ENGINE INSTALLATION & SERVICE HANDBOOK Caterpillar


Combustion Air Formulas

Usage:



If combustion air is to be drawn from the engine room, a slight modification is in order. Since the air used for combustion takes some engine room heat with it, it can be counted partially as ventilation air. This can be added into the calculation by adding about half of the combustion air required (1/2 Qc) resulting in the following equation:

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)]

Qc = Combustion air required in cfm (m3/min)

For combustion air requirement a good rule of thumb is to multiply the horsepower in the engine room by 2.5. Remember to include all engines in the engine room space for this calculation. If you need more exact combustion air figures then you can get that information from the TMI system. However, the 2.5 times rule is usually adequate for sizing purposes.

If the rule of thumb of 8 cfm/.22656 m3/min of air per installed horsepower is applied, the minimum duct cross sectional area (Av) per installed horsepower would be:

Av = 0.6 in2/Hp (3.87 cm2/kW) @

Va = 2000 fpm (609.6 m/min)

Av = 0.9 in2/Hp (5.81 cm2/kW) @

Va = 1200 fpm (365.8 m/min)

If you included combustion air into the ventilation system [used 9.25 cfm (.262 m3/min)]:

Av = 0.7 in2/Hp (4.52 cm2/kW) @

Va = 2000 fpm (609.6 m/min)

Av = 1.0 in2/Hp (6.45 cm2/kW) @

Va = 1200 fpm (365.8 m/min)

Remember air should enter the engine room freely. It is far better to have extra air than not enough. This installation parameter is second only to sufficient liquid cooling capacity in importance. If the rules of thumb are adhered to they will normally be sufficient, however, they are not overly conservative ... Don't Cheat!

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