Several throttling mechanisms for refrigeration equipment in HVAC engineering
A throttling mechanism is a device or component used to limit the flow of fluids, which can achieve the regulation and control of the flow rate by changing the cross-sectional area of the flow channel or introducing resistance. Throttling mechanisms are often used in fluid control systems to meet flow requirements under different operating conditions. The throttling mechanism has the throttling and depressurizing of the liquid refrigerant to ensure the pressure difference between the condenser and the evaporator, so that the liquid refrigerant in the evaporator can evaporate at the required low pressure to achieve the purpose of refrigerationRefrigerant flow to the evaporator is adjusted to accommodate changes in the heat load of the evaporator.
The types of throttling devices can be divided into:
1) Manual expansion valve;
2) Float Expansion Valve;
3) Thermostatic expansion valve;
4) Electronic expansion valve;
5) Capillary;
6) Throttling stubs;
7) Orifice plate throttling;
1) Manual expansion valve;
It is only used in ammonia refrigeration systems, experimental setups, bypass backups, etc.
A manual expansion valve is a device used to control the flow of fluids and consists of a valve body, disc, and handle. The valve body is the main part of the valve and is usually used to regulate the pressure and flow of liquids or gases in the pipeline. It can be operated manually in order to adjust the opening of the valve as needed. It is usually made of metal and has inlet and outlet pipe connections. The disc is the movable part of the valve that is connected to the valve body by a handle. By rotating the handle, the position of the disc can be changed and the opening of the valve can be adjusted.
The working principle of the manual expansion valve: it is to change the flow cross-sectional area in the pipeline by adjusting the position of the valve disc, so as to control the flow and pressure of the fluid. When the handle rotates, the disc moves accordingly, allowing the fluid to pass through the valve's passage. By increasing or decreasing the opening area of the channel, the flow rate of the fluid and the pressure of the valve can be adjusted.
2) Float Expansion Valve;
In addition to throttling and reducing pressure and regulating the flow rate, it can also maintain a certain liquid level in the evaporator. Suitable for evaporators with free liquid level, such as flooded evaporators;Low-pressure circulating liquid storage tank;Intercooler. The float expansion valve is simple in structure;The liquid level of the float chamber fluctuates greatly, and the impact force transmitted by the float to the valve core is also large, which is easy to be damaged. The float device is a critical part of the valve and usually consists of a float and a float rod. The float is located in the valve body and can float up and down with the flow of media. The float rod is connected to the float ball and extends to the outside of the valve body, which is manually operated by means of a handle.
The working principle of the float expansion valve is based on the buoyancy principle. As the medium in the pipe flows, the float moves up and down as the liquid level changes. When the liquid level rises, the float rises, and the float rod closes the valve disc by mechanical transmission, reducing the opening area of the channel, thereby reducing the flow of fluid. Conversely, when the liquid level drops, the float descends and the disc opens, increasing the opening area of the channel and increasing the flow of fluid.
3) Thermostatic expansion valve;
The thermostatic expansion valve controls the opening of the valve through the superheat of the gaseous refrigerant at the outlet of the evaporator. For non-flooded evaporators.
A thermostatic expansion valve usually consists of a valve body, a valve core, a temperature sensing element, and a regulating device. The valve body is connected between the evaporator and the condenser of the refrigeration system, which plays the role of diverting and regulating the flow rate. The spool is a movable part, controlled by a temperature sensing element, which changes the flow of refrigerant by adjusting the size of the valve port. The temperature sensing element is typically an expansion-sensitive device that automatically adjusts the position of the spool in response to temperature changes. The regulator can manually or automatically control the opening of the spool to meet the needs of the refrigeration system.
The working principle of the thermostatic expansion valve: based on the thermostatic expansion effect. As the refrigerant in the refrigeration system flows through the evaporator, its temperature rises and expands. The temperature sensing element senses this temperature change and moves the spool accordingly to increase or decrease the opening of the valve port. By varying the size of the valve port, the thermostatic expansion valve can control the flow of refrigerant between the evaporator and the condenser to achieve proper refrigeration.
4) Electronic expansion valve;
Electronic expansion valves typically consist of an electric spool, an electronic control unit, and sensors. The electric spool is the core part of the valve, and the position of the spool is driven by an electric motor, which changes the opening of the valve port. The electronic control unit is responsible for receiving signals from the sensors and controlling the movement of the motorized spool according to preset parameters and algorithms. Sensors can be temperature sensors, pressure sensors, or liquid level sensors, for example, to monitor the operating parameters of the refrigeration system.
Electronic expansion valve superheat adjustment: used for dry evaporator to set a temperature sensor or pressure sensor at the evaporator outlet to collect the superheat of the refrigerant at the evaporator outlet and feedback the opening of the control valveFeedforward plus feedback compound adjustment can eliminate the superheat control lag caused by the heat capacity of the evaporator tube wall and the sensor, improve the quality of system adjustment, and control the superheat within the target range in a wide evaporation temperature area.
By constantly adjusting the size of the valve port, the electronic expansion valve can control the flow of refrigerant in real time to adapt to changes in load and environmental conditions in the refrigeration system. The electronic control unit precisely calculates and adjusts the opening of the valve port based on the feedback signals from the sensors, so that the refrigeration system can maintain the best performance and efficiency under various operating conditions.
Electronic expansion valves offer a number of advantages. With higher accuracy and response speed, it can achieve more precise flow regulation and improve the control accuracy of the refrigeration system. Secondly, the electronic expansion valve has a wider operating range and can adapt to different working conditions. In addition, the electronic expansion valve is self-adapting and intelligent, which can automatically adjust and optimize according to changes in the system.
5) Capillary;
Diameter 07~2.5mm, length 06 6m slender copper tube, it is widely used in small fully enclosed direct cooling devices. The liquid supply capacity depends on the condition of the refrigerant at the capillary inlet (pressure, temperature) and the geometry of the capillary (length, inner diameter).
Capillary characteristics: simple structure, low quality;No moving parts;However, the inner diameter and length of the capillary have a significant impact on the regulation of fluid flow and pressure, and need to be selected and designed according to the needs of the specific application. The system does not need to be equipped with a liquid reservoir and has less refrigerant chargeAfter the compressor stops, the pressure can reach equilibrium quickly, reducing the starting load of the motorThe adjustment performance is poor, and the liquid supply volume cannot be adjusted with the change of working conditionsIt is suitable for occasions where the evaporation temperature changes little and the working condition is relatively stable.
Capillary phenomenon refers to the fact that when a liquid or gas flows in a small pipe, the liquid or gas forms a thin liquid or gas film on the inner wall of the pipe due to the action of surface tension. The smaller the inner diameter of the capillary, the greater the surface area of the liquid or air film, and the more pronounced the effect of surface tension becomes. This surface tension causes the fluid to create a pressure difference in the capillary, allowing the fluid to flow.
6) Throttling stubs;
The choke stub is a kind of throttling device with a fixed cross-section throttle orifice, which has been used in some automobile air conditioners, chillers and heat pump units. Main advantages: low cost, simple manufacturing, good reliability, easy installation, cancel the thermostatic expansion valve system used to determine the size of the refrigeration load increased by the temperature bag, etc., with good interchangeability and self-balancing ability. It is important to note that the relationship between the pressure drop and the flow rate of the throttle spool is not linear, and the relationship between the flow rate and the pressure drop will vary over different flow velocity ranges. Therefore, when designing and applying throttle stubs, it is necessary to make the correct selection and calculation according to the specific fluid properties and flow requirements.
The throttle spool works by placing a small hole in the pipe that causes an increase in the velocity and pressure of the fluid. When the fluid passes through the small hole of the short tube, the flow rate increases due to the limitation of the flow channel, resulting in a decrease in pressure. By controlling the pore size of the spool and the characteristics of the fluid, the flow rate can be adjusted.
7) Orifice plate throttling;
An orifice plate usually consists of a plate-like device with a specific geometry with a hole or aperture in the middle. Orifice plates are installed in pipes, and when a liquid or gas flows through the orifice, a pressure difference is created between the front and back of the orifice. Depending on the differential pressure of the fluid and the geometry of the orifice plate, the actual flow rate can be calculated by some empirical formula or flow coefficient.
For equipment with large refrigeration capacity, such as centrifugal chillers, the refrigerant circulation capacity is relatively large, so the capillary is obviously not suitable. When the pressure difference between the front and back of the pipeline is large, it is often achieved by adding a throttle orifice plateThe principle of the orifice plate is that when the fluid flows in the pipeline, the pressure of the fluid decreases and the energy is lost due to the local resistance of the orifice. This phenomenon is known in thermodynamics as throttling.
The use of the throttle orifice plate is simpler than the use of the control valve, but it must be properly configured, otherwise the liquid is prone to cavitation, affecting the safe operation of the pipeline, the function of the throttle orifice plate is to make the aperture smaller in the appropriate place of the pipeline, and the flow beam will be tapered or contracted when the liquid passes through the necking. The smallest cross-section of the flow beam occurs downstream of the actual necking, which we call the downsizing section. The flow velocity is greatest at the condensation section.
The working principle of orifice throttling: based on Bernoulli's principle and the continuity equation. As the fluid passes through the orifice, the flow rate increases and the pressure decreases, resulting in a pressure differential. The magnitude of the differential pressure is related to the fluid density, the flow rate, and the geometry of the orifice plate. By measuring the differential pressure, the flow rate of the fluid can be calculated.
Orifice throttling has the advantages of simple structure, high reliability and wide range of application. They are widely used in industrial process control, liquid and gas metering, water treatment and water supply systems, and more. However, there are also some limitations to orifice throttling, such as the potential for reduced accuracy at high velocity flow or low differential pressure, so it is necessary to select the appropriate throttling device for the specific situation in practical applications.