Thermostatic Expansion Valve: 27 Important Facts

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THERMOSTATIC EXPANSION VALVE DEFINITION

A thermostatic expansion valve is a component that is used in the refrigeration system or air conditioning system that helps to control the amount of refrigerant that is released into the evaporator. Hence a thermostatic expansion valve ensures that the superheat from the evaporator coils is released at a steady rate. Although it is termed a ‘thermostatic’ valve, it is not capable of controlling the temperature of the evaporator coils. The temperature in the evaporator depends on the pressure which is often controlled by adjusting the capacity of the compressor.

Thermostatic Expansion Valve
Thermostatic Expansion Valve

Image Attribution: MasterTriangle12, Thermostatic expansion valve, CC BY-SA 4.0

Thermostatic expansion valves are also known as metering devices though other devices might be referred to with a similar name such as a capillary tube. In abbreviated form, TX or TXV is used to refer to as the Thermostatic expansion valve.

THERMOSTATIC EXPANSION VALVE FUNCTION

The function of a TXV is to regulate the flow of refrigerant into the evaporator coils depending on the superheat required. The TXV consists ofa sensory bulb filled with gas that senses the evaporator pressure. A spring beneath the diaphragm of the valve also exerts pressure. Further, the lower section of the diaphragm exerts another pressure. If the pressure of the gas in the sensing bulb is higher than the combined pressures around the diaphragm; the valve opens.

Thermostatic expansion valve responds to changes in pressure. Though, three main forces are usually considered in the study of valve opening. Another force determines the opening and closing of the valves which the force exerted by the refrigerant.

THERMOSTATIC EXPANSION VALVE DIAGRAM

Thermostatic Expansion Valve
Thermostatic Expansion Valve Diagram

Image Attribution: Neurotronix, Thermostatic Expansion Valve PHT, CC BY-SA 4.0

THERMOSTATIC EXPANSION VALVE COMPONENTS

There are several designs of thermostatic expansion valve that are available in the market but the main components inside a TEV are the following

  • The main structure that holds the different components together is the valve body which is composed of an inbuilt orifice that restricts the refrigerant flow.
  • A thin flexible material which is made up of metal is the diaphragm which flexes to apply pressure on the pin.
  • The size of the orifice opening is adjusted using a pin or needle which controls the flow of refrigerant.
  • It consists of a spring that has a counter effect to the action of the pin.
  • It consists of a sensing bulb and a capillary line installed at the exit section of the evaporator which causes the valve to open and close.

THERMOSTATIC EXPANSION VALVE SPECIFICATIONS

The thermostatic expansion valve specifications vary from one design to another and depending on the refrigeration or air conditioning system. For example, in the Emerson series of thermostatic expansion valves itself, there is variation in the port valve design, the sizing, and the ranges of evaporation temperature.

Specification for Emerson TX7 series of Thermostatic expansion valve is tabulated below:

Maximum Working Temperature667 PSIG
Temperature range of refrigerant-130F to 1580F
Temperature to be stored at-220F to 1580F
Connection materialODF Copper
Emerson TX7 Specifications

THERMOSTATIC EXPANSION VALVE WORKING

The valve remains open during the normal functioning of the refrigeration system. The working of a thermostatic expansion is explained below:

  • When the cooling load on the refrigeration system is high, the evaporator temperature increases which is senses by the sensory bulb of the TEV. This indicates that more refrigerant needs to be provided for the refrigeration load. The gas in the sensory bulb increases and the spring of the TEV experience an increase in pressure P1. As a result of this, the diaphragm bends downward allowing more refrigerant to flow through the valve opening into the evaporator
  • It is noted that the pressure below the diaphragm P2 also increases with the increasing superheat in the evaporator coils of the refrigeration system. This increase in pressure closes the valve opening of the TEV. Another pressure P3 is exerted by the spring below the diaphragm which opposes the closure of the valve. The valve will open if P1 is much greater than P2 and P3 thereby allowing the entry of refrigerant.
  • When the cooling load reduces in the HVAC system, the pressure P1 is less than P2 and P3 which results in the closing of the valve partially allowing an only a limited amount of refrigerant to flow into the evaporator coils of the refrigeration system. In this way, the TEV helps in maintaining the flow of refrigerant into the evaporator coils based on the superheat which is senses by the sensory bulb located on the TEV.

WHERE IS THE THERMOSTATIC EXPANSION VALVE LOCATED?

The thermostatic expansion valve is located between the evaporator and condenser region of the refrigeration cycle. The main body of the valve is often made from brass and consists of an inlet and outlet valve. The inlet opening is at the bottom of the device while the outlet valve is situated at the lateral side of the valve. A removable cap at the adjacent side helps in adjusting the superheat of the refrigerant.

HOW TO INSTALL THE THERMOSTATIC EXPANSION VALVE ?

The steps to be followed during the installation of a Thermostatic expansion valve are given bellow: –

  • It is recommended to clean any dust or soldering particles in the valve fittings or any other parts that might interfere with the normal functioning of the refrigeration system.
  • It is essential to protect the TEV by wrapping the body of the valve with a wet cloth to protect thermal agents and it is recommended to keep the soldering torch away from the valve body. Further, it should be ensured that no excess solder should be used as there are chances that it might enter the valve and interfere with the refrigeration process.
  • The senor bulb of a TEV that is attached to the suction line controls the valve and keeps check of the system temperature. Further, the TEV is usually installed close to the coils of the evaporator. In case the TEV comprises an equalizing pressure system, then the suction line and pressure line should be connected and should be located after the sensor bulb of the valve.
  • The sensing bulb is usually located on the top of the suction line, especially in a small line. For systems with sensor bulbs outside the refrigeration system, special protection against ambient conditions is required. Further, the suction line should be insulated to one foot on both sides.
  • For HVAC systems having lines with large diameters, the TEV bulb is positioned at 5 or 7’ o clock direction at the lower portion of the suction line. It is recommended to install the bulb on a horizontal platform of a suction line.
  • The TEV bulb can be attached to the vertical or horizontal region of the suction line but should never be located on the elbow which could interfere with the proper functioning of the bulb in sensing temperatures.
  • TEVs are never located on the lower side of the cooling line as the oil flowing through the line acts as an insulator thereby interfering with the normal operation of the sensor bulb.
  • In a system with multi-evaporators installed with multiple TEVs; the TEVs should not be located at the common suction line. Instead, it should be clamped onto the suction line of each evaporator to obtain a clear indication of each evaporator’s operating condition.

HOW TO ADJUST THE THERMOSTATIC EXPANSION VALVE?

While adjusting TEV, it should be ensured that there is 20 minutes gap between each adjustment. TEVs are used for adjusting the flow of refrigerant into the evaporator coils. The valve consists of a pin or a needle that allows setting the coolant flow. The needle turned to a quarter is accounted to be one degree. Moreover, the needle should be adjusted only after every 20 minutes, as it is very sensitive. The steps to be followed while adjusting a TEV are as follows: –

  • Have a clear picture of whether the temperature reading should be increased or decreased in the TEV.
  • Locate the position of the needle/pin.
  • The needle should be turned one-quarter clockwise for every degree increase in temperature and vice-versa for every degree decrease in the temperature.

HOW TO CALIBRATE THE THERMOSTATIC EXPANSION VALVE?

There are not particular means of calibrating the Thermostatic Expansion Valve, but it can be adjusted as it is a valve with modulating options. On turning the stem of the valve clockwise, the built-in pressure increases will result in a higher superheat.

 While turning the stem anti-clockwise, the pressure in the spring decreases which reduces the superheat. The TXV loses its charge in the powerhead when the refrigeration system is turned off, but there is no chance that the valve is out of adjustment. It is recommended not to re-adjust a faulty valve; instead, it should be replaced. The new valve which will be replaced should be protected from overheating due to brazing.

TYPES OF THERMOSTATIC EXPANSION VALVE

There are two different types of Thermostatic expansion which are

  • Internally Equalized Thermostatic expansion valve
  • Externally Equalized Thermostatic expansion valve

An internally equalized Thermostatic expansion valve is used when the inlet pressure of the evaporator forces the valve to close. When an internally equalized TEV is used in a system with a large pressure drop across the evaporator, the pressure below the diaphragm is greater than the pressure exerted by the gas in the sensory bulb causing the valve to close and results in a superheat which is higher than that is required. This results in a starving condition.

An externally equalized TEV functions with the outlet evaporator pressure and flows to the same location as the valve temperature sensory bulb. It compensates for the pressure drop that occurs across the evaporator or refrigerant distributor. An externally equalized TEV is usually used on an evaporator with multiple circuits of refrigerant and distributor.

INTERNALLY EQUALIZED THERMOSTATIC EXPANSION VALVES

An internally equalized Thermostatic expansion valve is used when the inlet pressure of the evaporator forces the valve to close. When an internally equalized TEV is used in a system with a large pressure drop across the evaporator, the pressure below the diaphragm is greater than the pressure exerted by the gas in the sensory bulb causing the valve to close and results in a superheat which is higher than that is required. This results in a starving condition.

The internally equalized TEVs are usually used on large systems with a capacity greater than 1 ton and on any system that uses a distributor. It should be noted that an internally equalized TEV cab be replaced with an externally equalized TEV but not vice-versa.

EXTERNALLY EQUALIZED THERMOSTATIC EXPANSION VALVE

An externally equalized TEV functions with the outlet evaporator pressure and flows to the same location as the valve temperature sensory bulb. It compensates for the pressure drop that occurs across the evaporator or refrigerant distributor. An externally equalized TEV is usually used on an evaporator with multiple circuits of refrigerant and distributor. For an evaporator without a distributor if the pressure drop across the evaporator is noted to be greater than 3 psi, then an externally equalized TEV needs to be used.

PURPOSE OF EQUALIZING LINE IN THERMOSTATIC EXPANSION VALVE

In a refrigeration system, if the evaporator coils are composed of extremely long tubes or tubes with narrow internal diameter then there are higher chances for greater pressure drop between the inlet and the outlet. In case the pressure drop is too high, then the saturation temperature of the refrigerant at the evaporator outlet will be lower than the saturation temperature of the refrigerant at the evaporator inlet.This calls for the need increased amount of superheat to create a condition of equilibrium around the diaphragm or TXV. To offset the effects of this high pressure, drop across the evaporator, and externally equalized TEV needs to be installed.

This line connects the lower portion of the diaphragm to the evaporator outlet; thereby ensuring that the measured superheat is related to the saturation conditions at the evaporator exit. The externally equalizing line is not capable of reducing the pressure drop but ensures that the evaporator coil area is effectively used for evaporation thereby increasing the efficiency and performance of the refrigeration system.

ADVANTAGES OF THERMOSTATIC EXPANSION VALVE

The advantages of a thermostatic expansion valve are as follows:

  • The TEV can change its valve opening depending on the superheat condition in the coils of the evaporator.
  • It can maintain a varying refrigerant charge to adjust varying ambient conditions.
  • Its capability to adjust the valve opening by sensing the pressure increase which benefits the refrigeration system in increasing its performance and preventing damage to the compressor due to flooding.

Unless the need of the device is to provide fixed release of refrigerant or coolant, a thermostatic expansion valve is the device that is largely preferred over the other options in an HVAC system.

DISADVANTAGES OF THERMOSTATIC EXPANSION VALVE

The major disadvantage of using a thermostatic expansion valve is that if the pressure difference between the P1 (TEV sensing bulb) and combined pressures P2 (below the diaphragm) and P3 (the spring exerts a pressure (are not significant then the opening and closing of the valve will not work properly which will interfere with the proper release of the refrigerant as per the need of the heat loading. In such cases, it is recommended to install a balanced port or electronic expansion valve to cope up with the varying needs and limitations that may come up.

APPLICATION OF THERMOSTATIC EXPANSION VALVE

Thermostatic Expansion valves are largely used in the HVAC system especially in air-conditioning and refrigeration units. They are usually installed in units with larger capacities. Few areas where the thermostatic expansion valves are used are

  • Split AC
  • Refrigeration units used in industries
  • Central AC
  • Packaged Air conditioners

There are many more applications wherein the thermostatic expansion valve can be installed in the future depending on the requirements to be met.

DIFFERENCE BETWEEN CAPILLARY TUBE AND THERMOSTATIC EXPANSION VALVE

Both the TEV and Capillary Tube work towards a common goal of controlling the flow of refrigerant into the evaporator coils but the way it functions varies. The difference between the functioning of the capillary tube and thermostatic expansion valve are tabulated below:

Thermostatic Expansion ValveCapillary Tube
The valve opening is adjusted according
to the superheat which is
sensed by the sensory bulb of the TEV		It does not respond to the heat load changes
and the valve opening is fixed.
It provides better efficiency
as the refrigerant flow is adjusted
according to the heat load		Lower efficiency as the refrigerant flow
is not controlled by the heat load.
It is capable of functioning at a
broader range of ambient temperatures.
As the temperature is higher, the TEV will release more refrigerant.
A shortcoming of this capability
is slugging which can damage the compressor coils.		When the ambient temperature increases,
the system must work harder to provide the
required cooling
This type of valve can adjust itself to
varying need of refrigerant charge
thereby contributing to increased performance		It cannot accommodate varying needs of
refrigerant charge thereby impacting the
overall performance of the refrigeration system.
Thermostatic Expansion Valve V/s Capillary Tube

LIQUID EXPANSION THERMOSTATIC VALVE

This type of expansion valve is usually used in gas cookers. This expansion valve works on the principle that liquid expands when heated. It consists of a PHIAL usually made of copper which is filled with liquid. The PHIAL is connected to a bellow using a capillary tube. This valve is connected to the bellow. When the liquid expands due to the increased temperature, the bellow pushes the valve into its position. In this way, gas flow is stopped to the burner.

The liquid expansion thermostatic valve is adjusted by using a temperature adjustment bar which moves the valve either closer or away from its position. In this way, a higher or lower temperature is obtained before achieving the bypass rate.

BALANCED PORT THERMOSTATIC EXPANSION VALVE DEFINITION

There are 4 types of forces that are exerted on thermostatic expansion valve which are

  1. Pressure in the sensory bulb which an opening force.
  2. Pressure in the evaporator or the pressure exerted by the external equalizer i.e., a closing force.
  3. The spring below the diaphragm exerts a closing force.
  4. The refrigerant that flows through the needle exerts an opening force.

When the pressure exerted by the refrigerant is higher than the usual norm, the force exerted by this force will be greater which will result in an inflow of more refrigerant through the coil.

While when the liquid pressure is lower, this will result in less flow through the coil. These fluctuations in superheat will be unacceptable especially for systems with accurate feeding requirements for the evaporator.

A balanced TXV is a solution for this pressure fluctuation that is experienced due to the pressure exerted by the refrigerant. Here the pressure of the refrigerant is used for balancing the top and bottom part of the needle. The liquid pressure in this type of TXV is used as a balancing force which neither contributes to the closing or opening of the valve.

BIDIRECTIONAL THERMOSTATIC EXPANSION VALVE

When a thermostatic expansion valve is installed on a split system with two TXVs and two check valves. This unit is referred to as Bidirectional TXV It is recommended to install the Bi-directional TXV on the condensing unit and the tubing between the valve and the heat exchanger placed indoors needs to be insulated. To reduce the pressure, drop, it is essential to increase the insulation diameter.

ELECTRONIC THERMOSTATIC EXPANSION VALVE

The function of an electronic thermostatic expansion valve is like that of an ordinary thermostatic expansion valve. But using an electronic TEV ensures that the refrigerant flows in controlled in precise ratios or levels. The overheating that is required is calculated using a temperature sensor that is clamped onto the expansion valve and another one on the evaporator outlet.

The installation and control of the electronic expansion valve are simple and highly reliable. The valve is controlled using a centralized unit to controls the refrigerant flow through the entire system. It can improve the performance of the refrigeration system even at low condensing pressures. The plus point of electronic TEV is that it can enhance compressor performance without considering the evaporator load.

This type of TEV can improve the performance of the evaporation system and increasing the refrigeration capacity by around 15%. There are several designs of TEVs that are available in the market while most of the electronic TEVs are composed of a permanent magnet and copper coil inside the motor body to create an electromagnetic field. The motor is attached to the shaft which is linked to a thread. When the system is switched on, the shaft exerts pressure on the thread and thereby on the needle which is then pushed to its position. In this way, the electronic expansion valve functions.

ELECTRONIC EXPANSION VALVE VS THERMOSTATIC EXPANSION VALVE

The major difference between an electronic expansion valve and thermostatic expansion valves is that in a thermostatic expansion valve the opening is dependent on the pressure exerted while an electronic expansion valve operates using temperature sensors that calculated the required overheating. The electronic expansion valves enhance the performance of the refrigeration system to a greater extend when compared to that of an ordinary TXV due to the precise measurements

AUTOMATIC THERMOSTATIC EXPANSION VALVE

These types of TXVs are also referred to as constant pressure expansion valves as the pressure of the refrigerant is controlled in the refrigeration unit. It sends the refrigerant into the evaporator in a controlled and metered manner so that the pressure that is required to change the refrigerant from liquid to vapor is attained.

The valve body is made up of metal with a diaphragm inside the body. On the upper portion of the diaphragm, a spring is located which is always acted upon by pressure and is controlled by an adjustable screw. There is a seat beneath the diaphragm which is controlled by a needle linked to the diaphragm. The needle moves according to the diaphragm. Hence when the diaphragm moves down, the needle also moves down resulting in the opening of the valve.

DIFFERENCE BETWEEN AUTOMATIC EXPANSION VALVE AND THERMOSTATIC EXPANSION VALVE

The major difference between an automatic expansion valve and a thermostatic expansion valve is that the thermostatic expansion valve regulates the refrigerant flow depending on the headload that is exerted on the evaporator. While an automatic expansion valve functions according to outlet pressure; it releases the refrigerant into the evaporator coils based on the constant evaporator pressure.

A TXV can be used in varying ambient conditions, unlike AEV which can be used only in controlled conditions where the pressure in the evaporator is constant which is a limitation. This results in lower performance of refrigeration system installed with AEV in comparison to a refrigeration system that has TXV as a metering device of the refrigerant flow to the evaporator coils.

FREQUENTLY ASKED INTERVIEW QUESTION AND ANSWERS

1. Why is electronic thermostatic expansion valve preferred over the ordinary TEV?

An electronic TEV is superior to that of an ordinary TEV by releasing precise and accurate amounts of refrigerant into the system by calculating the overheating. But in ordinary TXV, the refrigerant release is carried out by sensing the pressure. The electronic expansion valves enhance the performance of the refrigeration system to a greater extend when compared to that of an ordinary TXV due to the precise measurements.

2. How does a TEV maintain the refrigerant flow in an HVAC system?

The function of a TXV is to regulate the flow of refrigerant into the evaporator coils depending on the superheat required. The TXV consists of a sensory bulb filled with gas which senses the evaporator pressure. A spring beneath the diaphragm of the valve also exerts pressure.

Further, the lower section of the diaphragm exerts another pressure. If the pressure of the gas in the sensing bulb is higher than the combined pressures around the diaphragm; the valve opens.

Thermostatic expansion valve responds to changes in pressure. Though, three main forces are usually considered in the study of valve opening. Another force determines the opening and closing of the valves which the force exerted by the refrigerant.

PROBLEM STATEMENT

1. In a refrigeration system that uses Thermostatic expansion valve for regulating the release of refrigerant. The pressure exerted on the valve are as follows

  • Pressure P1 in the sensory bulb – 5 psi
  • Pressure P2 below the diaphragm – 2 psi
  • Pressure P3 by the spring below the diaphragm – 2 psi

Based on the above information, is it expected for the TEV to open or close.

From the above information we know that

P1>P1+P2

5 psi > 4 psi (i.e., 2+2 psi)

i.e., the pressure in the evaporator is much higher than combined pressure exerted by the spring and the pressure below the diaphragm which concludes that more refrigerant is required for handling the heat load. Therefore, the TEV will open allowing the refrigerant to be released into the evaporator coils.

To read about Superheat in an HVAC system. Click Here