Valves are essential in many devices and play a very important role. In this issue, let’s take a look at some of the most common valves in Heating, Ventilation, and Air Conditioning system.
1. Four-way valve
Four-way valve, a hydraulic valve term, is a control valve with four oil ports.
(1) Working principle of four-way valve:
When the solenoid valve coil is in the power-off state, the pilot slide valve moves left under the drive of the right compression spring, and the high-pressure gas enters the capillary tube and then enters the right piston chamber. On the other hand, the gas in the left piston chamber is discharged. Due to the pressure difference between the two ends of the piston, the piston and the main slide valve move left, so that the exhaust pipe is connected to the outdoor unit pipe, and the other two pipes are connected, forming a refrigeration cycle.
2. Gate valve
A gate valve is a valve that uses a gate as a start and stop part and moves vertically along the axis of the valve seat to achieve the start and stop action.
(1) Features of gate valves:
A gate valve can only be fully opened and fully closed. The start and stop part is the gate. The movement direction is perpendicular to the fluid direction. The two sealing surfaces of the square I-shaped gate valve form a wedge. The wedge angle varies with the valve parameters, usually 5°, and 2°52′ when the medium temperature is not high. To improve the processability of the gate valve and compensate for the deviation of the sealing surface angle during the processing, this gate is called an elastic gate.
3. Thermal expansion valve
Thermal expansion valve controls the refrigerant flow rate entering the evaporator by controlling the superheat of the gaseous refrigerant at the outlet of the evaporator.
(1) Function of thermal expansion valve:
Thermal expansion valve realizes throttling from condensing pressure to evaporating pressure and controls the flow rate of refrigerant at the same time; the expansion valve ensures that when the operating environment changes (such as heat load changes), the evaporator is optimally supplied with the best liquid.
4. Pressure reducing valve
A pressure reducing valve is a special device that automatically reduces the working pressure of a pipeline. It can reduce the higher water pressure in the pipeline before the valve to the required level of the pipeline after the valve.
(1) Function of the pressure reducing valve:
The basic working principle of the pressure reducing valve is to reduce the water pressure by the local resistance of the flow channel inside the valve to the water flow. The range of water pressure drop is automatically adjusted by the inlet and outlet water pressure difference on both sides of the diaphragm or piston connecting the valve disc.
(2) Types of pressure reducing valve structure:
There are many types of pressure reducing valve structures. The most common ones in the past are diaphragm type, internal spring piston type, etc. Pressure reducing valves usually have various specifications such as DN50~DN100. The working pressures before and after the valve are <1MPa and 0.1~0.5MPa respectively, and the pressure adjustment range error is ±5%~10%.
5. Butterfly valve
(1) Definition of butterfly valve:
Also known as flap valve, it is a regulating valve, which means that the closing part (valve disc or butterfly plate) is a disc, which rotates around the valve axis to achieve opening and closing. It mainly plays the role of cutting off and throttling in the pipeline.
(2) Application of butterfly valve:
Butterfly valves are commonly used to control the on-off function of low-pressure pipelines. They are suitable for regulating and shutting off the flow of various corrosive and non-corrosive fluid media in engineering systems, including those involving power generation, gas supply, air distribution, chemical processing, metallurgy, and environmental protection.
(3) Classification of butterfly valves:
Handle butterfly valve, turbine butterfly valve, pneumatic butterfly valve, electric butterfly valve, etc.
6. Ball valve
(1) Definition of ball valve:
A ball valve is a valve that uses a ball with a circular through hole as the opening and closing part. The ball rotates with the valve stem to achieve the opening and closing action.
(2) Working principle of ball valve:
Solenoid valves can also be used for fluid regulation and control. Among them, the hard-sealed V-type ball valve features a strong shearing action between the V-shaped ball core and the metal valve seat coated with hard alloy. This makes it especially suitable for handling media that contain fibers or fine solid particles. The multi-port ball valve offers flexible control over flow convergence, diversion, and directional switching within the pipeline. Additionally, it allows for shutting off any one of the channels while maintaining a connection between the other two.
(3) Classification of ball valves:
Pneumatic ball valve, electric ball valve, manual ball valve.
7. Stop valve
(1) Definition of stop valve:
Stop valve, also known as gate valve, is a forced sealing valve and one of the most widely used valves.
(2) Principle of stop valve:
The stop valve relies on the pressure of the valve stem to make the valve disc sealing surface fit tightly with the valve seat sealing surface to prevent the flow of the medium. Since the medium is only allowed to flow in one direction, it has directionality when installed.
(3) Classification of stop valves
Stop valves can be divided into direct current stop valves, angle stop valves, plunger stop valves, upper threaded stem stop valves, lower threaded stem stop valves, etc. They are durable, have a small opening height, are easy to manufacture and maintain, and are suitable not only for medium and low pressures, but also for high pressures.
(4) Advantages of stop valve:
① Simple structure, easy to manufacture and maintain.
② Small stroke, short opening and closing time.
③ Good sealing performance and long service life. In addition, stop valves also have shortcomings, such as poor adjustment performance, large force required for switching, and unsuitable for media with particles, high viscosity, and easy coking.
8. Check valve
(1) Definition of check valve:
Check valve is also called one-way valve or non-return valve. Its main function is to prevent the backflow of medium in the pipeline. Its opening and closing parts rely on the force of the medium flow to open or close automatically. Check valve only allows the medium to flow from one direction to another, and does not allow backflow to prevent accidents. Therefore, check valve is also called non-return valve.
(2) Installation precautions:
The biggest difference between check valve and other valves in appearance is that check valve has no valve shaft and does not require manual operation or electric control. During installation, be careful not to let the check valve bear weight. Large check valves are independently supported; the direction of medium flow should be consistent with the direction of the arrow on the valve.
(3) Classification of check valves:
① Check valves can be divided into lift type, swing type, butterfly type and diaphragm type according to the structure.
② According to the connection method, they can be divided into threaded connection, flange connection and welding.
9. Solenoid valve
(1) Definition of solenoid valve:
A solenoid valve is a fundamental component in fluid automation systems and functions as an actuator. It is applicable to more than just hydraulic and pneumatic systems. In industrial settings, solenoid valves are primarily used to control the direction of hydraulic fluid flow. Since most mechanical equipment in factories is operated through hydraulic cylinders, solenoid valves play a key role in enabling that control.
(2) Working principle of solenoid valve:
Inside a solenoid valve, there is a sealed chamber with multiple ports positioned at various locations. Each port connects to a different oil pipeline. At the center of the chamber sits a valve spool, flanked on both sides by electromagnets. When one of the solenoid coils is energized, the valve spool is pulled toward the energized side. By moving the valve spool, certain oil outlet ports are either opened or closed, redirecting the flow. The oil inlet port remains open, allowing hydraulic oil to flow continuously, and depending on the position of the spool, the oil enters specific outlet lines. This pressurized oil then acts on the piston inside the hydraulic cylinder, causing it to move. The piston drives the piston rod, which in turn moves a mechanical component. In this way, the mechanical movement is precisely controlled by regulating the electrical current supplied to the solenoids.
(3) Application of solenoid valve:
A solenoid valve is an industrial device operated electromagnetically and serves as a fundamental element in fluid automation control. It functions as an actuator and is applicable to both hydraulic and pneumatic systems. In industrial control environments, solenoid valves are used to regulate various parameters of the working medium, such as direction, flow rate, and speed. By integrating with different electrical circuits, they offer precise and flexible control. A wide variety of solenoid valves are available, each designed to perform specific functions within different parts of a control system. The most commonly used types include check valves, safety valves, directional control valves, and flow control valves.
10. Safety valve
(1) Definition of safety valve:
A safety valve is a valve that protects the system from being damaged by excessive pressure. Once the medium pressure exceeds the set value, the safety valve will automatically open, thereby achieving the purpose of safety protection.
(2) Classification of safety valves:
According to the structure, safety valves can be divided into heavy hammer lever type, spring type and pulse type.
(3) Heavy hammer lever type safety valve:
It uses the principle of lever, using a small weight to obtain the force acting on the valve opening through the amplification effect of the lever, and changing the position of the weight to change the opening pressure acting on the valve, as shown in the left figure. Its advantages are simple structure and accurate adjustment, and it is suitable for high temperature and high pressure occasions such as boilers and high pressure containers. The disadvantage is that the heavy loading mechanism is prone to leakage due to vibration, and it is difficult to close and keep it tight after being opened by pressure.
(4) Spring micro-opening safety valve:
The force acting on the valve disc is balanced by the force of the compression spring. The compression amount of the spring can be changed by adjusting the nut on it, which can be used to adjust the opening pressure of the safety valve. The advantages are light and compact structure, high sensitivity, and no restrictions on installation location. It can be used on mobile pressure vessels. As shown in the left figure below, the disadvantage is that the elastic force of the spring increases with the increase of compression, so the pressure of the loading valve will increase with the increase of compression, which directly makes the safety valve unable to open quickly.
(5) Pulse safety valve:
It consists of a main valve and an auxiliary valve, and the pulse action of the auxiliary valve drives the operation of the main valve. Its application range is narrow and its structure is complex. It is only suitable for boilers and pressure vessels with large safety discharge volume. Among these three safety valves, the spring micro-opening safety valve is the most commonly used.
11. Electric control valve
(1) Definition of electric control valve:
The electric control valve consists of an actuator and a valve body located below the actuator. The actuator and the control valve are connected and assembled, and can only be used after installation and debugging.
(2) Principle of electric control valve:
By receiving the weak signal (0-10v or 4-20mA) sent by the industrial controller to change the valve opening, the medium flow, temperature, pressure and other parameters can be adjusted. Automatic adjustment is achieved to achieve energy saving.
(3) Precautions:
When selecting an electric control valve, it is best to choose one with manual control if possible, so as to prevent the valve from being unable to open or close normally due to circuit failure. The threaded disc at the top of the electric control valve in the figure is the manual adjustment disc. When using the electric control valve, pay attention to regular inspections to check whether the actuator has any faults and whether there are any abnormalities in the connection between the actuator and the valve shaft. This can ensure its safe and efficient operation. Generally speaking, it should be checked every half a month.
12. Balancing valve
(1) Definition of balancing valve:
A balancing valve is a specialized valve designed with favorable flow characteristics, a visible opening indicator, an opening lock mechanism, and integrated pressure measurement ports for flow monitoring. When used in conjunction with a dedicated intelligent instrument, the operator can input the valve model and current opening position, enabling the device to calculate and display real-time flow based on the measured differential pressure. By installing appropriately sized balancing valves at each branch and user inlet, and performing a one-time commissioning with the intelligent instrument, the flow to each end user can be precisely adjusted to meet the desired setpoint.
(2) Characteristics of balancing valve:
① Ideal adjustment performance;
② Excellent cut-off function;
③ Opening status display accurate to 1/10 circle;
④ The theoretical flow characteristic curve is an equal percentage characteristic curve;
⑤ There is a certain flow coefficient corresponding to each full circle. During debugging, as long as the pressure difference at both ends of the valve is measured, the flow through the valve can be easily calculated;
