Retrotec USACE - Manual (Page 256)

D126 ENERGY & PROCESS ASSESSMENT PROTOCOL
The temperature of the fl ue gas is a measure of the combustion effi ciency of
the furnace or oven. Temperatures inside this equipment can exceed 1,093 °C
(2,000 °F). Thus, the leaving temperature of the combustion gases can be very
hot. If the combustion gases are clean of particulate, this is an opportunity for
the use of an economizer, which would transfer heat from the fl ue gases to the
incoming combustion air. An economizer could also be used to preheat water
from some process need or for domestic uses. If the combustion temperature
is greater than 93 °C (200 °F), the use of an economizer to recover some of this
energy may be cost-effective (Figure D202).
D.7.8.12 More Than 20% Excess Oxygen in Flue Gases
(Ineffi ciency)
Figure D203. Combustion gas monitors
for sensing the oxygen and carbon dioxide
content of fl ue gases.
Furnaces and ovens need the proper amount of oxygen (air) for optimum com-
bustion and effi cient operation. If there is an excess air fl ow to the burner and
up the stack, the result would be wasted heating energy caused by raising the
temperature of the extra air. This condition can be the result of faulty furnace
and oven controls (Figure D203), inoperable dampers, or dirty burners. An
excess air of more than 20% is higher than necessary, and improvements can
be made to better control it.
D.7.8.13 Furnace or Oven Cycling On and Off at Low Loads
(Ineffi ciency)
Furnaces and ovens have a limited low operating level that they can still fi re
without supplying too much heat. This is called the “turndown ratio,” and a
value of 15–20% of the rated output is a common minimum fi ring rate. If the
heating demand is less than this percentage of the rated output, the furnace
and oven will cycle on and off to satisfy that demand. This mode of operation
is wasteful, since extra combustion air fl ows through the furnace and oven at
startup and on shutdown. This extra air fl ow gets heated in the furnace and
oven, reducing its temperature. Extra fuel is required to bring the furnace and
oven temperature up to operating levels when the unit is started again. This
condition is found when noticeably oversized equipment is being used.