An exposure results in 200,000 heat units. How long will it take the anode to cool by 50,000 heat units?

• A. 4 minutes
• B. 5 minutes
• C. 6 minutes
• D. 7 minutes

Answer: (B)

Cooling is treated as a steady, linear process in this kind of problem. That means the anode loses heat at a roughly constant rate after an exposure, so the time needed to shed a certain amount of heat is directly proportional to the amount you want to remove.

Here, to cool by 50,000 heat units, you apply the constant rate and do the math: time = heat to remove / cooling rate. If the rate is 10,000 heat units per minute, then 50,000 heat units take 50,000 / 10,000 = 5 minutes. So the anode will cool by 50,000 heat units in 5 minutes.

In other words, this relies on the common simplified linear cooling assumption used in practice problems. If cooling were nonlinear, you’d need a different model or data to determine the time.