We never got around to taking a look back at the totals for 2002, so now is as good a time as any, and I thought it would be a good idea to start with ASHRAE design conditions. To recap, the ASHRAE design outdoor air conditions are published in the 1997 ASHRAE Fundamentals Handbook for 1,459 locations around the world. These values are used to determine required heating and cooling equipment capacities for HVAC systems and other energy related processes. The 0.4%, 1.0%, and 2.0% design values for outside air dry bulb and wet bulb temperatures represent the temperatures that are exceeded 0.4%, 1.0% and 2.0% of the year on average, or 35, 88, and 175 hours per year respectively. Similarly, the outside air dry bulb temperature is less than the heating 99.6% and 99% design values 35 and 88 hours per year respectively.
Figures 2 and 3 indicate that the '02 cooling season was very demanding on air-conditioning systems all over the country. The expected 35, 88, and 175 hours for design dry bulb and wet bulb values were exceeded many times over in many locations with the exception of western cities. In some cases, the design wet bulb hours were off the chart. In Miami, there were 775 hours above 2.0% design. That's almost 4.5 times the average of 175 hours!
Higher wet bulb temperatures mean higher cooling ventilation loads. And as some of you have experienced, if the wet bulb temperature is continuously greater than the temperature that a cooling tower was sized for, it could be load-shedding time. Wet bulb temperature is very volatile year to year, and sometimes it's a good idea to take a look at some actual data when designing a ventilation system or selecting a cooling tower, especially in critical design applications.
On the heating side (Figure 4), the number of hours below the heating design values were lower than normal pretty much across the country. Not quite what I expected to see, given what seemed to be a long winter. Note however that Figure 4 reflects the '02 calendar year and does not include the early '03 winter months. ES
EDITOR'S NOTE: Some images associated with this article do not transfer to the Internet. To view the images, please refer to the print version of ES.