Thursday, July 7, 2011

Rules of Thumb

Over the years, Engineers develop “rules of thumb” for quick estimates of engineering issues. We manufacturers often do the same. Here are a few of the ones I use in looking at air distribution issues:

Buildings cost about $2/sf/year for energy. This is of course, a wild guess. I’ve been using it for 20 years. It is likely less than this in modern buildings, but I’m not getting much push back from engineers. If you take 1 watt/sf * 8670 hours/year, you get 8.6 kwh/sfy. At $0.10/kwh, that is less than $1/sfy. I said this was rough. I recently visited a building in Canada that claimed less than 1W/sf HVAC load.

Currently, a commercial building HVAC system’s first cost is about $30/sf. It was $15-20 when I started in this end of the industry, about 1980.

Building loads are typically designed at 1 cfm/sf. With 55F supply air in a conventional overhead mixing system, that comes to 22 BTUH/sf load. I suspect most office building loads are 1/3 of that most of the time.

The ASHRAE 62.1 default ventilation rate is 17 cfm/person. At 55F (typically required to maintain a minimum of 60%RH), that is likely more BTU’s than a person generates, and likely has enough left over to handle his computer.

Most diffuser reported NC’s are 5NC below what you should expect. There is an ASHRAE research project that will provide more definitive data, but catalog data is collected with 10 diameters of straight duct, not the typical flex inlet. In addition, it assumes 10dB room attenuation in all bands, which is also unlikely. I recommend adding 5 NC to all manufacturers ceiling diffuser data.

If I can just barely hear a diffuser in an operating environment, it’s about NC=35. Specifications requiring NC 25 or less are typically too conservative, except in very quiet environments. Remember, however, that you add 3NC each time you double the number of similar diffusers when they are all in hearing distance.

75fpm throw is affected by temperature by about 1% /degree delta-t. This rule of thumb allows calculation of jet projections at differing temperatures. Cold air falls further, hot air falls less, compared to isothermal throw, which is what most manufacturers catalog. Along a ceiling warm air travels further.

I’m sure I’ll think of some more. Stay tuned….

Authored by: Dan Int-Hout, Chief Engineer Krueger