To reduce the energy consumption of boilers:
Match capacity to demand, using a small boiler for periods of low demand, such as in the summer
Use a sequence controller to ensure the minimum number of boilers is fired at any time
Insulate the boiler and all piping and tanks
Maximise the recovery of hot condensate
Supply the correct amount of combustion air
Keep boilers free of fireside fouling and scale by cleaning
Minimise waterside scaling by treating feedwater correctly
Reduce heat loss by keeping blowdown to a minimum
Preheat combustion air using flue gases, air from the top of the boiler house, or air drawn over the boiler
Preheat feedwater using flue gases or heat recovered from top blowdown
Use a variable speed drive on the combustion air fan
If steam demand is irregular with occasional high peaks, consider using a steam accumulator and thereby
enable a smaller boiler to be used.
Consider replacing old inefficient boilers
In domestic and light commercial heating applications, consider a condensing boiler
Ensure boilers and piping are well insulated
Between 3 and 10% of boiler operational running costs can be saved by ensuring stringent energy
efficiency measures are adopted. These include
Sequence control & firing strategies
Variable speed drives
Boiler cladding and insulation of pipework and fittings
Combustion air pre-heat
Feedwater pre-heat with an economiser
Fuel to air ratio
Steam accumulators
Recovering the residual heat from blowdowns and
Condensate recovery
The thermal efficiency of a heating boiler decreases when it is operating under low thermal load due to the
increasing significance of fixed heat losses such as:
radiation and convection losses from the hot boiler casing
draught losses up the flue when the burner is not firing and
purge losses when the burner starts up and/or shuts down
For this reason, energy savings can be obtained in multi-boiler installations by ensuring that only the
minimum number of boilers is allowed to fire at any time. This can be most reliably achieved by installing an
electronic boiler sequence controller to regulate the operation of the plant in line with the prevailing heat
load. Usually, the output from the boiler sequence controller will simply turn the boiler plant on or off
electronically. While this prevents the boiler from firing, hot water will still circulate through the offline plant,
meaning that there will continue to be some standing heat losses.
If the load characteristic of a large boiler plant is variable, then substantial energy savings are possible
using variable speed drives (VSD’s)A VSD system will reproduce the operating characteristics of a fixed speed combustion air fan, as well as
reducing the average electrical demand of the fan motor by approximately 60%.
The use of a VSD system has been shown to be cost effective while at the same time maintaining good
combustion conditions and high boiler efficiency.
All boilers, piping and valves conveying hot water, steam and condensate should be properly insulated and
weatherproofed, and the supply should be turned off when there is no heat requirement.
Insulating large steam carrying pipework can pay for itself in a matter of weeks.
Please see our courses on insulation for more details.
Combustion air preheat is another potential energy saving technique worthy of consideration.
The usual sources for combustion air preheat include:
Heat remaining in flue gases
Higher temperature air drawn from the top of the boiler house and
Heat recovered by drawing air over or through the boiler casing to reduce shell losses.
For more information on a variety of devices used to recover heat and make it available for preheating
combustion air, see Waste Heat Recovery.
In addition to pre-heating the combustion air, we can also pre-heat the feedwater.
The boiler flue gases, having passed through the main boiler and the superheater, will still be hot. The
energy in these flue gases can be used to improve the thermal efficiency of the boiler. To achieve this, the
flue gases are passed through an economiser.
The economiser is a heat exchanger through which the feedwater is pumped. The feedwater consequently
arrives in the boiler at a higher temperature than would be the case if no economiser was fitted. Less
energy is then required to raise the steam. Alternatively, if the same quantity of energy is supplied, then
more steam is raised. This results in a higher efficiency. In broad terms a 10°C or 18°F increase in
feedwater temperature will give an efficiency improvement of 2%.
To achieve a high thermal efficiency and thereby minimise fuel costs, the amount of combustion air required
should be limited at all times to that necessary to ensure complete combustion of the fuel used.
In practice this means supplying air in excess of that which is theoretically necessary, but keeping the
excess air to a minimum.
The amount of excess air giving the optimum boiler efficiency will depend on the fuel used, the type of
boiler, its state of repair, the method of operation, and the combustion equipment. Boiler efficiencies can be
improved by balancing the fuel to air ratio, and by taking measures that reduce stack temperatures to a
safe minimum.
The most appropriate means of providing clean, dry steam instantaneously to meet a peak demand is to
use a method of storing steam so that it can be 'released' when required. Storing steam as a gas under
pressure is not practical due to the enormous storage volume required at normal boiler pressures.
A steam accumulator is, essentially, an extension of the energy storage capacity of the boiler(s). When
steam demand from the plant is low, and the boiler is capable of generating more steam than is required,
the surplus steam is injected into a mass of water stored under pressure.
The accumulator tank is about half-filled with cold water and steam is blown in from a boiler via a perforated
pipe near the bottom of the drum. Some of the steam condenses and heats the water. The remainder fills
the space above the water level. When the accumulator is fully charged, the condensed steam will have
raised the water level in the drum to about three-quarters full and the temperature and pressure will have
also risen.
Steam can be drawn off as required, either for heating or for process purposes, by opening a steam valve
on top of the drum. The pressure in the drum will fall but the reduced pressure causes more water to boil
and the accumulator can go on supplying steam, while gradually reducing pressure and temperature, for
some time before it has to be re-charged
Water removed from the boiler during blowdown contains a lot of heat. Any boiler with continuous surface
water blowdown exceeding 5 percent of the steam generation rate is a good candidate for blowdown waste
heat recovery. There is very little scope for “bottom” drawdown as the “mud” or “sludge” is too expensive totreat.
The residual heat in blowdown water can be recovered with a flash tank, a heat exchanger, or the
combination of the two. Water inside the boiler is normally kept in a liquid state by high pressure. A flash
tank allows the water to reach a lower pressure after the blowdown. The pressure change will cause some
or all of the water to rapidly evaporate or “flash” into steam. Heat recovery can then be applied.
When steam from the boiler has given up its heat in the process equipment
that it supplies, it condenses back to water.
This “condensate” (as it is called) is released through a steam trap and returns to the
boiler to be converted back into steam.
By maximising the percentage of condensate that is returned, the operator minimises the need to top up
the system with raw water. This saves on water-treatment chemicals, which in turn reduces the amount of
blowdown required as well as saving money. Reducing blowdown reduces energy losses. Furthermore, the
returned condensate contains heat and this saves on energy needed to preheat the boiler feedwater.
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