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hot and dry regions of the middle east and its effect on size and
type of cooling and heating systems have been not specially per-
formed. In such regions, humidity increase caused byWBT changes
hasmore undesirable results; in that, in the dry area, required cool-
ing for buildings can be provided by direct evaporative coolers,
considering very low energy, cannot meet the proper comfort-
able conditions in humid area. Correspondingly, high consumptionchillers, comparedto the evaporative cooling systems,have become
unavoidable.
In spite of variations from country to country, about 40% of
total energy demand is utilized in buildings. There are strongly
weather changes in recent years in this area. Due to considerable
variations of weather conditions in recent years, outdoor design
conditions, based for calculation of cooling and heating loads, dif-
fer significantly. Hence, selecting of appropriate design conditions
has a critical effect on design of HVAC systems.This paper presents how outdoor design conditions change in
recent 40 years (1967–2006) for Tehran as a most important city
in the hot and dry region of Iran. Since any documents according
to new version of ASHRAE in Iran had not been done, the outdoor
design conditions are obtained by the authors.
Moreover, the variations of someparameters suchas cooling and
heating degree-hours, design DBTs andWBTs for summer andwin-
ter during thementioned period are evaluated. The effects of these
parameters on energy consumption and design of cooling equip-
ment are also considered. In other words, considering the effect of
outdoor design conditions on selecting and sizing of cooling equip-
ment is the main goal of this paper. Finally, based on variations
obtained for last years, climatic change and energy demand for
further decades are predicted.
2. Climatic data
Tehran, the capital of Iran, locates in longitude 35.41◦E, latitude
51.19◦N with elevation 1190m. Based on outdoor design condi-
tions, there are five different regions in Iran (Hot & Dry, Hot &
Humid, Hot & Semi-Humid, Moderate & Dry, Moderate & Humid).
As shown in Fig. 1, Tehran is located in the hot & dry region, the
largest climatic zone of Iran. This city, with approximately eight
million populations, includes about 10% of the total number of
people living in Iran.
According to 2005 ASHRAE Handbook-Fundamentals, annual
simple climatic design values are listed as:
• 99.6% and 99% heating DBT;
• 0.4%, 1% and 2% cooling DBT;
• 0.4%, 1% and 2% evaporationWBT.
To calculate design conditions, long-term hourly weather data
are required. In this paper, hourly DBT and WBT from the Islamic
republic of IranMeteorological Organization (IRIMO)were used for
the calculation of design conditions.In the previous edition of ASHRAE Handbook [10], design con-
ditions were calculated using joint frequency matrices (including
DBT and dew-point temperature). In thismethod, DBT is not taken
into account where the dew-point temperature is not recorded.
Therefore, the 2005 edition of ASHRAE Handbook [11] used fre-
quency vectors with which calculate the distribution functions for
variables inpidually.
By counting the number of the hours (Nbin) within each tem-
perature interval (bin), frequency vector is formed. The considered
period is recent 40 years from 1967 to 2006 in four decades due to
more precise comparison. Based on binmethod, averaging over the
years considered, the numbers of the hours are obtained in eight
daily 3 h shifts (1–3, 4–6, 7–9, 10–12, 13–15, 15–17, 18–21, 22–24).
Distribution function is provided by the cumulative sumof the fre-