Building Services Engineering 5th Edition Handbook

162 Ventilation and air conditioning
40.0
35.0
30.0
Temperature (°C)
25.0
20.0
15.0
10.0
5.0
0.0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
predicted outdoor measured outdoor
Time (h)
measured indoor glass temperature
5.21 Temperature profiles: south-facing office, Sunday 27 June 1993.
outdoor air, indoor air and the internal surface temperature of the outer pane of the doubleglazed
window, hourly. Figure 5.21 shows that the measured outdoor air temperature follows
the general shape of the predicted sine wave. The thermocouple that was adhering to the glass
showed a combination of two factors: first, that the air temperature in the cavity between the
panes of double glazing rose to 35 ◦ C; second, that the glass absorbed some of the incident
solar radiation and was raised in temperature. The internal room air temperature was measured
in a shaded location at just above desk height. The room air temperature remained between
25 ◦ C d.b. and 31 ◦ C d.b. During normal use, the same office produced an internal air temperature
of 25 ◦ C d.b. when the outdoor air temperature peaked at 27 ◦ C d.b.
While such an example does not prove conclusively that all south-facing rooms in the UK need
to be air conditioned for human thermal comfort, it does give some evidence to strengthen the
argument in favour of mechanical cooling. Working in air temperatures that move above 24 ◦ C
d.b. in naturally ventilated spaces that have significant solar radiation can be noticeably uncomfortable.
Whether the performance of human productivity or effectiveness becomes impaired is
arguable. Low-cost cooling systems can be designed that make use of cool parts of a building
to lower the temperature of the areas that are exposed to solar radiation. Heat pump systems,
mechanical ventilation and evaporative water-cooling towers can be used to limit room
air temperatures, without the need to involve high-cost refrigeration equipment.
EXAMPLE 5.11
A south-facing office in Basingstoke has natural ventilation and a large area of glazing.
The maximum and minimum outdoor air temperatures are expected to be 26 ◦ C d.b. and
12 ◦ C d.b. on a summer day. Calculate the outdoor air temperature that is expected at
1600 h. Find the indoor air temperature that will be generated if solar radiation heat gains
raise the indoor air by 1.5 ◦ C from that of the outdoor air. Comment upon the thermal
comfort conditions that are provided for the office and make recommendations.