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Refrigeration Cycles and HVAC Systems
Published in S. Bobby Rauf, Thermodynamics Made Simple for Energy Engineers, 2021
Common refrigeration system expansion valves are also referred to as Thermal Expansion Valves or TXV’s. Operating principle of the thermal expansion valve is illustrated in Figure 11-4. A thermal expansion valve functions as a metering device for the high pressure liquid refrigerant; it allows small proportionate amounts of the high pressure liquid refrigerant into the discharge side. This permits the refrigerant to transform into low pressure liquid; ready to be converted to vapor phase as it absorbs heat from the warm ambient air passing through the heat exchanger coils. As the refrigerant evaporates to higher temperature, the temperature of the gas in temperature sensing bulb rises. The higher temperature gas develops higher pressure thus pushing the expansion valve open. The valve, in its metering function, stays open only until the temperature in the evaporator section drops. When the temperature in the evaporator section drops, the temperature and the pressure of the gas in the temperature sensing bulb drop, thus, resulting in the valve closure. This cycle repeats itself in a closed loop control fashion, continuously, in a typical refrigeration system.
Heat Pump Modeling
Published in Vasile Minea, Industrial Heat Pump-Assisted Wood Drying, 2018
A thermal expansion valve in heat pump systems uses a temperature sensing bulb to open a valve, allowing liquefied refrigerant to move from the high pressure side of the system to the low pressure side before entering the evaporator. As the evaporator temperature decreases, pressure on the bulb decreases, allowing a spring to close the valve.
Experimental study on temperature distribution in an ice-making machine multichannel evaporator
Published in Science and Technology for the Built Environment, 2019
Zhan Liu, Jia Yan, Penghui Gao, Haihui Tan
Driven by the screw compressor, the superheating refrigerant vapor (state 3') is inhaled into the intermediate intake of the compressor, then compressed from the intermediate pressure state (state 3') to high pressure state (state 4) with the temperature increasing to 35–40ºC.As the lubricating oil is mixed in the refrigerant gas, the high-temperature vapor flows in the oil separator firstly, where parts of lubricating oil are separated and returning to the compressor again. Afterward, the refrigerant vapor passes the four-way valve and inflows into the condenser, where the superheating refrigerant vapor is condensed to the saturated liquid (state 5). As the volume of the condenser is not large enough, the condensed liquid is inflowing into the liquid receiver and in storage for a time. While the liquid refrigerant flows out of the receiver and passes the dryer and the solenoid valve, it is divided into two parts before flowing into the economizer. A small part of liquid refrigerant passes a thermal expansion valve firstly, where the liquid refrigerant is throttled into two-phase flow, with the state of refrigerant fluid transferring from the saturation point (state 5) to two-phase flow(state 6). Afterward, both the large part of the refrigerant and the small part of two-phase flow are flowing into the economizer. In the economizer, two parts of fluids conduct the heat transfer with the reverse flow directions. With the cold capacity caused by the throttling of the small part of refrigerant, the large part of refrigerant is cooled to subcooling fluid (state 7). Absorbing the heat energy from the large part of refrigerant, two-phase flow is transferring to the saturation vapor (state 3) from state 6. While the subcooling liquid flows out of the economizer, it inflows in liquid distributor firstly and then is distributed into 14 parts. Before flowing into the refrigeration evaporation loop, each part passes a thermal expansion valve, where the subcooling fluid (state 7) is throttled into two-phase flow (state 8) with the fluid temperature reducing to –25 to –8ºC. As the two-phase fluid flows into the multichannel refrigeration evaporator, large amounts of cold capacity are released. Under the cooling caused by the two-phase fluid, water in the ice bath is transforming to ice cubes. While the sensible heat and the latent heat of the low temperature two-phase fluid have been burned out by the heat energy from water or ice in the ice bath, the two-phase fluid is heated to the saturation vapor (state 0). As it is not acceptable that the wet refrigerant vapor be inhaled into the compressor, the inlet gas is usually heated to the superheated state (state 1). While the superheating refrigerant vapor enters the compressor, it is compressed to the intermediate pressure from the initial pressure, with the state varying from point 1 to point 2. In the intermediate pressure the saturated vapor and the superheated vapor mix, with the final state 3' arrived. Thereafter, the mixing vapor is compressed again in the compressor from the intermediate pressure. Thus, one refrigeration cycle is introduced completely.