CATEGORIES FOR ARTICLE LINKS BELOW

The Major Factor Governing Cooling System Heat Transfer

Cooling system heat transfer is governed by a single major factor-the heat load to the cooling system. Under “steady-state” conditions, the heat load to the cooling system (the heat rejected by the engine to the cooling system) will be transferred to the cooling air by the radiator no matter how good or how poor the radiator. So, if both a “poor” radiator and a “good” radiator will both transfer the same heat load to the cooling air, how can we say that one radiator has better heat transfer performance than the other? The answer is that, under “steady-state” conditions, with a “good” radiator in the cooling system, the radiator inlet temperature (Radiator top tank temperature) will stabilize at a lower temperature than a “poor radiator” in place. The “poor radiator may be so poor that its coolant temperature may rise to the boiling point resulting in engine overheating.

Temperature Differential

The difference between the radiator average core temperature and the temperature of the cooling air is the driving force behind the transfer of heat from the coolant to the cooling air. When an engine starts and is run up to rated load, the coolant begins to heat up. When there is no thermostat in the system, the coolant flows from the engine through the radiator and back to the engine. Initially, the coolant and metal in the engine absorb the heat being produced and continue to do so until the temperature of these parts exceeds the cooling air temperature. At this point, heat transfer to the cooling air commences. The coolant temperature continues to rise until it reaches a temperature at which the difference between the radiator average core temperature and the incoming cooling air is great enough to transfer the entire heat load to the air. This then becomes a “steady-state” condition.

Heat Load to the Cooling System

The heat load to the cooling system is related to the flow through the radiator and the temperature drop through the radiator by the following expression:

Q = M * cp *dT

Where Q is the heat load BTU/min., M is the mass flow rate of the coolant in BTU per pound per degree F, dT is the temperature drop through the radiator in degrees F, and * indicates multiplication. Since a gallon of coolant weighs about 8.3 pounds, we can replace M in the expression by 8.3 times the coolant flow in gallons per minute, or GPM. The resulting expression is as follows:

Q = 8.3 * GPM * cp * dT

Since the specific heat of the coolant is essentially constant and the coolant flow rate

is constant at rated engine speed, the expression tells us something that surprises most people. That is, for a given heat load and coolant flow rate, the coolant temperature drop through the radiator will be constant, and nothing anyone can do to the design of the radiator can change that. Adding rows or fins or face area or whatever will not change the temperature drop through the radiator. As a general rule, cooling systems are designed to operate with a coolant temperature of about 190 degrees F at the radiator inlet and have about a 10 degree F temperature drop through the radiator at rated power and rated coolant flow. This will result in a bottom tank temperature of 180 degrees F.

Note that the coolant temperature drop through the radiator must be specified in degrees F or degrees C, not percent. Taking a percentage of the radiator inlet temperature will yield different results depending on whether the inlet temperature is given in degrees F or degrees C.

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