February 2020 — Many energy management opportunities (EMOs) for heating systems require little time and expense to implement. It may sound overly simple, but the first EMO to consider is regular maintenance. A properly maintained heating system will consume less energy, cost less money to operate, and last longer than an improperly maintained system.
Keep Boiler Tubes Clean (Water Side)
Scale formation is the fouling or lining on interior surfaces of heat transfer and water storage equipment caused by water deposits. Preventing scale formation inside the boiler drum can produce substantial energy savings. The exact savings amount will depend on how much scale is prevented from forming, the amount of scale that does form between cleanings, and the chemical composition of the formation.
Ideally, you should make periodic visual inspections to ensure that scale is not forming on boiler tubes. Signs that a boiler may have excessive scaling include reduced steam output, excessive fuel use, and/or an increase in stack temperature. If scale is present, consider modifying the feedwater treatment and/or the schedule of chemical additives. The cost of modification can vary widely, depending on such factors as the type of treatment facilities already available and the chemical problems present, if any. The advice of a water treatment consultant can often be helpful.
Use Blowdown Controls
In addition to chemical treatment, blowdown controls also reduce the scale that forms from concentrated impurities in the boiler water. Boilers are commonly equipped with either an intermittent manual blowdown or a continuous blowdown device that eliminates concentrations of dissolved and suspended solids and removes boiler sludge.
A manual blowdown system is more energy efficient than a continuous system since the operator can select the frequency of each blowdown based on need. Manual or sludge blowdown is necessary even when continuous blowdown devices are installed. How often sludge blowdown is scheduled depends on the amount of solids in the boiler makeup water and the type of water treatment used.
Continuous blowdown systems are automatic and produce a steady drain in energy because blowdown makeup water must be heated. Blowdown energy losses can be minimized, however, by installing automatic blowdown controls to monitor the conductivity of the water and blow down the boiler only when needed.
Additional savings are possible when the blowdown water is piped through a heater or a flash tank with a heat exchanger. In this way, heat from the boiler blowdown water can be used for other heating requirements, such as service water heating.
Other EMOs for Heating Systems
In addition to preventive maintenance measures, the following EMOs can be used to increase system performance and decrease operating costs.
Reset Heating Supply Water Temperature
Many buildings have their heating supply water temperature set to ensure comfort during the coldest periods, although they occur infrequently. When the heating supply water temperature is reset to match anticipated average maximum demands, heating energy costs are reduced up to 14 percent and occupant comfort is improved. Additional savings are also realized through reduced transmission line losses and efficient thermostatic control.
Isolate Off-Line Boilers
Unused boilers that are left idling needlessly consume energy. The heat loss from idling boilers is compounded by the continuous flow of air induced through the boilers and up the stack. You save energy and money by operating the appropriate number of boilers for the heating load requirement. During periods of low heating loads, only one boiler from a multiple boiler system may be needed to meet heating demands.
Unless a boiler is about to be used to meet an expected increase in load, you should secure and isolate it from the heating system and the stack by closing valves and dampers. Boilers can be fitted with a bypass valve or a regulating orifice to accomplish this purpose. These fittings permit the minimum waterflow required to keep the boiler warm and avoid thermal stress when it is brought back on line again. To avoid freezing the boiler, prevent airflow through the stack when the boiler water side is isolated.
Improve Combustion Efficiency
Heat loss is a common characteristic of boilers. Industrial boilers are likely to convert only 80 percent of their fuel heat into useful heat output.
A central furnace or boiler’s efficiency is measured by annual fuel utilization efficiency (AFUE). AFUE is the ratio of annual heat output of the furnace or boiler compared to the total annual fossil fuel energy consumed by a furnace or boiler. An AFUE of 90 percent means that 90 percent of the energy in the fuel becomes heat for the home, and the other 10 percent escapes up the chimney and elsewhere.
AFUE does not include the heat losses of the duct system or piping, which can be as much as 35 percent of the energy for output of the furnace when ducts are located in the attic, garage, or other partially conditioned or unconditioned space.
The Federal Trade Commission requires new furnaces or boilers to display their AFUE so consumers can compare heating efficiencies of various models. AFUE is a measure of how efficient the appliance is in converting the energy in its fuel to heat over the course of a typical year.
Retrofit Boiler Tubes
In a typical boiler, the heated gases within the boiler tubes cool and contract to the shape of the tube. This action forms a hot core that slides down the center of the flue tube, limiting the amount of heat transferred to the tube walls. Only the water along the hot combustion chamber and the upper rear flue tubes effectively absorb the heat, increasing energy losses out of the stack. A gas turbulation device can eliminate this problem.
Turbulation devices may reduce fuel consumption by up to 16 percent. A typical device consists of 14-gauge flat metal strips that are formed into opposing 30°, 45°, and 60° bends. Inserted into the boiler return tubes, these devices create turbulence by breaking up the direct flow of hot combustion gases and forcing more contact with the tube walls. The increased contact with the tube walls causes more heat transfer to the water side of the boiler.
Turbulation devices require no permanent boiler modification and are relatively easy to install and remove. By quickly spreading heat evenly across all heating surfaces, they increase boiler output and productivity and reduce boiler running time, cycle purges and purge sequences, and fuel consumption.
Replace Single Boilers with Modular Boilers
Most boilers achieve maximum efficiency only when running at their rated output. Full capacity is seldom required, however, since most of the time boiler heat load is only 60 percent of full load. As a result, large capacity boilers operate intermittently or at low output during most of the heating season, resulting in low seasonal efficiencies.
A modular or multiple boiler system consisting of two or more boilers increases overall seasonal efficiency since each boiler is fired near capacity as much as possible. As the heating load increases, separate boilers are brought on line in sequence until the load is satisfied. In a typical installation, replacing a single large-capacity boiler with modular boilers may improve seasonal efficiency by 5 to 10 percent. Consider this option when an existing boiler system is due for replacement.
Applying these EMOs to your facility should help increase the performance of your HVAC systems and help you realize a greater return on investment in your equipment. Basic maintenance and common sense go a long way in the proper management of energy.
This article is adapted from BOMI International’s Energy Management and Controls course, part of the SMA and SMT designation programs. More information regarding this course or BOMI International’s new High-Performance Sustainable Buildings credential (BOMI-HP™) is available by calling 1-800-235-2664. Visit BOMI International’s website, www.bomi.org.