Page 201 - Boiler plant and distribution system optimization manual

186 Boiler Plant and Distribution System Optimization Manual

1. The over-fire draft control system for a natu- firing No. 6 fuel oil which has a combustion
ral gas fired boiler with a very tall stack has gas oxygen level of 5% and the average net
failed and the change in the negative over- stack temperature has dropped from 600F
fire draft pressure has caused a problem. [316C] to 420F [204C]. How has this affected
The oxygen level in the exhaust combustion losses? See tables on pages 12 and 24.
gases has risen from 5% to 13%, with the net (Ans.: Flue gas losses have dropped from 18.7%
stack temperature remaining at 500F [260C]. to 14.5% with a 4.2% improvement)
What losses have been created by this condi-
tion? See Table 11-1. 3. The purpose of the following example is to
(Ans.: losses from 20.2% to 30.5%; efficiency demonstrate the loss of fuel dollars caused
loss 10.3%) by the gradual fouling of heat exchange sur-
faces.
2. In a boiler burning bituminous coal, the
damper doors have become warped and can The stack temperature has risen from 480F
not be shut. This and other problems have [249C] to 680F [360C] since the boiler firesides
caused the carbon dioxide in the flue gas to were last cleaned. How much is this costing in
drop from 12.3% to 6.6%. With a net flue gas terms of the monthly fuel bill?
exit temperature of 730F [388C], how is this
affecting efficiency? See Table 11-4.
(Ans.: Losses have gone up from 24% to 40.9% Fuel No. 2 Fuel Oil
with an efficiency change of 16.9%) Monthly fuel costs $400,000

Tuned Up Conditions:
REDUCING STACK TEMPERATURE Flue gas oxygen 2%
TO IMPROVE EFFICIENCY Stack temperature 480F [249C]
Air Temperature 80F [28C]
Because it represents a measure of the unre- Net stack temperature 400F [204C]
coverable heat being wasted to the atmosphere, Tuned up Efficiency 86%
the temperature of the stack gas is an important
indicator of boiler efficiency. Present Conditions:
Exit gas temperature, should be measured at Flue gas oxygen 2%
the boiler outlet, after the economizer or air heat- Stack temperature 680F [360C]
er depending on which is last before the gasses Air Temperature 80F [28C]
exit to boiler. If the measurements are not typical, Net stack temperature 600F [316C]
being cooled by wall losses of the duct or stack Efficiency 82.1%
or by the infiltration of cold air, the resulting heat
efficiency measurements will be erroneous. 1.Percent efficiency loss _______%
(Ans.: 3.9%)
1. In a natural gas fired boiler with 6% flue gas
oxygen, recent maintenance has resulted in 2. Percent fuel loss _______%
the net stack temperature going down from (Ans.: 4.75%)
600F [316C] to 400F [204]. How has this af-
fected losses? See Table 11-1 and 11-9. 3. Monthly dollar loss $_______
(Ans .: losses drop from 23 .1% to 18 .6% a 4 .5% (Ans. $19,000/Mo)
improvement)
4. Annual dollar loss $_______
2. An economizer has been installed in a boiler (Ans.: $228,000/Yr)