Handbook of air conditioning and refrigeration / Shan K

3.46 CHAPTER THREE
3.12 HEAT EXCHANGE BETWEEN THE OUTER BUILDING
SURFACE AND ITS SURROUNDINGS
Because atmospheric temperature is lower at high altitudes, there is always a radiant heat loss from
the outer surface of the building to the sky vault without clouds. However, it may be offset or partly
offset by reflected solar radiation from the ground on a sunny day. Radiant heat loss from the building
needs to be calculated during nighttime and included in year-round energy estimation.
In commercial and institutional buildings using glass, concrete, or face brick on the outside surface
of the building envelope, the migration of moisture through the glass pane is rather small.
Because of the heavy mass of the concrete wall, the influence of the diurnal cyclic variation of the
relative humidity of outdoor air on moisture transfer through the building envelope is also small.
Therefore, for simplicity, the moisture transfer between the building envelope and the outside air
can be ignored.
The heat balance at the outer building surface, as shown in Fig. 3.17, can be expressed as
follows:
Q sol � Q ref � Q os � (Q rad � Q at) � Q oi
(3.91)
In Eq. (3.91), Q sol represents the solar radiation absorbed by the outer surface of the building envelope,
in Btu/h (W). It can be calculated as
Q sol � � os [A s (I D � I d) � A sh I d]
(3.92)
where � os � absorptance of the outer surface of the building envelope. From Eq. (3.61), the reflection
of solar radiation from any reflecting surface to the outer surface of the building and absorbed
by it, or q ref, Btu/h (W), is given by
Q ref � A� os I ref � A� s� os F sr (I D � I d)
(3.93)
where A � total area of the outer surface of the building envelope, A � As � Ash, ft . The
term qos indicates the convective heat transfer from the outer surface of the building outward, in
2 (m2 )
FIGURE 3.17 Heat balance at the outer surface of a building.