Let the Engineers Have Power Over the Codes
This letter references Julius Ballanco's June 2002 column. As an ambidextrous engineer, P.E. mechanical and electrical, I would not imagine allowing job protection rules into a document like the NEC that is concerned with safety. I believe it is time for engineers to demand that any plumbing or hvac code committee either be composed of a majority of engineers in the field, or that engineers be given veto power over codes.Walter E. Wallis, P.E.
Wallis Engineering
unclgene@pacbell.net
Managing a Boiler Plant
This is in reference to Ted Wilinski's letter in the July 2002 issue. Ted, I noticed your letter inPM Engineerand thought I would answer the question Don Holohan didn't. Like you, I question some of the conventional wisdom, and I'm most emphatic about alternating systems and similar programs being a poor way to manage a boiler plant. It isn't just a matter of wear. As for valving off one boiler and bringing another on only when needed, hooray! That makes sense and it saves energy; you must be another real energy engineer.Everyone fails to consider radiation losses when discussing boiler efficiency. It's one of those things that's very difficult to measure but, on the other hand, supposedly so small that it's insignificant. Radiation losses run less than 3%, and some get as low as 1% of the modern boiler's full load input, but some old designs go as high as 8% in smaller boiler sizes.
Allow me to use 2% in this example; you can substitute the manufacturer's rating to get actual values. When two boilers are serving a load that one could serve, then they both have to be operating at less than 50% of full load. Radiation losses don't really care about load, they're pretty much the same regardless so, at 50% of full load those radiation losses now account for 4% of the boiler input.
That isn't what really happens, though; two boilers end up serving a load equal to less than half the capacity of one boiler. That's because the boiler was sized over a design value that included a safety factor and ignored the presence of people, equipment and lighting in the building, plus the normal low temperature is considerably higher than the design low. In my experience, the two boilers are running close to one-eighth of full load, and radiation losses account for about 16% of the boiler's input. Also, running two boilers on loads significantly lower than design ensures they will be cycling most of the time.
Once a boiler starts cycling, its efficiency really begins to deteriorate. Check out my website (www.KEH-EE.com) for my spreadsheets on cycling efficiency. You can download them and get a good idea of what the efficiency is at each outdoor temperature given a particular load. If you only know Excel, then I'll apologize in advance because Excel doesn't seem to be able to handle the macro, you will have to fix it. You can use the results combined with bin values to figure the potential savings of running one boiler. Unless most of the sites I've examined are unique, you'll also find that the boilers are 4 to 6 times bigger than they should be, and you'll be able to show a significant ROI from installing a smaller boiler.
Ken Heselton
KEH Energy Engineering
Joppa, MD
