2Air Conditioning System.docx

1、2Air Conditioning SystemLesson two Air Conditioning SystemAir conditioning for people is the control of temperature, humidity, air movement and air cleanliness, heat radiation sometimes e.g. by chilled ceiling , normally with mechanical means, to achieve human thermal comfort.Air conditioning system

2、s can be categorized according to the means by which the controllable cooling is accomplished in the conditioned space. They are further segregated to accomplish specific purposes by special equipment arrangement.In selecting a suitable air conditioning system for a particular application, considera

3、tion should also go to the following:- System constraints : Cooling load, Zoning requirements, Heating and ventilation- Architectural Constraints : Size and appearance of terminal devices, acceptable noise level, Space available to house equipment and its location relative to the conditioned space,

4、acceptability of components obtruding into the conditioned space- Financial Constraints : Capital cost, Operating cost, Maintenance costThere are four basic system categories:1 Central chilled water air conditioning systems - All Air Systems1.1 Single zone1.2 Reheat1.3 Variable Air Volume1.4 Dual Du

5、ct1.5 Multizone2 Central chilled water air conditioning systems - Air-and Water Systems2.1 Induction2.2 Fan Coil2.3 Two-pipe2.4 Three-pipe3 Central chilled water air conditioning systems - All Water Systems, including cooling towers which can also be applied to systems 1, 2 above3.1 Fan-coil units3.

6、2 Central chilled water air conditioning system with fan coils and other devices3.3 Water cooling tower4 Direct expansion Systems i.e. direct expansion of refrigerant, without the chilled water cooling medium 4.1 Window air conditioners4.2 Unitary and Rooftop Air Conditioners4.3 Split type and packa

7、ge air conditioning systems4.4 Heat pumps1. Central chilled water air conditioning systems - All Air SystemsAn all-air system provides complete sensible and latent cooling capacity in the cold air supplied by the system. Heating can be accomplished by the same air stream, either in the central syste

8、m or at a particular zone. All-air systems can be classified into 2 categories:-Single duct systems-Dual duct systemsSystem Advantages The central plant is located in unoccupied areas, hence facilitating operating and maintenance, noise control and choice of suitable equipment. No piping, electrical

9、 wiring and filters are located inside the conditioned space. Allows the use of the greatest numbers of potential cooling seasons house with outside air in place of mechanical refrigeration. Seasonal changeover is simple and readily adaptable to climatic control. Gives a wide choice of zonability, f

10、lexibility, and humidity control under all operating conditions. Heat recovery system may be readily incorporated. Allows good design flexibility for optimum air distribution, draft control, and local requirements. Infringes least on perimeter floor space. Adapts to winter humidification.System Disa

11、dvantages Requires additional duct clearance which can reduce the usable floor space. Air-balancing is difficult and requires great care. Accessibility to terminals demands close cooperation between architectural, mechanical and structural engineers.Distribution systems have a number of important co

12、mponents: 1. The Air Handling Unit is a cabinet that includes or houses the central furnace, air conditioner, or heat pump and the plenum and blower assembly that forces air through the ductwork. 2. The Supply Ductwork carries air from the air handler to the rooms in a house. Typically each room has

13、 at least one supply duct and larger rooms may have several. 3. The Return Ductwork carries air from the conditioned space back to the air handler. Most houses have only one or two main return ducts located in a central area. 4. Supply and Return Plenums are boxes made of duct board, metal, drywall

14、or wood that distribute air to individual ducts or registers. 5. The Ductwork is a branching network of round or rectangular tubes generally constructed of sheet metal, fiberglass board, or a flexible plastic and wire composite material located within the walls, floors, and ceilings. The three most

15、common types of duct material used in home construction are metal, fiberglass duct board, and flex-duct. 6. Flex-duct is installed between the register and plenum box, or plenum box and air handler, usually in a single, continuous piece. While flex-duct has fewer seams, the inner lining and outer in

16、sulated covering can tear or be pinched closed. Also longer flex-duct runs can restrict the flow of air; proper design and installation is very important. 7. Both metal and fiberglass duct board are rigid and installed in pieces. Fiberglass duct board, like flex-duct, is made of an insulation materi

17、al. Ducts are built of sections of the duct board. The seams in the duct board should be carefully sealed with mastic or high quality duct tape. 8. Rectangular metal duct, especially the kind used for plenums and larger trunk runs, is often insulated on the inside with fiberglass duct liner. If it i

18、s not insulated on the inside, metal ducts should be insulated on the outside using a fiberglass batt with an attached metal foil vapor retarder. The insulation should be at least two inches thick, and the vapor barrier installed on the outside of the insulation facing away from the duct. The seams

19、in the insulation are usually stapled together around the duct and then taped. All of the seams should be sealed before insulation is installed. All return and supply ducts located outside the conditioned space, in attics, crawlspaces, or basements, for example, should be sealed and insulated. 9. Du

20、ctwork Joints join pieces of ductwork. 10. Elbows are manufactured pieces of duct used for turns. 11. Boots connect ductwork to registers. 12. Registers and Grilles are the coverings for duct openings into the conditioned space.These components will bring about:- -Heat balance: The amount of heat ex

21、tracted out of the air conditioned room (by the cooling system, exhaust air systems, building leakage, must be equal to the amount of heat generated inside the room (by human being, electrical appliances, etc.) and transferred into the room (by conduction through the building envelope, radiation via

22、 the glass, hot air leakage into the room through gaps in windows, doors, fresh air introduced into the room, etc.) i.e. Total kW going into room = Total kW going out of the room.-Air balance: The mass flow rate of the air going into the room = The mass flow rate of air going out of the room. Fresh

23、air coming into the room : 2.5 l/s per person, non-smoking, 5 l/s per person for smoking accommodation, good indoor air quality (IAQ) is important.1.1 Single Zone SystemThe all-air single-zone air conditioning system is the basic central system which can supply a constant air volume or a variable ai

24、r volume at low, medium or high pressure. Normally, the equipment is located outside the conditioned space but can also be installed within the conditioned area if conditions permit. Typical applications include:-Space with uniform loads-Small spaces requiring precision control-Multiple systems for

25、large areas1.2 Reheat SystemThe reheat system is a modification of the single-zone system. It provides:-Zone or space control for areas of unequal loading.-Heating or cooling of perimeter areas with different exposures.-Close control for process or comfort applications. In the reheat system, heat is

26、 added as a secondary process to either preconditioned primary air or recirculated room air. The heating medium can be hot water, steam or electricity. Advantages : Closely controls space conditionsDisadvantages : Expensive to operate1.3 Variable Air Volume SystemThe variable air volume system compe

27、nsates for varying cooling loads by regulating the volume of cooling air supplied through a single duct.(a) Simple Variable Air Volume (VAV)Simple VAV systems typically cools only and have no requirement for simultaneous heating and cooling in various zones. (b) Variable Air Volume ReheatIt integrat

28、es heating at or near the terminal units. It is applied to systems requiring full heating and cooling flexibility in interior and exterior zones. Heating is turned on when the air flow reaches a predetermined minimum.Advantagesa) When combined with a perimeter heating system, it offers inexpensive t

29、emperature control for multiple zoning and a high degree of simultaneous heating-cooling flexibility.b) Capital cost is lower since diversities of loads from lights, occupancy, solar and equipment of as much as 30% are permitted. c) Virtually self-balancing.d) It is easy and inexpensive to subdivide

30、 into new zones and to handle increased loads with new tenancy or usage if load does not exceed the original design simultaneous peak.e) No zoning is required in central equipment.f) Lower operating cost becauseFans run long hours at reduced volumeRefrigeration, heating and pumping matches diversity

31、 of loadsUnoccupied areas may be fully cut-offg) Reduced noise level when the system is running at off-peak loads.h) Allows simultaneous heating and cooling without seasonal changeover.1.4 Dual Duct SystemThe dual-duct system employs two air ducts to supply cold air and warm air to a mixing terminal

32、 unit which proportions the cold and warm air in response to a thermostat located in the conditioned space. The system is well suited to provide temperature control for individual spaces or zones.Advantages (in addition to those common to all air systems) Systems with terminal volume regulation are self-balancing. Zoning of central equipment is not required. Instant temperature response is ach