Weather Report: February 2002
Design Conditions In 2001Continuing with our evaluation of the last year's data, it's always interesting for design engineers to see how closely (or not) actual data measures up to the design conditions defined by the 1997 ASHRAE Fundamentals. Figure 2 provides the number of hours in 2001 that either meet or exceeded the cooling, wet bulb, and heating design conditions.
In general, the data shows that Southern and Western cities experienced fewer cooling design hours than normal and major cities in the northern and central regions experienced above-normal design cooling seasons. The design heating hours were low in most cities. In fact, the 99.6% heating design threshold was never reached in six of the reported cities.
The data also indicates a glaringly high number of wet bulb design hours in 2001 compared to ASHRAE design for most of the country. This would particularly impact the cooling capacity and operational performance of cooling equipment selected using the 1.0% and 2.0% design conditions.
For example, let's say an engineer has selected a cooling tower for a project in Miami using the ASHRAE 2.0% wet bulb design criteria of 78 degrees F. The engineer assumes the system would experience approximately 175 hours per year above design because 2.0% of 8,760 total hours/year is 175.
However, in 2001 there were 1,134 hours above 2.0% design in Miami. That's 650% more hours than normal, or 6.75 weeks in total. In fact, 175 of those hours (normally 35 of 8,760) were actually above the 0.4% threshold of 80 degrees and there is a big difference between 78 degrees and 80 degrees wet bulb when selecting cooling towers.
For such a system, 2001 could likely have been a long, hot, and humid year. The design conditions defined in ASHRAE are based on the average of multiple years of data, and it is important to know how drastically different actual data can be from the design data from year to year. If cooling towers are undersized, potentially disastrous problems can occur when warm condenser water is delivered to centrifugal chillers trying to maintain the desired chilled water temperature setpoint.
For systems that heat and cool large amounts of outside air, more hours of higher-than-expected temperature and humidity can result in cooling capacity and IAQ problems. It wouldn't be a bad idea to take look at actual data from recent years as a sanity check, especially when dealing with close tolerances. Such a task is much more fun during the design rather than after the project is built. ES
EDITOR'S NOTE: The images associated with this article do not transfer to the Internet. To review the figures, please refer to the print version of this issue.