Modern Cooling System Theory - Flexxaire - #1

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Modern Cooling System Theory Summary The demands on engine cooling systems have risen dramatically over the past two decades. Factors that have driven this rising demand are an increase in the number of fluids that need cooling, a push for higher power output, and the implementation of stricter emissions regulations. As both initial capital costs and operating costs continue to increase for cooling systems, so has the awareness of the importance of properly optimizing these systems. Cooling system designers must ensure that engines do not overheat during peak ambient conditions when running at full power, while considering the cost and parasitic power lost to the cooling system. Flexxaire produces variable pitch fan systems that provide total airflow control, allowing cooling system designers to meet the cooling requirements of peak ambient conditions in addition to minimizing the parasitic losses inherent to standard cooling fans. Heat Transfer Fundamentals The science of heat transfer provides the basis for cooling system design. The basic formula of heat transfer is written as: q = m! "T !Cp Where: q = heat transferred m = mass !T = difference in temperature p C = specific heat A fundamental implication of the above formula is that the amount of heat that can be transferred from one thing to another is directly proportional to the difference in temperature between the two things ( !T ). For radiators, as the difference between ambient air and engine coolant temperatures increases, the volume of air required to cool the fluid decreases. This law has a dramatic effect on airflow requirements as the temperature of the air approaches the temperature of the fluids being cooled by the radiator. The heat transfer formula from above can be re-written to apply directly to a radiator as: T Q cfmr ! = Where: r cfm = the required airflow generated by the cooling fan Q = the required amount of heat rejected into the air to maintain proper engine temperature coolant ambient "T = T -T Figure 1 illustrates the relationship between A and !T . As the ambient temperature approaches the coolant temperature, !T decreases and airflow requirements increase. 0 0 20 40 60 80 100 120 140 160 Delta T (°F) Required Airflow (cfm) Figure 1: Relationship between r cfm and !T