1、 CENRTRAL COOLING AND HEATING The energy used to heat and cool many buildings often comes from a central location in the facility. The energy input may be combination of electricity , oil ,gas , coal ,solar ,geothermal ,etc. This energy is typically converted into hot or chilled water or steam that
2、is distributed throughout the facility for heating and air conditioning(cooling) . Centralizing this function keeps the conversion equipment in one location and distributes the heating and cooling in a more readily usable form. Also ,a central cooling and heating plant provides higher diversity and
3、generally operates more efficiently with lower central location and a potentially large distribution system. This lesson addresses the design alternatives that should be considered when centralizing the cooling and heating sources in a facility. The economics of these systems require extensive analy
4、sis. Boilers ,gas , steam ,and electric turbine-driven centrifugal refrigeration units； and absorption chillers may be installed in combination in one plant . In large buildings with core areas that require cooling and perimeter areas require heating , one of several heat reclaim systems may heat th
5、e perimeter to save energy . The choice of equipment for a central heating and cooling plant depends on the following : Required capacity and type of use Cost and types of energy available Location and space for the equipment room A stack for any exhaust gases and its environment impact Safety requi
6、rements for the mechanical room and surrounding areas Type of air distribution system Owning and operating costs Many electric utilities impose very high charges for peak power use or ,alternatively ,offer incentives for off-peak use . This policy has renewed interest in both water and ice thermal s
7、torage .The storage capacity installed for summer load leveling may also be available for use in the winter ,making heat reclaim a more viable option. With ice storage, the low-temperature ice water can provide colder air than that available from a conventional system that supplies air at 10 to 13 .
8、 The use of a higher water temperature rise and a lower supply air temperature, lowers the required pumping and fan energy and , in some instances, offsets the energy penalty due to the lower temperature required to make ice. Refrigeration Equipment The major types of refrigeration equipment in larg
9、e systems use reciprocating compressors, helical rotary compressors, centrifugal compressors, and absorption chillers. These large compressors are driven by electric motors, gas and diesel engines, and gas and steam turbines. The compressors may be purchased as part of a refrigeration chiller that i
10、ncludes the compressor, drive, chiller, condenser, and necessary safety and operating controls. Reciprocating and helical rotary compressor units are frequently field assembled, and include air-cooled or evaporative condensers arranged for remote installation. Centrifugal compressors are usually inc
11、luded in packaged chillers. Absorption chillers are water-cooled. They use a lithium bromide/water or water/ammonia cycle and are generally available in the following four configurations(1) direct fired, (2) indirect generated by low-pressure steam of hot water, (3) indirect generated by high-pressu
12、re steam of hot water, and (4)indirect generated by hot exhaust gas. Small direct-fired chillers are single-effect machines with capacities of 12 to 90kW. Larger, direct-fired, double-effect chillers in the 350 to 7000kW range are also available. Low-pressure steam at 100kPa of low temperature hot w
13、ater heats the generator of single-effect absorption chillers with capacities from 180 to 5600kW. Double-effect machines use high-pressure steam up to 1000kPa or high-temperature ht water at an equivalent temperature. Absorption chillers of this type are available from 1200 to 7000kW. In large insta
14、llations the absorption chiller is sometimes combined with steam turbine-driven centrifugal compressors. Steam from the noncondensing turbine is piped to the generator of the absorption machine. When a centrifugal unit is driven by a gas turbine of an engine, an absorption machine generator may be f
15、ed with steam of hot water from the engine jacket. A heat exchanger that transfers the heat of the exhaust gases to a fluid medium may increase the cycle efficiency. Cooling towers Water from condensers is usually cooled by the atmosphere. Either natural draft or mechanical draft cooling towers or s
16、pray ponds are used to reject the heat to the atmosphere. Of these, the mechanical draft tower, which may be of the forced draft, induced draft, of ejector type, can be designed for most conditions because it does not depend on the wind. Air-conditioning systems use towers ranging from small package
17、 units of 20 to 1800kW to field-erected towers with multiple cells in unlimited sizes. If the cooling tower is on the ground, it should be at least 30m away from the building for two reasons: (1)to reduce tower noise in the building； and (2)to keep discharge air from fogging the building windows. To
18、wers should be kept the same distance from parking lots to avoid staining car finishes with water treatment chemicals. When the tower is on the roof, its vibration and noise must be isolated from the building. Some towers are less noisy than others, and some have attenuation housings to reduce noise
19、 levels. These options should be explored before selecting a tower. Adequate room for airflow inside screens should be provided to prevent recirculation. The bottom of many towers, especially larger ones, must be set on a steel frame 1 to 1.5m above the roof to allow room for piping and proper tower
20、 and roof maintenance. Pumps below the tower should be designed for an adequate net positive suction pressure, but they must be installed to prevent draining the piping on shutdown. The tower must be winterized if required for cooling at outdoor temperatures below 2 . Winterizing includes the abilit
21、y to bypass water directly into the tower basin or return line (either automatically of manually, depending on the installation) and to heat the tower pan water to a temperature above freezing. Heat may be added by steam of hot water coils of by electric resistance heaters in the tower pan. Also, an
22、 electric heating cable on the condenser water and makeup water pipes and insulation on these heat-traced sections is needed to keep the pipes from freezing. Special controls are also required when a cooling tower is operated near freezing conditions. Where the cooling tower will not operate in free
23、zing weather, provisions for draining the tower and piping must be made. Draining is the most effective way to prevent the tower and piping from freezing. Careful attention must also be given to water treatment to minimize the maintenance required by the cooling tower and refrigeration machine absorbers and /on condenser.
