AHU – Air Conditioner

To determine the system’s air-handling requirement, the designer must consider the function and physical characteristics of the space to be conditioned and the air volume and thermal exchange capacities required. Then, the various components may be selected central system—equipment must be adequate, accessible for easy maintenance, and not too complex in arrangement and control to produce the required conditions. Further, the designer should consider economics in component selection. Both initial cost and operating costs affect design decisions. The designer should not arbitrarily design for a 2.5 m/s face velocity, which has been common for selection of cooling coils and other components. Filter and coil selection at 1.5 to 2 m/s, with resultant lower pressure loss, could produce a substantial payback on constant volume systems. Chapter 35 of the 1999 ASHRAE Handbook—Applications has further information on energy and life-cycle costs.

Figure 1 shows a general arrangement of the air-handling unit components for a single-zone, all-air central system suitable for year-round air conditioning. With this arrangement, close control of temperature and humidity are possible. All these components are seldom used simultaneously in a comfort application. Although Figure 1 indicates a built-up system, most of the components are available from many manufacturers completely assembled or in subassembled sections that can be bolted together in the field. When selecting central system components, specific design parameters must be evaluated to balance cost, controllability, operating expense, maintenance, noise, and space. The sizing and selection of primary air-handling units substantially affect the results obtained in the conditioned space.

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