Plate heat exchanger (PHE) is an efficient and compact heat exchange equipment, which is superior to shell and tube heat exchanger in many aspects. Under the same heat exchange load, the volume of the plate heat exchanger is 1/3~1/6 of the tube and shell type, and the weight is only 1/2~1/5 of the tube and shell type. The required refrigerant charge The volume is only about 1/7 of the shell and tube type. In terms of water heat exchange, under the same load and the same water speed, the heat transfer coefficient K of the plate heat exchanger can reach 2000~4650W/(㎡·K), which is the 2~5 times, because of the advantages of high heat transfer efficiency, small size, light weight, convenient area and process combination, plate heat exchangers have been widely used in the refrigeration industry, such as chillers and air-cooled heat pumps. The application of heat exchangers such as evaporators, condensers, heat recovery heat exchangers (superheat coolers), liquid subcoolers, and oil coolers of screw compressors in hot water units is increasing. The plate heat exchanger is used as an evaporator during cooling and as a condenser during heating.
1 Basic requirements of plate heat exchangers for refrigeration equipment
(1) Plate heat exchangers used in refrigeration equipment have high pressure on the refrigerant side (up to about 25 bar) and strong penetration, so high-pressure brazed plate heat exchangers specially produced for the refrigeration industry should be preferred when modeling .
(2) The heat transfer coefficient is an important indicator to measure the heat exchange effect of the heat exchanger. With the increase in energy requirements per unit volume of refrigerators, the heat exchange area per unit volume of plate heat exchangers used in refrigeration equipment should also increase accordingly. The water-cooled and glycol-cooled units use a highly efficient and compact stainless steel brazed plate heat exchanger, which has a larger increase in resistance efficiency than the shell and tube heat exchanger, and the scaling rate is only the same as that of the shell condenser. 1/10[2]. .
(3) A large pressure drop in the condenser will reduce the condensation temperature of the steam, resulting in a reduction in heat transfer temperature difference; a large pressure drop in the evaporator will cause excessive steam superheat at the outlet. Both cases will increase the heat exchange area, which is unfavorable for heat exchange. For the condenser, it is recommended that the refrigerant (such as R22) side pressure drop ≤0.01~0.03Mpa2.
(4) For air-cooled heat pump units, the water-side heat exchanger is used as an evaporator in summer and as a condenser in winter. The two should be selected and calculated separately in the design, and the larger area shall prevail.
2 As a plate heat exchanger for evaporator
As shown in Figure 1, in the air-cooled heat pump unit, the refrigeration cycle
At that time, the plate heat exchanger is used as an evaporator. The evaporator must be kept vertically, and the gas and liquid two-phase refrigerant intercepted by the expansion valve enters from the bottom of the evaporator to ensure uniform distribution of the refrigerant, and at the same time avoid liquid refrigerant entering the compression chamber when evaporation is incomplete "Liquid hit" phenomenon. The action of gravity on the droplets prevents them from going up with the gas vapor before complete vaporization, which can ensure sufficient heat exchange, so that the refrigerant can completely intensify or even produce a certain degree of superheat when leaving the evaporator; from this point, it can be seen that the high The number of plates of the plate heat exchanger of the plate is less than that of the low plate, which can greatly reduce the degree of unevenness of the refrigerant in the heat exchanger. At the same time, because the volume flow rate of the gas refrigerant at the outlet of the evaporator is more than 30 to 60 times the volume flow rate of the two-phase mixture at the inlet, the size of the refrigerant outlet joint of the plate heat exchanger should not be too small.
Because the plate spacing of the plate heat exchanger is small and the plates are thin, the antifreeze protection of the plate evaporator is particularly important. For chillers without frost protection, the outlet temperature should be controlled above the freezing point. Of course, there are many antifreeze measures for the plate evaporator. For example, the unit is equipped with a low-pressure control switch, a refrigerant-side antifreeze switch, a water-side temperature-controlled antifreeze switch, and a flow switch.
As the evaporation temperature rises, the average temperature difference of the evaporator will decrease, so the cooling capacity of the evaporator will decrease, but for the compressor, the situation is reversed. The cooling capacity of the compressor increases with the evaporation temperature。
If a smaller evaporator is selected, the operating point will move along the compressor curve toward a lower evaporating temperature , which not only causes a decrease in the output of the whole machine, but also the evaporating temperature at the operating equilibrium point is close to the freezing temperature, farewell It is more likely to freeze the plate evaporator. If a larger evaporator is selected, which not only avoids the possibility of reduced output and freezing, but also reduces the pressure drop on the water side, which reduces the energy consumption of the pump during operation. Of course, choosing a larger evaporator will increase the initial investment, so it should be considered comprehensively.
It is worth mentioning that when the air-cooled heat pump unit enters the defrosting phase, due to the high water temperature on the low pressure side (plate heat exchanger side) and the low temperature on the high pressure side (fin heat exchanger side), the high and low The difference is very small, resulting in a relatively small flow rate of the expansion valve (because the flow rate of the expansion valve is proportional to the root mean square of the pressure difference before and after the valve), and at the same time, the refrigerant stock in the plate heat exchanger is less, and the evaporation temperature here is higher. High, so there will be a phenomenon of low-pressure evacuation of the air-cooled heat pump unit during defrosting, and this evacuation is extremely unfavorable for the cooling and oil return lubrication of the compressor, and it will cause damage to the compressor coil and moving parts for a long time.
In response to this situation, we can optimize the system， a capillary tube to be opened and closed by the solenoid valve 9 is connected in parallel at both ends of the refrigeration expansion valve 4, as shown in FIG. 3. When vacuuming occurs in the heat pump unit, the opening and closing of the opening solenoid valve 9 must be able to be reasonably controlled. The author of this article has done a lot of exploration work in this aspect and achieved relatively satisfactory results.
3 Plate heat exchanger as condenser
The plate heat exchanger used as an evaporator in the refrigeration cycle of the heat pump unit is used as a condenser during the heat pump cycle. In both the refrigeration cycle and the heat pump cycle, the refrigerant flow direction is opposite, while the water flow direction is unchanged. Since the load of the condenser is greater than the load of the evaporator, the design should consider that the plate heat exchanger works as an evaporator for co-current heat exchange and when used as a condenser for counter-current heat exchange. The determination of the inlet and outlet water temperature of the cooling water of the plate heat exchanger should be based on the local meteorological conditions (mainly referring to the calculation of the temperature of the air-conditioning wet bulb in summer) and a comparison of the investment and operating costs. In general, the inlet temperature of the cooling water is 4 to 6 degrees higher than the calculated temperature of the local summer air-conditioning wet bulb, and the temperature difference of the cooling water is 4 to 6 degrees.
For plate condensers, there is generally no need to have a condensing section and a supercooling section coexisting, because the heat exchange of the supercooling section is to exchange sensible heat, which is much lower than the latent heat exchange efficiency of the condensing section. If supercooling is required, in principle, a subcooler should be set separately.
Since the condensation heat transfer coefficient is generally smaller than the water side heat transfer coefficient, in order to make the two as close as possible, the water flow rate should be smaller than that of the water-to-water heat exchanger, which can be 0.3~0.6m/s. It is worth noting that the water flow speed should not be too small, otherwise it will cause low turbulence, which will lead to unsatisfactory heat transfer efficiency and self-cleaning effect.
The heat transfer in the condenser basically occurs through film condensation, so the plate heat exchanger must rotate vertically. The refrigerant enters the plate condenser from the top in the form of superheated gas, cools, condenses after superheating and exits from the bottom in the form of supercooled liquid.
It should be pointed out that since the internal volume of the plate condenser is very small and cannot store liquid, the heat pump system with the plate heat exchanger as the water side heat exchanger must be provided with an additional liquid reservoir.
4 Plate heat exchanger for heat recovery unit
The heat recovery unit uses condensing heat and tap water to heat it into domestic water or process hot water, so as to take away the condensation heat generated by the refrigeration system, that is, directly send tap water that meets the amount of hot water to the heat recovery heat exchanger and discharge it into the atmosphere. The plate heat exchanger used as the heat recovery heat exchanger is installed between the condenser and the compressor, as shown in the flow chart of the LSFMH type heat recovery unit produced by Changzhou Aisite Air Conditioning Equipment Co., Ltd.
Experiments show that the load of the superheat cooler is usually 20% of the condenser load. With the increase of the inlet water temperature of the heat recovery heat exchanger, the recovery of latent heat of condensation has been reduced, and then the condensed sensible heat is mainly used to continue to further heat the preliminary heated hot water to 50~60 ℃ high temperature water stored in Water storage tank for use.
In the design and selection of heat recovery plate heat exchangers, in order to ensure the normal cooling function of the unit and obtain a higher energy utilization coefficient, the energy distribution relationship between heat recovery and cooling capacity should be considered comprehensively. It is worth noting that due to the high temperature in the heat recovery plate heat exchanger, in order to prevent the scaling of the water in the heat recovery plate heat exchanger, the tap water must be softened.
In addition, due to the fluctuation or huge impact of the compressor exhaust, it may cause pressure damage to the brazed plate heat exchanger. Therefore, some manufacturers install a buffer section between the compressor and the superheat cooler.