Designing efficient, effective boilers
Several codes and standards regulate boiler specification, plus energy efficiency and efficacy of these boiler systems.
Boilers for heating and domestic hot water systems are used in many nonresidential buildings and across campuses. With the advent of steam engines and boilers, the Industrial Revolution changed the way people lived by introducing machinery to daily life. This machinery allowed goods to be manufactured, transportation to move at higher speeds, and homes to be heated. However, boiler design and construction in the 19th century was not regulated, and resulting boiler explosions led to substantial rates of injury and loss of lives.
In reaction, the American Society of Mechanical Engineers (ASME) formed and issued a standard regulating the construction of stationary boilers. Since then, several codes and standards regulating the specification, construction, and design of boilers have been adopted by states and municipalities. With the adoption of more stringent energy conservation codes, engineers are under pressure to specify more efficient boiler system components as well as engineer a more efficient system. The success of these standards has led to widespread use of boilers for HVAC and domestic hot water systems in nonresidential heating systems and across campuses. To design a safe and efficient heating system, engineers need to be familiar with these codes, standards, and efficiency ratings.
ASME Boiler Pressure Vessel Code
The ASME Boiler Pressure Vessel Code, initially issued in 1914 under another name, is used in the United States and Canada to specify boiler construction requirements, and is often adopted by state and municipality building and mechanical codes, and boiler and pressure vessel codes through the legislative process. The code is organized into 12 sections, including requirements for the nuclear power industry. An overview of the sections relevant to commercial boiler construction requirements follows.
Section I—Power Boilers covers electric boilers, miniature boilers, and high-temperature boilers for stationary service and power boilers for portable service. Section II—Materials covers specifications for ferrous and nonferrous materials. Section IV—Heating Boilers covers rules for design and construction of a heating boiler, defined as a steam boiler with design pressure less than 15 psi, or a hot water boiler with design pressure less than 160 psi and design temperature less than 250 F. Requirements for high-pressure power boilers are more stringent and are discussed in Section I.
Section V—Nondestructive Examination contains examination methods required by other code sections to detect discontinuities in parts and components. Section VI—Recommended Rules for the Care and Operation of Heating Boilers and Section VII—Recommended Guidelines for the Care of Power Boilers include guidelines and recommendation for maintaining plant safety. Section VIII—Pressure Vessels contains construction requirements for constructing pressure vessels. Section IX—Welding and Brazing Qualifications has rules for qualification of welding and brazing procedures and welders and brazers.
To receive an ASME Code Symbol stamp on the boiler shell ensuring compliance with ASME Code requirements, a boiler must be tested and inspected by a certified third-party inspector.
ASME Performance Test Code
The ASME PTC 4-2013 Fired Steam Generators, updated in February 2014, contains two methods of determining overall boiler efficiency: the direct method and indirect method. The direct method, otherwise known as input-output method, measures the heat addition to steam divided by the gross heat in the fuel. The benefit is that plant operators can evaluate boiler efficiency quickly, requiring few parameters and little instrumentation for measurement/monitoring. However, the direct method does not give the operator any indication of why efficiency may be low.
The indirect method, or “by loss method,” measures several losses in the flue gas, including, but not limited to: loss to dry flue gas, loss due to hydrogen in fuel, loss due to moisture in fuel, loss due to moisture in air, loss due to carbon monoxide, and loss due to surface radiation. To measure these losses, flow measurements, fuel gas stoichiometric analysis, temperature measurements, pressure measurements, and water chemistry analysis are required under controlled laboratory conditions.
When specifying burners and oxygen trim controls for high-pressure steam boilers, design engineers should specify which method manufacturers should use to rate component efficiency. For example, a typical steam boiler specification states, “Burners shall be capable of minimum 8:1 turndown ratio … Minimum input to output efficiency of 82% based on stack loss method shall be maintained for 100% through 25% firing range,” and “Provide oxygen trim control system capable of calculating and displaying boiler efficiency using ASME By Losses Method.”
Gas-fired low-pressure steam and hot water boilers with capacities between 300,000 and 12.5 million Btu (defined as a steam heating boiler operating at less than 15 psi or a water heating boiler operating at temperatures less than 250 F and pressures less than 160 psi) are design-certified by an accredited testing laboratory in accordance with ANSI Z21.13-2013. Combustion efficiency as measured by this standard is a steady state efficiency rating that does not reflect actual operating conditions. The unit is operated at full-fire for 30 min to “soak” the metal with heat and operated at full-fire at 100% capacity for the entire 30-min test period. In addition, the method uses 80 F entering water temperature and a 100 F temperature rise on the water-side. Several consequences of this method cause efficiency ratings not to translate directly to fuel savings.
First, some of the boiler’s capacity will be required to “soak” the heat exchanger surface before useful heat transfer will begin, which leads to efficiency losses at each start or cycle. Second, most boiler plants are oversized to provide sufficient heating on a design day with a safety factor as a cushion. Boilers with limited turndown or 100% on/off capabilities resort to cycling to meet part-load conditions, which can lead to dramatic efficiency loss. Finally, efficiency increases as inlet water temperature drops, with a dramatic increase at temperatures less than 135 F due to condensing gases releasing latent energy.
Less expensive noncondensing boilers are often constructed of cast iron parts and must operate with an entering water temperature above 140 F to avoid corrosive condensate in the flue stack/heat exchanger and probable breaches in warranty. If a noncondensing boiler is operated at 140 F inlet temperature to maintain warranty and protect the boiler, real-world operating efficiency would decrease significantly when compared to rated efficiency.
When discussing boiler efficiency and comparing published boiler ratings, efficiency must be defined and the parameters used to rate efficiency clearly qualified. ASHRAE Standard 90.1-2013: Energy Standard for Buildings Except Low-Rise Residential Buildings, lists minimum efficiency requirements for hot water and steam boilers. Minimum efficiencies are combustion efficiency or thermal efficiency for boilers with input greater than 300,000 Btu/h and annual fuel utilization efficiency (AFUE) for boilers with input less than 300,000 Btu/h. The 2012 ASHRAE Handbook–HVAC Systems and Equipment defines boiler efficiency as follows:
Combustion efficiency: “Input minus stack loss (flue gas outlet) divided by input.” Combustion efficiency equals the total heat released in combustion, minus heat lost in the stack gases divided by total heat released. According to the 2012 ASHRAE Handbook–HVAC Systems and Equipment, combustion efficiencies range from 75% to 80% when in the noncondensing mode for most noncondensing boilers operating at return water temperature above 140 F, and 88% to 95% for condensing boilers operating at return water temperatures less than 130 F. If a condensing boiler is operated at return water temperatures higher than 140 F, efficiencies would approach standard efficiencies.
Overall efficiency: “Gross energy output divided by energy input.” Overall efficiency, also known as fuel-to-steam efficiency or thermal efficiency, is the combustion efficiency minus the heat lost due to convection and radiation.
Seasonal efficiency or AFUE: “Actual operating efficiency that the boiler will achieve during the heating season at various loads.” Combustion and overall efficiency refer to efficiency of boilers when operating at steady state condition, often at unrealistic operating conditions that could potentially void boiler warranties. Figure 1 shows the relationship between efficiency ratings. ASHRAE Standards Project Committee 155P has a working draft titled “Method of Testing for Rating Commercial Space Heating Boiler Systems” that seeks to define test measures to determine a seasonal efficiency based on the boiler’s efficiency as a function of load, the building’s load profile, realistic design entering and leaving water temperatures, the use of supply temperature reset, and boiler blow-down and purge.
Heating accounts for 35% of total energy consumption in this country’s commercial buildings according to the U.S. Energy Information Administration (EIA), and because a large fraction of a facility’s energy consumption and cost is associated with the boiler system, single-digit efficiency increases have a significant impact on reducing facility and nationwide energy cost in absolute terms. Several technologies and upgrades discussed below increase system efficiency beyond the minimum values prescribed in ASHRAE Standard 90.1.