Three important performance considerations pertain to boilers: fuel, emissions, and efficiency. All three have important impact on boiler performance, and can affect long-term boiler operating costs.
From an operating perspective, fuel costs typically account for approximately 10% of a facility's total operating budget. Therefore, fuel is an important consideration. Normally, the fuel of choice is natural gas, propane, or light oil. Increasingly stringent emissions standards have greatly reduced the use of heavy oil and solid fuels such as coal and wood. Of the fossil fuels, natural gas burns cleanest and leaves the least residue; therefore less maintenance is required.
It can be advantageous to supply a boiler with a combination burner that can burn two fuels independently - for example, oil or natural gas. A combination burner allows the customer to take advantage of "peak time" rates, which substantially reduces the cost of a therm of gas when operating "off peak" by merely switching to the backup fuel. Dual fuel capability also is beneficial if the primary fuel supply must be shut down for safety or maintenance reasons.
Some waste streams can be used as fuel in the boiler. In addition to reducing fuel costs, firing an alternate fuel in a boiler can greatly reduce disposal costs. Waste streams are typically used in combination with standard fuels to ensure safe operation and to provide additional operating flexibility.
Emissions standards for boilers have become very stringent in many areas because of the new Clean Air regulations. The ability of the boiler to meet emissions regulations depends on the type of boiler and burner options. Cleaver-Brooks has options to meet 5ppm NOx regulations, as well as 1 ppm CO regulation at 30 ppm NOx out of the box. We can also custom-engineer Selective Catalytic Reduction (SCR) for more rigorous emissions controls.
Combustion efficiency is the effectiveness of the burner only and relates to its ability to completely burn the fuel. The boiler has little bearing on combustion efficiency. A well-designed burner will operate with as little as 15% to 20% excess air, while converting all combustibles in the fuel to thermal energy.
Thermal efficiency is the effectiveness of the heat transfer in a boiler. It does not take into account boiler radiation and convection losses - for example, from the boiler shell, water column piping, etc.
The term "boiler efficiency" is often substituted for combustion or thermal efficiency. True boiler efficiency is the measure of fuel-to-steam efficiency. Cleaver-Brooks guaranteed boiler efficiencies are fuel-to-steam efficiencies. Fuel-to-steam efficiency is calculated using either of two methods, as prescribed by the ASME Power Test Code, PTC 4.1. The first method is input-output, which is the ratio of BTU output divided by BTU input x 100. The second method is heat balance, which considers stack temperature and losses, excess air levels, and radiation and convection losses. Therefore, the heat balance calculation for fuel-to-steam efficiency is 100 minus the total percent stack loss and minus the percent radiation and convection losses. Fuel-to-steam efficiency is important, but that efficiency calculation does not take into account cycling and purge losses, operating levels, and other variables.
Fuel-to-steam efficiency is important, but that efficiency calculation does not take into account cycling and purge losses, operating levels, and other variables. More importantly, in-service efficiency—the amount of fuel you have to use to get the needed output, may be a truer measurement of the value of your system. We supply the highest in-service efficiencies on the market. Whether modular, a single boiler, or a hybrid, our cutting-edge designs combined with advanced controls provide the absolute lowest operating cost available.
Stack Temperature and Losses
Stack temperature is the temperature of the combustion gases (dry and water vapor) leaving the boiler. A well-designed boiler removes as much heat as possible from the combustion gases. Thus, lower stack temperature represents more effective heat transfer and lower heat loss up the stack. The stack temperature reflects the energy that did not transfer from the fuel to steam or hot water. Stack temperature is a visible indicator of boiler efficiency. Any time efficiency is guaranteed, predicted stack temperatures should be verified.
Stack loss is a measure of the amount of heat carried away by dry flue gases (unused heat) and the moisture loss (product of combustion), based on the fuel analysis of the specific fuel being used, moisture in the combustion air, etc.
Excess air provides safe operation above stoichiometric conditions. A burner is typically set up with 15% to 20% excess air in higher firing ranges. Higher excess air levels result in fuel being used to heat the air instead of transferring it to usable energy, increasing stack losses and significantly decreasing efficiency. Boilers with lower excess air throughout the operating range have higher efficiencies.
Radiation and Convection Losses
Radiation and convection losses will vary with boiler type, size, and operating pressure. The losses are typically considered constant in BTU/hr, but become a larger percentage loss as the firing rate decreases. Boiler design factors that also impact efficiencies of the boiler are heating surface, flue gas passes, and design of the boiler and burner package.
Heating surface is one criterion used when comparing boilers. Boilers with higher heating surface per boiler horsepower tend to be more efficient. Traditional packaged boilers are offered with 5 square feet of heating surface per boiler horsepower as an optimum design for peak efficiency, but new design breakthroughs allow Cleaver-Brooks EX boilers to have increased efficiency using more effective heating surface and reducing the size of the footprint.
Flue Gas Passes
The number of passes that the flue gas travels before exiting the boiler has been a good criterion when comparing boilers. As the flue gas travels through the boiler it cools, and therefore changes volume. Multiple pass boilers increase efficiency because the passes are designed to maximize flue gas velocities as the flue gas cools. Cleaver-Brooks has developed new design technologies in our EX boilers allowing for comparable efficiencies in fewer passes, resulting in smaller boiler systems that will fit in tighter quarters.
Integral Boiler/Burner Package
Ultimately, the performance of the boiler is based on the ability of the burner, the boiler, and the controls to work together. When specifying performance, efficiency, emissions, turndown, capacity, and excess air all must be evaluated together. The efficiency of the boiler is based, in part, on the burner being capable of operating at optimum excess air levels. Burners not properly designed will produce CO or soot at these excess air levels, foul the boiler, and substantially reduce efficiency. In addition to the boiler and burner, the controls included on the boiler (flame safeguard, oxygen trim, etc.) can enhance efficiency and reduce overall operating costs for the customer. A complete packaged boiler design includes the burner, boiler, and controls as a single, engineered unit, and it is this design that Cleaver-Brooks has focused on for more than 80 years.
For more information, refer to the ABMA Firetube Engineering Guide, the ASHRAE Handbook, or contact your local Cleaver-Brooks authorized representative.