ASHRAE Journal - February 2020 - 44

ASHRAE - CELEBRATING 125 YEARS

FIGURE 10 Valve cross charge.

FIGURE 11 (Left) Cap tube. (Right) Piston (small).

140
120
Pressure (psig)

100
80
60
40

Bulb Charge Curve

20
0
-40

R-507A Curve
-31

-22

-13

-4
0
14
Temperature (°F)

simply not realistic.
Seasonal temperature changes have a profound effect
on system performance, as does increasing the heat load
inside. For example, summer brings warmer weather,
which increases the condensing pressure of the refrigerant in the air conditioner. Since the throttling device acts
as a type of dam within the system, a fixed orifice device
will not open more when the load increases to allow
more refrigerant through, which increases the superheat
to the compressor. Additionally, once the load decreases
(perhaps at night), the back pressure drops, which significantly increases the risk of liquid refrigerant making
it to the compressor, which would likely damage it.
A TXV will modulate open or closed based on changing conditions and will adjust to maintain a constant
superheat. This modulation ensures that efficiency is
optimized and that the compressor is protected against
damage from liquid refrigerant. Unless the system is in
a laboratory and only tested under one condition, a TXV
will always be the better choice.
While TXVs were originally invented for use in refrigerators, today they are used in a wide range of applications
from refrigeration to air conditioning. Because TXVs
are able to better regulate the flow of refrigerants based
on the needs of a system, compared to other throttling
devices, systems with TXVs have shorter pulldown times.

the longest life possible, with many valves testing above
200,000 cycles.
Another common point of failure is the sensing bulb
capillary tube (Figure 13). Once again, stainless steel
offers flexibility and durability not available with other
metals. Stainless steel sensing bulb coils can be bent and
flexed over and over without fear of cracking, as is typical with copper capillary tubes.
Finally, TXVs are often used to upgrade a system from
a fixed-orifice device, so a TXV that is easy to install is
most ideal. Retrofitting a fixed-orifice device system
with a TXV is simple, especially when the valve comes
packed with the most common fittings to attach to the
evaporator distributor, like flare, Aeroquip, or Chatleff,
making upgrades hassle-free.

Factors That Led to the TXV's Popularity in the Market
The two major factors that have led to TXVs' popularity:
performance and price. As discussed above, TXVs outperform fixed-orifice throttling devices while also better
protecting against liquid refrigerant from flowing back
into the compressor. However, there is a device that can
even outperform a TXV: an electronic expansion valve
(EEV). As will be discussed later, EEVs are more precise
than TXVs but are currently much more expensive. With
TXVs in in the middle, both in terms of price and performance, it is easy to see why so many have chosen TXVs
for their cooling system.

Not All TXVs Are Created Equal5

The Key Role of TXVs in the High-Efficiency Market

TXVs are not just about efficiency and compressor
protection-another important factor is reliability. The
power element is the most common part of a TXV to
fail, so having a power element that is made from highquality materials will extend the life of the valve. Laserwelded, stainless steel power elements (Figure 12) offer

In the United States

ASHRAE JOURNAL

ashrae.org

FEBRUARY 2020

In the US, there are two significant energy efficiency
governing bodies: ASHRAE Standard 90.16 and the US
Department of Energy (DOE).
90.1 sets the standard of what constitutes a high-efficiency building, including setting minimum efficiency

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ASHRAE Journal - February 2020

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