7.15.3 Evaporative Cooling

Evaporative cooling may be used where the facility in question is eligible for air-conditioning, and evaporative cooling can provide the required indoor design conditions based on the appropriate outdoor design conditions. For special applications where close temperature or humidity control is required, two-stage evaporative cooling or indirect evaporative cooling should be considered in life-cycle cost analysis as a supplement to, not in lieu of, a primary cooling system.

7.16 Ventilation and exhaust systems

The design of all systems should comply with the ASHRAE handbook, ASHRAE Standard 62, and the requirements of NFPA Standards Nos. 90A, 90B, and 91. Motorized low-leakage dampers, with blade and jamb seals, should be provided at all outside air intakes and exhausts.

7.16.1 Supply and Exhaust Fans

Exterior wall and roof-mounted supply or exhaust fans should be avoided; connect interior fans with ductwork and louvers.

Except for interior wall-mounted propeller units, all fans should be centrifugal type and connected directly to weatherproof louvers or roof vents via ductwork.

• Fans larger than 2000 CFM (944 1/s) should be provided with V-belt drives.

• Care should be taken to prevent the noise level generated by exhaust fans and associated relief louvers from being transmitted to the exterior of the building. Any in-line fans located outside the main mechanical and electrical areas should be provided with acoustical enclosures to inhibit noise transmission to the adjoin­ ing occupied spaces, depending on occupant use.

Where possible, exhaust fans in all buildings in housing, recreational, hospital, and administrative areas should be of the centrifugal type, discharging through louvers in the side wall of the building using ductwork, as necessary. Roof-mounted fans of the low-silhouette type may be used.

Centrifugal type roof exhausters should be used in shop, flight line, or ware­house areas. Where exhaust ventilating fans or intakes are provided in buildings, a positive means (gravity dampers are not acceptable) of closing the fan housing or ducts should be provided to prevent heat loss in cold weather, except as prohibited by NFPA Standard 96.

7.16.2 General Items

Incorporate the following:

• Ventilation for VAV systems will ensure proper ventilation rates at low and high system airflow.

• Year-round supply (makeup) air should be provided to equal the total quantity of all exhaust hoods.

• Where desirable, incorporate a purge mode into system design. This mode could be used, for example, to purge the building with outside air during off-hours or to purge the affected zone during building maintenance, such as painting.

• The toilet rooms and janitor closet should be exhausted at a rate of 2 CFM/ft² (10 1/s/m²) by insulated in-line fans to maintain a negative room pressure. The required makeup air for the exhaust system should be from undercut doors or, if necessary, through door grilles. Exhaust registers, in lieu of grilles, should be pro­ vided in areas with rigid ceilings.

• Shower areas have a 2.5 CFM/ft² (131/s/m²) exhaust rate to maintain a negative room pressure.

• Where practical, photocopiers, laser printers, and print equipment should be located in a separate room. Copy rooms with photocopiers and laser printers should not be directly conditioned, but should be maintained at a negative pres­ sure relative to adjacent areas by exhausting air from these adjacent areas directly to the outdoors. All conditioned supply air to the room should be exhausted and not returned to the air-handling unit system due to contaminants.

• Mechanical and electrical equipment rooms should be ventilated and cooled with outside air by thermostatically controlled fans set to operate when the tempera­ ture exceeds 85°F (29°C).

• The boiler room should be ventilated and cooled with outside air at a minimum rate of 20 air changes/h by a thermostatically controlled supply or exhaust fan set to operate when temperature exceeds 85°F (29°C). Supply fans should be used when atmospheric burners are permitted.

The fire protection room should be ventilated and cooled with outside air by

a thermostatically controlled fan set to operate when the temperature exceeds

85°F (29°C).

Provide exhaust fans in laundry rooms sized for a minimum of 3-min air changes.

Automotive maintenance shops must be provided with a suitable engine exhaust

ventilating system. General ventilation should be provided at 1.5 CFM/ft²

(81/s/m²) of outside air.

Battery rooms should be ventilated at a rate of four air changes per hour.