Application of air source heat pump chiller and hot water unit

1. Overview
  According to the “Technical Language Standards for Heating, Ventilation and Air Conditioning” (GB50155-92), a heat pump is defined as a refrigerator that can convert the functions of an evaporator and a condenser. We can also call a heat pump a unit that works based on the reverse Carnot cycle principle and can be used for both cooling and heating. There are many types of heat pumps. If they are classified according to the heat exchange medium of the evaporator and condenser of the same heat pump, heat pumps can be divided into: air-air heat pump, air-water heat pump, water-water heat pump, water-air heat pump, soil -Air heat pump and soil-water heat pump, etc. Among them, air-water heat pump units, that is, air heat source heat pump chillers and hot water units, are more widely used in engineering.
  The research, production and application of heat pump units only began to embark on a path of sound development in developed countries such as the United States in the 1970s. In 1980, China independently designed and produced the first air-water heat pump unit with a compressor power of 55KW and used it as a working substance in Shanghai, and put it into practical engineering application. It was not until the end of the 1980s that the research, production and application of air source heat pump chillers and hot water units had rapid development in China. At the beginning, the application of heat pump projects was mainly for buildings without the conditions to install boilers, or the land price was too expensive, or there was no space to install a refrigerating machine room. Only a few imported brand units were available for selection. The unit form was relatively simple, mostly piston compression. Type air-cooled heat pump unit. In the past few years, heat pump projects have achieved great breakthroughs in terms of geographic or architectural function and scale. The choice of brands and types of heat pump units has greatly expanded. There are not only many imported brands, but also many. Domestic brands, there are piston compression heat pump units, screw type units, integrated units, and modular heat pump units. The cooling capacity of a single unit ranges from 3RT-400RT, everything, and the cooling and heating performance and quality of the unit , Reliability, etc. have been significantly improved.
  2. Heat pump characteristics
  air source heat pump hot and cold water units have the following characteristics: 
  1. The air-conditioning system combines cold and heat sources and is placed on the roof of the building. There is no need to set up a special refrigeration room, boiler room, and no need for chimneys and cooling. Building space occupied by water pipes.
  For buildings in prosperous urban areas with little land and money, or buildings with unconditional boiler rooms, air source heat pump cold and hot water units are undoubtedly a more suitable choice.
  2. No cooling water system, no cooling water system power consumption, no cooling water loss.
  If the air-conditioning system adopts water-cooled chillers, the loss of tap water includes not only evaporation loss, floating water loss, but also sewage loss, winter antifreeze drainage loss, system flushing loss during summer operation, chemical cleaning dilution loss, etc., all of these losses are total The sum is equivalent to 2-5% of the circulating water volume of the cooling water. According to different types of chillers, the loss per unit cooling capacity is 2-4t/100RT·h. This is a considerable amount for some of our cities that are seriously lacking. In addition, a considerable part of the project remains unchanged in the cooling water circulation under partial load conditions. Or according to the number of hosts running, only adjust the corresponding number. Our previous economy paid little attention to this.
  3. Because there is no boiler, no corresponding fuel supply system, no flue gas, no cooling water, the system is safe, sanitary and simple.
  For heating professionals, the boiler room is most likely to have safety hazards. In addition, many cases of Legionella infection caused by cooling water pollution have been reported. From the perspective of safety and health, air source heat pumps have obvious advantages.
  4. The system equipment is small and centralized, and the operation, maintenance and management are simple and convenient. Some small systems can control the switch of the heat pump unit through the start and stop of the indoor fan coil.
  5. The stand-alone capacity ranges from 3RT to 400RT, with complete specifications and strong engineering adaptability, which is conducive to the detailed division of the system, and the system can be independently set up in layers, blocks, and user units.
  6. The COP value of running in summer is lower than that of water-cooled units, and it consumes more electricity, while running in winter saves energy consumption. For general buildings in a city with cold winters and hot summers like Nanjing, the annual energy consumption of heat pump systems is lower than that of water-cooled units and boilers. However, at current energy prices, the annual operating cost of heat pump systems is higher than that of water-cooled units and boilers. plan.
  7. Higher cost. As the equipment investment for the cold and heat sources of the air-conditioning system, the cost of the air-source heat pump cold and hot water units is higher, which is 20-30% more expensive than the comprehensive system cost of the water-cooled unit plus boiler. For example, only the cold and heat source equipment, heat pumps The price is about 1.5-1.7 times that of water chiller + boiler.
  8. Air source heat pump hot and cold water units are exposed to the outdoors all the year round, operating conditions are worse than water-cooled chillers, and their service life is correspondingly shorter than water-cooled chillers.
9. The noise of the heat pump unit is relatively loud, which has a certain impact on the environment and adjacent rooms.
  The heat pump is usually placed directly on the podium or the top roof, and the effect of vibration isolation and sound insulation directly affects the use of adjacent rooms and some surrounding rooms. With reasonable location and vibration isolation and sound insulation measures in place, the influence of heat pump noise can be basically eliminated.
  10. The performance of the air source heat pump changes significantly with the outdoor climate. When the outdoor air temperature is higher than 40-45℃ or lower than -10~-15℃, the heat pump unit cannot work normally.
  3. Heat pump output and climate
  under rated working conditions, the temperature is 35℃, the effluent is 7℃, the air source heat pump’s summer refrigeration performance coefficient COP value is about 3.0, in winter (air 7℃, effluent 45℃), if defrost loss is not included, the system The thermal coefficient COPH value is also around 3.0. The cooling and heating performance of the air source heat pump is directly related to the outdoor climate. The cooling capacity of the air source heat pump chiller and hot water unit decreases with the increase of outdoor temperature, and the power consumption of the unit increases with the outdoor climate. The increase in ambient temperature increases. When the outdoor air temperature increases to 40°C, the cooling capacity generally decreases by about 5-7%. The upper limit temperature for normal cooling of air source cold and hot water units is generally 40-45°C, and individual brands have a gradual control system for condenser fan speed, and the maximum allowable outdoor temperature can reach about 50°C. It should be pointed out that, unlike cooling towers, the relative humidity under refrigeration conditions has no adverse effect on the air source heat pump. On the contrary, the relative humidity is large, which is beneficial to cooling. The relative humidity in Nanjing is relatively high in summer, so in fact, the difference in cooling effect between air-cooling and water-cooling is smaller than people think.
  The heating characteristics of the air source heat pump chiller and hot water unit are more complicated. When the surface temperature of the coil is lower than the dew point temperature of the air, the air will condense. At this time, phase transformation heat occurs on the surface of the coil, which is beneficial to improve the heat pump unit’s performance. Heating capacity, but when the surface temperature of the coil is lower than the freezing point temperature of the air (below 0℃), if the relative humidity in the air reaches a certain level at the same time, the surface of the coil will be frosted. If it is not defrosted in time, the frost layer It will become thicker and thicker, which will affect the actual flow of air and hinder the heat exchange on the coil. In the worst case, it will freeze and the compressor will shut down under low pressure protection. Corresponding to different front wind speeds and climatic conditions, there are three states of humid air on the outdoor side air coil of the heat pump unit, namely, frosting zone, condensation zone, and dry-cold zone (no frost or condensation). The frosting transition curve is close to the isothermic bulb temperature line on the enthalpy-humidity diagram. When the face-to-face wind speed is 2.5M/S, the ambient temperature is 0°C, and the relative humidity is 73%, frost begins to form on the coil. If the face-to-face wind speed is increased to 4M/S and the ambient temperature is 0°C, the relative humidity is up to At 82%, the coil begins to form frost. Increasing the wind speed can slow down the frost. When the surface wind speed is 2M/S, the outdoor air dry bulb temperature is 0-5℃, and the relative humidity is >85%. The frosting is the most serious. When tw<-5℃, the frosting rate slows down. This is due to the air at this time. The moisture content has been significantly reduced.
  Frosting on the coil of the heat pump unit will affect the normal and effective heating of the unit, so it must be defrosted regularly. At present, most units use reverse cycle to defrost. At this time, not only this part of the compressor stops heating operation, but also performs cooling operation, so the heat supply of the system is obviously affected. When the frosting is severe, the average defrosting time is half an hour, and the time for one defrosting is about 5 minutes. The heat supply reduced by the defrosting is about 17%. In addition, when the outdoor temperature drops, the output of the heat pump unit is significantly reduced. Under the condition of 0℃, the actual output rate of the heat pump unit is about 70% of the rated working condition. At -6°C, the output is only about 62% of the rated operating condition, and the heat supply at -10°C is only about 55% of the rated operating condition. Rain, snow, and cold weather have a significant impact on the output of the heat pump, and in the worst case, it affects the normal operation. Some users use methods such as artificially extending the defrosting time and pouring warm water to remove the frost. When the ambient temperature is lower than -10℃— -15℃, the heat pump unit generally cannot operate normally.
  Nanjing is hot in summer, cold in winter, and high in humidity. In 1993, the temperature in winter was below -5°C for 69 hours, and during the day from 8:00 to 18:00, the temperature was below -5°C for 7 hours. In 1994, the summer temperature was higher than 37℃ for 10 hours. In 1993 and 1994, the winter heat pump was in the frosting area for 1613 hours and 1527 hours respectively (the wind speed on the windward side of the heat exchanger was 2.0m/s). And during the daytime from 8:00 to 18:00, in 1993 and 1994, the winter heat pump was in the frosting area for 711 hours and 653 hours respectively (the wind speed on the windward side of the heat exchanger was 2.0m/s). On average, Nanjing area The frosting time of the air source heat pump unit is about 1500 hours. If it is only operated during the day, the annual frosting time is about 680 hours. If the front wind speed of the coil is increased to 3M/S, the frosting time of the air source heat pump unit is 1300 If it only runs during the day, the annual frost time will accumulate to about 600 hours.
 Fourth, heat pump application
  Nanjing is a city with hot summers, cold winters, and high humidity. Many people only supply heat for heat pumps in Nanjing in winter. There is a certain degree of skepticism about reliability and rationality, but due to some of the above-mentioned advantages of air source heat pumps, the development of air source heat pump chillers and hot water units in Nanjing is also quite fast. In the early 1990s, there were projects in Nanjing that began to use air source heat pump chillers and hot water units. The number of air source heat pumps put into use from 1995 to 1998 has increased significantly. According to the information we currently have, there are currently about 250 projects in Nanjing that use air-source heat pump chillers and hot water units as the air-conditioning system’s cold and heat sources. Among them, a certain design institute has selected about 35 projects using heat pumps as cold and heat sources in recent years, accounting for about 30% of the number of air-conditioning projects in the institute. A certain engineer has used air-source heat pump chillers and hot water units as the air-conditioning cold and heat source in nearly 10 projects. Among the projects we know about that use air source heat pump chillers and hot water units as cold and heat sources include shopping malls, office buildings, office buildings, hotels, factories, and complex buildings.
  It needs to be pointed out that the import here refers to wholly foreign-owned assembly or original imported products. In addition, the higher domestic share (the number of engineering projects) is related to the better sales performance of a Taiwan-funded enterprise in Nanjing during a certain period. Recently, The market share of imported products for engineering heat pump units is on the rise. Although after years of digestion, the engineering heat pump unit market has not been the same as household air conditioners, and domestic companies have not achieved a dominant position. This is a problem worthy of our consideration.
  According to the results of the investigation, air source heat pump chillers and hot water units have developed rapidly in Nanjing in recent years, and the heat pump air-conditioning systems of most projects can basically meet the required cooling and heating requirements. Here are some introductions to several typical projects.
  The Yangtze River Trade Building was designed in 1991 and completed and put into use in 1994. It is a modern office building with a building area of ​​about 35,000 m2 and a total building height of 95 meters. Among them, there is one underground floor and 23 floors above ground. The outer protective structure is an all-glass curtain wall. The building uses 6 American York company AWHC-200 heat pump units, the installed rated cooling capacity is 3672KW (1044RT), the area cooling index is 105W/m2, and the heat pump rated heating capacity is 107W/m2. The heat pump unit is placed on the top roof of the main building. The system is equipped with 8 water pumps, each with a circulating water volume of 200m3/h and a head of 32mH2O. The heat pump and the water pump are connected in parallel and then connected in series. The inlet and outlet pipes of each heat pump are directly connected to the manifold, and the water pump is placed indoors.
  The heat pump unit adopts a spring shock absorber to reduce vibration, and the water pump also adopts a spring shock absorber base to reduce vibration. The end of the air-conditioning system is designed for variable water volume, and the host is controlled by fixed water volume. According to on-site survey and measurement, the building staff are highly satisfied with the air conditioner. One day in summer, when the inspiratory temperature is 40℃ (partially ruled out short circuit)

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