Boilers provide hot water or steam for industrial processes, for heating spaces and for hot water. A wide range of types and sizes of boilers meet the varied needs of industrial and other facilities.
Most boilers have three main parts: a burner that converts the fuel to heat, a heat exchanger that transfers the heat to steam or water, and a boiler vessel. A chimney stack draws off the combustion by-products (flue gases), and the hot water or steam flows through a distribution system to its end uses. Figure 1 shows a schematic of the combustion process.
Natural gas and oil are the most common fuels used in boilers. Propane, electricity, coal and biomass are also used. Electric boilers are generally found where combustion boiler fire hazards pose safety risks and where it is important to reduce air pollution.
Figure 1: Boiler Configuration
Boiler life is approximately 25 years, so it is essential to consider both long-term fuel and maintenance costs along with initial capital costs when buying or retrofitting. Fuel costs for a new high-efficiency model can be up to 40 percent lower than for a conventional one. Over 25 years, this can be a great saving. In many cases, simply retrofitting an existing boiler can improve efficiency by 20 percent or more.
Boilers with heat outputs of 300 000 Btu/hr to 2 500 000 Btu/hr are rated by Thermal Efficiency.
We are interested in the steady-state Thermal Efficiency – i.e., after the flue gas temperature has warmed up and reached equilibrium. Many combustion systems do not operate in steady-state equilibrium: they cycle up and down, taking a significant time to reach equilibrium, if at all. Nearly all transient systems are significantly less efficient than ones that operate in the steady state.
Thermal Efficiency is a steady-state measure only and does not include the effects of heat loss caused by on-off cycling or transient operation. This measure is different from the Annual Fuel Utilization Efficiency (AFUE) rating, which measures the average efficiency of a system over a year. The AFUE rating takes into account the cyclic on/off operation and associated energy losses of the heating unit as it responds to changes in the load, which in turn is affected by changes in weather and occupant controls.
Standards and Regulation
In Canada, gas and oil boilers under 300 000 Btu/hr are regulated under the Energy Efficiency Act. No similar Canadian standards exist for boilers over 300 000 Btu/hr. In the United States, boilers over 300 000 Btu/hr are subject to standards under the National Energy Policy Act. Under this Act, large gas-fired boilers must have a steady-state Thermal Efficiency of at least 80 percent, and large oil-fired boilers must have a steady-state Thermal Efficiency of at least 83 percent.
Heat Losses in Combustion
Losses result from:
- Dry flue gas loss: These heat losses are due to the temperature of the flue gases and are a function of excess air (which wastes energy by carrying heat up the stack) as well as the flue and combustion air temperatures.
- Hydrogen loss: This is caused by the latent heat loss due to the boiling of water vapour by the combustion of hydrogen in the fuel.
- Moisture-in-fuel loss: This is latent heat loss due to boiling of water in the fuel. Significant levels of water are found naturally in biomass, garbage and lignite.
- Casing loss: This is the radiant heat loss from the furnace casing.
- Incomplete combustion.
- Other losses: moisture in the air, air or flue gas leaks, or heat in the ash.
How Can the Combustion System Be Improved?
Flue gases are the single most important cause of energy loss. As much as 18 to 22 percent of available energy goes up the chimney. Heat radiation and convection from boiler walls raise heat loss another 1 to 4 percent.
There are four main ways of reducing flue gas energy losses:
- by improving the efficiency of converting the fuel to heat (improved combustion system efficiency)
- by requiring less air for satisfactory combustion
- by ensuring that the boiler casing is tight, so that there is no air/heat entering or leaving the casing through leaks
- by improving the efficiency of transferring the heat to the steam or hot water (improved heat exchanger efficiency)
Improved Combustion System Efficiency
New boilers generally incorporate several new technologies. These same technologies can also be applied when retrofitting older boilers. The most important new technologies are as follows.
Fan-assisted combustion: Originally, boilers and furnaces relied on natural draft, i.e., the buoyancy of the hot air in the flue, to draw the air into the firebox and up the flue. A draft hood limited condensation in the flue and ensured that the burner and flame were isolated from outside air pressure fluctuations by adding "dilution" air to the flue. At the same time, the dilution air lowered the vapour pressure at which the flue gases would condense and cause damage to the flue. However, efficiency was lost because of the loss of heated interior air up the chimney.
Newer-technology fan-assisted burners eliminate the draft hood and are better at mixing fuel and air. As a result, excess air is reduced. Fan-assisted burners also diminish losses by reducing the amount of hot air going up the chimney.
The fan also improves the heat transfer inside the boiler by improving combustion gas flow through the heat exchanger.
Two types of fan-assisted systems are available: a forced-draft system uses a fan to blow the fuel and air mixture into the boiler; an induced-draft system has the fan located at the outlet end of the heat exchanger passages.
Motorized dampers: Motorized dampers stop heat from escaping up the chimney by automatically closing the flue when the boiler is idle.
Electric ignition: Older gas boilers have pilot flames that remain lit whether the boiler is firing or idle. Electric ignitions or other intermittent ignition devices eliminate this waste of fuel. A control circuit energizes the ignitor and, if the burner does not fire on the first try, the ignitor re-fires until the burner is lit.
Sealed combustion: Sealed combustion controls the combustion process more carefully by preventing boilers from inducing infiltration into the building. In a sealed combustion boiler, air is drawn directly from outside through a sealed venting system, ensuring that heated indoor air is not mixed with the outside air during the combustion process.
- Pulse combustion: Instead of a continuous flame, pulse systems create discrete, rapid combustion pulses in a sealed chamber. This intensely turbulent process results in a highly efficient heat transfer to the heat exchanger and allows for flue gas condensation in condensing boilers.
High-efficiency condensing boilers feature additional advanced heat exchanger designs and materials that extract more heat from the flue gases before they are exhausted. The temperature of the flue gases is reduced to the point where the water vapour produced during combustion condenses back into liquid form, releasing the latent heat, which improves energy efficiency. With some 12 percent of the energy of a gas-fired boiler tied up as latent heat, this represents a significant energy-savings potential. A side effect is that this condensate is usually acidic and has to be piped to a drain.
Modern condensing boilers have energy efficiencies of 90 to 96 percent. New conventional non-condensing models have energy efficiencies of only 70 to 85 percent. Many boilers over 20 years old typically operate at only 60 to 70 percent efficiency, making them good candidates for upgrading or replacement. A number of natural-gas-fired condensing boilers are available, but very few oil-burning ones are on the market.
An important point is that for the water vapour in the flue gases to condense, the temperature of the flue gas must be reduced to below the water dew point of the flue gas. For this to occur, the return water temperature to the boiler proper must be below 60°C. If there are no heat exchange surfaces at the back of the boiler below this dewpoint, condensing will not occur, and this energy opportunity will be lost, even if the boiler is a “condensing” boiler.
In retrofit applications where you wish to retain your existing boiler, boiler efficiency can be improved by adding an economizer, which is a heat exchanger that utilizes the waste heat from the flue gas to preheat the boiler feedwater. A condensing economizer improves the effectiveness of reclaiming flue gas heat by cooling the flue gas below the dewpoint. The condensing economizer thus recovers both the sensible heat from the flue gas and the latent heat from the moisture that condenses. You do have to ensure, however, that the condensate does not enter the boiler, as the condensate is highly corrosive.
Oil condensing boilers are more expensive, and it is much harder for them to actually achieve condensing because:
- Sulphur in the oil turns the condensed water into sulphuric acid that must be neutralized. The heat exchanger must be of very high quality to prevent corrosion by the acid.
- Oil has 50 percent less energy tied up in latent heat, compared with natural gas.
- The dew point for oil is low – 47°C compared with 60°C – for natural gas, making the water vapour in the flue gas very difficult to condense.