Unit 9 Air Conditioning SystemWord文件下载.docx

1、1. The meaning of air conditioningAn air conditioning system is an assembly of various components which operate in a controlled manner to produce a specified condition of the air within a space or building. To provide complete air conditioning a system must accomplish all the following: heat, humidi

2、fy, cool, dehumidify, ventilate, filter, and circulate.1.1 Comfort conditioningThe design specification for a comfort conditioning system is intended to be the framework for providing a comfortable environment for human beings throughout the year, in the presence of sensible heat gains in summer and

3、 sensible heat losses in winter. Dehumidification would be achieved in summer but the relative humidity in the conditioned space would be allowed to diminish as winter approached. There are two reasons why this is acceptable: first, human beings are comfortable within a fairly large range of humidit

4、y, from about 65 percent to about 20 percent and, secondly, if single glazing is used it will cause the inner surfaces of windows to stream with condensed moisture if it is attempted to maintain too high a humidity in winter.1Thus, a system might be specified as capable of maintaining an internal co

5、ndition of 22 dry-bulb, with 50 percent saturation, in the presence of an external summer state of 28dry-bulb, with 20 wet-bulb, declining to an inside condition of 20 dry-bulb, with an unspecified relative humidity, in the presence of an external state of -2 saturated in winter.1.2 Industrial condi

6、tioningHere the picture is quite different. An industrial or scientific process may, perhaps, be performed properly only if it is carried out in an environment that has values of temperature and humidity lying within well defined limits. A departure from these limits may spoil the work being done. I

7、t follows that a choice of the inside design condition is not based on a statistical survey of the feelings of human beings but on a clearly defined statement of what is wanted.Thus, a system might be specified to hold 21 0.5, with 50 percent saturation 2.5 percent, provided that the outside state l

8、ay between 29.5 dry-bulb, with 21 wet- bulb and 4 saturated.2. System CategoriesAir conditioning systems may be classified as central-station or unitary systems. A central-station system is one in which the components of the system are all grouped together in one central mechanical room and the cond

9、itioned air is sent from there to the spaces to be conditioned through extensive ductwork.Unitary systems make use of factory-assembled and tested air conditioning units. These are usually installed in or immediate adjacent to a zone or space to be conditioned. The source of the heat and the refrige

10、ration may be in each air conditioning unit or in a “central” mechanical room.Another possible classification divides air conditioning systems into cooling systems and heating systems. Cooling systems may in turn to be classified in terms of the cooling medium and cooling surface used. Heating syste

11、ms are classified as warm-air systems or radiant-heat system.The basic systems and their several variants will be discussed in the following section.2.1 Central-station System A central-station year-round air conditioning system is one which usually serves a single large building or space. The term

12、“central-station” is ordinarily not applied to systems of less than about 25 tons of cooling capacity, circulating about 10,000 cfm of conditioning air. Buildings whose demand is less than this are usually more economically air conditioned by unitary equipments, all field-assembled. A general arrang

13、ement of the component parts of a central system suitable for year-round air conditioning with close control of temperature and humidity is illustrated in Figure 9.1.It usually consisted of following components:（1） Coils for cooling and dehumidifying, either for chilled water or for direct expansion

14、 ofthe refrigerant（2） Coils for heating, supplied with steam or hot water（3） Blower and driving motor（4） Sprays for cooling and dehumidifying or for washing the air（5） Air-cleaning equipment such as filters, electrostatic precipitators, odor-removing equipment, and germicidal lamps（6） A wide variety

15、 of control devices.Fig.9.1 Equipment Arrangement for Central Systems The central-station air conditioning plant must be supplied with either chilled water or refrigerant for the cooling coils and with hot water or steam for the heating coils. Refrigeration condensing units and steam or hot-water bo

16、ilers for these purposes may be in the same mechanical room with the air conditioner or may be in an adjoining room or in the basement.2.2 Capacity considerations. It will be recalled that heat transfer between air and metal surfaces requires rather large surfaces areas. Heating and cooling coils handling air at face velocities of around 500 fpm must p