Regenerators in refrigeration systems typically have three important functions: (1) to increase the coefficient of performance of the refrigeration system; (2) to supercool the liquid leaving the condenser to avoid vaporization before throttling; (3) to allow vapor to leave the evaporator The entrained liquid vaporizes the vapor compression refrigeration system with the regenerator before entering the compressor, in which the refrigerant liquid leaving the condenser is further cooled by the regenerator, thereby effectively preventing the pre-throttle At the same time, the liquid entrained in the cold gas leaving the evaporator is also completely vaporized before entering the compressor, thereby effectively preventing the occurrence of liquid hammer.
The efficiency of the regenerator regenerator transfers part of the energy of the high temperature liquid to the low temperature gas, the transfer capacity of which depends on the heat exchange area and structure. In the performance analysis of the regenerator, its efficiency is an important parameter. The efficiency of the regenerator can be defined as: the subscript value of the temperature corresponds to the point. By means of the regenerator, by exchanging heat with the vapor at the outlet of the evaporator, the refrigerant leaving the condenser is further reduced in value before entering the expansion valve, so that the temperature of the high-pressure liquid can be further lowered, the cooling capacity per unit mass is increased, and The possibility of flashing gas before the expansion device is reduced. At the same time, the regenerator increases the suction temperature of the low-pressure side compressor and reduces the pressure of the refrigerant, which will increase the specific volume of the refrigerant, thereby reducing the mass flow rate of the refrigerant and the gas flow of the compressor. the amount. Therefore, the efficiency of the regenerator has a great influence on the performance of the system.
Ignoring the effect of the regenerator on the cooling capacity of the system when the mass flow correction is neglected, it can be seen from Fig. 2 that when there is no regenerator in the system, the unit mass cooling capacity is h; when there is a regenerator, it becomes 4, and Compared with the regenerator, the unit mass cooling capacity is increased by h4. If the correction of the mass flow rate is neglected, the cooling capacity of the system will always increase after the regenerator is installed, but the degree of cooling increase depends on the refrigerant. The type, the efficiency of the regenerator and the operating conditions of the system. The influence of the regenerator on the cooling capacity can be analyzed according to the relative cooling capacity index (RCI). The cooling capacity of the system is assumed to be stable, and the theoretical calculation and analysis are obtained without considering the pressure loss of the regenerator. For different refrigerants, when the evaporation temperature is -20 ° C, the condensation temperature is 40 ° C, the relationship between the efficiency of the regenerator and the relative cooling capacity index.
For all refrigerants, the cooling capacity of the system can be increased by further cooling the refrigerant at the outlet of the condenser. It can also be seen from the figure that the efficiency of the regenerator and the relative cooling capacity index are approximately linear. However, the relative cooling capacity index is affected by both the evaporation temperature and the condensation temperature.
The effect of regenerator on the cooling capacity of the system during mass flow correction. Relative Clearance Volume As the efficiency of the regenerator increases, the degree of superheat obtained by the refrigerant entering the compressor also increases, which not only reduces the density of the refrigerant, but also reduces the volumetric efficiency of the compressor. The pressure loss on the low pressure side of the regenerator further reduces the density of the refrigerant entering the compressor. Therefore, when the efficiency of the regenerator is increased, the mass flow rate of the refrigerant is reduced, and sometimes the system cooling amount is negatively affected. At this time, although the unit mass cooling capacity is also increased, due to the pressure loss in the regenerator, at the state point 2, the suction density decreases due to the temperature increase and the pressure drop, and the mass flow rate itself decreases.
In addition to considering the effect of the regenerator on the cooling capacity, it is also necessary to consider the effect on the COP, which requires an understanding of the effect of the performance of the regenerator on the compressor power. Threckeld puts forward that the compressor's compression process is a variable process, and proposes the approximate expression of the compressor's ratio indicating work. P -- the absolute pressure of the compressor inlet -- the absolute pressure of the compressor outlet -- - the specific volume of the refrigerant at the inlet of the compressor n - the variable process index can be used to calculate the input electric power of the compressor by the formula (6): where η -- the product of the mechanical efficiency η of the compressor and the motor efficiency η The theoretical unit air volume per compressor is only a function of the motor speed and is not affected by the regenerator; the compressor suction and discharge pressure is controlled by special methods and equipment, and thus is independent of the regenerator, and assumes a variable process. The index n is also independent of the regenerator, and the input electric power of the compressor is independent of whether or not the regenerator is installed, on the premise of negligible pressure drop.
Since the change in system COP is directly related to the amount of cooling, the amount of change in COP and cooling capacity is consistent under the assumption that the pressure drop can be ignored. Due to the increase of the suction air temperature of the compressor inlet, the mass flow rate of the refrigerant is reduced. For different refrigerants, the effect of the regenerator on the COP is consistent with the influence on the cooling capacity.
The pressure loss in the regenerator affects the pressure loss. If the pressure loss is considered, the relative cooling capacity index RCI and COP values ​​of the system will decrease. The amount of pressure loss in the regenerator depends on the design of the regenerator. On the basis of a large number of studies, Domanski and Didion give an approximate formula for the correction of the results. Δh --- evaporation pressure, vapor ratio 焓 difference (h --- evaporation temperature, refrigerant saturated liquid constant pressure Specific heat capacity --- condensing temperature --- evaporation temperature ε --- the efficiency of the regenerator pressure loss is different for the liquid and gas lines. In the liquid line, the pressure loss on the cooling capacity and COP The effect is much smaller than the effect on the suction line at the same pressure loss. The pressure loss on the liquid line reduces the pressure of the refrigerant flowing into the expansion device, assuming that the pressure loss is small enough that it does not appear before entering the expansion valve. Under the premise of flashing gas, since the liquid refrigerant has incompressible characteristics, the pressure loss has little effect on the relative cooling capacity, and when the regenerator passes through, the liquid refrigerant decreases, so the flashing is reduced. The possibility of generating gas.
The pressure loss of the regenerator on the gas line (low pressure side) has a large influence on the cooling capacity and COP. The pressure loss reduces the density of the refrigerant entering the compressor, resulting in a decrease in the mass flow rate of the refrigerant, which in turn causes a decrease in the amount of refrigeration. In addition, in order to increase the pressure of the condenser, the specific work of the input compressor is increased, and the volumetric efficiency is decreased. However, the influence of pressure loss on the cooling capacity and the compressor power is different, so when considering the pressure loss of the regenerator, the relative change amount of the system COP is not necessarily equal to the relative change amount of the cooling capacity.
When there is fluid pressure loss in the regenerator, the cooling capacity Q should be less than the cooling capacity Q without pressure loss; likewise, the COP with pressure loss should be less than the COP without pressure loss. Refrigeration with regenerator The system was analyzed for Q and COP/COP at different temperature differences (difference between condensing temperature and evaporating temperature), regenerator efficiency, and pressure loss. The pressure loss in the regenerator is dimensionless, that is, the dimensionless number is the ratio of the pressure loss ΔP of the regenerator on the low pressure side line to the absolute pressure P in the evaporator.
The relationship between dimensionless pressure loss and COP/COP is shown in Fig. 6. The pressure loss leads to the decrease of refrigerant mass flow rate, and leads to the decrease of cooling capacity. The decrease of refrigerant mass flow rate will lead to the decrease of compressor input electric power. And the input power of the compressor caused by the increase in the pressure ratio caused by the pressure loss is greatly increased. The size of /COP can be determined based on the dimensionless pressure loss and the temperature difference between the condensing temperature and the evaporation temperature.
When analyzing the effectiveness of a regenerator in a refrigeration system, consideration should be given to changes in refrigerant mass flow in the system. The performance of the refrigeration system with regenerator can be evaluated for a particular refrigerant using the above method.
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