CATEGORIES FOR ARTICLE LINKS BELOW

Effects of Radiator Design on the Cooling System

A cooling system whose heat load and coolant flow rate results in a 10 degree F coolant temperature drop through the radiator will have that same coolant temperature drop w

hether the radiator has a very small face area and flat fins or a very large face area and louvered fins. The difference is that the large louvered fin radiator will be more effective than the small radiator at transferring heat to the cooling air, meaning that it can do it with a much lower difference in temperature between the core and cooling air. The small radiator may require such a high difference in temperature between the core and the cooling air and the core that the coolant may reach boiling temperature before the core is able to transfer all of the heat load to the cooling air. While both radiators would have the same coolant temperature drop through the radiator, we would say that the larger radiator had better heat transfer performance if its top tank temperature (Inlet coolant temperature) stabilized at, say, 180 degrees F while the smaller radiator stabilized at 220 degrees F.

Improving an Overheated Cooling System

Armed with this understanding of how a cooling system works what recommendations should we make for a cooling system that is overheating? Suppose we have an engine and cooling system that, in stock condition, produced a rated 200 hp and ran at rated ambient temperature with a top tank temperature of 190 degrees F and a 10 degree F temperature drop through the radiator. Now suppose the engine were modified to produce 240 hp, a 20 percent increase. We would find that at 240 hp the core temperature drop had increased by 20 percent to 12 degrees F and the top tank temperature had increased, let’s say to the point where it was just overheating. Now suppose we take this system and reduce the power to the point where the radiator inlet, or top tank temperature is steady at 190 degrees F. (Guess what? It’ll be producing 200 hp! Funny, how that works). So we check coolant temperature drop and find it is back to 10 degrees F, as we would expect, meaning the average core temperature is 185 degrees F. Now we want to make improvements to the system in order to lower the top tank temperature to the point where we can then go back to 240 hp without the engine overheating.

Coolant Flow Rate

Looking at the previous expression, we can see that slowing the coolant down is the wrong way to go. If the heat load is constant, lowering the flow will increase the temperature drop through the radiator, making the bottom tank, or radiator outlet, temperature less than before. If the bottom tank temperature goes down, the top tank temperature must go up to maintain approximately the same average core temperature so that the heat load may be transferred to the cooling air. At the reduced power setting it would rise above 190 degrees F and at 240 hp the engine would be overheating worse than before. In fact, because the lower flow rate results in lower coolant velocity and less “scrubbing action” in the tubes, the average coolant temperature must rise slightly in order to transfer the heat load from the coolant to the cooling air, making matters even worse.

What would happen if we increase the coolant flow? Will it go through the radiator so fast that there won’t be time for cooling to take place? Not at all, from the expression, we can see that if the heat load is constant, increasing the coolant flow rate will reduce the coolant temperature drop through the radiator, resulting in a higher bottom tank temperature. If the bottom tank temperature is increased, the top tank temperature must go down to maintain approximately the same average core temperature. This is what we were hoping to achieve. With the top tank temperature now less that 190 degrees F at the reduced power point, we can expect that the system will be better able to run at 240 hp without overheating, In fact, because the increased coolant flow rate results in a higher coolant flow velocity and better “scrubbing action” in the tubes, the average coolant temperature decreases slightly while transferring the same heat load to the cooling air, further lowering the top tank temperature, resulting in better cooling performance.

From this we see that increasing the coolant flow rate will result in better heat transfer performance.There are some cautions to be observed in increasing coolant flow rate, however. Going too far may result in aeration and foaming of the coolant, possible damage to the radiator by overpressure, cavitation of the pump, due to excessive pressure drop through the radiator, and erosion of the radiator tubes. The ideal coolant flow rate is one that will provide optimum coolant flow velocity through the radiator tubes in the range of 6 to 8 feet per second. Flow velocities above 10 feet per second should be avoided.

- IMPROVEMENT RULE # 1 -Anything you can do to increase the coolant flow rate, within limits described, will improve heat transfer and cooling performance. Anything you do to restrict or reduce the coolant flow rate will hurt cooling performance

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