HEAT LOSS THROUGH CONVECTION

Convection is the transfer of heat from solid surfaces by moving liquids and gases—our pie cooled faster because the air surrounding it moved up after being heated and the replacement air was cooler and therefore absorbed more heat from the pie. There are two types of convective transfer: free, or natural, convection and forced convection. Free convection is due to the fluid’s change in density as its temperature changes. Air warmed over a hot surface, such as a radiator, becomes less dense and rises. Forced convection takes place when a fluid is forced, by a fan or pump, past a surface at a different temperature than the fluid. Forced convection is the mechanism of heat transfer in an automobile heater. A fan blows air over tubes heated by water from the car’s engine. Forced convection can move more heat than free convection for a given temperature difference, which is why small electric space heaters have a fan.

The equation describing convective heat transfer is similar to that describing conductive heat transfer, but we use another coefficient instead of “k,” and there is no thickness to consider.

The equation is as follows.

Qc = hc ■ A (Ts – Tf)

Where

Qc = the rate of heat transfer hc = the average convection heat transfer coefficient A = the surface area in contact with the fluid Ts = the surface temperature Tf = the fluid temperature

For air in free convection the convective heat transfer coefficient is between 5 and 30 watts 2 2 per meter per degree Celsius and about 3 BTUs per second per feet per degree Fahrenheit.

Table 7 to this article list some convective heat transfer coefficients.

TABLE 7

Convective heat transfer

Wind Speed (MPH)

Convective Heat Transfer Coefficient

(Btu/(hr x sq ft x °F))

0

1

2

1.6

4

2.2

6

2.8

8

3.4

10

4

12

4.6

14

5.2

16

5.8

So why do we care? One example is heat loss from a swimming pool. A comfortable temperature for the water might be 25 °C. The pool might be exposed to moving air at 10 °C, corresponding to a cool breeze blowing over it. We will take the heat transfer coefficient to be 25 W/m °C. The following equation gives us the heat lost per second from each square meter of water surface.

q/A = 25 (25 – 10) = 375 Watts/square meter

The water surface area of a small swimming pool might be 10 meters by 5 meters. In this case, the pool loses 18.75 kilowatts to the wind—equivalent to about 187 lightbulbs—which is why swimming pool heaters use much energy. The situation is actually worse because only about half the energy from a pool is lost by convection. Convective heat transfer is also important when estimating heat loss from a window or when calculating the efficiency of a solar collector.

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