MACS Monthly Newsletter - 2020 - SEP1

©

Total Vehicle Climate and Thermal ManagementTM

September 2020

By Gene Dianetti and Mike Milliman

Function and Operation of TXVs (Part 1)
Welcome to part one of a two part MSR series covering thermal expansion valves. The material contained
within is based on a popular TXV class that Mike and Gene have developed and presented over the years at
the annual MACS Training Event and Trade Show. In fact, there is so much information here, that we're splitting it into two issues! Part one explains TXV function, design and operation (including the power element,
superheat and MOP), while part two looks at specialty valves and troubleshooting.
The TXV (thermostatic expansion valve) is an important component in mobile air conditioning systems. Do they control pressure, temperature? Let's
take a closer look at the basic principles of how
TXVs control refrigerant flow in the system to optimize system performance.

changer designed to remove heat from the
high temperature, high pressure vapor exiting the compressor. The refrigerant condenses from a vapor to a high-pressure liquid as it
exits the condenser.
*	Receiver Drier: The receiver drier performs a vital function in the mobile air conditioning system. This device removes moisture from the refrigerant which may have
been introduced in the system during assembly or ingression of moisture though rubber
seals or hoses in system connection assemblies. Filtration of the refrigerant is also provided which is beneficial for long system life.
In addition, the receiver function provides
refrigerant storage under low system thermal loads to prevent liquid refrigerant from
backing into the condenser which reduces its
efficiency. The receiver drier is located as an
integrated component into the condenser or
as a separate component between the condenser and the expansion device.

Basic Mobile Air Conditioning Systems

Before we learn about the operation and function
of the TXV, let's start with the basic system components in a mobile air conditioning system.
*	 Expansion Device: Expansion devices
separate the high-pressure side from the lowpressure side of the air conditioning system.
The main function is to regulate the flow of
liquid refrigerant to the evaporator in order
to remove heat from the cabin of the vehicle.
*	 Evaporator: The evaporator is a heat exchanger designed to remove heat and moisture from the air inside the cabin of the vehicle to create the desired level of comfort
to the passengers. As refrigerant enters the
evaporator through the expansion device, it
boils at a low pressure and temperature as it
removes heat from the cabin air as it passes
through the coils.
*	 Compressor: The compressor increases the
system refrigerant from a low pressure, low
temperature vapor to a high pressure, high
temperature vapor creating refrigerant mass
flow in the system.
*	 Condenser: The condenser is a heat ex-

The TXV performs an important function in the
mobile air conditioning system (Figure 1). As the
warm air from inside the vehicle flows through the
evaporator coil, the air gives up its heat to the liquid refrigerant flowing through the evaporator. This
transfer of heat lowers the air temperature creating
comfort in the vehicle and causes the refrigerant in
the evaporator to boil and change state from a liquid to a vapor.

MACS Service Reports is the official technical publication of the Mobile Air Conditioning Society Worldwide, Inc., P.O. Box 88, Lansdale, PA 19446. The material published in MACS Service Reports expresses the views of the contributors
and not necessarily that of MACS. Every attempt has been made to ensure

the accuracy of the content of MACS Service Reports. MACS, however, will
not be responsible for the accuracy of the information published nor will MACS
Worldwide be liable in any way for injury, labor, parts or other expenses resulting
from the use of information appearing in MACS Service Reports.

September 2020	

Function of Expansion Devices

1	

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