Thermostatic Expansion Valve (TXV) in HVAC

         In this post, we will see about  Thermostatic Expansion Valve (TXV) in HVAC.

Thermostats & Expansion Valves

A thermostat is a device that senses temperature changes inside of a system and adjusts the amount of cooling/heating power flowing into or out of the system. An expansion valve regulates the flow of refrigerant gas entering the evaporator coil. A thermostat turns the compressor off if the temperature reaches a certain level, while an expansion valve opens up wider and lets more fluid pass through to speed up the evaporation rate. These devices make sure that the air conditioning unit works properly and efficiently at all times.

1. Thermostat Expansion Valve - Thermostatic Controls

A thermostatic control is a device consisting of a bimetallic strip which controls the temperature of a system by responding to changes in temperatures. There are several types of thermostats including differential and single-point, and they can either open or close based on the set point temperature. A closed position indicates normal operating conditions while an open position signals higher than normal temperatures. A thermostatic expansion valve (TXV) is a device installed in a heating water vessel to reduce pressure fluctuations caused by thermal expansion. TXVs work by using a diaphragm to separate high-pressure steam from low-pressure steam. In a typical application, the TXV operates on a cycle of hot/cold cycles throughout the day. When the TXV senses a change in the temperature, it closes its vents until the pressure rises back to a safe level.

2. Function of Thermostatic Expansion Valves - Why do we need them?

Thermostatic expansion valves work by reducing pressure fluctuations caused by thermal expansions. As steam cools, it expands and creates more pressure inside the boiler. If the boiler is not pressurized correctly, the pressure fluctuates due to the expansion and contraction of water. The result is a rapid increase or decrease in pressure which could damage the equipment. The TXV works by opening a vent whenever the pressure reaches a dangerous level.

3. Types of Thermostatic Expansion Valve - Differential & Single-Point

There are different types of thermostatic expansion valves that operate differently. Most common among these are the differential and single point. A differential TXV senses the difference between the top and bottom pressures and opens its vent accordingly. A single-point TXV only detects the pressure at the lowest point of the system.

4. Benefits of Using Thermostatic Expansion Valve - Reduce Pressure Fluctuations

The use of a TXV provides several advantages over conventional methods of controlling pressure fluctuations. A traditional method uses a regulator to adjust the flow rate of steam being sent to the boiler. However, since the steam is already under pressure, the pressure does not fluctuate. On the contrary, a TXV reduces the pressure fluctuations caused by thermal variations.

5. Thermostatic Expansion Valve Installation Tips - How to install?

When installing a TXV, make sure to follow instructions carefully. Ensure that the TXV is seated properly and tighten down the nuts to prevent accidental disassembly. You should also check whether the TXV has been installed correctly on both sides.

            An expansion valve is a device designed to reduce pressure fluctuations inside a heating/cooling system by gradually opening or closing valves based on temperature changes. TXVs work by using a thermostat to control the flow of refrigerant through small channels in a diaphragm. When the thermostat senses that the temperature of the refrigerant entering the compressor is above a certain threshold, the thermostat sends a signal to open one or more valves on the suction side of the compressor. If the temperature of the refrigerating fluid drops below a certain point, the reverse happens. The thermostat closes the intake valves.

Thermal expansion valves work by restricting air flow towards the return side of the system. The restriction causes the temperature at the low pressure side of the system to rise above the set point value, causing the vaporizer to shut off. The higher the set point, the lower the risk of overheating.

The thermostat operates based upon the difference between the temperature of the two sides of the system. The lower the differential, the more likely the system will turn on. If the temp in the room rises beyond the setpoint, the valve will close, cutting off airflow out of the house.

Thermostat Expansion Valves work by sensing the temperature inside the room and adjusting the flow rate of air to keep it at a constant temperature. When the temperature rises above a set point, it shuts off the incoming airflow to the heating system and opens the return airflow. In winter when the temperature drops below a set point, it switches the system back on to maintain the desired temperature. These valves have two types of operation; thermal type (thermal expansion) and vacuum type (vacuum expansion). Vacuum valves sense the pressure of the air being drawn through, whereas thermal valves sense the actual temperature of the air passing through.

Thermostatic Expansion Valve (TEV) is an electro-mechanical device that controls refrigerant flow into the evaporator coil based upon the difference between the temperature of the outlet side of the air conditioning system's condenser coil and the temperature of the refrigerant entering the compressor.

The function of TEV is to maintain proper cooling capacity while preventing overloading of the compressor. Overloaded compressors cause excessive wear on the internal components.

1. Thermostatic Expansion Valve consist of three basic elements:- Valves, Sensors & Actuators

Valve: A small ball shaped metal diaphragm that opens and closes according to changes in temperature.

Sensors: Two thermistors are attached to the valve body and their output signals are fed into the control board where they are processed.

Actuator: An electromagnetic actuator is used for opening and closing the valves.

2. Operation Principle

Temperature Difference : When the temperature at the exit of the evaporator coil changes, the liquid refrigerant will expand causing the valve to open. As the liquid refrigerant enters the compressor, the volume increases, thus reducing the pressure. If there is no further change in temperature the valve remains closed.

Duct Pressure : Due to the increase in volume, the pressure inside the evaporator coil decreases. The decrease in pressure causes the valve to close.

3. Types of Thermostatic Expansion Valve

a. Single stage - Single stage TEV consists of only one expansion valve. In single stage TEV, two thermistors are connected across the solenoid and the output is taken from the junction box.

b. Dual stage -Dual stage TEV consists of two valves, each having its own thermistor circuit and output terminal. The two outputs are then joined together before being sent to the controller.

c. Triple stage - Triplestage TEV consists of three valves. One valve is located at the suction line, a second valve at the discharge line and a third valve at the return line. Each valve has its own independent thermistor circuits and each of the three output terminals is connected to the controller.

d. Quadruple stage - Quadrulestage TEV consists four valves. Four valves are placed in series in the suction line. Two valves are located in the discharge line, two valves in the return line. All four valves have their own independent thermistor circuits.

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