SECTION 4 - Ventilation

4-1. Outdoor Installations - Air Cooled Units

• Air cooled unit draws cooling air from different ends of the unit to cool the system, dependent upon the units cooling system design. Check with the generator's manufacturer to determine the optimal cooling method for the system. Factors such as climate and direction of prevailing winds must be considered in an outdoor installation.

• If your generator is expected to be in temperatures lower than -20^oF(-29^oC) consult the generator sets factory, a cold weather package may be required.

The following general rules apply:

• Where strong prevailing winds are anticipated, face the engine end away from the wind.

• Plan the installation carefully to prevent the cooling air vents on the generator from becoming clogged by leaves, grass, snow, etc.

4.2  Indoor Installations - Air Cooled Units

When a generator is installed and operated in an indoor environment, adequate ventilation for heat dissipation and combustion is required. Ventilation is typically done through the use of an air inlet, air outlet/exhaust fan, and/or other ventilation openings.

The following rules apply:

• When ever possible, face the generator air inlet openings away from the wind. The wind can prevent the air intake louver from opening on start up.

• The air inlet must be capable of moving enough air through the room to provide the correct minimum CFM (cubic feet per minute) cooling for generator as specified by the generator's manufacturer. (This means the generator's air inlet opening size will be greater than the generator's room exhaust fan outlet.)

• Locate air inlet, ventilation and air outlet openings in a structure so that already exhausted air will not be drawn back into the building.

• Louvers, screening, expanded metal and other materials used to cover air openings are a restriction to air flow. This restriction must be compensated for by making the air opening size proportionally larger.

• When possible, position the engine end of air cooled generators in line with the air inlet per the manufacturer's recommendation.

• When possible, position liquid cooled engines with the engine end in line with the exhaust outlet per the manufacturer's recommendation. The reason or this recommendation is that the air moving through a liquid cooled system is typically pulled past the engine and through the generator's radiator. The generator's radiator is placed so that the air is ducted out of the generator's room.

4-3. Ventilation Exhaust Fans and Air Inlet Louvers

Some indoor installations may require the use of one or more exhaust fans, to provide adequate ventilation during generator operation.

The following rules apply:

• Exhaust fans must have the proper capacity for the specific application. In addition, the fans must be located so that engine exhaust gases will not re-enter the building.

• Power is required to operate the exhaust fan. Typically, AC is provided from an DC/AC inverter or UPS to operate the exhaust fan and open air inlet louvers.

4-4. Ducting of Air

The engine cooling fan moves a large volume of air during operation. This heated air must be expelled to the outside of any structure housing a generator.

The following rules apply to the ducting of heated engine air out of a building:

• Whenever possible, use no ductwork at all. Simply position the inlet air duct so that air will be drawn directly over the generator and expelled horizontally to the building exterior (outdoors).

• If duct work must be used between the generator installation location and the building air outlet opening, keep such ductwork as short as possible with a minimum number of bends.

• Construct air outlet duct work of self-supported sheet metal.

• Never locate the air outlet opening of a structure close to adjacent buildings or walls as noise is amplified when air is expelled in large volumes.

Figure 7.  Typical Indoor Larger Generator Installation

Roof mount radiators on liquid cooled units is sometimes done where the installation location of the unit can not dissipate the heat generated by the unit during operation.  Roof mount radiators is beyond the scope of this publication.

4-5. Louvers, Screening And Expanded Metal

When louvers, screening or expanded metal are used to cover air openings in buildings housing generators, consideration must be given that these materials do not restrict the free flow of cooling air. Any restriction must be compensated for by making the actual opening size proportionately larger.

Louvers: Either fixed or movable louvers may be installed on the air openings in a structure that houses a generator. The installer must make sure that the total square inches of free air inlet opening is sufficient to limit the heat rise in the room to prevent the room temperature from exceeding the generator 's operating temperature.

Figure 8.  Louver

Find the actual free air opening as follows:

• Multiply the height of a single louvered opening by its width, to find the opening area of one louvered opening.

• Multiply the opening area of the single louvered opening by the total number of louvered openings to find total free air inlet area.

Figure 9.  Screening and Expanded Metal

Screening and Expanded Metal: Screening and expanded metal may be used to cover air inlet and outlet openings in a structure that houses the generator. These materials also offer a restriction to the free flow of cooling air, which must be compensated for by making the actual air opening in the structure proportionally larger. Screening and expanded metal are usually assigned a "free air inlet area" value by the manufacturer, which is given as a percentage.


To find the actual size of the air inlet opening needed, proceed as follows:

• Find the area of the generator's inlet air duct, by multiplying the air duct height by its width.

• Divide the inlet air duct area by the percentage of free air inlet area for the particular screening or expanded metal to be used. The result is the required size of the air inlet opening in the building.

Example 1: If the required inlet air opening area on a particular generator is 400 square inches and the building air opening is to be covered with screening having a 70% "free air inlet area":

Divide 400 square inches by 0.70, to obtain 576 square inches. Actual air inlet opening size in the building should be equal to or greater than 576 square inches. (An opening that measures 24 X 24 inches (576 square inches) would be adequate in this case to achieve the required 1125 Cu. ft/minute (31.86 Cubic Meters/Min.).

Example 2: The required inlet air opening area on a particular generator is 500 square inches and the building air opening is to be covered with screening having a 70% "free air inlet area":

Divide 500 square inches by 0.70, to obtain 714 square inches. Actual air inlet opening size in the building should be greater than 714 square inches. (An opening that measures 27 X 27 inches (729 square inches) would be adequate in this case to achieve the required 2000 Cu. ft/Minute (39 Cubic Meters/Min.).

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Tips on Hooking up a Generator (genset) to a Uninterruptible Power System (UPS)