapor like a
pot of boiling water-the liquid does not instantaneously
become a gas once the boiling point is reached, but
changes into steam erratically. Similarly, the bulb might
sense vapor one instant and liquid the next. In this scenario, a bulb with a universal charge will rapidly open
and close the valve, a process called hunting. Hunting
reduces the system's efficiency and increases the risk
of liquid refrigerant making its way to the compressor,
which will damage the compressor.
To avoid hunting, some TXVs have a ballast added to
the bulb, creating what is known as an anti-hunt charge.
The ballast dampens the rate of expansion within the
bulb, stabilizing the bulb pressure against the diaphragm by dampening the rate of phase change to the
bulb charge compared to the rate of temperature change
of the suction line. This stabilization ensures that the
TXV operates more in line with the dynamics of the system and better protects the compressor.

Internally Equalized
TXV

Externally Equalized
TXV

FIGURE 8 Conventional versus balanced.

Pb
Pe

Pb
Evaporator

Pe
Evaporator

Ps
Conventional Port TXV

Balanced Port TXV

FIGURE 9 Bulb charges.

Universal Charge

Anti-Hunt Charge

the TXV to perform similarly regarding the change in
opening degree for a given change in superheat across a
range of evaporator temperatures.

TXVs Compared to Other Throttling Devices4
TXVs earned the place as one of the most common
types of modulating-orifice throttling devices in air-conditioning and refrigeration systems worldwide by being
both efficient and affordable. But how do TXVs compare
to other throttling devices? Fixed-orifice devices like
capillary tubes or pistons are cheaper, so why are TXVs
so popular?

TXVs Vs. Fixed Orifices
Bulb Charge Fluid
There are different ways to approach the charging of
a TXV bulb. The one Danfoss recommends is called a
cross charge. Cross-charged bulbs mix a combination of
different refrigerants with gases to flatten the pressuretemperature (P-T) curve (Figure 10). Cross charges enable
42

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Fixed-orifice devices are ideal throttling devices if
conditions never change. If the load on the system is
constant and the ambient temperature remains steady,
then a simple capillary tube or piston (Figure 11) would
be preferable to a modulating-orifice device like a TXV.
However, outside of a laboratory, static conditions are

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ASHRAE Journal - February 2020

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