ASHRAE Journal - February 2020 - 40

CELEBRATING 125 YEARS

History of Thermostatic
Expansion Valve
BY JORDAN C. LARSEN; BRIGITTE BUSCH

A thermostatic expansion valve (TXV)-an example of which can be seen in Figure 1-
is a refrigeration and air conditioning throttling device that controls the amount of
refrigerant liquid injected into a system's evaporator based on the evaporator outlet
temperature and pressure, called the superheat.¹ Figure 2 shows the different phases
and pressures the refrigerant goes through as it is pumped through the system,
moving through the evaporator, the compressor, the condenser, and the throttling
device which injects liquid refrigerant into the evaporator before it evaporates and
moves into the compressor.
TXV Parts
While there are different types of TXVs, each one has
some things in common: a diaphragm, a power element, a setting spring and an oriﬁce (Figure 3).

TXV Operation
There are three different forces at work in a TXV: bulb
pressure, spring pressure and evaporator pressure
(Figure 4). Bulb pressure comes from the bulb that is
mounted at the outlet of the evaporator. The bulb senses
the suction temperature, which generates an internal
pressure in the bulb that drives the diaphragm down
when the temperature increases. Spring pressure is
constant and pushes up against the diaphragm, counter
to the bulb pressure. The spring pressure is calibrated
when the valve is set by the equipment manufacturer
or the installer. The evaporator pressure pushes the
diaphragm up when the suction pressure increases and

comes from the evaporator load on the system, which
varies according to different operating conditions,
such as changing load conditions. Based on the balance
between these three pressures, the valve will either
open or close.

Factors That Led to the Development of the TXV
Many early refrigeration systems required a human
operator2 to constantly adjust the ﬂow of refrigeration
to prevent liquid refrigerant from ﬂowing back into the
compressor, which would cause irreparable damage. To
properly address this challenge, an automatic refrigerant control was needed. Harry Thompson ﬁled the ﬁrst
patent3 for a TXV on August 24, 1927 (Figure
( igure 5).
5). With
Thompson's invention, refrigeration systems now had an
automated throttling device that also protected against
liquid refrigerant ﬂowing back into the compressor.
Thompson's invention was so successful that he

Jordan C. Larsen is a technical writer and editor for Danfoss North America. Brigitte Busch is a product manager for Danfoss Cooling.
40

ASHRAE JOURNAL

ashrae.org

FEBRUARY 2020

https://www.ashrae.org/

ASHRAE Journal - February 2020

Table of Contents for the Digital Edition of ASHRAE Journal - February 2020

Contents
ASHRAE Journal - February 2020 - Intro
ASHRAE Journal - February 2020 - Cover1
ASHRAE Journal - February 2020 - Cover2
ASHRAE Journal - February 2020 - Cover2a
ASHRAE Journal - February 2020 - Contents
ASHRAE Journal - February 2020 - 3
ASHRAE Journal - February 2020 - 4
ASHRAE Journal - February 2020 - 5
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ASHRAE Journal - February 2020 - Cover3
ASHRAE Journal - February 2020 - Cover4
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