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The weather data in use earlier, was not the "coincident" data. As an example, for any outside design Dry Bulb temperature, the corresponding Wet Bulb temperature was no the Wet Bulb temperature occurring simultaneously at that Dry Bulb temperature. In all probability, it was also a maximum Wet Bulb temperature for the particular season in question.

For years, Indian HVAC engineers have been using legacy weather data for Indian cities whose origins are not known. In 1999, the Indian Society of Heating, Refrigerating and Air-Conditioning Engineers (ISHRAE) http://www.ishrae.org.in, in association with Tata Energy Research Institute (TERI) http://www.teriin.org/ , developed a set of Indian weather data, based on the concept of "coincident" temperatures, in line with and in the format as published in the ASHRAE 1997 Fundamentals Handbook.

The set of weather data was created in TMY2 format, a commonly used format in the industry, and was developed for use in Building hourly load, and Building energy simulation programmes.

In 2004 the weather data set was converted to the EPW format as is used by EnergyPlus, an energy simulation engine, and uploaded to and made available to EnergyPlus users.

The "mean coincident Wet Bulb temperature", on the other hand, means the wet bulb temperature occurring at that particular dry bulb temperature, and in that particular 5 degree F interval.

For the first time, the research carried out by ISHRAE and TERI provided users with the "coincident" temperatures.

In addition, the design weather data was classified into three categories, viz; 2%, 1% and 0.4%. annual frequency of occurrance.

The design values represent the value that is exceeded on average by the indicated percentage of the total number of hours in a year (8,760). In other words, the 0.4%, 1.0%, and 2.0% values are exceeded 35-, 88-, and 175-hrs per year respectively.

The weather data for a particular city, at 2% annual frequency of occurance means that for 2% of the hours in a year, (i.e., (2/100) * 24 * 365 = 175 hours), the temperature will be above the temperature in question for 175 hours in a year. (A year has 8760 hours).

For example, for the city of Mumbai, the legacy Summer outside design conditions being considered so far have been 95 deg F and 83 deg F. Compare this with the 2% Dry bulb temperature, and Mean coincident wet bulb temperature from the new weather data, which is 92.3 deg F and 75.2 deg F, figures which are considerably lower than the legacy data.

It is also understood that the air-conditioning system would now be designed such that it was likely that for around 2% of the days in a year, the outside conditions would be higgher than the design conditions. This is acceptable practice for comfort air-conditioning and could result ni considerable savings in initial equipment costs and running loads.

If slightly more stringent conditionas are desired, then the 1% values could be considerd, and for very stringent industrial applications, the 0.4% values could be considered.

While earlier, outside design conditions were provided under the sub-heads of "Summer" "Monsoon" and "Winter", these sub-heads do not exist in the new classification based on annual occurances. Instead for calculating instantaneous space loads for Summer outside conditions, one would usually use the Cooling DB temperature / Mean coincident WB temperature while in Monsoon one would use the Cooling WB temperature / Mean Coincident DB temperature. (For Winter one would use the Heating Db temperature / Mean coincident Wet bulb temperature.

Merely following the above over-simplified rule blindly, however, could lead to serious errors in design.

Consider the situation for a 100% outside air application, (or for a treated air application, or even for an application where the outside air ventilation loads are high):

Assuming that the design is being carried out for the city of Mumbai, we list out the enthalpies of air for both the outside design conditons under investigation.


Design Conditions Comparison

City: Mumbai
Annual frequency of occurance: 2%

Design DB with Mean Coincident WB Design WB with Mean Coincident DB



In the first case for Design DB and mean coincident WB, the enthalpy of the outside air is 38.53 Btu/lb, and would be exceeded for a period of 175 hours as explained earlier.

In the second case for Design WB and mean coincident DB, the enthalpy of the outside air would exceed for a far greater number of hours than in the first case.

Had the outside design conditions been selected as the DB temperature and mean coincident WB temperature, there would have been serious under-sizing of the equipment and the systemw would have been pushed to its limits for a far greater number of hours.

Rajeev Kakkar