Boilers are pressure vessels designed to heat water or produce steam, which can then be used to provide space heating and/or service water heating to a
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Boilers Introduction How Boilers Work Types of Boilers Key Components of Boilers Safety Issues Best Practices for Efficient Operation Best Practices for Maintenance References Introduction Boilers are press ure vessels designed to heat water or produce steam, which can then be used to provide space heating and/or service water heating to a building . In most commercial building heating applications, the heating source in the boiler is a natural gas fired burn er. Oil fired burners and electric resistance heater s can be used as well . Steam is preferred over hot water in some applications , includ ing absorption cooling, kitchens, laundries, sterilizers, and steam driven equipment. Boilers have several strength s that have made them a common feature of buildings. They have a long life, can achieve efficiencies up to 95% or greater , provide an effective method of heat ing a building, and in the case of steam systems , re quire little or no pumping energy. However, f uel costs can be considerable, regular maintenance is required, and if maintenance is delayed, repair can be costly. Guidance for the construction , operation, and maintenance of boilers is provided primarily by the ASME (American Society of Mechanical En gineers) , which produces the following resources: Rules for construction of heating boilers, Boiler and Pressure Vessel Code , Section IV – 2007 Recommended rules for the care and operation of heating boilers, Boiler and Pressure Vessel Code , Section VII – 2007 B oilers are often one of the largest energy users in a building . For every year a boiler system goes unattended, boiler costs can increase approximately 10% (1) . Boiler operation and maintenance is therefore a good place to start when looking for ways t o reduce energy use and save money. How Boilers Work Both gas and oil fired boiler s use c ontrolled combustion of the fuel to heat water. The key boiler components involved in this process are the burner, combustion chamber, heat exchanger, and control s .
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Figure 1 : Firetube Boiler ( image source: www.hurstboiler.com ) The burner mixes the fuel and oxygen together and , with the assistance of an ignition device , provides a platform for combustion . This combustion takes place in the combustion chamber , and the heat that it generate s is transferred to the water through the heat exchanger. Controls regulate the ignition, burner firing rate, fuel supply, air supply, exhaust draft , water temperature, steam pressure, and boiler pressur e . Hot water produced by a boiler is pumped through pipes and delivered to equipment throughout the building , which can include hot water coils in air handling units, service hot water heating equipment, and terminal units. Steam boilers produce steam t hat flows through pipes from areas of high pressure to areas of low pressure, unaided by an external energy source such as a pump. Steam utilized for heating can be directly utilized by steam using equipment or can provide heat through a heat exchanger tha t supplies hot water to the equipment . The discussion of different types of boilers , below, provide s more detail on the designs of specific boiler systems. Types of Boilers Boilers are classified into different types based on their working pressure and tem perature, fuel type, draft method, size and capacity, and whether they condense the water vapor in the combustion gases . Boilers are also sometimes described by their key components, such as heat exchanger materials or tube design. These other characteri stics are discussed in the following section on Key Components of Boilers . Two primary types of boilers include Firetube and Watertube boilers . In a Firetube boiler , hot gases of combustion flow through a series of tubes surrounded by water. Alternativel y, in a Watertube boiler,
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water flows in the inside of the tubes and the hot gases from combustion flow around the outside of the tubes. A drawing of a water tube boiler is shown in Figure 2 . Figure 2 : Watertube Boiler Firetube boilers are more common ly available for low pressure steam or hot water applications, and are available in sizes ranging from 500,000 to 75,000,000 BTU input (5). Watertube boilers are primarily used in higher pressure steam applications and are used extensively for comfort hea ting applications. They typically range in size from 500,000 to more than 20 ,000,000 BTU input (5). Cast iron sectional boilers (figure 3 ) are another type of boiler commonly used in commercial space heating applications. sections that have water and combustion gas passages. The iron castings are bolted together, similar to ng steam or hot water, and are available in sizes ranging from 35,000 to 14,000,000 BTU input (2). Cast iron sectional boilers are advantageous because they can be assembled on site, allowing them to be transported through doors and smaller openings. Th eir main disadvantage is that because the sections are sealed together with gaskets, they are prone to leakage as the gaskets age and are attacked by boiler treatment chemicals.
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Figure 3 : Cast Iron Sectional Boiler (image source: chestofbooks.com) Wo rking Pressure and Temperature Boilers are classified as either low pressure or high pressure and are constructed to meet ASME Boiler and Pressure Vessel Code requirements . Low – pressure boilers are limited to a maximum working pressure of 15 psig (pound – force per square inch gauge) for steam and 160 psig for hot water (2) . Most boilers used in HVAC applications are low – pressure boilers. High – pressure boilers are constructed to operate above the limits set for low – pressure boilers, and are typically used for power generation. Operating water temperatures for hot water boilers are limited to 250 o F (2) . Fuel Type In commercial buildings , natural gas is the most common boiler fuel, because it is usually readily ava ilable, burns cleanly, and is typically le ss expensive than oil or electricity . Some boilers are designed to burn more than one fuel (typically natural gas and fuel oil). Dual fuel boilers provide the operator with fuel redundancy in the event of a fuel supply interruption . They also allow t he customer to utilize the fuel oil during for natural gas. In times when the rates for natural gas are greater than the alternate fuel, t his can reduce fuel costs by using the cheaper alternate fuel and limiting natural gas use to occur only . Electric b oiler s are used in facilities with requirements for a small amount of steam or where natural gas is not available . Electric boilers are known for being clean, quiet, and easy to install, and compact. The lack of combustion results in reduced complexity in design and operation and less maintenance. H eating elements are easily replaced if they fail . These types of boilers can be used to produce low or high pressure steam or water, and may be good alternatives f or customers who are restricted by emissions regulations. Sizes range from 30,000 to 11 ,000,000 BTU input with overall efficiency generally in the range of 92% to 96% ( 2 ).
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Draft Methods The pressure difference between the boiler combustion chamber and th e flue (also called the exhaust stack) produces a draft which carries the combustion products through the boiler and up the flue . Natural draft boilers rely on the natural buoyancy of hot gasses to exhaust combustion products up the boiler flue and draw fresh air into the combustion chamber . Mechanical draft boilers include: Forced Draft, where air is forced into the combustion chamber by a fan or blower to maintain a positive pressure; and Induced Draft , where air is drawn through the combustion chambe r by a fan or blower to maintain a negative pressure. Size and Capacity Modular Boilers are sma ll in size and capacity and are often intended to replace a large single boiler with several small boilers. These modular boilers can easily fit through a stan dard doorway , and be t ransported in elevators and stairways. The units can be arranged in a variety of configurations to utilize limited space or to accommodate new equipment. Modular boilers can be staged to efficiently meet the demand of the heating lo ad. Condensing Method Traditional hot water boilers operate without condensing out water vapor from the flue gas . This is critical to p revent corrosion of the boiler components. Condensing Boilers operate at a lower return water temperature than traditio nal boilers, which causes water vapor to condense out of the exhaust gasses . This allows the condensing boiler to extract additional heat from the phase change from water vapor to liquid and increas es boiler efficiency . Some carbon dioxide dissolves in t he condensate and forms carbonic acid. While some condensing boilers are made to handle the corrosive condensation, others require some means of neutralizing the condensate. Traditional non – condensing boilers typically operate in the range of 75% 86% c ombustion efficiency, while condensing boilers generally operate in the range of 88% to 95% combustion efficiency ( 2 ). Key Components of Boilers The key ele ments of a boiler include the burner, comb ustion chamber, heat exchanger, exhaust stack, and contro ls. Boiler accessories including the flue gas economizer are also commonly used as an effective method to recover heat from a boiler and will be discussed briefly in the section Best Practices for Efficient Operation. Natural gas boilers employ one of two types of burners , a tmospheric burners , also called natural draft burners a nd f orced draft burners , also called power burners . Due to more stringent federal and state air quality regulations, low N O x burners and pre – mix burners are becoming more commonly used and even required in some areas. By ensuring efficient mixing of air and fuel as it enters the burner, these types of burners can ensure that NOx emissions are reduced.
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Figure 4 : Forced Draft Boiler ( image source: www.Hurstboiler.com) The combusti on chamber , usually made of cast iron or steel, houses the burners and combustion process. Temperatures inside the combustion chamber can reach several hundred degrees very quickly. H eat exchangers may be made from cast iron , steel tube bundles, or , in th e case of some smaller boilers , copper or copper – clad steel. The exhaust stack or flue is the piping that conveys the hot combustion gasses away from the boiler to the outside. Typically this piping is made of steel, but in the case of condensing boiler s it needs to be constructed of stainless steel to handle the corrosive condensate. Another consideration i s whether the exhaust stack will be under a positive or negative pressure. This can determine how the joints of the exhaust stack must be sealed. Boiler controls help produce hot water or steam in a regulated, efficient, and safe manner. Combustion and operating controls regulate the rate of fuel use to meet the demand . The main operating control monitors hot water temperature or steam pressure and sends a signal to control the firing rate , t he rate at which fuel and air enters the burner. Common burner firing sequences include on/off, high/low/off and modulating . Boiler safety controls include high pressure and temperature, high and low gas/oi l pressure, and high and low water level and flame safeguard controls . These controls are considered safeties or limits that break the electrical circuit to prevent firing of the boiler. For example, in the event pressure in the boiler exceeds the pressure limit setting , the fuel valve is closed to prevent an unsafe, high pressure condition. The safety circuit of a flame safeguard control system typically includes switch contacts for low water cutoff, high limits, air proving switches, redundant safety and operating controls, and flame detectors. Flame detectors often consist of flame rods, and ultraviolet or infrared scanners to monitor the flame condition and deactivate the burner in the event of a non ignition or other unsafe condition . Flame safeguard controls are programmed to operate the burner and cycle it through the stages of operation.
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oxygen, then incomple te combustion will occur resulting in other products of combustion including carbon monoxide and unburned fuel . When incomplete combustion occurs, the chemical energy of the fuel is not completely released as heat and the combustion efficiency is reduced. This is also a safety concern as unburned fuel could ignite in the stack and cause an explosion. Boilers must be tuned to achieve complete combustion. One strategy to ensure complete combustion is to provide some amount of excess air. However, as shown in the figure below, a small amount of excess air will improve combustion efficiency, but a large amount will reduce efficiency. Figure 5 : Combustion Efficiency vs. Excess Air For high overall boiler efficiency, the heat released by combustion must be ef ficiently transferred into the working fluid. Any heat not transferred into the fluid will be lost through the boiler shell or the flue gas . The temperature of the flue gasses in the boiler stack is a good indicator of this heat transfer and thus the eff iciency . There are practical limits to how low the stack temperature can be. The temperature will be higher than the working fluid in the boiler. In non – condensing boilers, it must be high enough so that the water vapor in the exhaust gas does not conde nse and bath e the heat transfer surface in the corrosive condensate . Condensing natural gas boilers are designed and built with materials designed to resist corrosion . As such, they may have exhaust temperatures less than 150 °F . Capturing the heat from the condensate can result in combustion efficiencies of greater than 90%.
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Figure 6 : Combustion Efficiency Chart for Natural Gas ( image source: Federal Energy Management Program, U.S. Department of Energy ) Figure 6 is a combustion efficiency chart for nat ural gas fuel with power burners that shows the relationship between excess air , flue gas temperature , and combustion efficiency. As an example, tracing the Step 1 line, at 9% flue gas oxygen (equivalent to about 67% excess air as seen in the graph) and 5 00 o F flue gas temperature rise, the corresponding combustion efficiency is about 76.5%. Using the same 500 o F flue gas temperature rise, Step 2 illustrates that dropping to 2% flue gas oxygen results in an improved combustion efficiency of about 81.5%. Th is is shown as Step 2 in Figure 6 above. As percent flue gas oxygen decreases, less heat is transferred to the excess oxygen, and the combustion efficiency increases. As combustion efficiency increases, more heat is transferred to the feedwater instead o f the flue gas, and therefore the flue gas temperature decreases. Use Boiler Controls for Optimized Air – to – Fuel Ratio T o ensure that complete combustion occurs, extra air is introduced at the burner. But too much will result in air being wastefully heate d and exhausted out of the boiler flue , penalizing combustion efficiency , and creating a safety issue . When a boiler is tuned, the goal is to maximize combustion efficiency by providing just enough excess air to assure complete combustion but not too much to reduce efficiency . How much excess air is enough to assure complete combustion? That varies with the
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design and condition of the burner and boiler , as well as with the different firing rates of the burner , but is typically considered to be between 2% – 3% . Excess air must also be adjusted to allow for variations in temperature, density, and humidity of the boiler combustion air through out any daily and seasonal variations . firing range. The important idea to remember is that complete combustion is critical to ensuring efficient boiler operation. Incomplete combustion of the fuel can significantly reduce boiler efficiency by 10% or more, while increasing excess air by 10% may only impact boiler efficiency by about 1%. Signs of incomplete combustion are a smoky exhaust, a yellow flame, flame failures, and soot y boiler tubes. It is a good idea to tune up a boiler annually to ensure the combustion process is optimized. T ypically, excess air of around 10% for a natural gas boiler is optimal to ensure complete combustion and peak efficiency. This corresponds to excess O2 of around 2% to 3%. O perating with excess air beyond 10% is undesirable, as it can result in reduced e fficiency and higher emissions. Therefore maintaining the optimum level of excess air across the entire firing range is preferred . This can be accomplished with the use of burner controls including parallel positioning controls, cross – limiting controls, and oxygen trim controls. These types of controls are superior alternatives to traditional mechanical jackshaft controls. A brief description of each burner control type is provided below (3) : Mechanical jackshaft control is the simplest type of modulati ng burner control, typically used on smaller burners. Also called single point control because one mechanical linkage assembly controls both air and fuel. These controls cannot measure airflow or fuel flow. The range of control is limited, resulting in excess ive levels of excess air to ensure safe operation under all conditions and firing rates. Slop in the linkages makes accurate and repeated control difficult, and requires regular maintenance and adjustment. Parallel positioning controls use separate motors to adjust fuel flow and airflow allowing each to be adjusted over the entire firing range of the boiler. During setup, many points are typically 10 to 25 points, to create a curve of airflow and corresponding fuel flow. The air – fuel rati o can therefore vary across the entire firing range to provide the optimal ratio under all firing conditions . Also , with the use of electronic servo – motors, this method of control is highly repeatable. Cross – limiting control s, usually applied to larger bo ilers, use controls to sense and compensate for some of the factors that affect optimum air to fuel ratio. Air flow and fuel flow are measured and adjusted to maintain the optimum value determined during initial calibration. Oxygen trim control is used in conjunction with standard parallel positioning or cross – limiting controls . It analyzes the oxygen in the flue gas and adjusts the air – fuel ratio accordingly to maintain a set amount of excess oxygen . These controls are usually installed on larger boiler s with high annual fuel usage, and can increase energy efficiency by one or two percent beyond what is achieved with the standard control alone .
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Monitor Boiler Gauges It is possible that a leak will develop in the hot water distribution loop . Such leaks w ill increase the Hot water and steam distribution systems should be provided with make – up water to replace any steam or water that is lost through a leak in the system. This will provide an easy way to ensure the system is fully charged with water at all times. It is best practice to install a meter on the make – up line to the system. The meter should be read weekly to check for unexpected losses of water from the system . In stea m systems, i t is a best practice to monitor make – up water volume daily . As steam leaks from the system, additional make – up water is required to replace the loss. Monitoring the make – up water will ensure that you are maximizing the return of condensate, t hereby reducing the need for make – up water. Seasonal Operation If a steam or hot water system is not used for a portion of the year, shutting the system down can result in significant savings. Maintaining a boiler at its operating temperature consumes e nergy equivalent to its standby losses. In the case of a hot water system, energy use may also include pump operation. Operating Multiple Boiler Plants B oiler loads in commercial buildings vary greatly from summer to winter, from day to night, and from weekday to weekend. With a single boiler it is difficult to efficiently supply these varying loads. When the building heating needs drop below the heat supplied by the boiler at its lowest firing rate, the boiler cycles off. Cycling a boiler on and off is very inefficient because there is a pre – ignition purge and a post – ignition purge that draw heat out of the boiler with each cycle. Also, in the case of a non – modulating boiler, at part load and steady firing rate when combustion efficiencies are at their best. If a facility has multiple boilers, it may be possible to sequence the boilers to avoid frequent cycling. If using non – modulating boilers , it may be better to stage subsequent boilers on once the prima ry boiler has reached full capacity , rather than cycling multiple boilers on and off to meet the load. On the other hand, with modulating boilers , boiler efficiency increases at part load conditions. Therefore it may be advantageous to operate multiple b oilers simultaneously at part load conditions rather than one boiler at 100% output. Figure 7 below shows the relationship between firing rate and efficiency for a boiler with the ability to modulate both airflow and fuel input.
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