As a rule of thumb, the installer should provide ventilation air flow of about 8 cfm (.22656 m3/min) per installed horsepower. If combustion air is to be drawn from the engine room increase that figure to 91/4cfm (.26196 m3/min).
If you wish to compute more exact engine room air requirements it is necessary to determine the following factors:
H = Heat radiated to the engine room
This data is available from the TMI system for Caterpillar engines. Add in 4 Btu/min per generated 0.07032 kW for the normal maximum auxiliary generator load. Miscellaneous heat loads from other sources (pumps, motors, etc.) can be ignored if they are not exceptional.
Ta = Maximum ambient air temperature the vessel is expected to operate in during its whole life. [Usually assume 110°F (43.3°C).]
Sa = Density of the air at the maximum ambient air temperature.
Density of Air at Various Temperatures
dT = Maximum desired air temperature in the engine room. (Usually assume 10°F (5.6°C) rise above ambient)
When these factors have been determined, the ventilation air requirements in cubic feet per minute (cfm) can be calculated by the following formula:
Qa = Volume of inlet air required in cfm (m3/min)
H = Radiated heat [btu/min (kW)]
Sa = Inlet air density [lbs/cu. ft. (kg/m3)]
0.24 = Specific heat of air (btu/lbs/°F)
0.017 = Specific heat of air (kW·min/kg·°C)
dT = Temperature rise from ambient air to engine air [°F(°C)]