Air cooled heat pump chiller and hot water

Air cooled heat pump chiller and hot water unit is a new type of air conditioning unit which has been applied in China since 1990s. This kind of unit can provide both cooling and heating. It saves the boiler room and cooling water system, and is flexible and convenient to install. The operation of the unit is controlled by microcomputer with high reliability. Therefore, it is widely used in many air conditioning projects in the Yangtze River Basin. However, due to the different climatic conditions in different places and the lack of experience in engineering design. Therefore, many problems have been found in use. According to the author's engineering experience in recent years, the author of this paper talks about some experiences for the reference of colleagues. square duct machine In the design process of a project, if the local climate and environment permit, and after technical and economic analysis and comparison, it is determined that the air-cooled heat pump unit is used for the cold and heat source of the air-conditioning system of the project, then the designers should start to analyze and compare the products of relevant manufacturers at home and abroad, and select a more economical and reasonable heat pump product for users. The main content of the selection is the overall performance analysis of the unit, which includes the cooling capacity, heating capacity, COP value, noise, external dimension, operating weight and other parameters of the heat pump unit. Secondly, the internal configuration of this kind of heat pump is analyzed, including compressor type, condenser structure and layout, thermal expansion valve configuration, evaporator type, energy regulation mode, defrosting mode, safety protection and automatic control items, etc. After the above analysis and comparison, we can choose a more ideal unit. The next work is to arrange the equipment. In this process, we must consider the reasonable spacing between the equipment, the configuration of auxiliary heat sources and multiple setsThe influence of heat pump operation noise on the surrounding environment. The following several aspects of the above issues are described. Performance analysis of air-cooled heat pump cold and heat capacity of air-cooled heat pump: these two parameters are the most critical parameters to determine the normal use of the air-cooled heat pump, which refers to the cooling capacity or heating capacity of the air-cooled heat pump under the design conditions. It can be found from the product samples provided by relevant manufacturers. However, it is also found in the design that the sample parameters provided by some manufacturers have not been tested, but copied from the relevant samples of other manufacturers. This brings some difficulties to the designer in the correct selection. Therefore, the author suggests that designers can obtain the off design performance curve of the compressor from the compressor manufacturer according to the compressor model of the air-cooled heat pump provided by relevant manufacturers, and check the cooling capacity and heating capacity of the compressor under the designed working condition of the heat pump according to the design condition of the heat pump, so as to judge the authenticity of the parameters provided by the sample. COP value of air-cooled heat pump: this value is an important parameter to determine the performance of air-cooled heat pump, and its value directly affects the power consumption in the use of air-cooled heat pump. Therefore, the unit with high COP value should be selected as far as possible. At present, China's national standard is that COP value is 2.57, cop of most imported or joint venture brands is about 3, and that of some imported brands of high-efficiency units can reach 3.8. Noise: noise is also an important parameter to measure an air-cooled heat pump unit, which is directly related to the impact of heat pump operation on the surrounding environment.According to the engineering measurement, the noise level of all kinds of imported heat pumps has been divided into three levels, the first level is above 85 dB, the second level is between 75 and 85 dB, and the third level is below 75 dB. In the process of engineering design and selection, we should give priority to units with noise below 80dB. External dimension: most of the air-cooled heat pump units are arranged on the outdoor roof, and there are clear requirements for the distance between the equipment and the surrounding wall and between the equipment in the equipment layout. Therefore, we must consider whether the size of the selected equipment meets the size requirements of the equipment layout when selecting the equipment. On the premise of the same performance, the smaller size unit should be preferred to reduce the floor area of the equipment. Operating weight: since most of the air-cooled heat pump units are arranged on the roof, the bearing capacity of the roof must be considered when selecting the type. If necessary, the bearing capacity of the roof should be enhanced through consultation with the structural specialty. But in the selection of equipment, we should give priority to the unit with light operation weight. System analysis of air-cooled heat pump the so-called system analysis of air-cooled heat pump is to compare the cooling capacity, heating capacity, COP value, noise, operating weight, external dimension and other parameters of the air-cooled heat pump, as well as the compressor type, condenser type and layout, thermal expansion valve configuration, evaporator type, and other parameters Frost mode, energy regulation mode, automatic control and safety protection of heat pump system are analyzed, and their advantages and disadvantages in system configuration are compared. Type of compressor: currently used for air-cooled heat pumpThere are three types of compressor: piston type, scroll type and screw type. According to the characteristics of heat pump, such as long operation time and large compression ratio, the author thinks that scroll compressor and screw compressor will become the mainstream of heat pump compressor. The reasons are as follows: 1. Scroll compressor and screw compressor have less transmission parts than piston compressor, so that the friction loss of compressor is reduced correspondingly, and the efficiency of the whole machine is improved accordingly. 2. Due to the large compression ratio of the heat pump unit, the volumetric efficiency of the piston compressor decreases under the same clearance volume, resulting in the decrease of the overall efficiency. There is no such problem in scroll compressor and screw compressor. 3. The working environment of the compressor used for air-cooled heat pump is worse than that of other compressors working under ordinary air-conditioning conditions. The running time of each compressor is longer, and the range of working conditions is also large. Therefore, the reliability of the compressor is required to be higher. Scroll compressor and screw compressor have the characteristics of less parts and compact structure, so a is especially suitable for heat pump units. 4. The current air-cooled heat pump units generally use hot gas defrosting method to remove the accumulated frost on the air side heat exchanger under heating conditions in winter. At the beginning and the end of defrosting, the system needs to run in reverse direction. The liquid refrigerant accumulated in the original condenser coil suddenly decreases to the suction pressure, which causes the wet stroke of the compressor. This is not a big problem for scroll compressor and screw compressor, but it is easy to cause valve and connection for piston compressor The damage of the poleBad. 5. In addition, for the heat pump compressor itself, the noise of scroll type and semi closed screw type is lower than that of piston type. The type and arrangement of the condenser at present, there are mainly two types of fins used in the condenser: windowed fin and corrugated fin, which have high heat exchange efficiency, so they are often used in heat pump units produced in the previous two years. However, due to the poor air quality in China's cities, such fins are easy to deposit ash, and it is difficult to clean them up. Therefore, the current heat pump condenser with corrugated sheet with internal thread copper tube, which has the characteristics of high heat transfer efficiency, not easy to deposit ash, small wind resistance and so on. The fin spacing of the condenser is also very particular. As a condenser, it is better to have a higher fin ratio and a large number of heat transfer systems, so it is better to have a smaller fin spacing. However, when it is used as an evaporator, the heat transfer effect will be greatly reduced when the fins are frosted, so it is hoped that the fin spacing is larger; generally, the fin spacing should be 3mm. The arrangement of condenser is directly related to its heat transfer effect and external dimension. Generally, the condensing coil of heat pump is arranged into three types: straight coil, v-coil and W-type coil. However, in the larger space between v-coil units, there are no other parts except axial flow fan, so the space utilization rate is low. Although the compressor, four-way valve, evaporator and other system components are centrally arranged in the straight coil unit, due to the high coil height, uneven windward surface velocity, low heat transfer efficiency of the condenser, unsatisfactory air distribution and large air resistance. The W-shaped arrangement overcomes the above shortcomings and can not only improve the air distributionThe heat transfer efficiency is improved and the air resistance is reduced. Moreover, the heat transfer area of the condensing coil is increased and the space utilization rate is high under the same space conditions, so the overall size of the unit is reduced. Thermal expansion valve configuration now there are two ways to use single expansion valve and double expansion valve in the heat pump refrigeration system. The so-called double expansion valve means that one expansion valve is used for heating and cooling conditions. If an expansion valve is used in the system and the model is selected according to the standard refrigeration conditions, the heating capacity of the system will decrease with the decrease of the ambient temperature when the heat pump system is operating under the heating condition. At this time, the heating capacity of the expansion valve will also decrease, but the decrease range is smaller than that of the system heating capacity. In this way, with the decrease of ambient temperature, the expansion valve is too large for the system. If the expansion valve is too large, the evaporator will supply too much liquid, the evaporation pressure will rise, and the heat exchange with outdoor air will be reduced, which will lead to the reduction of heat supply of heat pump. At present, many heat pump units of many manufacturers adopt the double expansion valve type, and the refrigeration expansion valve is selected according to the standard refrigeration conditions. If the thermal expansion valve is selected according to the standard heating conditions, the expansion valve will appear too large when operating at low temperature. Therefore, according to the author's own experience, it is suggested to select the thermal expansion valve according to the ambient temperature of - 7 ℃, the hot water inlet temperature of 40 ℃, and the outlet temperature of 45 ℃. The selected expansion valve can operate normally at the ambient temperature of no less than - 15 ℃. At present, plate heat exchanger and dry type evaporator are commonly used in air-cooled heat pump unitsShell and tube heat exchanger. Plate heat exchanger is mostly used in small air-cooled heat pump, it has the characteristics of high heat transfer efficiency and difficult to accumulate oil in evaporator; especially, the new plate with built-in distribution device solves the key problem of refrigerant distribution uniformity between plates, which can raise the evaporation temperature by 15-2 ℃ under the same outlet water temperature and improve the refrigeration efficiency. Dry shell and tube evaporator is mostly used in large and medium-sized air-cooled heat pump. At present, high-efficiency tube has been widely used in its heat transfer tube, so the heat transfer efficiency has been greatly improved. But generally speaking, it is not as good as plate heat exchanger. Moreover, it is relatively difficult to return oil, which often accumulates at the bottom of the heat exchanger. If a return pipe is set at the bottom to connect with the suction pipe, the superheat of the refrigerant will be unstable due to the liquid refrigerant brought in, which will affect the operation of the expansion valve and the refrigeration capacity of the system. Axial fan configuration the configuration of axial-flow fan should first meet the heat exchange requirements of condenser (air side heat exchanger). According to experience, the ratio of air volume of axial fan equipped with air-cooled heat pump unit to standard cooling capacity (ambient temperature 35 ℃, outlet temperature 7 ℃) is about 0.071-0.095/kj. In addition, the wind speed on the windward side of the condenser should be ensured, because this is related to The higher the wind speed on the windward side of the gas side heat square duct machine exchanger is, the more difficult it is to frost in winter. However, the power consumption of the fan will increase when the air volume passes through the fan, and the noise will also increase. Therefore, in general, the wind speed on the windward side is 3-5M / s. In addition, the noise should be considered when the fan is configured. At present, the axial flow fan with large diameter, low speed and small blade torsion angle is generally selected to reduce the noiseFan noise. Energy regulation mode at present, the energy regulation methods commonly used in air-cooled heat pump unit include compressor number control, compressor clearance operation, cylinder unloading adjustment (piston type), frequency conversion speed regulation (scroll type), slide valve stepless regulation (screw type). From the energy regulation mode, we can see that the number control, compressor clearance acl duct machineoperation, cylinder unloading adjustment are all step-by-step regulation, while variable-frequency speed regulation and slide valve stepless regulation belong to stepless regulation. Stepless regulation has the advantages of energy saving, low noise and vibration, good starting performance, and also reduces the interference to the power supply system. From this point we can also see the optimal heat of scroll and screw compressors. Defrosting method the defrosting methods of the units produced by different factories are basically the same, and most of them adopt hot steam defrosting method; the difference is the control technology of defrosting. The common methods include pressure difference control, temperature difference control and temperature time control, among which the temperature time control method is the most common. In this control technology, the setting of defrosting parameters is the most important. Defrosting parameters include defrosting temperature, defrosting time and defrosting interval. The defrosting temperature is the liquid temperature after throttling through the temperature sensing element behind the expansion valve. Generally, the setting value is - 5 ℃, and the defrosting time interval is controlled by a timer, which is generally set at 4min. The defrosting time is also controlled by a timer, generally less than 10 minutes. When the temperature of the heat pump drops to - 5 ℃ and the interval between the last defrosting time is 40 minutes, the unit will enter the defrosting mode. If the defrosting time is more than 1010min, and the liquid temperature in the coil still does not rise to + 5 ℃Group B should stop defrosting and resume heating. In the above three parameters, the defrosting interval is directly affected by the environment. However, at present, most manufacturers still use a fixed value for the defrosting interval. This practice leads to the phenomenon that the defrosting can not be carried out due to the failure to reach the set time in the case of severe frosting in low temperature and high humidity areas, resulting in the phenomenon that the frost layer is too thick or even frozen, and the unit is shut down due to low pressure protection. This problem should be paid attention to in the unit commissioning. Therefore, the author suggests that on the one hand, the defrosting parameters of heat pump should be set according to local conditions. On the other hand, it is not suitable to use heat pump units in low temperature and high humidity areas. Safety protection and control at present, the protection and control of air-cooled heat pump units in China are mostly controlled by computer, which includes programmable control and microcomputer control, and their control principles are roughly the same. The safety protection system of an air-cooled heat pump should at least include the following aspects 1) low suction pressure protection 2) over discharge pressure protection 3) oil pressure protection 4) low cold water temperature protection 5) water cut-off protection of water side heat exchanger 6) compressor start time interval protection 7) compression The internal motor overheat protection 8) motor overload protection 9) low power supply voltage protection 10) three phase power phase loss protection 11) oil temperature control air cooled heat pump control should at least include 1) defrosting control 2) sequence control of multiple compressors 3) energyAdjustment 4) fault shutdown and display 5) remote control interface (used to set operation parameters remotely and control unit startup and shutdown, and display unit operation parameters and fault contents on control terminal) interface