Tuesday, January 19, 2010

Calculating heat loss from a box or cylinder

A student writes: "I was thinking back to a lab we had at the ITC facility where we used a formula to determine the amount of energy from a cardboard box that had a 60 watt lamp underneath it. Do you have the specifics for that exercise and the formula used to obtain the results? I was wondering if I could use the formula to calculate temperatures under other types of materials (metal enclosures)."

The “watt loss” or “mystery box” lab uses a cardboard cylinder with a light bulb inside. We change the bulb occasionally, using anything from 25 to 75 watts or so. We also use a spaghetti pot as a cylinder.

If you measure the area of the box or cylinder and also measure the average surface temperature, you can calculate the total wattage. This assumes that there is no loss through the bottom. Not quite true, but we insulate the bottom of the cylinder to minimize heat losses. Since we perform this experiment in still air, we assume all natural convection (not forced). The formulas used can be found in any standard heat transfer or engineering handbook. This information and experiment is covered in our Level II and Level III classes.

The purpose of this lab is to simulate an oil filled circuit breaker, or similar device, that is heating up. Any heat losses through the bottom or by wind simply means that the device is actually generating more watts than you have calculated.

Once you calculate the power in watts, if you measure the current you can calculate the internal resistance. For OCB’s this is measured in micro-ohms.

Bob Madding created an application to calculate the wattage and internal resistance. This has been included on the Utility or Resource CD that we pass out in class. He also wrote a paper on this. It was presented at the 2001 InfraMation conference. The proceedings are available at the ITC web store.

Infrared Training Center