Selection and Engineering Design of Air Cooled Heat Pump Chiller
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September 10 10:06:02, 2022
Air-cooled heat pump hot and cold water unit is the nineties in China began to apply a new type of air-conditioning host, such units can both cold and heat, save the boiler room and cooling water system, installation flexibility. Unit operation using microcomputer control, high reliability. Therefore, many air-conditioning projects in the Yangtze River Basin are widely used. However, due to the different climatic conditions in various places, there is also a lack of experience in engineering design. So in use also found a lot of problems. The author of this article based on my own experience in recent years to talk about a few experiences, for the majority of peers reference. During the design of a project, if the local climate and environment allow, at the same time after technical and economic analysis to determine the project air-conditioning heat source using air-cooled heat pump units, then designers should proceed to the relevant domestic and foreign manufacturers of products for analysis and comparison , For the user to choose a more economical and reasonable heat pump products. The main content of the selection is firstly the overall performance analysis of the unit, which includes the heat pump unit cooling capacity, heating capacity, COP value, noise, dimensions, operating weight and other parameters. Secondly, the paper analyzes the internal configuration of this type of heat pump. It includes the compressor type, the structure and layout of the condenser, the configuration of the thermal expansion valve, the type of evaporator, the mode of energy regulation, the way of defrosting, the safety protection and automatic control projects. After doing the above analysis and comparison, we can choose an ideal unit. The next task is to arrange the equipment. During this process, we must consider the reasonable spacing between the equipment, the configuration of the auxiliary heat source, and the overall operation noise of multiple heat pumps The impact on the surrounding environment. The following are some of the above issues are described. Air-cooled heat pump performance analysis Air-cooled heat pump heat and cooling: These two parameters are the most critical parameters that determine the normal use of air-cooled heat pump, it refers to the air-cooled heat pump inlet temperature, inlet and outlet temperatures in the design conditions It has the cooling capacity or heating capacity. It is available from the manufacturer's product samples found. However, the current design is also found in such a situation, that is, some manufacturers have provided the sample parameters have not been tested but copied from other manufacturers of the relevant samples. This gives designers the correct selection has brought some difficulties. Therefore, I suggest that under the conditions of the designer can be based on the manufacturer's air-cooled heat pump configuration of the compressor model from the compressor manufacturer to get the compressor performance changes in the performance curve, according to the heat pump design conditions Check the compressor in the heat pump design conditions cooling capacity and heating capacity, to determine the sample provided by the authenticity of the parameters. COP value of air-cooled heat pump: This value is to determine the performance of air-cooled heat pump is an important parameter, the value of the level of a direct impact on the use of air-cooled heat pump power consumption, therefore, should try to choose a high COP value of the unit. At present, China's national standard is a COP of 2.57, most of the imported or joint-venture brands have a COP of about 3. Some imported brands have a capacity of 3.8. Noise: Noise is also a measure of an air-cooled heat pump unit, an important parameter, which is directly related to the heat pump operation on the surrounding environment. Relevant domestic experts have been based on engineering measurement of various types of imported heat pump noise is divided into third gear, the first gear at 85dB or more, the second gear at 75 ~ 85dB, the third gear at 75dB or less. We should carry on engineering design selection should give priority to the noise below 80dB units. Dimensions: Most of the air-cooled heat pump units are arranged in the outdoor roof, it is in the layout of the equipment on the device and the surrounding wall spacing, the spacing between devices have clear requirements, so we have to consider when selecting equipment Whether the size of the device meets the size requirements of the device layout. In the same premise of the same performance should give priority to the use of smaller units to reduce the equipment area. Operating weight: As most of the air-cooled heat pump units are arranged on the roof, the load-bearing capacity of the roof must be taken into account when selecting the type of the roof. If necessary, professional consultation should be made with the structure to enhance the load bearing capacity of the roof. However, in the selection of equipment we should give priority to the operation of light weight units. System analysis of air-cooled heat pump The so-called air-cooled heat pump system analysis, that is, in the selection process of air-cooled heat pump in addition to their respective cooling capacity, heating capacity, COP value, noise, operating weight, dimensions and other parameters, Analyze their compressor type, condenser type and arrangement, configuration of thermal expansion valve, evaporator type, defrosting mode, energy adjustment mode and the automatic control and safety protection of heat pump system, etc. Compare their respective system Configuration advantages and disadvantages. Compressor type: Currently used for air-cooled heat pump compressor piston type, scroll, screw three types. According to the heat pump is characterized by long running time, large compression ratio, etc., I believe that scroll and screw compressors will become the mainstream of heat pump compressors. The reason is: 1, scroll and screw compressors than the piston compressor with less transmission, so that the corresponding reduction of friction loss of the compressor, the overall efficiency of the corresponding increase. 2, due to the heat pump unit compression is relatively large, so for the piston compressor volume loss in the same clearance volume decreased, resulting in a decline in the efficiency of the machine. The scroll and screw compressors do not have this problem. 3, the compressor for air-cooled heat pump Its working environment is worse than other compressors working under normal air-conditioning conditions, each running time is longer, the working conditions change range is also larger, so the reliability of the compressor Sexual requirements are higher. Scroll and screw compressors with fewer parts, compact structure, it is especially suitable for heat pump units. 4, the current use of air-cooled heat pump units are generally used hot gas defrosting method to exclude the accumulation of frost on the air side heat exchanger in winter heating conditions. At the beginning and at the end of the defrost the system is run in reverse and the liquid refrigerant accumulating in the original condensing one of the coils is flooded to the compressor due to a sudden drop in pressure therein to the suction pressure causing the compressor's wet stroke, This is not a big problem for scroll and screw compressors, which can easily cause damage to the air valve and connecting rod for piston compressors. 5 In addition to the heat pump compressor itself, the vortex and semi-closed screw-type piston than the noise is lower. Condenser Type and Arrangement The type of fin used in the condenser mainly includes two types of windows and corrugated sheets. The heat-transfer efficiency of the window-opening sheet is high, so heat pump units produced in the previous two years are often used. However, due to poor urban air quality in our country, such fins are very easy to foul and harder to clean up. As time goes by, the heat exchange effect greatly decreases. Therefore, the current heat pump condenser multi-use corrugated sheet with internal thread brass, which has a higher heat transfer efficiency, easy to fouling, wind resistance and other small features. Condenser fin pitch is also very particular, as the condenser to use the ribbed ratio, the number of large heat transfer system is good, so I hope the smaller chip better. However, when used as an evaporator, the fins frost, the use of the heat transfer effect will be greatly reduced, so I hope the larger chip distance; general film to 3mm is appropriate. The layout of the condenser with its heat transfer effect and dimensions have a direct relationship. Condenser coil heat pump is usually arranged in a straight coil, V-coil, W-coil three types. However, there is no other components in the larger space between the V-type coils besides the axial fan, so the space utilization rate is low. Although the straight-type coil arranged between the compressor, four-way valve, evaporator and other system-related components, but due to the coil height is higher, the wind speed is uneven, the condenser heat transfer efficiency is low, and the air flow is not organized Ideal, air resistance larger. The W-type layout to overcome the above shortcomings, not only can improve the air flow to improve the efficiency of heat transfer and reduce air resistance, and because of the same space conditions, the condensing coil heat transfer area increases, space utilization is higher, thus reducing the unit Dimensions. Thermal expansion valve configuration Heat pump refrigeration system now has a single expansion valve and dual expansion valve in two ways, the so-called dual expansion valve is the heating conditions and cooling conditions using an expansion valve. If the system uses an expansion valve, according to the standard cooling conditions selection, the heat pump system in the heating conditions when the system heat with the decline in ambient temperature will also decline. At this time expansion valve heating capacity will decline, but the extent of its decline is less than the system heating capacity decline. This heating conditions with the decline in ambient temperature, the configuration of the system expansion valve is too large. Excessive expansion valve will cause the evaporator too much liquid supply, evaporation pressure increases, and outdoor air heat reduction, resulting in reduced heat pump heat. Many current heat pump manufacturers use double expansion valve type, refrigeration expansion valve according to the standard refrigeration conditions to choose. If the thermal expansion valve according to the standard heating conditions to choose, then when operating under low temperature conditions the expansion valve will appear too large, so according to my own experience suggest that the thermal expansion valve according to the ambient temperature -7 ℃, hot water inlet temperature 40 ℃, the outlet temperature of 45 ℃ to select, according to such conditions after the selected expansion valve can be no less than -15 ℃ ambient temperature in normal operation. Evaporator type At present, the evaporator commonly used in air-cooled heat pump units is mainly a plate heat exchanger and a dry shell and tube heat exchanger. Plate heat exchanger used in small air-cooled heat pump, it has high heat transfer efficiency, the evaporator is not easy oil production characteristics; especially the new plate with a built-in distribution device to solve the uniformity of refrigerant distribution between the plates A key issue is to increase the evaporation temperature by 15 to 2 ° C at the same effluent temperature to improve the refrigeration efficiency. Dry shell and tube evaporator used in large and medium-sized air-cooled heat pump, the heat pipe has been widely used efficient tubes, so the heat transfer efficiency has greatly improved. But generally not as plate heat exchanger. And its back to the oil is relatively difficult, often accumulated in the bottom of the heat exchanger. If the return pipe and the suction pipe are connected at the bottom, the superheat degree of the refrigerant is unstable due to the liquid refrigerant brought in, which affects the operation of the expansion valve and the cooling capacity of the system. Axial fan configuration Axial fan configuration must first meet the condenser (air side heat exchanger) heat transfer requirements, according to the experience of air-cooled heat pump unit with axial fan air flow and the standard cooling capacity (ambient temperature 35 ℃, water Temperature 7 ℃) ratio of about 0.071 ~ 0.095 / kJ between, in addition to ensure that the condenser windward wind speed, because it relates to the winter operation of the air side heat exchanger frosting speed, the windward windward winter The less likely it is to run frost. However, excessive air flow fan fan power consumption should also increase, but also increase the noise, so the wind speed under normal winds take 3 ~ 5m / s. In addition, the fan configuration should also consider the noise, the current general selection of large diameter, low speed, and the blade angle smaller axial fan to reduce fan noise. Energy adjustment methods Currently used in air-cooled heat pump units commonly used energy regulation compressor number control, compressor clearance operation, cylinder unloading regulation (piston), frequency control (scroll), stepless adjustment of the valve formula). From the energy regulation we can see that the number of units control, compressor clearance operation, cylinder unloading adjustment are all level regulation, and variable frequency speed regulation and stepless adjustment of the valve are infinitely adjustable. Stepless adjustment with energy saving, noise and vibration is small, good starting performance also reduces the interference to the power supply system. From this point can also be seen vortex and screw compressors of the heat. Defrosting methods The production of various units of the defrost unit is basically the same, most of the use of steam defrosting method; the difference is the defrost control technology. Common pressure control method, temperature control method, temperature and time control