Convection heat loss

DoExercise B before continuing with this sub-section.

Exercise B

1. How much air flows up the chimney per hour from an open fire if the speed of the flow is 1 m/s and the chimney opening is 0-3 m x 0-1 m?

2. How much air flows per hour through a 0-01 m gap under a door 0-8 m wide if it flows at 6 m/s in a high wind? (Think of the feel of the air against you when walking or cycling, to estimate its speed.)

3. How much air flows per hour through an open window 1 m×0-4 m at 1 m/s?

4. If the flows in Questions 1-3 occur in a room 5m×4 m by 2-5 m high, how many times would you estimate the air in the room changes per hour in each case?

Fig. 1.1 Dwelling house Scale 1cm represents 1-43cm

Air flows into a house through open windows, gaps between floor boards around ill-fitting doors, etc., and out through chimneys, ventilators and windows. Warmed air rises, and the air in a house must be warmed for comfort. The flow of warmed air through a house is known as convection and the warm air that flows out causes convection loss of heat. It is difficult to estimate the total flow of air through a house; in an exposed site there may be (on the average) 3 changes of air per hour throughout the house and in a sheltered site, 2 changes.

Example 3

How much heat per hour will be required to heat the air flowing through a house 9-1m×7-6m ×5-2m in which there are 3 air changes per hour and the average difference of inside and outside temperature is 10 degC?

F = 9-l×7-6×5-15m³,

so L = 1-4× (9-1×7-6×5-2) ×3×10 = 14900 mj/h.

Exercise C

1. Estimate the total heat loss per hour by convection from a building 2mx6mx5m, if there are 2 air changes per hour and the difference between inside and outside temperatures is 10 degC.

2. Estimate: {a) the convection loss; (b) the conduction loss of heat from your classroom per hour.

3. Repeat Question 2 but for your house, under the same temperature conditions.

4. When a heating appliance is bought it should be able to supply sufficient heat to meet the worst reasonable conditions. How many kJ/h would be needed to meet a temperature difference of 20 degC instead of the 10 degC allowed for in Questions 2 and 3? What percentage increase is this?

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Conduction heat loss	\|	Radiation losses

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