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Piping Design
Published in Herbert W. Stanford, Adam F. Spach, Analysis and Design of Heating, Ventilating, and Air-Conditioning Systems, 2019
Herbert W. Stanford, Adam F. Spach
It is usual to find solenoid valves (thermal expansion valves), filter dryers, strainers, and/or sight glasses in liquid lines, each of which adds pressure loss to the circuit. Since thermal expansion valves are rated and selected on the basis of 100% liquid at the valve inlet, the presence of “flash gas” in the supply reduces the valve capacity and results in loss of system capacity. To ensure that little or no flashing occurs due to pressure reduction resulting from pressure losses in the liquid line, subcooling of the liquid is utilized. Thus, pressure losses will result in some heat gain, but the refrigerant will remain in the liquid state.
Air Conditioning Systems
Published in Samuel C. Sugarman, HVAC Fundamentals, 2020
However, once the entire vapor has been changed to liquid, the liquid can reject additional heat. As the refrigerant, which is now a high-pressure, high temperature liquid (120F @ 260 psig) flows through the liquid line to the pressure reducing device it continues to give up heat. This is called “subcooling.” The liquid refrigerant will enter the expansion stage’s pressure reducing device (metering device) at approximately 90F. The liquid was subcooled 30F. Only liquids can be subcooled and only vapors can be superheated or desuperheated. When the liquid refrigerant goes through the metering device the pressure on the refrigerant is reduced to 70 psig. This reduction in pressure (from 260 psig to 70 psig) reduces the boiling point of the liquid refrigerant to 40F. However, the temperature of the liquid refrigerant at 90F is above the new boiling point (40F). Because the liquid refrigerant is hotter than its boiling point a part of the liquid refrigerant begins to boil off. This boiling off of the liquid refrigerant is called flashing. The liquid refrigerant which is boiled off or flashed, changes state to a vapor or gas. This vapor is called “flash gas.” When a part of the liquid refrigerant is flashed, it removes heat from the remaining liquid. This flashing continues until the remaining liquid refrigerant is cooled down to the boiling point which corresponds to the pressure on the liquid (40F @ 70 psig). About 18% of the liquid is flashed off to a vapor and is not available to pick up heat (i.e., latent heat of vaporization) but can pick up sensible heat in the evaporator stage. The vapor and the remaining liquid (82%) enters the evaporation stage and the cycle starts over. The AHU has taken 1200 cfm of mixed air at 78F and cooled it down to 55F supply air.
Vapor Compression-Cycle Systems
Published in Neil Petchers, Combined Heating, Cooling & Power Handbook: Technologies & Applications, 2020
Thus, the actual latent heat of evaporation available for cooling duty is lower than the ideal heat of vaporization. Therefore, it is beneficial for a refrigerant to have a low specific heat in its liquid state to minimize flash gas and, thus, the flow rate of refrigerant required for cooling duty.
Performance of a computerized refrigeration cycle test rig for varying concentrations of (CuO-Al2O3/MO) hybrid nano-lubricants and R600a refrigerant charges: an energetic and exergetic approach
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2023
Ankit Kumar, Satish Pal Singh Rajput
In present study, a Computerized Refrigeration Cycle Test Rig Model: RAC11 based on VCR system is utilized for experimental work shown in Figure 1. This test rig operates on the principle of a vapor compression refrigeration system. Vapor Compression refrigeration test rig is made up of four elementary components: an evaporator, a compressor, a condenser, and an expansion valve. The main function of the evaporator is to extract the amount of heat from the desired space or location so that the temperature of the space reduces. The refrigerant inside the evaporator changes its phase from liquid to gaseous phase and then goes into the compressor. The compressor, which is primarily driven by an electric motor, raises the pressure and temperature of the refrigerant. It is assumed in this study that at the entry to the compressor refrigerant is dry saturated. It means the compression is 100% dry compression and no amount of liquid particles inside the compressor. The high pressure and high temperature refrigerant is cooled inside the condenser with the help of a cooling fan (forced convection), which is also driven by an electric motor. The expansion process inside vapor compression test rig takes place with the help of an expansion valve and the process is isenthalpic process. The isenthalpic process is an irreversible process and is accompanied by an increase of entropy. In the expansion process, the refrigerant changes from liquid to the mixture of saturated liquid and saturated vapor, i.e. flash gas. The process again repeated continuously to carry out experiment.