Handbook of air conditioning and refrigeration / Shan K

Summer and Winter Outdoor Design Conditions
In Table 4.10, note the following:
● Summer outdoor design dry-bulb temperature for a specific locality To,s, in °F (°C), is the rounded
higher integral number of the statistically determined summer design temperature To,ss such that
the average number of hours of occurrence of outdoor temperatures To higher than To,ss annually
is on average 0.4 percent (35 h), 1 percent (88 h), or 2 percent (175 h).
● The summer outdoor mean coincident design wet-bulb temperature T�o, s, in °F (°C), is the mean
of all wet-bulb temperatures occurring at the specific summer outdoor design dry-bulb temperature
To,s during the summer.
● The 1.0 percent summer design wet-bulb temperature T�o1 is the design value having an average
annual occurrence of T�o � T�o1 of 88 h. Variable T�o1 is used for evaporative cooling systems,
cooling towers, and evaporative condensers, which are covered in later chapters.
● The mean daily range, in °F (°C), indicates the mean of the difference between the daily maximum
and minimum temperatures for the warmest month.
● The winter outdoor design dry-bulb temperature To,w, in °F, is the rounded lower integral value of
the statistically determined winter outdoor design temperature To,ws, such that the annual average
number of hours of occurrence of outdoor temperature at values To � To,ws should be equaled or
exceeded 99.6 or 99 percent of the total number of annual hours. The annual average number of
hours in winter when To � 0.99To,ws is 35 h, and To � 0.99To,ws is 88 h.
● The number of degree-days is the difference between a base temperature and the mean daily outdoor
air temperature for any one day Tbase � To,m, both in °F. Annually, the total number of heating
degree-days with a base temperature of 65°F, or HDD65, is
HDD65 � � (65 � To, m)
(4.32)
n�1
where n � number of days whose To,m � 65°F per annum. The total number of cooling degreedays
with a base temperature of 50°F, or CDD50, is
CDD50 � � (To, m � 50)
(4.33)
m�1
where m � number of days whose To,m � 50°F per annum. Heating and cooling degree-days with
different base temperatures have been used as climatic parameters to calculate the energy flux
through building envelope, or to determine the U value and the configuration of the building
envelope.
● In the last two columns, MWS/MWD to DB 99.6% indicates mean coincident wind speed
(MWS)/mean coincident wind direction (MWD), i.e., most frequently occurring with the heating
dry-bulb 99.6 percent. In MWD, wind direction is expressed in degrees; 270° represents west, and
180° south.
The Use of Outdoor Weather Data in Design
INDOOR AND OUTDOOR DESIGN CONDITIONS 4.39
During the design of air conditioning systems, the following parameters are often adopted:
1. Indoor and outdoor design conditions are used to calculate the space cooling and heating loads.
2. Summer outdoor dry-bulb and coincident wet-bulb temperatures are necessary to evaluate the
coil load. The summer outdoor wet-bulb temperature is used to determine the capacity of the
evaporative coolers, cooling towers, and evaporative condensers.