The search for energy efficiency in institutional and commercial buildings is never-ending. For maintenance and engineering managers, the quest involves pursuing energy-conservation efforts that improve system efficiency while also reducing operating costs.
An excellent place to focus on improving efficiency is to consider improved maintenance practices on boilers and water heaters, which frequently represent 60-90 percent of annual fuel consumption in facilities. It is not uncommon to see overall system efficiency between 60-80 percent with older boiler systems. Boilers and water heaters not only are large energy consumers. They are historically poor performers.
By implementing energy conservation measures (ECMs) targeting boilers and water heaters, managers can improve both efficiency and savings.
In a typical heating system, the total energy input for boilers and water heaters includes the fuel, fan power, and water preheating and pumping. The system’s energy output includes:
- desired steam or hot water — 85 percent of the total output
- stack losses — 15-20 percent
- blowdown losses — 3-4 percent
- feedwater losses — 2-3 percent
- radiant losses — 2 percent.
Boilers are often commissioned at the time of installation. After that, attention often fades to a bare minimum to maintain operation. This dangerous mentality prevents many technicians and operators from targeting boilers for improvements. But during normal operation, boiler efficiency degrades unless workers pay continued attention to both the water side and air side of heat transfer. On the most basic level, operators first should validate that boiler doors securely shut and that the gaskets are in good condition.
Air-side considerations include monitoring the build-up of soot from products of combustion on the heat-transfer surface, either inside on fire-tube boilers or outside on water-tube boilers.
Operators can estimate the relative cleanliness of this convection heat-transfer section by determining the differential pressure between the burner section and the breeching. Trending this draft loss indicates the level of cleanliness of air-side surfaces. While this film is thin, the layer of soot significantly impedes the conductive heat transfer. The typical range is a 1-3 percent increase in energy use.
Water-side considerations are similar regarding the inhibition of heat transfer. As water vaporizes, steam generated is removed for heating or process work. City-supply make-up water contains impurities that do not vaporize and remain in the boiler water.
These impurities build up over time, becoming total solids, which consist of suspended and dissolved solids. Suspended solids can be removed by filtrations and blowdown.
Blowdown is the intentional discharge of boiler water to eliminate the build-up of impurities. Dissolved solids are impurities that remain in suspension in water. Coagulants are required to enable their removal via filtration or blowdown. Both suspended and dissolved solids inhibit heat transfer and, as a result, reduce boiler efficiency. Boiler blowdown removes these impurities, while chemical treatment can mitigate scale.
A comprehensive water-treatment program can control water-side impurities. By working with a water-treatment professional, managers can improve the performance of boilers and water heaters by optimizing heat transfer and extending the useful life of the equipment. Reducing water-side fouling can result in a 2 percent increase in boiler energy efficiency.
Boiler Optimization to Improve Energy Efficiency
One of the most significant variables impacting boiler optimization to improve energy efficiency is ensuring the complete combustion of fuel. Boiler combustion air blowers have a few functions, including introducing turbulence in the firebox, which promotes complete combustion and maintains metal surface temperatures in the firebox and boiler tubes. Without the blower, tubes would overheat and rupture.
The ideal fuel-air mixture is the stoichiometric balance point at which all the fuel is used for combustion. Too little air results in incomplete combustion of the fuel, which wastes a valuable commodity purchased with the department’s operating budget.
Too much air results in more heat removed from the boiler than is needed, resulting in removing energy from the boiler that will need to be re-introduced. The typical value to ensure complete combustion without excessive loss is 10 percent excess air — more air than required for stoichiometric combustion — throughout the entire firing range.
As the boiler load varies throughout the day, the needed mix of fuel and air changes. Continuous monitoring of oxygen (O2) is the best way to optimize this ratio. Operators can use carbon dioxide (CO2) or carbon monoxide (CO) as an indicator of available oxygen in the products of combustion. Tracking O2 content in the products of combustion minimizes fuel input required to satisfy the heating load. Optimized excess air — the O2 level — can reduce annual fuel use by 5 percent on average.
Another significant variable to control is the energy leaving the boiler in products of combustion through breaching. A high stack temperature indicates energy is not transferring to the water.
If operators trend stack temperatures and find they are rising over time, this can indicate heat-transfer surfaces that need cleaning.
High stack temperatures — combustion products of more than 600 degrees — result from fouled heat-transfer surfaces or a process that provides more combustion air than required. Stack temperatures of 350-450 degrees are more typical. Cleaning heat transfer surfaces via punching tubes and proper water chemistry can reduce stack temperatures and the parasitic losses associated with it.
A third control strategy involves optimizing the purge cycle and modulating the burner at a lower continuous level rather than cycling the boiler on and off frequently. Pre- and post-purging of the boiler’s firebox is required to safely ignite burners, but operating the burner continuously at a lower setting can minimize the number of purge cycles.
Boiler operation can be as simple as on/off or high/low/off, but the number of purge cycles can be significant. A burner’s effective range of firing has been improved from 4:1 to 20:1 with the advent of direct digital controls and technology enhancements. The savings associated with continuous operation, with limited purge cycles, are in the range of 1-2 percent of annual fuel consumption. It is a low-cost measure for a worthwhile energy efficiency upgrade.
The final control strategy involves reducing steam pressure or scheduling heating hot water based on outside air temperature. To implement one of these measures, operators need to validate the load requirements.
What is the actual thermal load of critical systems? This figure includes both pressure and energy rate in Btus. In hospitals, the autoclaves used for instrument sterilization can be a critical load. Elevated temperature and pressure figures at the point of use are required.
To determine if this ECM is viable in a particular setting, operators need to verify a building’s critical loads and available steam quality at the point of use. Lowering steam pressure by 75 psig results in a 1 percent savings on annual fuel input.
If a heating plant generates hot water, operators can establish critical-load requirements in a similar manner.
If the current control strategy is to supply a constant 180 degrees throughout the heating season to satisfy comfort conditions, then scheduling this heating water based on outside-air temperature is a simple ECM.
Operators can start with 180 degree water at peak heating load — say, 0 degree outside air — and 120 degree water for the minimum heating load — say, 50 degree outside air.
Operators can further refine this schedule depending on the thermal mass of the building and its internal loads. Savings from hot water reset can be 5-15 percent of annual values.
New HVAC Equipment and Technologies Improve Energy Efficiency
The third phase of ECMs to improve the energy efficiency of boilers and water heaters involves burner replacements and heat-recovery systems.
New-generation burners have incorporated DDC and flame-shaping technologies to greatly improve operating efficiency throughout a unit’s range of operation. Operators can use heat-recovery ECMs on both the air side — in the form of a stack economizer — or the water side, in the form of blowdown heat recovery.
Burner replacement is a higher-cost measure, but it yields attractive payback periods. Facilities in Europe have incorporated the technology advances in boiler burner design for years, validating their effectiveness. DDC technology has replaced mechanical linkages, providing significantly greater control of fuel-air mixtures. New burners control the shape of the flame and the turbulence imparted to the fuel-air mixture, enhancing fuel-to-water efficiency.
To account for varying load, variable-speed drives on the fuel valve and blower fan enable turndown rates of 20:1. Improved turndown means higher efficiencies at all but peak-firing rates, which happens for only a fraction of the heating season. Burner replacements have the greatest impact on reducing the use of heating fuel. While replacing burners represents a major investment, they offer simple payback periods of two-four years, depending on the burner-tip fuel rate and boiler use.
Energy is extracted from the products of combustion and directed to preheat combustion air or boiler feed water. Candidates for economizers have a high load factor — average load divided by maximum load — and stack temperatures greater than 500 degrees. The economizer includes two heat coils — one in the breaching and one in the make-up air — and a small circulating pump.
Combustion air typically can be heated to 300-400 degrees, depending on the burner’s construction. Installing a heat-recovery system lowers the stack temperature, and it increases the make-up-air temperature. Boiler efficiency increases by about 2 percent for every 100 degree decrease in the stack temperature.
A second method of heat recovery involves diverting energy contained in hot blowdown in order to preheat make-up water or for some other low-grade thermal energy use in the central plant. Candidates use continuous boiler blowdown and have a continuous load that can use this waste energy.
This system consists of shell-and-tube or plate-and-frame heat exchangers and a small pump to circulate water between the heat source and the sink. Boiler efficiency improves by about 1 percent for every 10 degree increase in make-up water. Heat recovery of 20-30 degrees is typical. Overall efficiency improvement is 2-3 percent.
Sidebar: Fine-Tuning the Energy-Efficiency Process in HVAC Equipment
Maintenance and engineering managers can employ numerous methods to improve the efficiency of boilers and water heaters. One essential, early step is engaging design professionals, equipment vendors and service providers to evaluate system conditions. Validating savings projections with a third party also is prudent. Managers should be leery of savings of 101 percent of annual heating costs that result from adding the savings from potential ECMs. There may be interaction between the measures, and vendors also frequently use ideal conditions, which is not the case in all systems.
But real, substantial savings are achievable. A trusted technical advisor with expertise in energy analysis can help navigate the options. After you have implemented the ECMs
One important facet many managers overlook is training the engineering staff regarding the new sequence of operation or equipment. With better understanding of the reasons changes were made, operators and technicians can make better decisions regarding operation of their facility.
As managers begin identifying and engineering ECMs, the need to keep in mind that this is the easy part. The challenge often is the communication — presenting a compelling case that resonates with the top management. Stiff competition for funds exists in every business. In health care, for example, should the hospital replace the 50-year-old boiler or install a new magnetic resonance imaging machine?
The struggle for funds between revenue-generating and revenue-consuming investments is a constant battle. When presenting ECMs for review, managers should avoid limiting the discussion to energy conservation. Include information on the return on investment for the project, and identify the way it benefits stakeholders and supports the organization’s mission, if possible.
Managers can proactively schedule a brief meeting with the CFO or controller to establish criteria for evaluating capital projects. Most importantly, managers must keep in mind that it takes a team to identify and implement these ECMs. To succeed, they need to engage a team that includes maintenance staff, trusted technical advisors and a representative of building occupants.