In principle, the heating and cooling loads
are calculated to maintain the indoor design conditions when the outdoor weather data do
not exceed the design values.
2.1 Outdoor Design Conditions
It is not economical to choose either the
annual maximum or annual minimum values of the outdoor weather data in determining the
outdoor conditions. The outdoor design data is usually determined according to the
statistical analysis of the weather data so that 1 to 5% of the total possible operating
hours is equalled or exceeded the outdoor design values.
2.1.1 Summer Design Condition
The recommended summer design and
coincident wet bulb temperature, when chosen as being equalled to or exceeded by 2.5% of
the total number of hours (i.e. 2928 hours) in June, July, August and September, are
(i) 23 oC dry bulb temperature, and
(ii) 28 oC wet bulb temperature
Figure 2 shows the outdoor dry bulb
temperature and wet bulb temperature curves for a typically hot summer day in Hong Kong.
Usually the maximum temperature of 33 oC occurs at 2 p.m. and the minimum temperature of
28 oC occurs just before sunrise. The daily range of dry bulb temperature is about 5 to 6
oC, and the daily mean dry bulb temperature is 30.5 oC.
2.1.2 Winter Design Condition
The recommended winter design and
coincident relative humidity, when chosen as being equalled to or exceeded by 1% or 2.5%
of the total number of hours (i.e. 2160 hours) in December, January and February, are
(i) 9 oC dry bulb temperature, and
(ii) 50% relative humidity
Minimum temperature occurs at 6 a.m. or 7
a.m. before sunrise and the daily range is about 6 to 8 oC during very cold winter days.
2.2 Indoor Design Conditions
For most of the comfort air-conditioning
systems used in the commercial and public buildings, the recommended indoor temperature
and relative humidity are as follows:
(i) Summer: 23.5 - 25.5 oC dry bulb
temperature, 40 - 60 % relative humidity
(ii) Winter: 21 - 23.5 oC dry bulb
temperature, 20 - 30 % relative humidity