How Do I Measure Efficiency

How Do We Measure Efficiency?

As you read this I will be on the briny sailing from New York City.  Having grown up around this wonderful city and taken for granted the Statue of Liberty so many times (I visited the monument only once in all those years), I have always wanted to sail past the Statue on my way somewhere and that time has finally arrived.  I’ll glance out at this wonderful symbol of America’s freedom with a deep respect for all those who have preceded me.

When I first got into this business lo these many years ago, boilers were not as efficient as they are now.  To add to that, there were still many boilers firing coal and heavy oils and because of the high sulfur contents of these fuels, flue gas exit temperatures from an economizer were relatively limited.  The old literature I remember when I first started spoke of efficiency gains from 5-8%.  Using the old rule of thumb that each 40 degree drop in exit gas temperature equaled one percent gain in efficiency you can see that boiler exhaust temperatures were quite hot back then.  Now we look more at 3-5% gains, this despite the fact that the predominant fuel being fired on package boilers is now clean-burning natural gas.

With the advent of natural gas also came the ability to squeeze out additional heat recovery through grabbing extra BTU’s available with condensing economizers.  For more detail see the following link: Condensing Economizer White Paper

 This was particularly true in the middle of the last decade when gas prices shot up to $14.00/dekatherm and higher.  With the vagaries of heat transfer, as latent heat is recovered, the rule of thumb of a 40 degree drop in gas temperature being equivalent to one percent efficiency gain goes away.  Jamie Tighe put together for me the following graph that illustrates this better than I can write it.

Note that as the gas temperature goes down from 300 to 135 degrees F., the slope of the curve is consistent but at approximately 135 degrees F., the water dew point is reached and true condensation occurs and the curve flattens out.

I do hope this is helpful to all of you.

And finally, my saying for this blog is vacation related:

We hit the sunny beaches where we occupy ourselves keeping the sun off our skin, the saltwater off our bodies, and the sand out of our belongings.  ~Erma Bombeck