Energy Efficiency Opportunities: Operations and Maintenance

Workshop on Efficient Operation & Maintenance of Boilers
ENERGY EFFICIENCY OPPORTUNITIES

The newest gas-fired condensing boiler technologies have efficiencies of over 92
percent, compared to efficiencies of 75 to 85 percent for older, standard boilers. If
you are purchasing a new boiler, choosing a high-efficiency boiler will save you
significantly on future fuel bills. For existing boilers, many opportunities for big
energy savings require only minimal expenditures.
Operations and Maintenance

Boilers combustion controls are designed to automatically operate the burners
efficiently and match the steam or hot water demands on the boiler. For example,
larger boilers may employ stack sensors to provide an indication of combustion
efficiency as the concentration of oxygen in the exhaust gas. Records of important
boiler operating information should be kept in a boiler log.
Examining the following factors can indicate if a boiler is being run to maximize its
efficiency:
Stack Temperature
The stack temperature should be low as possible. However, it should not be so low
that water vapor in the exhaust condenses on the stack walls. Stack temperatures
greater than 230°C indicates potential for recovery of waste heat.
Target Flue gas Concentration
Low oxygen concentration or high CO 2 concentration should be maintained in flue

gas for minimum excess air. The Table 1 gives oxygen/carbon dioxide concentration
for excess air level.
Table 1 Stack Gas Concentrations: Oxygen (O2) or Carbon Dioxide (CO2).
Fuel Target Flue Gas Excess Air Efficiency

Concentrations
Oxygen Carbon
Dioxide
Natural Gas 1-2 11.9 - 12.3 % 5 - 10 % 85.2 - 85. 4 %
Fuel Oil 2-3% 15.4 - 15.8 % 10 - 15 % 89.0 - 89.2 %
Coal (Stoker) 3.5 - 5 % 17.2 - 17.4 % 20 - 30 % 81.0- 82.0 %
Coal 3 - 3.5 % - 15 - 20 %
85.0 - 86.5 %
(Pulverized)
Energy Saving Measures

Economizers
Heat is transferred from hot stack gases to water being fed to the boiler. It is a heat
exchanger installed in the exhaust stack for preheating boiler feed water.
National Productivity Council, Gandhinagar

An economizer removes additional heat or waste heat from the stack gasses by
circulating the deaerated boiler feedwater through a series of bent tubes in the stack.
This translates into a "free" source of energy from the boiler operation. Finned tube
economizers are less costly and more efficient as the "fins" are a source of heat
transfer as well as the tubes. Economizers in watertube boilers typically increase the
efficiency of the boiler 4-10%, which is usually less than a one year payback. Due to
the higher efficiencies of firetube boilers the payback is usually longer and therefore
economizers are not used as
frequently on them. An economizer
can also be a useful means of
increasing the steam capacity of a
boiler.

The use of high sulfur oils, is very
corrosive on the economizer tubes.
This can be improved by increasing
the temperature of the feedwater to
the economizer and the use of soot Economiser
blowers but the life of an economizer in
that environment is limited to about 2-3 years. A bare tube economizer is easier to
keep free of the corrosive sulfur but requires more tubes to achieve the same
efficiency as a finned tube economizer.
Air Preheaters
Heat is transferred from hot stack gas to air that

is to be mixed with fuel for combustion. This Every 20OC rise in
device saves energy by increasing the combustion air temperature
temperature of the mixture of fuel and air prior to increases efficiency by 1%.
combustion, so that more heat of combustion is
available to heat water.
Oxygen Trim Controls
Oxygen trim controls measure excess oxygen in the flue gas and modulate air intake
to the burners for optimizing fuel/air ratio.
Insulate boilers and boiler piping
Insulation reduces heat loss through boiler wall Addition of 2.5 cm of insulation
and piping by repairing or adding insulation. can reduce heat loss by 80-
Automatic Blow down control 90%
Uncontrolled continuous blowdown is very wasteful. Add automatic

blowdown controls that sense and respond to A 10% blow down in a 15
boiler water conductivity and pH. kg/cm2 boiler results in 3%
efficiency loss.

Keep Heat Transfer Surfaces Clean
In oil- and coal-fired boilers, soot buildup on tubes acts as an insulator against heat
transfer. Any such deposits should be removed on a regular basis. Elevated stack
temperatures may indicate excessive soot buildup.
Reduce Boiler Steam Pressure
If process allows, reduce boiler steam pressure to reduce fuel consumption and
stack temperatures
Eliminate standby boiler use
Sometimes boiler is operated at very low load for standby purpose. About 1.5-2.0%
of the rated boiler fuel input is lost in such cases.

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Workshop on Efficient Operation & Maintenance of Boilers

ENERGY EFFICIENCY OPPORTUNITIES

Operations and Maintenance

Low oxygen concentration or high CO 2 concentration should be maintained in flue

Table 1 Stack Gas Concentrations: Oxygen (O2) or Carbon Dioxide (CO2).

Fuel Target Flue Gas Excess Air Efficiency

Energy Saving Measures

National Productivity Council, Gandhinagar

Heat is transferred from hot stack gas to air that

Oxygen Trim Controls

Insulate boilers and boiler piping

Uncontrolled continuous blowdown is very wasteful. Add automatic

National Productivity Council, Gandhinagar

Keep Heat Transfer Surfaces Clean

Reduce Boiler Steam Pressure

Eliminate standby boiler use

National Productivity Council, Gandhinagar

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