According to the "Language Standard for Heating Ventilation Air-Conditioning Technology" (GB50155-92), a heat pump is defined as a refrigerator that enables the function of the evaporator and the condenser to be switched. We can also call the heat pump a unit that works on the principle of inverse Carnot cycle, both for cooling and heating. There are many types of heat pump, heat pump 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, air - water heat pump units, air heat pump water chiller units in engineering applications more widely. The research, production and application of heat pump units began to take the road of sound development in developed countries such as the United States in the 1970s. In 1980, China designed and produced the first air-to-water heat pump unit with R12 as working fluid and compressor power of 55KW in Shanghai and put it into practical application. Until the end of the eighties of the twentieth century, air-source heat pump water chiller research, production, application in our country had a rapid development. At the beginning, the application of heat pump project mainly to set the condition of the boiler or the land price is too expensive or no space for the construction of freezer room, available for selection only a few imported brands of units, the unit is relatively simple, mostly for the overall piston compression Air-cooled heat pump. In recent years, the use of heat pump project, both in the area or in the building function and size have a great breakthrough, heat pump unit brand, type of choice greatly expanded the space, there are many imported brands, there are many Domestic brands, there are piston compression heat pump units, there are screw-type units, there are integral units, there are modular heat pump units, single unit cooling capacity from 3RT-400RT, everything, and the unit's cooling, heating performance, quality , Reliability, etc. have significantly improved. Heat pump characteristics Air source heat pump chiller has the following characteristics: 1, air-conditioning system is a combination of hot and cold, and placed on the roof of the building, do not need to set up a special refrigeration room, boiler room, but also eliminates the chimney and cooling water pipe Occupied building space. For the construction of bustling city lots of land, or unconditional boiler room construction, air-source heat pump hot and cold water unit is undoubtedly a more appropriate choice. 2, no cooling water system, no cooling water system power consumption, no cooling water loss. Air-conditioning systems such as water-cooled chillers, tap water loss not only evaporation loss, drift loss, as well as blowdown losses, loss of winter antifreeze drainage, system washoff when the summer is enabled, chemical cleaning dilution loss, etc., all of these losses And equivalent to about 2-5% of the amount of cooling water circulating water, according to the different nature of the chiller, the equivalent unit of cooling capacity loss 2-4t100RT · h. This is a considerable amount for some of our severely missing cities. In addition, a considerable part of the project under partial load cooling water circulation remained unchanged. Or according to the number of host operating units, only for the appropriate number of adjustments. Our previous economy paid much less attention to this point. 3, because there is no boiler, no corresponding fuel supply system, no flue gas, no cooling water, system safety, sanitation and conciseness. For the professional heating, the boiler room most likely to have potential safety problems, in addition, the formation of cooling water contamination caused by Legionella infections have been reported in many cases, from a health and safety point of view, air-source heat pump has obvious advantages. 4, system equipment and less concentrated, operation, maintenance and management simple and convenient. Some small systems can be used to control the opening and closing of indoor fan coil heat pump unit switch. 5, stand-alone capacity from 3RT to 400RT, complete specifications, strong adaptability of the project, which will help the system to refine the division, can be layered, sub-block, sub-user units set up independent systems. 6, COP COP value lower than water-cooled units in summer, more power consumption, saving energy consumption in winter operation. For a typical building in Nanjing, a hot and cold winter city, the annual energy consumption of a heat pump system is lower than that of a water-cooled unit plus a boiler. However, at current energy prices, the annual operating cost of the heat pump system is higher than that of a water- Program. 7, higher cost. As a cold and heat source of air-conditioning system investment in equipment, air-source heat pump water heater unit cost is higher than the water-cooled unit plus the program of the boiler system cost more than 20-30% of the cost, such as only cold and heat source equipment, heat pump The price is about 1.5-1.7 times of water-cooled machine + boiler. 8, air-source heat pump chiller units exposed to the outdoors all year round, operating conditions worse than water-cooled chillers, their life is also shorter than the water-cooled chillers. 9, heat pump unit noise, the environment and adjacent rooms have a certain impact. Heat pump usually placed directly on the podium or roof, noise isolation effect, directly affect the neighboring rooms and the surrounding use of some rooms. Reasonable location and isolation noise isolation measures in place, the impact of heat pump noise can be basically eliminated. 10, air-source heat pump performance with the obvious changes in outdoor climate. Outdoor air temperature is higher than 40 ℃ -45 ℃ or below -10 ℃ ~ -15 ℃  heat pump unit can not work properly. Heat pump output and climate In the rated conditions, the temperature 35 ℃, water 7 ℃, air-source heat pump summer cooling coefficient COP of about 3.0, winter (air 7 ℃, water 45 ℃) Coefficient of heat of COPH is also about 3.0, air-source heat pump cooling, heating performance and outdoor climate has a direct relationship  air-source heat pump chiller units with the outdoor temperature cooling capacity increases, the unit power consumption Increase with outdoor temperature. When outdoor air temperature is increased to 40 ℃, the cooling capacity will generally drop about 5-7%. Air-source chiller normal cooling maximum temperature is generally 40 ℃ -45 ℃, individual brands with condenser fan speed control system, the maximum allowable outdoor temperature of up to about 50 ℃. It should be pointed out that with the cooling tower is not the same, the relative humidity in cooling conditions on the air source heat pump did not adversely affect the contrary, the relative humidity, the cooling is beneficial. Nanjing summer relatively high humidity, so in fact the difference between the cooling effect of air-cooled and water-cooled, smaller than people imagine. Air-source heat pump water heating unit more complex heating characteristics, when the coil surface temperature is below the air dew point temperature, the air will be dew condensation, coil surface heat transfer occurs at this time, help to improve the heat pump unit Heating capacity, but when the coil surface temperature is below the freezing temperature of air (below 0 ℃), if the relative humidity in the air at the same time reach a certain level, the coil surface will frost, if not timely defrosting, frost Will be more knot the thicker, affecting the actual flow of air, and hindered the heat exchange on the coil, the heavy will freeze, the compressor appears low voltage protection downtime. Corresponding to the different oncoming wind speed and climatic conditions, the heat pump unit outside the air coil on the air there are three states, namely the frosting area, condensation area, dry and cold  结 do not frost nor condensation. Frosting transition curve, which is close to the wet-bulb temperature line on the psychrometric chart. When the head-on wind speed is 2.5m / s, the ambient temperature is 0 ℃ and the relative humidity is 73%, the frosting begins on the coil. If the headwind speed is increased to 4m / s and the ambient temperature is 0 ℃, the relative humidity Up to 82%, the coil began to frost, improve wind speed can reduce the accumulation of frost. When the frontal wind speed is 2m / s, the outdoor air dry-bulb temperature is 0 ℃ -5 ℃ and the relative humidity is more than 85%, the frosting is the most serious. When tW <-5 ℃, the frosting rate will slow down, Moisture in the air has been significantly reduced. Heat pump unit frosting appears on the coil, will affect the unit's normal and effective heating, it must be defrosted. At present, most of the units use reverse circulation to defrost. At this time, not only the compressor stops heating operation, but also the cooling operation, so the system heat supply is obviously affected. Frosting serious, an average of half an hour of cream, a defrost time of about 5 minutes, due to defrost reduced heat about 17%. In addition, the outdoor temperature decreases, the output of heat pump significantly reduced. 0 ℃ conditions, the actual rate of heat pump units under rated conditions of about 70%. -6 ℃ case, the output is only about 62% under rated conditions, -10 ℃ under the conditions of heat supply is only about 55% under rated conditions. Cold weather, rain and snow have a significant impact on the heat pump, while affecting the normal operation of heavy, some users to extend the defrost time, pouring warm water and other methods to remove frost. Ambient temperature is below -10 ℃ ~ -15 ℃, the heat pump unit generally can not be normal operation. Nanjing summer hot, cold winter, high humidity  1993 winter temperatures below -5 ℃ a total of 69 hours  8: 00-18: 00 during the day when the temperature is below -5 ℃ for 7 hours. The temperature in summer 1994 was higher than 37 ° C for a total of 10 hours. 1993, 1994,  winter heat pump in the frosting work area, respectively 1613 hours and 1527 hours  wind speed of 2.02m / s wind heat exchanger, such as only 8: 00-18: 00 daytime hours  1993, 1994  winter heat pump in the frosted work area, respectively, 711 hours and 653 hours  wind speed of 2.02m / s. The average  Nanjing area air source heat pump frost formation time of about 1500 hours, if only in the daytime operation, the annual frost accumulation time in a total of about 680 hours. Such as increasing the coil head-to-head speed to 32m / s, air-source heat pump frost-forming time of about 1300 hours, such as only operating in the daytime, the annual frost accumulation time of about 600 hours. Heat Pump Application Nanjing is a hot, cold winter and humid city in winter. Although many people are skeptical about the reliability and rationality of winter heat pump heating in Nanjing, due to some of the above advantages of air source heat pump, air source Heat pump hot and cold water unit in Nanjing's development is quite fast. In the early nineties of the twentieth century, Nanjing began to use air-source heat pump chillers. The number of air source heat pumps put into service in 1995-1998 increased significantly. According to the information we currently have, there are currently about 250 projects in Nanjing that use air-source heat pump chillers as the hot and cold source of air conditioning systems. Among them, a design institute in recent years the selection of heat pump for the cold source of about 35 as many projects, accounting for the hospital air-conditioning project number of 30%. An engineer one person has nearly 10 projects using air-source heat pump hot and cold water units as a hot and cold air conditioning. In our understanding of the air source heat pump hot and cold water units as a hot and cold source projects in shopping malls, office buildings, office buildings, hotels, factories, complex buildings.  It should be pointed out that the import referred to here is solely owned or imported by foreign investors. In addition, the higher proportion of domestic shares (the number of construction projects) is related to the better sales performance of a Taiwan-invested enterprise in Nanjing during a certain period. Recently, the market share of imported heat pump units has risen. Although after years of digestion, engineering heat pump unit market did not like domestic air conditioners, domestic enterprises did not gain a dominant position, which is worth our consideration. From the survey