ASHRAE CLIMATIC DESIGN CONDITIONS (2009)

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CLIMATIC DESIGN CONDITIONS

Annual Design Conditions

Annual Heating and Humidification Design Conditions.

Annual Cooling, Dehumidification, and Enthalpy Design Conditions.

Extreme Annual Design Conditions.

Monthly Design Conditions

Temperatures, Degree-Days, and Degree-Hours.

Monthly Design Dry-Bulb, Wet-Bulb, and Mean Coincident Temperatures.

Mean Daily Temperature Range.

Clear-Sky Solar Irradiance.

The top part of the table contains station information as follows:

Annual Design Conditions

Annual climatic design conditions are contained in the first three sections following the top part of the table. They contain information as follows:

Annual Heating and Humidification Design Conditions.

Annual Cooling, Dehumidification, and Enthalpy Design Conditions.

Extreme Annual Design Conditions.

Monthly Design Conditions

Monthly design conditions are divided into subsections as follows:

Temperatures, Degree-Days, and Degree-Hours.

Monthly Design Dry-Bulb, Wet-Bulb, and Mean Coincident Temperatures.

These values are derived from the same analysis that results in the annual design conditions. The monthly summaries are useful when seasonal variations in solar geometry and intensity, building or facility occupancy, or building use patterns require consideration. In particular, these values can be used when determining air-conditioning loads during periods of maximum solar radiation. The values listed in the tables include

For a 30-day month, the 0.4, 2.0, 5.0 and 10.0% values of occurrence represent the value that occurs or is exceeded for a total of 3, 14, 36, or 72 h, respectively, per month on average over the period of record. Monthly percentile values of dry- or wet-bulb temperature may be higher or lower than the annual design conditions corresponding to the same nominal percentile, depending on the month and the seasonal distribution of the parameter at that location. Generally, for the hottest or most humid months of the year, the monthly percentile value exceeds the design condition for the same element corresponding to the same nominal percentile.

A general, very approximate rule of thumb is that the n% annual cooling design condition is roughly equivalent to the 5n% monthly cooling condition for the hottest month; that is, the 0.4% annual design dry-bulb temperature is roughly equivalent to the 2% monthly design dry-bulb temperature for the hottest month; the 1% annual value is roughly equivalent to the 5% monthly value for the hottest month, and the 2% annual value is roughly equivalent to the 10% monthly value for the hottest month.

Mean Daily Temperature Range.

These values are useful in calculating daily dry- and wet-bulb temperature profiles. Three kinds of profile are defined:

Clear-Sky Solar Irradiance.

Clear-sky irradiance parameters are useful in calculating solar-related air conditioning loads for any time of any day of the year. Parameters are provided for the 21st day of each month. The 21st of the month is usually a convenient day for solar calculations because June 21 and December 21 represent the solstices (longest and shortest days) and March 21 and September 21 are close to the equinox (days and nights have the same length). Parameters listed in the tables are

Temperature is the measure of the intensity or level of heat.

Dry Bulb Temperature (DB, db, DBT, dbt) is the temperature registered by an ordinary thermometer. db represents the measure of sensible heat, or the intensity of heat.

Wet Bulb Temperature (WB, wb, WBT, wbt) is the temperature registered by a thermometer whose bulb is covered by a wetted wick and exposed to a current of a rapidly moving air having a velocity of around 5 m/s. WB is measured by a sling psychrometer which has a set of dry and wet bulb thermometers. The psychrometer is whirled at such revolutions per second that the velocity of the bulb will be 5 m/s approx (in still air).

Relative Humidity (RH, rh); (expressed in percentage) is the ratio of actual partial pressure of water vapour to its saturation pressure corresponding to the same db. Alternate definitions are – ratio of amount of moisture present in the air to the amount the same air holds at saturation at the same temperature, It indicates the ability of air to absorb additional moisture.

Dew Point (DP, DPT) is the temperature at which water vapour in moist air starts condensing when it is cooled.

Humidity Ratio is the weight of water contained in the air per unit of dry air. This is often expressed as kgs of moisture per kg of dry air or grams of moisture per kg of dry air (g/kg).

Specific Volume is the cubic meter of moist air per kg of dry air represented as m3/kg.

Enthalpy is the heat energy content of moist air. It is expressed in kJ/kg and represents the heat energy due to temperature and moisture in the air. Lines of constant enthalpy (OH in Figure A) run diagonally downward from left to right across the chart. Lines of constant enthalpy and constant wet-bulb temperature lie close to each other. Accordingly, they coincide in many elementary charts, but with both values being indicated on the common line.