How much does YOUR air weigh?
Winter is here, and we are in the “THICK” of it. (pun
intended) Those combustion systems in
service with the cold weather are working even harder to produce the required
heat. When your system operator walks up to you with the complaint, “The oven
is just not getting up to temperature with the same process load today!” Or, your belt speed on that conveyor has to
be slowed down to keep from running the process out of spec. Loss of production
and loss of money!
With the outside air being in the low teens or even the
single digit level, its density is greater or could be said, “Thicker” then it
was during the summer. Airplane pilots are keenly aware of this, and calculations
to pilot their aircraft safely depend a lot on what is termed “Density
Altitude” conditions. In the warmth of Summer, the warmer thinner, lighter air
requires more room (cubic feet per lb) as compared to the air we breathe now in
the cold months of winter. Your combustion systems are the same. In the summer,
those combustion air dampers and butterfly valves are adjusted to give the
required cubic feet or pounds of air per cubic feet-pounds of fuel. Since the
air of summer is warmer, and many combustion systems draw their combustion air
from an outside duct, that change in density can be a real factor in the fuel
air mixture. If those same burners are adjusted during the cold winter, and
operate into summer conditions, the reverse effect occurs, lighter and less density,
the combustion system is getting less pounds or cubic feet of air per
pound-cubic feet of fuel.
Air at 60F is compared to the same air at 600F.
Looking at this from a burner adjustment level, you may need
to cut back on the combustion air to regain that balance of air to fuel ratio.
A typical burner of 3.5 million btu/hr capacity running on a
8000hour/year schedule with just the temperature change from around an 85F
combustion air temperature that drops to 55F (That is just a 30 degree drop in
temperature) could cost almost $1,000.00 added running cost in the winter if
not checked. To add to this problem, the colder denser air can increase the %
excess air from let’s say 3% O2 to 5% O2 (just a two point increase). That
changes your % excess air from about 15% to about 30%. That is double! And the
added cost of that winter operation to $2,500.00 over normal. That’s only one
burner, and if there are more, that cost can be even higher.
Things that you may want to do? Check those systems for
operational efficiency when there are big changes in weather conditions like
winter/summer. You may even want to think about where that combustion air is
drawn from or consider pre-heating your combustion air. Pre-heating can help
even in the summer on some systems and make real pay back value. So, the question is, How much are you willing
to lose? That is, how much are you “over weight” on combustion air?
For more information on the effects of temperature and
altitude on combustion, check out the Eclipse Engineering Guide and check out Exothermics for tips and explanations on
heat exchangers and pre-heating combustion air.