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130 M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 Fig. 18. System with low-pressure receiver. a pressure-regulating valve (A), that controls the high-side pressure, and an electronic or thermostatic expansion valve (B), that regulates liquid flow to the evaporator. The receiver pressure may either be supercritical or subcritical. In case of subcritical receiver pressure, the outlet from the pressure-regulating valve (A) will be on the saturation line during steady-state operation. The receiver pressure will adjust itself to this point, since vapor cannot escape. Adjustment of the valve opening temporarily moves the end-point of the throttling away from the saturation line, and the resulting imbalance between the mass flow rates through the two valves gives a transfer of mass to or from the receiver, thereby affecting high-side charge and pressure. In case of supercritical receiver pressure, the refriger- ant mass in the buffer is regulated by changing the buffer pressure, thereby modifying the density of the com- pressed fluid. The pressure can be controlled between the compressor discharge pressure and the critical pressure. A large receiver volume may be necessary in order to obtain the necessary range of high-side charge variation. Another system with intermediate-pressure buffer is shown in Fig. 20 [11]. Here, the receiver is located in parallel to the flow circuit, connected to the high and low sides by valves. These two valves and the expansion valve are operated to control high-side charge and pressure. 3.2.2. Systems with high-side volume control Instead of varying the mass, the pressure in the high side can be regulated by adjusting the internal volume of the high-side part of the circuit. For a given volume change, the largest pressure variation will be obtained at the lowest possible temperature (highest density). This makes the gas cooler refrigerant outlet the ideal location for a volume-control device. This device may be constructed in a number of ways, including bellows arrangement inside a pressure vessel or a cylinder where the displacement of a piston defines the refrigerant-side volume. The buffer design must consider factors like lubricant trapping and means for adjusting the expansion valve, temporarily changing the balance between compressor mass flow rate and valve flow rate. By reducing the valve opening, a temporary reduction in the valve mass flow rate gives refrigerant accumulation in the high side, and the pressure rises until a new balance point between valve flow rate and compressor flow rate is found. The vapor fraction at the evaporator outlet may temporarily rise while pressure is rising, and the additional high-side charge is transferred from the low-side buffer. Conversely, increased valve opening will reduce the high-side charge and pressure, and the excess high-side charge is deposited as liquid in the buffer. In practice, such systems will in most cases need a liquid bleed from the receiver in order to return lubricant to the compressor and to maintain the evaporator outlet slightly wet. The liquid surplus may be an advantage when the high- side pressure is raised, to avoid drying up the evaporator. By installing an internal (suction line) heat exchanger, the liquid is evaporated before the compressor inlet, and the COP is improved at high heat rejection temperature. The use of internal heat exchange is discussed elsewhere in the paper. Systems with medium-pressure buffer. Fig. 19 shows a system where the buffer is kept at an intermediate pressure [11]. An in-line receiver is located between Fig. 19. System with in-line medium-pressure receiver. Fig. 20. System with medium-pressure receiver.PDF Image | CO2 Vapor Compression Systems
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