Designing efficient, effective boilers
Historically, burners consisted of a single linkage rod connected to air dampers and fuel valves driven by a common motor varying fuel firing rate and fuel-to-air ratio. Several disadvantages reduced boiler efficiency and decreased boiler life. Because the linkage was mechanical in nature, boilers with dual-fuel requirements were set up to burn the backup fuel, which often compromised efficiency when firing the primary fuel.
With a linear cam actuator, the linkage system was adjusted for maximum fire or low fire, which leads to reduced efficiency in mid-range applications. In addition, hysteresis and inaccuracies lead to wasteful excess air at low firing rates. Other disadvantages include a low burner turndown ratio. Oversized boilers and antiquated burner turndowns of 3:1 lead to excessive burner cycling, purge cycles, and boiler thermal shock.
With the introduction of programmable linkageless burners, direct digital control, and individual stepper motors for each fuel and air function, fuel-to-air ratio can be optimized for multiple fuels over the entire firing range. Turndown can reach 8:1 to 15:1, which leads to reduced boiler start/stop cycles, better load control, and significant fuel savings.
Oxygen trim and blower VFDs
Air is 79% inert (nitrogen, argon, and water vapor), so 5% excess air contains roughly 1% oxygen. In an ideal world, excess air would be limited to avoid heat loss in the flue gas; however, excess air provides a safety factor to avoid incomplete combustion. Operating at excess air fractions less than 9% results in unburned combustibles, and operating at high excess air fractions results in heat loss due to heating inert parts of the combustion air.
Well-designed natural gas-fired systems can operate on 12% to 15% excess air or 3% to 5% excess oxygen. An oxygen “trim” system provides feedback to burner controls to automatically minimize excess air for the range of fuel firing rates and inlet air conditions, without increasing NOx or CO emissions. Air-to-fuel ratios and the excess air fractions are based on mass, so oxygen “trim” systems require forced draft power burners with blower variable frequency drives (VFDs) to vary the airflow based on temperature and barometric conditions.
Boilers combust fuel and oxygen contained in air to release the fuel’s heat energy. The products of combustion of natural gas include carbon dioxide and water vapor. In a conventional boiler, the flue gas temperature is kept above the flue gas dew-point (roughly 140 F) to prevent the water vapor from condensing in the boiler or exhaust stack and avoid corrosion of of cast-iron, steel, or copper components. Water vapor contains a significant amount of latent heat energy—roughly 1000 Btu per pound of water. Roughly 9% of the fuel’s total heat energy is contained in the water vapor.
Condensing boilers are equipped with high-efficiency heat exchangers designed to extract the latent heat energy of water vapor at return water temperatures below 135 F normally lost in conventional boilers and sensible heat energy from flue gas temperature change, leading to efficiencies in the mid- to high-90th percentile. Because the condensed flue gas is highly corrosive, condensing boilers must be constructed from stainless steel and aluminum alloys in place of carbon steel. This increase in efficiency comes at the price of higher first cost when compared to a noncondensing boiler.
Condensing boiler plants have the opportunity to significantly reduce energy consumption and costs. The Green Proving Ground (GPG) program evalulates new technologies in association with independent researchers and subjects selected technologies to real-world measurement and verification in GSA’s real estate portfolio. As part of its “Condensing Boiler Assessment: Peachtree Summit Federal Building, Atlanta, Georgia,” Pacific Northwest National Laboratory (PNNL) monitored data from a modular condensing boiler plant and found that, after normalizing results based on weather, the condenser boiler plant had a reduced natural gas consumption of 14% when compared to a nominal 80% efficient boiler plant.
Durkin (2006 ASHRAE Journal) analyzed 10 schools after converting from low-pressure steam to low-temperature hot water condensing boiler plants. Energy cost savings due to the conversion averaged 68%. Energy cost savings of 10 schools that converted from a conventional 180 F hot water heating system to a low-temperture modular condensing boiler system averaged 49%. Changing from a standard 180 to 150 F system to a 140 to 110 F system would require evaluation and replacement of existing coils to increase surface area and heat transfer.
Economizers and turbulators
Boiler stack economizers preheat boiler feed-water by exchanging heat between hot flue gas and boiler feed-water. Because boiler economizers are inserted into the boiler stack, the interior of the heat exchanger must be constructed to withstand the corrosive effects of condensing gases. Heat exchanger fins are typically constructed of stainless steel. Without an economizer, exhaust gas temperature can be 450 to 650 F. A well-designed economizer can reduce flue gas temperatures to 170 F. As a rule of thumb, every 40 F reduction in flue gas temperature increases boiler efficiency by 1%.
In a fire-tube boiler, hot combustion gases enter the tubes in a turbulent flow regime, but degrade to a laminar flow regime within a few feet. A boundary layer of cooler gas forms around the tube walls, which reduces heat transfer. Inserting baffles, metal strips, or spiral blades, known as turbulators, can break up the laminar boundary and increase turbulence. DOE estimates turbulators can be installed for $10 to $15 per tube, which provides a lower cost option when compared to other energy-efficiency improvements.
Two fundamental canons consulting engineers must abide by include “holding paramount the safety, health, and welfare of the public” and “acting for each employer or client as faithful agents or trustees.” Boiler codes and standards were developed and will continue to evolve to regulate boiler specification and construction, striving for safe and reliable boiler systems. Energy standards and codes are increasing minimum efficiency requirements, causing owners and engineers to seek solutions and technologies beyond those used in the past. In light of these trends, and to best serve our clients, consulting engineers must be familiar with codes and standards to design and specify safe, efficient, and cost-effective boiler and domestic water heating systems.
Dylan M. McWhirter has 5 years of mechanical engineering experience with ccrd in Houston. His primary project expertise is mechanical system design in the laboratory and health care industries, and he is also heavily involved in the ccrd energy group, which is focused on developing energy models for U.S. Green Building Council LEED projects and target energy usage intensity metrics for Architecture2030 Challenge.