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Heating Ventilation and Air Conditioning Full Course - HVAC Course (Part 28)


            In this part of Full HVAC course, we are going to learn about Expansion Devices and their types in HVAC.

Expansion Devices

There are many different types of expansion devices in HVAC systems. Depending on what type of system you have, there may be several different components that need to be expanded. Expansion devices are used to increase the volume of air being pulled through your ductwork. There are three basic types of expansion devices: dampers, diffusers, and registers. Dampers and diffusers control airflow rate while registers are typically used to direct the flow of air in certain directions. However, both types of expansion devices vary greatly in their effectiveness based on how they are installed. 

What do expansion valves do?

Expansion valves regulate the flow of refrigerant gas (in a vapor-compression air conditioning system) over time and maintain a constant pressure in the return line. In an HVAC system, expansion valves control the amount of refrigerant flowing back to the compressor. If there’s too much liquid in the refrigerant, it will not compress properly, leading to reduced performance.

 What does the valve look like?

A typical expansion valve looks similar to a small ball bearing. When the valve opens, it expands outward, opening the refrigerant line to the compressor. When the valve closes, its shape contracts and seals off the refrigerant line. Expansion valves can be vented or nonvented. Vented valves allow a certain percentage of the refrigerant to escape out of the system. Nonvented valves force the entire amount of refrigerant out of the system. A venting valve should remain open until the temperature reaches its set point.

 How does the expansion valve work?

An expansion valve consists of a spring-loaded piston inside a chamber. When the pressure builds up in the system, the spring pushes the piston toward the outlet port, increasing the size of the orifice. As the refrigerant flows past the piston and through the orifice, it cools down and drops pressure. At some point, the drop in pressure causes the piston to reverse direction and move toward the inlet port; this reduces the size of the orifce, thus reducing the flow rate of the refrigerant.

 Why would I need an expansion valve?

Expansion valves prevent refrigerant from being trapped in the system at low temperatures, which could cause freezing damage to components. There are two types of expansion valves: venting and nonventing. Venting valves allow some refrigerant to pass through them while keeping the rest sealed off. Nonventing valves completely block the flow of refrigerant from the system. Both types should be used if condensing units are installed.

What is the difference between an electric and electronic expansion valve?

Electronic expansion valves use electronics to determine when to open and close. This type of valve uses microcontrollers and sensors to monitor and adjust the valve position automatically. Electronic expansion valves don't require any external power source.

 Is there a way to check to make sure my expansion valve is working correctly?

You can find a list of symptoms that indicate your expansion valve isn't working properly. Check these items before calling a professional to repair your unit.

a. Refrigerant leaks around the expansion valve.

b. Your thermostat isn't registering a change in temperature.

Next, we will see all the different types of Expansion devices that are available in HVAC.

Manual Expansion Valves

A manual expansion valve is a device that controls the rate at which refrigerant is released into the system. These valves usually regulate the flow rate based on temperature changes in the air flowing through them, rather than pressure differences.
                    A manual expansion valve is installed between the supply air and exhaust air chambers of a furnace. When the temperature inside the chamber rises above a certain level, the valve opens to let out some of the heated air. When the temperature falls below a certain point, the valve closes to prevent cold air from entering the furnace.

Constant Pressure or Automatic expansion valve

An automatic expansion valve (AEV) performs the same function as a manual expansion valve. However, instead of being manually adjusted, it adjusts itself automatically. An AEV operates the same way as a manual expansion valve except that, rather than having an external thermostat, an AEV is connected directly to the blower motor control board.

What is Automatic expansion valve?

An automatic expansion valve (AEV) is a device designed to control pressure losses in air distribution systems. It does this by maintaining a constant supply of air at a set temperature by closing off sections of pipe to reduce airflow until the desired pressure is reached. The AEV may be either active or passive. An active AEV uses a motorized pilot valve to open and close a solenoid-actuated check ball valve with precision timing. An example of an active AEV system would be an HVAC unit powered by a central control panel where the heating/cooling coils are connected to the AEV via pipes. An example of a passive AEV system would be a floor mounted fan coil unit with a separate blower and ductwork. In this case, the fan controls the flow of air using a check valve and adjustable dampers mounted directly to the blower outlet.

Why do we need AEV's?

AEVs were originally developed for commercial buildings prior to the widespread use of computer automation. Early building designs relied upon manual thermostats and manually operated ventilators. As buildings became larger and had longer distances between individual zones, the number of vents required increased dramatically. Manual controls proved cumbersome and prone to human error. There was a real need to design a control system that could automatically regulate a plurality of vents, yet still provide centralized monitoring and control of each zone.

How does an AEV work?

The AEV consists of two parts - the actuator and the valve assembly. When activated, the AEV opens or closes the valve.

In some cases, the AEV is attached to the supply side of the HVAC system and is called an “inlet” AEV. In this case, when the HVAC unit operates, the AEV is closed. If the HVAC runs continuously, the AEV remains closed and the air supply is maintained at the preset temperature. If the HV AC turns off, the AEV opens and allows fresh outside air to enter the supply line. When the HVAC unit returns, the AEV closes again.

If the AEV is attached on the return side of the HV AC system and is called an "outlet" AEV, it works exactly the same way. However, instead of operating on the supply side, the AEV is opened when the HVAC returns. Depending on how many outlets are required, this type of AEV can be used on any supply side of the H VAC system.

Types of AEVs

There are several types of AEVs depending on their location in the HVAC system. All AEVs have three basic components: 1) an external actuator; 2) a valve body; and 3) a spring mechanism.

Inlet AEVs operate on the supply side of the system and are located near the supply registers. Outlet AEVs operate at the return side of the system and connect to the return register. There are four different ways in which these devices can be installed:

• Flapper – The flapper valve is a simple mechanical device consisting of a cylindrical housing with a flat disc on its end. The disc moves up and down inside the cylinder, thereby opening and closing the port.

• Rotary disk – The rotary disk AEV is similar to the flapper in appearance, however the moving part is a rotating circular disk. As the disk rotates, it covers and exposes ports in the cylinder.

• Ball check – The ball check AEV is similar to both the flapper and rotary disk. It is a combination of a ball and a cylinder. As the ball rolls around inside the cylinder, it opens and closes various ports.

Thermostatic expansion valve

The thermostatic expansion valve (TEV), also known as a variable-capacity expansion valve, is designed to regulate the flow rate of return air to maintain a constant room temperature. Usually located at the return air plenum, TEVs have two internal adjustable dampers that provide increased capacity at low temperatures. Their adjustment is controlled by a single temperature sensor.

In HVAC systems, expansion valves regulate the flow rate of refrigerant gas from the compressor outlet back to the compressor inlet. As the temperature drops, the pressure inside the evaporator coil increases. When the refrigerant reaches a certain point, the valve opens, allowing the refrigerant to circulate freely. If the temperature continues to drop, the valve closes again, preventing further loss of refrigerant.

The thermostat-controlled expansion valve contains a sensing bulb (or diaphragm) inside that changes shape according to the temperature around it. A spring pushes down on the sensing bulb until it touches the bottom of its cavity. At low temperatures, the sensing bulb becomes thinner and the spring force is less, causing the bulb to bulge outwards slightly. When the bulb is fully expanded, the spring forces it to snap back into place.

There are two types of thermostatic expansion valves: single-stage and double-stage. Single-stage valves have only one opening position. Double-stage valves, however, have two positions—one at high temperature and another at low temperature.

A typical thermostatic expansion valve has three components:

• Sensing bulb

• Compression spring

• Pivot rod

Sensing bulbs are the heart of the valve. They work much like a thermometer in a car’s radiator. When the temperature rises, the bulb expands and pushes the compression spring upwards. When the temperature drops, the bulb contracts and the spring moves downwards.

The way the bulb works is similar to the way a piston operates in an automotive engine. This makes sense because the bulb sits inside a cylinder where the refrigerant travels.

Compression springs connect the sensing bulb and pivot rod. The spring creates a powerful force that compresses the bulb. Once the bulb is compressed enough to open the valve, the spring releases its energy and returns the bulb to its original shape.

Pivot rods help move the sensing bulb in relation to the valve body. The pivot rods provide the connection between the sensing bulb and the valve body.

Capillary Tubes

Capillary tubes are small thin-walled tubing used in a variety of applications including plumbing and air conditioning. Their small diameter makes them useful in maintaining the flow rate of fluids at low pressures. Because they have a high surface area, they are often used in humidifiers to add moisture to the air.
                A capillary tube Expansion valve prevents air flow from continuing until the pressure inside the line decreases sufficiently.

 An expansion valve is a specialized type of valve designed to control the flow of compressed gas. Its function is to maintain a constant pressure while controlling the volume of flowing gas. The typical application for an expansion valve is on a gas pipeline where maximum capacity is desired. In such applications, a variable-opening pilot control valve is often installed upstream of the expansion valve to supply a small amount of pressure to keep the expansion valve closed.

Capillary tubes are hollow cylinders that transmit liquid, gases, or both. They are commonly used in medical, chemical, and related industries. These types of tubes are used in laboratory procedures, gas chromatography, and analytical chemistry to transfer fluids, liquids, or vapor. Typical sizes range from 1/8 inch to 4 inches in diameter.

 An expansion chamber is a passive way of regulating the flow rate of a fluid through a pipe or duct. It consists of two chambers separated by a sealable wall. The first chamber contains high-pressure gas acting as a pressure reservoir; the second chamber contains low-pressure gas acting as the working fluid. The two chambers are connected via a series of holes called the expansion orifice. As the high-pressure gas expands into the low-pressure gas, it creates enough force to push open the expansion orifice.

 Expansion valves are devices used to control the flow of pressurized gas through piping systems. They consist of a cylindrical body that houses a diaphragm or other mechanism to adjust the size of the opening between the chambers. Gas entering the system travels through the inlet port, passes through an orifice in the diaphragm, then flows through a hole in the body and out the outlet port.

Expansion valves may be either manual or automatic. Manual valves require operator intervention to change their settings. Automatic valves do not need human attention to operate. They have a spring-loaded diaphragm that maintains the position of the orifice between the chambers. When the pressure increases, the spring forces the diaphragm toward the side of the cylinder, closing the orifice and stopping the flow of gas. As pressure drops, the spring reverses direction and opens the orifice again, allowing gas to pass through.

A capillary tube (or capillary) is a thin, long tube used in medicine, biology, engineering, physics, and many other fields. Capillaries are generally made of glass and plastic, although some are made of metal, ceramic, or other materials. These tubes can be straight or curved, single or double-ended, and are usually manufactured in diameters ranging from less than a millimeter to several centimeters in length. They are produced in various lengths, including short segments, long tubes, and even microtubes.

This is sometimes referred to as a “diaphragm” expansion valve. It is similar to a pressure regulator valve except that instead of using a flexible piece of rubber, it uses a small diameter copper tube. Similar to a diaphragm in a car tire, the copper tube flexes with changes in fluid pressure. This movement results in a corresponding displacement of the valve body that controls refrigerant flow into the evaporator.


Orifice expansion valves (OEV) in HVAC systems are designed to control air flow in order to maintain constant temperature and humidity level throughout the facility. These devices are used to prevent over-pressurization or under-pressure conditions in the HVAC system. OEV’s are installed at various locations in HVAC systems including supply dampers, return dampers, diffusers, and registers.

Benefits of having an OEV in your HVAC system

• Improves air circulation

• Prevents excessive moisture build-up inside ductwork

• Reduces noise levels

Installation of an OEV in HVAC systems

There are two types of OEVs: manual OEV and automatic OEV. Manual OEV is activated manually through a handwheel mechanism. Automatic OEV uses a spring force and electronic actuator to open and close the damper.

 Types of OEV

Manual OEV

• Hand wheel type

Automatic OEV

• Cable operated mechanism

• Electronic actuators

Orifice expansion valve (OEV) is a type of variable air volume (VAV) system that uses an adjustable orifice plate to control airflow in heating, ventilating, and air conditioning systems. OEVs allow for precise control over the rate at which air flows through an HVAC system.
                Most commonly used in residential applications, OEVs are designed to operate automatically under normal conditions and shut off when the temperature reaches setpoint. Typical OEV designs use a thermostat to activate the motorized valve plate. The plate is either opened or closed based on the current temperature inside the space being cooled or heated.

- An advantage of using an OEV is that it helps keep the HVAC system cool in summer and warm in winter. However, it may increase the amount of condensation on windows and cooling coils.

Float type Expansion Valve

In HVAC systems, float type expansion valves (FTEVs) are used to regulate the flow rate of refrigerants. FTEVs generally have two positions, fully open and partially closed. When the valve is in the fully open position, the refrigerant can freely pass through the valve. However, when the valve is in the partially closed position, the valve restricts the flow rate of the refrigerant. Thus, the FTEVs provide protection to the compressor by preventing overloading the system if the refrigeration load increases suddenly.

Float Type Expansion Valve Types

There are different types of FTEVs based on their construction. A typical FTEV consists of three parts: a body; a shaft; and a disc. In general, the disc rotates along with the shaft, thereby controlling the opening and closing of the FTEV. The disc may rotate using various mechanisms, including linear motion, ball screw, rack-and-pinion, and worm gear. To reduce noise caused by rotation of the shaft, some manufacturers use electric motors to drive the shaft. The motor may be integrated with the shaft or separate from the shaft.

 Material Selection for FTEVs

The material selection for FTEVs should consider certain factors. These factors include the type and size of the FTEV, the operating temperature range of the FTEV and its environment, and the amount of vibration expected from the FTEV. In addition, the material selection for FTEV should take into consideration the cost associated with selecting the right material.

Design Considerations

The design considerations for FTEVs should include the following:

• Mechanical reliability

• Thermal stability

• Vibration control

• Cost effectiveness

Float type expansion valves are commonly used in HVAC systems, where they are designed to control the flow of air/gas (thermal) from the system. These may be either electro-mechanical or pneumatic types.

Pneumatic type expanders utilize compressed gas to open and close the valve body, whereas electro-mechanical type uses electrical current to operate.

 A differential pressure across the valve causes a flow of air through its small orifice.

 In HVAC applications, the size of the orifice is determined based on the design of the ductwork or piping around the unit being serviced.

Float valves are fitted with a float, which moves up and down due to changes in the water level inside the tank. This movement in turn creates a change in the amount of pressure on the top of the valve causing the valve to move towards the closed position.

 Differential pressure across the valve causes air to pass through the orifice. As the water level rises, the air moves out of the tank into the plumbing and air vents. Once the water level drops below the bottom end of the float, the float begins to go back down, which forces the valve to go back to its full open position.

The advantage of using float valves is that they are simple in construction, less prone to leaks and maintenance requirements than mechanical valves.

 There are two basic designs of float valve - single action and double action. Single action floats have only one opening through which fluid flows. Double action floats have two openings - one for incoming and one for outgoing.

 Float valves work best with air rather than liquid, although they can handle both.

 When selecting a float valve for any application, it should be noted whether the application requires continuous or intermittent operation. Continuous means the valve is always opening, while intermittent means the valve opens and closes at various times.

 If the application calls for intermittent operation, the valve should be selected with a smaller orifice. This will reduce the chance of clogging the orifice over time.

 The size of the orifice determines the maximum operating temperature of the valve.

 Determining the right size orifice for a particular application involves some trial and error. However, if the valve is not operated continuously, the number of starts and stops per day should be taken into consideration.

 Another factor to consider is the pressure drop across the valve. A higher value indicates that the valve is working harder, which could mean that the orifice size is too big, and therefore reducing efficiency.

Electronic Expansion Valve

 Why do I need an electronic expansion valve?

If you have experienced sudden changes in home temperatures, you probably know what happens if you forget to switch off the HVAC system. If the compressor is not switched off, it continues running even after the temperature reaches the set point or lower limit. Running continuously increases the internal pressure of the system, causing damage to the evaporators and the entire plumbing system. To avoid this problem, an electronic expansion valve is installed between the condenser and the evaporator. This prevents the unit's operation at higher pressures than recommended. An electronic expansion valve also helps prevent over-cooling or under-cooling of the house by preventing the compressor from operating at full capacity. By regulating the speed of the motor and/or the opening degree of the damper, the unit adjusts the airflow rate while delivering maximum cooling performance.

 Where should the expansion valve be located?

The ideal location is between the furnace and the evaporator. But, if space does not permit, it could be placed anywhere in the ductwork that connects the boiler and the condenser.

 How does an electronic expansion valve work?

When the thermostat calls for heating or cooling, the control module sends a signal to the relay board, which in turn switches the power supply to the fan motor. As soon as the fan starts turning, the fan begins moving air across the fins of the condenser coil, thereby increasing its surface area. Because of increased evaporation, the vapor cools down and returns to the condenser coil. At the same time, the refrigerant expands and moves towards the top of the evaporator, thus reducing the pressure inside the evaporator. When the pressure drops below a certain level, electrical contacts open and close, thereby shutting off the blower motor. Now the compressor stops working, and the flow of refrigerant is blocked. Since the evaporator still remains cold, no further cooling takes place. That is how an electronic expansion valve works.

An electronic expansion valve (EEV) is a device that regulates the pressure in a system that uses air. A typical EEV works by using the principle of venturi flow control. Venturi flow control consists of a restriction at the inlet side, causing low-pressure liquid flow, which creates a larger velocity, and therefore higher static pressure at the outlet than the supply pressure. When operating correctly, the difference between supply and discharge pressures results in increased fluid flow through the expander. The effect is similar to the action of a vacuum pump. If the supply pressure exceeds the demand, the flow will decrease. In addition, if the supply pressure decreases below the demand, the flow increases to maintain the desired differential across the valve.

An electronic expansion valve (EEV) is a device that regulates pressure based on temperature. Its purpose is to prevent over-pressure buildup inside the system if the gas flow drops below the minimum demand. By reducing the gas flow at low temperatures, excess liquid or ice is prevented from building up in pipes and tanks. EEVs are installed in industrial and commercial applications where high-temperature gases are present and regulate the flow of cool air using electric motors. These units are installed in HVAC systems that use cooling towers to maintain comfortable conditions inside buildings.

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