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g1. Use the thermal balance equation to discuss the principles of heat transfer.

Thermal balance occurs when the sum of the different types of heat flow in and out of the

building is zero. That is, the building loses as much heat as it gains and therefore the building is

said to be in thermo equilibrium. The thermal balance equation is given by:

Q_c + Q_v + Q_s + Q_i + Q_e = 0

Where

Qc –Conduction Gains

Qv – Ventilation Gains

Qs – Solar Gains

Qi - Internal Gains

Qe - Evaporative Loss

Qc - Conduction Gains

This occurs when heat from outside flows through external building envelop. This occurs mostly

by conduction and to some extent convection. The external envelops are walls, windows, doors

and roofs. Heat flow depends on characteristic of the material making the wall, surface area and

also the temperature difference between the inside and outside. The total heat flow at any

particular time is therefore given by:

Q_c = U A \delta T

where:

Qc = the total instantaneous heat flow in Watts (W),

DT = the instantaneous temperature difference between the inside and outside

A = the surface area of the building element, and

U = the U-value of the building element.

QV- Ventilation gains

This occurs when inside air enters the building. This can be through natural leakage through the

building fabric (infiltration) or by intentional door and window (ventilation)

where:

Qv = Total ventilation gains in watts total

N =the number air changes per hour within the zone,

V = the total internal volume of the zone, and

DT =the temperature difference between the inside and outside.

Qs - Solar Gains

Qs - Solar gains

Solar gains refer to the additional heat flows generated by the sun within the building either

directly through the window or through opaque elements. The sun acts to heat up the exposed

surfaces in case of opaque elements thus increasing the amount of heat flowing though the fabric.

Direct Gains

This is the gains through transparent elements given by:

Q_s = G A sgf

where:

Qs = total direct solar gain in Watts (W),'

G = the total solar radiation incident on the specified window (W/mÃ‚Â²),

A = the surface area of the window in mÃ‚Â², and sgf = the solar gain factor.

The solar gains factor is a function of the type of window and represents the amount of the heat

that actually makes it through the element and into the zone, usually fraction between 0 and 1.

Indirect Gains

This is the gains through opaque elements usually slightly complex because the incident of solar

radiation acts first to increase the external surface temperature. The surface temperature that

results called the sol-air temperature. As this increases the DT value of the conduction gain

component, more heat will flow from outside to in. To isolate indirect gains, it is convenient to

deal with the increased temperature due to solar radiation separately from the outside air

temperature. Therefore; only sol-air excess temperature is used

Q_s = U A (G abs R_{so})

where:

Qs = total direct solar gain Watts (W),

U = the U-V value of the specified element.

G = the total solar radiation incident on a specified window (W/mÃ‚Â²),

A = the surface area of the opaque element in mÃ‚Â²,

abs = the surface absorption of the element and

Rso = the outside air-film resistance.

Surface absorption is determined by color and material, which affects the absorption of solar

radiation. This is simply a fraction between 0 and 1.

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g1. Use the thermal balance equation to discuss the principles of heat transfer. Thermal balance occurs when the sum of the different types of heat flow in and out of the building is zero. That is, the building loses as much heat as it gains and therefore the building is said to be in thermo equilibrium. The thermal balance equation is given by: Q_c + Q_v + Q_s + Q_i + Q_e = 0 Where Qc –Conduction Gains Qv – Ventilation Gains Qs – Solar Gains Qi - Internal Gains Qe - Evaporative Loss Qc - Conduction Gains This occurs when heat from outside flows through external building envelop. This occurs mostly by conduction and to some extent convection. The external envelops are walls, windows, doors and roofs. Heat flow depends on characteristic of the material making the wall, surface area and also the temperature difference between the inside and outside. The total heat flow at any particular time is therefore given by: Q_c = U A \delta T where: Qc = the total instantaneous heat flow in Watts (W), DT = the instantaneous temperature difference between the inside and outside A = the surface area of the building element, and U = the U-value of the building element. QV- Ventilation gains This occurs when inside air enters the building. This can be through natural leakage through the building fabric (infiltration) or by intentional door and window (ventilation) where: Qv = Total ventilation gains in watts total N =the number air changes per hour within the zone, V = the total inte ...
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