User Guide

BULLETIN 10-9 / Page 9
floodback during compressor startup, reduces the load on the
compressor after startup, and permits rapid pulldown. Since
the majority of low temperature systems operate at or near
a specific evaporating temperature, the TEV can be set for
optimum superheat at the design temperature permitting
the system to operate as efficiently as possible.
The Types ZP Charges (FZP, VZP, SZP, RZP, and PZP) are
gas-cross charges having the same operating range as the
Type Z Charges. A typical superheat characteristic curve
of the ZP Charge is illustrated in Figure 6. The Z and ZP
Charges are essentially the same with the exception of the
ZP Charge providing an MOP. Type ZP Charges are not
intended as replacements for Z Charges. Each should
be selected for its unique purpose. A ZP Charge
should only be used for low temperature refrigera-
tion systems where it is necessary to limit evaporator
pressure during pulldown.
During and after a hot gas defrost cycle or after a shutdown
period, evaporator pressure may rise to a level the compres-
sor motor cannot handle. In such cases, a pressure limiting
charge is often effective in limiting suction pressure at the
compressor. For systems employing long suction lines, a
crankcase pressure regulating (Sporlan CRO type) valve
may be required to limit suction pressure at the compressor
quickly. While a pressure limiting charge can be used with
a CRO valve, pulldown time may be adversely affected if
the charge MOP and the CRO valve setting are close to one
another. Therefore, Sporlan does not recommend a CRO
valve and a pressure limiting TEV be used on the same
system.
Type X Charge — The charges listed under the extremely
low temperature refrigeration are known as the X Charges.
The X Charges are liquid-cross charges having an operating
range from an evaporating temperature of –40°F to –100°F.
This curve is similar to the Z Charge curve since the perfor-
mance characteristics of the Z Charges previously discussed
apply very well to extremely low temperature refrigeration.
Contact Sporlan for assistance in selecting TEVs for applica-
tions requiring the X Charge.
Special Selective Thermostatic Charges and
Elements
Sporlan manufactures a number of special thermostatic
charges and elements designed for specific applications. A
few of these are described below:
Type N Charge — This charge is an adsorption type charge
which has a superheat characteristic curve similar to the
C Charge but tends to be less responsive. The N Charge is
a noncondensable charge, and it has no MOP feature. The
N Charge is used on special medium and high temperature
applications such as chillers which are located outdoors and
must operate while exposed to cold temperatures.
Hydraulic Elements — These thermostatic elements are
specially designed double diaphragm elements which pro-
vide a pressure limiting feature without the problems associ-
ated with charge migration from the bulb when the element
becomes cooler than the bulb. The hydraulic element is
often used on chillers which require a TEV with an MOP
type charge, but experience problems with charge migra-
tion caused by cold ambient temperatures. For additional
information on the hydraulic element, contact the Sporlan
Division of Parker.
Mechanical Pressure Limit Elements — These thermo-
static elements may use either liquid or liquid-cross charges,
and they employ a mechanical means to limit suction pres-
sure (PL-type). A collapsible member is used to limit evapora-
tor pressure when it exceeds a specified value. This method
of limiting evaporator pressure is considered obsolete, and
replacement valves and thermostatic elements are no longer
available. A cross reference is available from the obsolete PL
element to the thermostatic element with the MOP charge,
please refer to Bulletin 210-10-17.
Special Refrigerants — Thermostatic charges for use
with special refrigerants are available. These refrigerants
include: R-13, R-23, R-13B1, R-124, and R-503. Contact the
Sporlan Division of Parker for assistance in valve selection
for special refrigerant applications.
Desuperheating Charges — Special thermostatic charges
have been developed for applications requiring suction gas
desuperheating. The subject of hot gas bypass and desuper-
heating TEVs is discussed on Page 12.
THERMOSTATIC EXPANSION VALVE
APPLICATIONS
Due to its superior operating characteristics, the TEV is
currently used on a wide variety of applications. These
applications include both large and small capacity air con-
ditioning and heat pump systems; commercial refrigeration
systems including refrigerated display cases, ice cubers, and
soft drink dispensers; and low temperature refrigeration
systems.
Most air conditioning and refrigeration systems use some
method of capacity reduction to match the capacity of the
system to a reduced heat load condition, commonly referred
to as partload operation. The simplest method of capacity
reduction is cycling the compressor, usually in response
to a thermostat. Other methods of capacity reduction
include using compressors equipped with cylinder unload-
ers, bypassing hot gas, or some combination of the above.
A discussion on these capacity reduction methods and their
effect on TEV operation is presented later in this section.
The thermostatic expansion valve is a modulating type flow
control device with the capability to adjust to low load condi-
tions and maintain reasonable refrigerant flow control. The
range of effective TEV control, however, has limits and may
not be capable of operating properly on a system requiring a
high degree of capacity reduction. As a result, systems using
capacity reduction methods require the use of proper design
and installation practices.
System Design Factors
Predicting TEV performance at reduced system capacities is
difficult due to the many influencing design factors present
in any system. These factors include: TEV sizing, refrigerant
distribution, TEV setting, evaporator coil design, suction
line piping, and bulb location. General recommendations
which address these factors are provided below. By observ-
ing these recommendations, a conventional TEV can be
expected to operate satisfactorily down to approximately 35
percent of its rated capacity. The Types (E)BF, SBF, EBS,
and O valves, featuring the balanced port design, can be
expected to operate satisfactorily down to approximately 25
percent of its rated capacity.
Valve Size — The TEV should be sized as close as possible to
the system’s maximum designed heat load condition. A valve
with a capacity rating up to 10 percent below the full load con-
ditions may be selected if the system is to operate at reduced