Download AG31-004 - School HVAC Design Manual by McQuay PDF

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Be careful not to oversize the terminal device with VAV systems. Figure 36, Central System Psychrometrics To calculate the required supply air in the space, the internal heat gain must first be calculated. Figure 36 shows the psychrometric process for a typical classroom. Although the supply air is cooled to 55°F off the coil, the fan work (assuming a draw-through unit) will raise the supply air temperature delivered to the classroom to about 57°F. This leaves an 18°F delta T to absorb the sensible heat in the classroom.

Outdoor Air Psychrometrics Since the outdoor air load represents a large portion of the school heating and cooling load, and it has the potential to introduce large amounts of moisture into the school, the psychrometrics must be clearly understood. For example consider a schoolroom located in Miami, Florida. The ASHRAE design conditions are 90°F db and 79°F wb. 5°F wb (50% RH). 5 Ton C ooling U nit OA R /A EA 75 F 50% R H 32 Application Guide AG 31-004 Figure 24, Outdoor Air Supplied at 75° F db One way to achieve the design conditions is to cool the outdoor air to 75°F db with either DX or chilled water coils.

Alternatively, reheat coils at the VAV boxes can be used. Care should be take to ensure condensation along the ductwork won’t occur if the supply air is not heated until the VAV boxes. In colder climates, coil freeze-ups are another issue, particularly with rooftop equipment. Antifreeze, pumped coils and face and bypass coils are options that can be considered to protect the coils from freezing. Larger air handling systems will require a return fan to maintain proper building pressurization. For rooftop equipment, the return fan should be integral to the unit.

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