Selection and Engineering Assumption of Air Cooled Heat Pump Cold and Hot Water Units

The air-cooled heat pump cold and hot water unit is a new type of air conditioning unit that began to be applied in China in the 1990s. This type of unit can provide both cooling and heating, eliminating the need for boiler rooms and cooling water systems, making installation flexible and convenient. The operation of the unit adopts microcomputer control, with high reliability. Therefore, it has been widely adopted in many air conditioning projects in the Yangtze River Basin. However, due to different climatic conditions in different regions, coupled with a lack of experience in engineering design. Therefore, many problems have also been found in use. The author of this article provides some insights based on his engineering experience in recent years, for the reference of colleagues.
During 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 conditioning cold and heat source of the project adopts air-cooled heat pump units, the designer should start analyzing and comparing the products of relevant domestic and foreign manufacturers, and select a more economical and reasonable heat pump product for users. The main content of selection is first the overall performance analysis of the unit, which includes the cooling capacity, heating capacity, COP value, noise, external dimensions, operating weight and other parameters of the heat pump unit. Secondly, analyze the internal configuration of this type of heat pump, which includes compressor type, condenser structure and layout, configuration of thermal expansion valve, evaporator type, energy regulation method, defrosting method, safety protection and automatic control items, etc. After conducting the above analysis and comparison, we can choose a more ideal unit. The next step is to arrange the equipment. In this process, we must consider the reasonable spacing between the equipment, the configuration of auxiliary heat sources, and the impact of the overall operating noise of multiple heat pumps on the surrounding environment. Below, we will elaborate on the above issues separately.
Performance analysis of air-cooled heat pumps
The cooling capacity or heating capacity of an air-cooled heat pump: These two parameters are the most critical parameters that determine the normal use of the air-cooled heat pump. They refer to the cooling capacity or heating capacity of the inlet and outlet air temperature and water temperature of the air-cooled heat pump under design conditions. It can be found from product samples provided by relevant manufacturers. But currently, it has also been found in the design that some manufacturers provide sample parameters that have not been tested but are copied from relevant samples from other manufacturers. This has brought certain difficulties to the correct selection of designers. Therefore, the author suggests that designers can obtain the variable operating condition performance curve of the compressor from the compressor manufacturer based on the compressor model configured by the relevant manufacturer’s air-cooled heat pump, and check the refrigeration capacity and heating capacity of the compressor under the design working condition of the heat pump based on the design working condition of the heat pump, in order to determine the authenticity of the parameters provided by the sample.
COP value of air-cooled heat pump: This value is an important parameter for determining the performance of air-cooled heat pumps, and its value directly affects the power consumption in the use of air-cooled heat pumps. Therefore, it is recommended to choose units with high COP values as much as possible. At present, the national standard in China our story is a COP value of 2.57. Most imported or joint venture brands have a COP value of around 3, and some imported high-efficiency units can reach a value of 3.8.
Noise: Noise is also an important parameter for measuring an air-cooled heat pump unit, which directly affects the impact of the heat pump on the surrounding environment during operation. Domestic experts have classified the noise of various imported heat pumps into three levels based on engineering measurements, with the first level above 85dB, the second level between 75-85dB, and the third level below 75dB. We should prioritize selecting units with noise levels below 80dB in engineering design selection.
External dimensions: Most air-cooled heat pump units are located on the outdoor roof, and there are clear requirements for the spacing between the equipment and the surrounding walls and between the equipment during equipment layout. Therefore, when selecting equipment, we must consider whether the selected equipment size meets the size requirements of the equipment layout. Given the same performance, smaller units should be prioritized to reduce the footprint of the equipment.
Operating weight: Due to the fact that most air-cooled heat pump units are arranged on the roof, the load-bearing capacity of the roof must be considered when selecting the type. If necessary, consultation with structural professionals should be conducted to enhance the load-bearing capacity of the roof. But when selecting equipment, we should prioritize selecting units with lighter operating weights.
System Analysis of Air Cooled Heat Pump
The system analysis of air-cooled heat pumps refers to the comparison of their respective cooling capacity, heating capacity, COP value, noise, operating weight, external dimensions and other parameters during the selection process. In addition, it is also necessary to analyze their respective compressor type, condenser type and layout, thermal expansion valve configuration, evaporator type, defrosting method, energy regulation method, as well as the automatic control and safety protection of the heat pump system, Compare their respective advantages and disadvantages in system configuration. Compressor types: Currently, there are three main types of compressors used for air-cooled heat pumps: piston type, vortex type, and screw type. According to the characteristics of heat pump operation, such as long operating time and high compression ratio, the author believes that vortex and screw compressors will become the mainstream of heat pump compressors. The reason is:
1. Vortex and screw compressors have fewer transmission components compared to piston compressors, resulting in a corresponding reduction in friction losses and an improvement in overall efficiency.
2. Due to the large compression ratio of heat pump units, the volumetric efficiency of piston compressors decreases under the same clearance volume, resulting in a decrease in overall efficiency. Vortex and screw compressors do not have this issue.
3. The working environment of compressors used for air-cooled heat pumps is worse than other compressors working under ordinary air conditioning conditions, with longer operating times and a larger range of operating conditions. Therefore, the reliability requirements for compressors are higher. Vortex and screw compressors have the characteristics of fewer components and compact structure, making them particularly suitable for heat pump units.
4. The currently used air-cooled heat pump units generally use the method of hot air defrosting to eliminate the accumulation of frost on the air side heat exchanger under winter heating conditions. At the beginning and end of defrosting, the system needs to run in the opposite direction. The liquid refrigerant accumulated in the original condensing coil suddenly decreases to the suction pressure, causing a large amount of surge towards the compressor, resulting in wet stroke. This is not a major problem for scroll and screw compressors, but for piston compressors, it is very easy to cause damage to the gas valve and connecting rod.
5. In addition, in terms of the heat pump compressor itself, the noise of the vortex type and semi enclosed screw type is lower than that of the piston type.
Type and arrangement of condenser
At present, there are two main types of fins used in condensers: window fins and corrugated fins. The window fins have high heat transfer efficiency, so they are often used in heat pump units produced in the past two years. However, due to the poor air quality in urban areas of our country, such fins are prone to dust accumulation and are difficult to clean. Over time, their heat transfer efficiency greatly decreases. So currently, condensers for heat pumps mostly use corrugated plates with internally threaded c145 c14500 tellurium copper pipes, which have the characteristics of high heat transfer efficiency, less ash accumulation, and low wind resistance.
The fin spacing of the condenser is also very particular. When used as a condenser, it is better to have a high rib ratio and a large number of heat transfer systems, so it is better to have a smaller fin spacing. But when it is used as an evaporator, once the fins frost, the heat transfer effect will be greatly reduced during use, so it is hoped that the fin spacing will be larger; The general spacing is 3mm.
The layout type of the condenser is directly related to its heat transfer effect and external dimensions. Usually, the condensation coils of heat pumps are arranged in three types: straight coils, V-coils, and W-coils. However, the large space between V-shaped coils has no other components except for axial fans, resulting in low space utilization. Although the compressor, four-way valve, evaporator, and other related components of the system are centrally arranged in the straight coil room, due to the high height of the coil, uneven windward surface velocity, low heat transfer efficiency of the condenser, and unsatisfactory airflow organization, the air resistance is high. The W-shaped layout overcomes the above shortcomings, not only improving airflow organization, improving heat transfer efficiency, and reducing air resistance, but also reducing the overall size of the unit due to the increased heat transfer area of the condensing coil under the same spatial conditions, resulting in higher space utilization.
Thermal expansion valve configuration
Nowadays, there are two methods used in heat pump refrigeration systems: single expansion valve and double expansion valve. The so-called double expansion valve refers to the use of one expansion valve for both heating and cooling conditions. If the system adopts an expansion valve and is selected according to standard refrigeration conditions, the heating capacity of the heat pump system will also decrease with the decrease of environmental temperature when operating under heating conditions. At this point, the heating capacity of the expansion valve will also decrease, but the magnitude of the decrease is smaller than the decrease in the heating capacity of the system. In this way, as the ambient temperature decreases under heating conditions, the expansion valve configured for the system appears too large. An excessively large expansion valve can cause excessive liquid supply to the evaporator, increase evaporation pressure, and reduce heat exchange with outdoor air, resulting in a decrease in heat pump supply.
At present, many manufacturers’ heat pump units mostly adopt the double expansion valve type, and the refrigeration expansion valve is selected according to standard refrigeration conditions. If the heating expansion valve is selected according to the standard heating conditions, it will appear too large when operating at low temperatures. Therefore, based on the author’s own experience, it is recommended to select the heating expansion valve according to the ambient temperature of -7 ℃, the inlet temperature of hot water of 40 ℃, and the outlet temperature of 45 ℃. After calculating according to this condition, the selected expansion valve can operate normally at an ambient temperature of not less than -15 ℃.
Evaporator type
At present, the commonly used evaporators in air-cooled heat pump units are mainly laser cutting ta2 titanium plate heat exchangers and dry shell and tube heat exchangers. Plate heat exchangers are commonly used in small air-cooled heat pumps, which have the characteristics of high heat transfer efficiency and less oil accumulation in evaporators; Especially the new plate with built-in distribution device solves the key problem of refrigerant distribution uniformity between plates, which can increase the evaporation temperature by 15-2 ℃ at the same outlet temperature and improve refrigeration efficiency. Dry shell and tube evaporators are commonly used in large and medium-sized air-cooled heat pumps. Currently, their heat transfer tubes have widely used high-efficiency tubes, resulting in a significant improvement in heat transfer efficiency. But overall, it is not as good as plate heat exchangers. Moreover, its return oil is relatively difficult and often accumulates at the bottom of the heat exchanger. If a return oil pipe is installed at the bottom to connect with the suction pipe, liquid refrigerant may be introduced, resulting in unstable refrigerant superheat and affecting the operation of the expansion valve and the cooling capacity of the system.
Configuration of axial flow fans
The configuration of the axial flow fan must first meet the heat exchange requirements of the condenser (air side heat exchanger). Based on experience, the ratio of the air volume of the axial flow fan equipped with the air-cooled heat pump unit to the standard cooling capacity (ambient temperature 35 ℃, outlet temperature 7 ℃) is approximately 0.071~0.095/kJ. In addition, it is necessary to ensure the wind speed on the windward side of the condenser, as this is related to the frosting speed of the air side heat exchanger during winter operation, The higher the wind speed on the windward side, the less prone it is to frost during winter operation. However, if the air volume is too high, the power consumption of the fan will also increase, and the noise will also increase. Therefore, in general, the wind speed on the windward side is taken as 3-5m/s. In addition, noise should also be considered when configuring the fan. Currently, axial flow fans with large diameter, low speed, and small blade torsion angle are generally selected to reduce fan noise.
Energy regulation method
At present, the commonly used energy regulation methods in air-cooled heat pump units include compressor number control, compressor gap operation, cylinder unloading regulation (piston type), variable frequency speed regulation (vortex type), and slide valve stepless regulation (screw type). From the energy regulation method, we can see that the number control, compressor clearance operation, and cylinder unloading regulation all belong to stepless regulation, while variable frequency speed regulation and slide valve stepless regulation belong to stepless regulation. Stepless regulation has the advantages of energy conservation, low noise and vibration, good starting performance, and reduced interference to the power supply system. From this point, it can also be seen that the advantages of vortex and screw compressors are superior.
Defrosting method
The defrosting methods of the units produced by each production plant are basically the same, and most of them use the hot steam defrosting method; The difference is the control technology of defrosting. Common methods include pressure difference control, temperature difference control, and temperature time control, among which temperature time control is the most common.