User Guide

BULLETIN 10-9 / Page 3
THE REFRIGERATION SYSTEM
To understand the function of the thermostatic expansion
valve, a short discussion of the refrigeration system is nec-
essary. The refrigeration system can be defined as a closed
system in which the process of absorbing and rejecting heat
is performed by flowing a refrigerant in a vapor compression
cycle. In its simplest form, the refrigeration system consists
of five components: the compressor, condenser, evaporator,
expansion device, and interconnecting piping.
The heart of the system is the compressor since it causes the
refrigerant flow. Its function is simply to receive low pres-
sure (and temperature) refrigerant vapor from the evapora-
tor and compress it into high pressure (and temperature)
refrigerant vapor. The high pressure vapor is then converted
to a liquid phase in the condenser. The condenser performs
this function by removing heat from the vapor and rejecting
the heat to the air, or to water in the case of a water cooled
condenser. The liquid, which remains at a high pressure,
passes through the expansion device and becomes a low
pressure two phase (liquid and vapor) mixture. This refriger-
ant mixture returns to its vapor phase in the evaporator by
absorbing heat from the medium being cooled.
The selection of the expansion device is of particular impor-
tance to the operation of the refrigeration system because it
regulates refrigerant flow into the evaporator. An expansion
device which is misapplied or incorrectly sized will ordinar-
ily result in operational difficulties and poor system perfor-
mance. For example, an undersized expansion device will
prevent sufficient refrigerant from flowing into the evapora-
tor causing a reduction in the design cooling capability of the
system. An oversized expansion device may allow too much
refrigerant into the evaporator causing liquid refrigerant to
flow back to the compressor. The latter condition is referred
to as floodback. Both conditions will invariably result
in compressor damage if not quickly remedied. Therefore,
the expansion device requires attention to its selection and
application.
TYPES OF EXPANSION DEVICES
Expansion devices can be divided into four general categories:
the fixed area restrictor, the automatic (constant pressure)
expansion valve, the thermostatic expansion valve, and the elec-
tric expansion valve. The fixed area restrictor expansion device
is simply a precisely formed restriction through which liquid
refrigerant flows. Two common examples of this type of device
are the capillary tube, or cap tube, and the short tube restrictor,
or plug orifice. These devices are typically used on certain small
air conditioning and refrigeration systems where operating con-
ditions permit moderately constant evaporator loading and con-
stant condenser pressures. The drawback associated with these
devices is their limited ability to efficiently regulate refrigerant
flow in response to changes in system operating conditions,
since they are sized based on one set of conditions.
Like the fixed area restrictor, the automatic expansion valve
(AEV) is best suited for applications having moderately
constant evaporator loading. The AEV regulates refrigerant
flow by simply maintaining a constant evaporator or valve
outlet pressure. As the heat load on the evaporator rises, the
AEV decreases refrigerant flow to maintain evaporator pres-
sure at the valve’s setting. Conversely, the AEV increases
refrigerant flow when the evaporator heat load decreases
to maintain evaporator pressure at the valve’s setting. As a
result, the AEV starves the evaporator at high load condi-
tions, and overfeeds it at low load conditions.
The thermostatic expansion valve provides an excel-
lent solution to regulating refrigerant flow into a direct
expansion type evaporator. The TEV regulates refrigerant
flow by maintaining a nearly constant superheat at the
evaporator outlet. As superheat at the evaporator outlet
rises due to increased heat load on the evaporator, the
TEV increases refrigerant flow until superheat returns
to the valve’s setting. Conversely, the TEV will decrease
refrigerant flow when superheat lowers as a result of a
decreased heat load on the evaporator. The effect of this
type of regulation is it allows the evaporator to remain as
nearly fully active as possible under all load conditions.
The concept of superheat, and the proper method of mea-
suring it is further explained on Page 14, TEV Operation
and Performance.
The thermostatic expansion valve provides an additional
benefit when charging the system with refrigerant. When
a TEV is used, the system refrigerant charge is usually not
as critical as with the other expansion devices. The proper
operation of a fixed restriction and, to a lesser extent, an
automatic expansion valve depends on having an exact
amount of refrigerant in the system.
The electric expansion valve (EEV) provides a means by
which applications can be designed with sophisticated
system control functions. This type of valve is controlled
by an electronic circuit which is often designed to allow
the valve to control some aspect of system operation in
addition to superheat at the outlet of the evaporator. For
example, evaporator discharge air temperature or water
temperature from a chiller could be monitored by the EEV’s
controller. See Bulletin 100-9 for details on electric valves
for refrigerant control or contact the Sporlan Division of
Parker for additional information.
HOW THE THERMOSTATIC EXPANSION
VALVE WORKS
Basic Operation
In order to understand the principles of thermostatic expansion
valve operation, a review of its major components is necessary.
A sensing bulb is connected to the TEV by a length of capil-
lary tubing which transmits bulb pressure to the top of the
The thermostatic expansion valve (TEV) controls the flow
of liquid refrigerant entering the direct expansion (DX)
evaporator by maintaining a constant superheat of the
refrigerant vapor at the outlet of the evaporator. Superheat
is the difference between the refrigerant vapor temperature
and its saturation temperature. To measure the superheat
the TEV controls, the difference between the actual tem-
perature at the sensing bulb and the saturation temperature
corresponding to the suction pressure at the sensing bulb
location is determined. By controlling superheat, the TEV
keeps nearly the entire evaporator surface active while not
permitting liquid refrigerant to return to the compressor.
The ability of the TEV to match refrigerant flow to the rate at
which refrigerant can be vaporized in the evaporator makes
the TEV the ideal expansion device for most air conditioning
and refrigeration applications.
SPORLAN THERMOSTATIC EXPANSION VALVES