Abstract: This paper describes the selection of air-cooled heat pump chiller plant should pay attention to several aspects of the problem, and its engineering design of several precautions should be taken to talk about the author's own experience.
Keywords: air-cooled heat pump chiller COP noise condenser evaporator air-cooled heat pump chiller is a new type of air-conditioning unit began to be used in China in the nineties, such units can be both cold and hot, eliminating the boiler room and cooling water system, flexible and convenient installation. Unit operation using microcomputer control, high reliability. Therefore, many air-conditioning projects in the Yangtze River Basin to be widely used. However, due to the different climatic conditions around the world, coupled with a lack of experience in engineering design. Therefore, many problems have been found in the use. In this paper, the author according to their own engineering experience in recent years to talk about a few points for the reference of the majority of colleagues.
In the design process of a project, if the local climate environment allows, and after a technical and economic analysis and comparison to determine the project air conditioning heat and cold source using air-cooled heat pump units, then the designers should begin to analyze and compare the domestic and foreign related manufacturers of products, for users to choose a more economical and reasonable heat pump products. Selection of the main content is first of all the unit's overall performance analysis, which includes the cooling capacity of the heat pump unit, heat production, COP value, noise, external dimensions, operating weight and other parameters. Secondly, the analysis of the internal configuration of the heat pump, which includes the compressor type, condenser structure and arrangement, thermal expansion valve configuration, evaporator type, energy regulation, frost melting mode, safety protection and automatic control projects and so on. After the above analysis and comparison we can choose a more ideal unit, the next job is to carry out the equipment layout, the process we must consider the reasonable spacing between the equipment, the configuration of auxiliary heat sources as well as more than one heat pump running noise on the surrounding environment. The following on the above aspects of the problem are described.
Air-cooled heat pump performance analysis of air-cooled heat pump heat and cold: these two parameters is to determine the normal use of air-cooled heat pump is the most critical parameter, which refers to the air-cooled heat pump air inlet temperature, inlet and outlet temperature of the water temperature in the design of the working conditions when it has the cooling capacity or heat production. It can be found in the product samples provided by the manufacturer. However, in the design of the current situation is also found, that is, some manufacturers provide sample parameters have not been tested but copied from other manufacturers of the relevant samples. This gives the designer the correct selection has brought some difficulties. Therefore, I suggest that under the conditions of the designers can be configured according to the manufacturers of air-cooled heat pump compressor model, from the compressor manufacturer to obtain the compressor's variable performance curve, according to the design of the heat pump to find the compressor in the design of the heat pump under the conditions of the refrigeration capacity and heat, so as to determine the authenticity of the parameters provided by the sample.
Air-cooled heat pump COP value: the value is to determine the air-cooled heat pump performance is an important parameter, the value of its value directly affects the use of air-cooled heat pump power consumption, therefore, should try to choose COP value of high units. At present, China's national standard is COP value of 2.57, most of the imported or joint venture brand COP in 3 or so, individual imported brands of high-efficiency units can reach the value of 3.8. Noise: Noise is also a measure of an air-cooled heat pump unit is an important parameter, which is directly related to the heat pump operation of the impact on the surrounding environment. Domestic experts have been measured according to the project of various types of imported heat pump noise is divided into three grades, the first grade in the 85dB or more, the second grade between 75-85dB, the third grade in the 75dB below. We should give priority to the selection of engineering design in the selection of noise below 80dB unit.
Outside dimensions: most of the air-cooled heat pump unit is arranged on the outdoor roof, it is arranged in the equipment on the equipment and the spacing of the surrounding walls, the spacing between the equipment have clear requirements, so we must consider the equipment selection must consider the size of the selected equipment in line with the size of the equipment layout requirements. Under the premise of the same performance should be prioritized to choose the smaller size of the unit, in order to reduce the footprint of the equipment.
Operating weight: Because most of the air-cooled heat pump units are arranged in the roof, so in the selection of the roof must be considered load-bearing capacity, if necessary, should be negotiated with the structure of the professional, to enhance the load-bearing capacity of the roof. But in the selection of equipment we should prioritize the selection of lighter operating weight units.
Air-cooled heat pump system analysis of the so-called air-cooled heat pump system analysis, that is, in the air-cooled heat pump selection process, in addition to comparing their respective refrigeration capacity, heating capacity, COP value, noise, operating weight, dimensions and other parameters, but also their respective compressor type, condenser type and arrangement, thermal expansion valve configuration, evaporator type, defrosting, energy regulation, and heat pump system automatic control and safety protection and so on. Self-control and safety protection of the system and so on to analyze and compare the advantages and disadvantages of their respective configurations in the system. Compressor type: currently used for air-cooled heat pump compressor type mainly piston, scroll, screw type three types. According to the work of the heat pump is characterized by long running time, compression ratio, etc., I believe that the scroll and screw compressor will become the mainstream of the heat pump compressor. The reason is: 1, scroll and screw compressor than piston compressor with less transmission parts, so that the compressor friction loss is reduced accordingly, the efficiency of the machine is improved accordingly.
2, due to the heat pump unit compression is relatively large, so for the piston compressor in the same clearance volume under its volumetric efficiency decline, resulting in a decline in efficiency of the machine. And scroll and screw compressors do not exist in this regard.
3, used for air-cooled heat pump compressor working environment than in other ordinary air conditioning conditions to work in the compressor to be harsh, each running time is also longer, the range of changes in conditions is also larger, so the reliability of the compressor requirements are higher. Scroll and screw compressors have fewer parts, compact structure, so especially suitable for heat pump units.
4, the current use of air-cooled heat pump units generally use hot air defrosting method to remove the winter heating conditions on the air side of the heat exchanger accumulated frost. At the beginning and end of the defrost, the system to run in reverse, in the original condensing side of the liquid refrigerant accumulated in the coil due to which the pressure is suddenly reduced to the suction pressure and a large number of surge to the compressor, resulting in the compressor's wet stroke, which is not a big problem for the scroll and screw compressor, but it is very easy to cause damage to the piston compressor valves and connecting rods.
5, in addition to the heat pump compressor itself on the scroll and semi-hermetic screw type than the piston type noise is lower.
The condenser type and arrangement of the condenser used fin type currently has two main open window and corrugated sheet, open window heat transfer efficiency is higher, so the first two years of production of heat pump units are often used. However, due to the poor quality of China's urban atmosphere, and these fins are very easy to accumulate ash, and more difficult to clean up, the use of a long period of time, the heat transfer effect is greatly reduced. So the current heat pump condenser with corrugated sheet with internal threaded copper tube, which has a higher heat transfer efficiency, not easy to accumulate dust, wind resistance and other characteristics.
The condenser fin spacing is also very concerned about, as a condenser when used to ribbing than the number of high heat transfer system is good, so I hope that the distance between the pieces of small better. But when it is used as an evaporator, fins a frost, the use of the heat transfer effect will be greatly reduced, and therefore hope that the distance between the pieces of large; general piece distance to 3mm is appropriate.
The arrangement of the condenser with its heat transfer effect and dimensions have a direct relationship. Usually the condensing coil of the heat pump is arranged into a straight coil, V-coil, W-coil three types. But V-type coil in addition to the larger space between the axial fan and no other parts, low space utilization. Straight type coil, although the centralized layout of the compressor, four-way valve, evaporator and other system components, but due to the high height of the coil, the velocity of the wind surface is not uniform, the condenser heat transfer efficiency is low, and the organization of the airflow is not ideal, the air resistance is greater. W-type arrangement to overcome the above shortcomings, not only to improve the airflow organization to improve heat transfer efficiency, reduce air resistance, and because in the same space conditions, the condensing coil heat transfer area increases, the space utilization rate is higher, thus reducing the unit size.
Thermal expansion valve configuration of the heat pump refrigeration system now has a single expansion valve and double expansion valve two ways, the so-called double expansion valve is the heating condition and refrigeration condition using an expansion valve. If the system uses a single expansion valve, according to the standard refrigeration conditions for selection, because the heat pump system in the heating condition when the system operates under the system's heating capacity with the drop in ambient temperature also declines. At this time, the thermal capacity of the expansion valve will also decline, but the magnitude of its decline is less than the decline in the system's heating capacity. Thus, in the heating condition with the drop in ambient temperature, the system is configured for the expansion valve appears to be too large. Too large an expansion valve will cause the evaporator supply too much liquid, evaporative pressure rise, and outdoor air heat exchange is reduced, resulting in a reduction in heat pump heat supply.
Currently, many manufacturers of heat pump units use double expansion valve type, refrigeration expansion valve according to the standard refrigeration conditions to choose. Thermal expansion valve, if the standard heating conditions to choose, that is, in low-temperature operating conditions when the expansion valve will appear too large, so according to the author's own experience suggests that the thermal expansion valve according to the ambient temperature -7 ° C, hot water inlet temperature of 40 ° C, the outlet temperature of 45 ° C to select the type, according to the calculation of such a condition of the selected valve can be calculated in the ambient temperature is not less than -15 ° C under normal operation.
Evaporator type is currently used in the air-cooled heat pump unit evaporator is mainly plate heat exchanger and dry shell and tube heat exchanger. Plate heat exchanger used in small air-cooled heat pump, it has a high heat transfer efficiency, the evaporator is not easy to accumulate the characteristics of the oil; in particular, the new plate with a built-in distribution device to solve the key problem of uniformity of refrigerant distribution between the plate, can be in the same temperature of the water to increase the evaporating temperature 15 ~ 2 ℃, improve the refrigeration efficiency. Dry shell and tube evaporator used in large and medium-sized air-cooled heat pumps, the heat transfer tube has been widely used in high-efficiency tube, so the heat transfer efficiency has improved greatly. But in general, less than plate heat exchanger. And its return oil is relatively difficult, often accumulated at the bottom of the heat exchanger. If the bottom of the oil return pipe and suction tube connected to the liquid refrigerant, due to the liquid refrigerant brought in, resulting in refrigerant superheat instability, affecting the work of the expansion valve and the system's refrigeration capacity.
Configuration of the axial fan axial fan configuration should first meet the condenser (air-side heat exchanger) heat transfer requirements, according to the experience of the air-cooled heat pump unit equipped with an axial fan air volume and the standard refrigeration capacity (ambient temperature of 35 ℃, water temperature of 7 ℃) than about 0.071 to 0.095/kJ between, in addition to ensure that the condenser windward side of the wind speed, because it relates to winter Operation of the air side of the heat exchanger frost rate, the windward side of the larger the wind speed of winter operation is less likely to frost. But the air volume is too large fan power consumption should also increase, while the noise should also increase, so in general the windward side of the wind speed to take 3 ~ 5m / s. In addition, the fan configuration should also be considered when the noise, the current general selection of large-diameter, low-speed, and blade torsion angle of the axial fan in order to reduce fan noise.
Energy regulation in the air-cooled heat pump unit in the energy regulation commonly used in the number of compressor control, compressor gap operation, cylinder unloading regulation (piston), frequency control (scroll), slide valve stepless regulation (screw). From the energy regulation, we can see that the number of compressor control, compressor gap operation, cylinder unloading regulation belongs to the graded regulation, while frequency control and slide valve stepless regulation belongs to the stepless regulation. Stepless regulation has energy saving, noise and vibration, good starting performance and also reduces the interference of the power supply system. From this point can also be seen scroll and screw compressor superior heat.
Defrosting method of each production plant units produced by the defrosting method is basically the same, most of the hot steam defrosting method; the difference is the defrosting control technology. Common differential pressure control method, temperature control method, temperature time control method, which is the most common temperature time control method. This control technology in the defrost parameter setting is the most critical. Defrost parameters include defrost temperature, defrost time, defrost interval. Defrost temperature is located in the expansion valve temperature-sensitive components to sense the temperature of the liquid after throttling, the general set value of -5 ℃, in addition to the defrost time interval is a timer control, generally set at 4min. defrost time is also controlled by the timer, generally no more than 10min. heat pump hair temperature drops to -5 ℃, and from the last defrost time interval is enough to 40min, the unit enters the defrost mode. If the defrost time exceeds 1010min and the liquid temperature in the coil still has not risen to +5℃, the unit should also stop defrosting to resume heating.
In the above three parameters in the defrost time interval is directly affected by the environment, but at present the majority of manufacturers of the defrost time interval is still using a fixed value, this practice leads to a serious frost in low-temperature and high humidity areas, due to the lack of a set time and can not be defrosted, resulting in a thick layer of frost or even freezing, the unit of low-pressure protection and shutdown of the phenomenon. This problem should be noted in the unit commissioning. Therefore, I suggest that on the one hand, the defrost parameters in the heat pump should be set according to local conditions, can not be generalized. On the other hand, it has been mentioned in the low temperature and high humidity in the region should not use the heat pump unit.
Safety protection and control of the current domestic air-cooled heat pump unit protection and control of computer control, including programmable control and microcomputer control, the two control principles are generally the same.
An air-cooled heat pump safety protection system should include at least the following aspects 1) suction pressure is too low protection 2) exhaust pressure is too high protection 3) oil pressure protection 4) cold water temperature is too low protection 5) water side of the heat exchanger water protection 6) the compressor start interval protection 7) compressor built-in motor overheating protection 8) motor overload protection 9) power supply voltage is too low protection 10) three-phase phase protection 11) oil temperature control? Protection 11) oil temperature control? Air-cooled heat pump control should at least include 1) defrost control 2) multiple compressor sequence control 3) energy regulation 4) fault shutdown and display 5) remote control interface (for remote setup of operating parameters and control of the unit start and stop, unit operating parameters and faults displayed in the control terminal)
Air-cooled heat pump engineering design of air-cooled heat pump layout: the air-cooled heat pump chiller-heaters are used to varying degrees, there is a phenomenon of the air-cooled heat pump chiller-heaters are used to varying degrees. To varying degrees, there is a phenomenon of insufficient cooling capacity in summer and insufficient heating capacity in winter. The reasons for this phenomenon are manifold, here in addition to the factors of the equipment itself, there are also engineering design problems. The main reason is that the equipment is not reasonably arranged to cause airflow short circuit, high temperature exhaust air unit in summer is re-absorbed, resulting in high air temperature condensing pressure rise, resulting in a decline in unit cooling capacity; winter is melting frost unit discharged wet air is next to the unit being heated inhalation of inhalation of air humidity is too high, exacerbated by the heating unit of the speed of the frost, so that the frost melting time to extend the time of heating time to reduce the unit, thus making the unit The heating capacity of the unit is reduced.
So the air-cooled heat pump should be arranged as far as possible in the outdoor, air inlet should be smooth, exhaust air should not be blocked. Avoid causing airflow short circuit. If there is an obstruction, it should meet certain requirements. Many production and other units to provide the design manual for the spacing between the unit and the distance between the unit and the wall have clear requirements, roughly as follows: the distance between the unit should be maintained at a distance of more than 2 meters, the unit and the main building (or the height of the daughter wall) should be maintained at a distance of more than 3 meters. In addition, in order to avoid exhaust short-circuit in the upper part of the unit should not be set up to block the canopy and other shelters. If the unit must be arranged indoors, should be taken to improve the static pressure of the fan approach, connected to the duct will be discharged to the outdoors. Exhaust air velocity should be large (7 m / s), so that it has a certain range, while the inlet velocity should be small (2 m / s), inlet and outlet vertical height difference should be as large as possible, in order to avoid short-circuiting the airflow.
Auxiliary heat source configuration air-cooled heat pump winter heat supply is with the decline in outdoor air temperature and reduce the outdoor air temperature per 1 ℃, the heat supply is about 2% lower; and with the decline in outdoor air temperature, indoor heat demand but need to increase, so we should consider setting up an auxiliary heat source, the auxiliary heat source can be an electric boiler, oil-fired boiler, gas boiler, steam-water heat exchanger and so on. According to engineering experience, air-cooled heat pump unit per 1RT refrigeration is configured 0.6kW auxiliary heat source is more stable, so that the configuration can fully guarantee the whole building in the winter air conditioning effect. Of course, many projects for investment considerations often do not configure the auxiliary heat source, which is also many buildings using heat pumps in the winter air conditioning effect is not good for one of the reasons.
The main reason for the impact of air-cooled heat pump winter heat supply is the relative humidity of the outdoor air in winter, especially in areas where the relative humidity of the outdoor air is greater than 75% of the air-cooled heat pump frost faster; defrosting to stop the heat supply, so that the unit's total heat supply decreased, increased power consumption. Therefore, I recommend that the average relative humidity of the outdoor air in winter is higher than 75% of the region should not use such units. If there are other reasons and must choose the heat pump unit, should consider configuring the auxiliary heat source.
Project noise control: air-cooled heat pump air conditioning project noise control is first of all in the stage of equipment selection should be given priority to choose a lower noise brand, the current single air-cooled heat pump noise is generally between 65 ~ 85dB, each increase in a unit, the overall noise will be an increase of 3dB, when the number of heat pumps in a project is more than the number of noise is more difficult to control. Therefore, the number of units in the selection of heat pump projects should not be too much, in other words, the heat pump should not be used in large-scale air conditioning projects, the general situation of a project should not be more than 5 units of heat pump.
In addition, in addition to the arrangement of the unit should be considered in addition to exhaust ventilation, to avoid exhaust air reflux, in the base of the unit and the inlet and outlet pipes must be installed vibration-damping devices, vibration isolation efficiency to meet the design requirements. In the cooling, heating station in the air-conditioning water trunk piping should be installed with shock absorbing hangers or brackets to prevent vibration of the unit and pumps through the pipeline to other places.Again, under the conditions of the unit should be arranged in the main building roof as far as possible, to reduce its noise on the main building itself and the impact of the surrounding environment.