建筑环境与设备工程专业(Heating Ventilation and Air Conditioning)英语教材

Lesson 9 Boiler System (锅炉系统)I. Text[1] Boiler are used to supply steam or hot water for heating, processing, or power purposes. This chapter is primarily concerned with a description of the low-pressure steam and hot-water space heating boiler used in the heating systems of residences and small buildings.[2]The basic construction of both low-pressure steam and hot-water space heating boiler fired by fossil fuels consists of an insulated steel jacket enclosing a lower chamber in which the combustion process takes place; and an upper chamber containing cast-iron sections or steel tubes in which water is heated or converted to steam for circulation through the pipes of the heating system.Steam and Hot Water Boiler Similarities and Differences [3]Steam and hot-water space heating boiler are very similar physically, but there are sone important differences:Steam boiler operate only about three-fourths full of water, whereas hot-water boiler operate completely filled with water. Steam boilers in residential steam heating systems operate at 13.79 kPa pressure or slightly more, where as residentialhot-water boilers operate at approximately six times that pressure.Steam boilers are equipped with a low-water cutoff device to protect the appliance from burning out if it should run out of water. Only large hot-water space heating boilers with a capacity exceeding 422.04 MJ/h are presently required by code to be equipped with low-water cutoffs. (Note: Many HV AC contractors who install the smaller residential hot-water boilers strongly recommend the addition of a low-water cutoff device to these appliances to prevent burn out if the boiler loses its water.) Steam boilers require makeup feed to replace water lost through evaporation and the production of steam during normal operation. Hot-water boilers can operate with little or on need for makeup water under the same normal operating conditions.[4]The design and construction of the lower chamber depends upon the type of fuel used to fire the boiler. It serves as a combustion chamber for coal-fired and oil-fired boilers and as a compartment for housing the gas burner assembly on gas-fired boiler. These gas burner assemblies are commonly designed for easy removal so that they can be periodically cleaned or serviced.[5]Oil burners are externally mounted with the burner nozzle extending into the combustion chamber. This is also true of gas conversion burners. Gas burner assemblies, on the other hand, are located inside the lower chamber of the boiler.[6]The cast-iron sections or stell tubes in the upper chamber of the boiler contain water that circulates through the pipes in the heating system in the form of either steam or hot water.The heat from the combustion process in the lower chamber of the boiler is transferred through the metal surface of the cast-iron sections or steel tubes to the water contained in them, causing a rise temperature. The amount of water contained in these passages is one of the ways in which steam boilers and hot-water space heating boilers are distinguished from one another.In hot-water space heating boilers these passages are completely filled with water; whereas in low-pressure steam boilers only the lower two-thirds are filled. In steam boilers the water is heated very rapidly, causing steam to form in the upper one-third. The steam, under pressure, rises through the supply pipes connected to the top section of the boiler.[7]A boiler jacket contains a number of different openings for pipe connections and the mounting of accessories. The numberand type of openings on a specific boiler jacket depends upon the type of boiler (i.e. Steam or hot water ). Among the different openings to be found on a boiler jacket are the flue connections, water feed (supply )connection, inspection and cleanout tapping, blow down tapping, relief valve tapping, control tapping, drain tapping, expansion tank tapping, and return tapping. These are also gas and oil burner connections Fig.1 illustrates the arrangement of tapping in a Weil-McLain oil-fires boiler.[8]Most (but not all ) of the controls on low-pressure steam and hot-water space heating boilers fired by the same fuel are similar in design and function, but there are exceptions. For example, a few boiler controls and fittings are designed to be specifically used on steam boilers; other are found only on hot-water space heating boilers.Boiler Rating Method[9]The construction of low-pressure steel and cast-iron heating boilers is governed by the requirements of the ASME Boiler and Pressure Vessel Code. This is a nationally recognized code used by boiler manufacturers, and any boiler used in a heating installation should clearly display the ASME stamp. State and local codes are usually patterned after the ASME Code.[10]The location of the identification symbols used by the ASME is specified by the code and determined by the type of boiler. For example, on a water-tube boiler, it appears in a head of the steam-outlet drum near and above the manhole opening. On vertical fire-tube boilers, the stamp bearing the identification symbol should appear on the shell above the fire door and handhole opening. Other types of boilers (e.g. Scotch marine and super heaters ) have their own specified location for the identification symbol stamp.[11] The ASME Boiler and Pressure Vessel Code applies only to boiler construction, specifically to maximum allowable working pressures, not to uts heating capacity. A number of different methods are used to rate the heating or operating capacity of a boiler, the boiler manufacturers have developed their own ratings, but these are generally used along with rating methods available from several professional and trade associations. [12]The Steel Boilers Institute no longer exists, but its SBI rating is still found on many existing stell boilers. The I=B=R (or IBR ) logo was created by the now defunct Institute of Boiler and Radiator Manufacturers to indicate the gross output (s) at 100 percent firing rate for most sectional cast-iron boilers. The I=B=R rating logo is now used by the Hydronic InstituteDivision of the Gas Appliance Manufacturers Association (GAMA).[13]The Mechanical Contractors Association of America has devised a method for rating boilers not covered by ether the SBI or I=B=R codes. Finally, fas-fired boilers are rated in accordance with methods developed by the American Gas Association.[14]Other rating logos appearing on boilers and in their installation and operation manuals are the Underwriters Laboratories, Inc. (UL) and the Underwriter’s Laboratories of Canada logos.[15]In terms of its heating capacity, the rating of a boiler can be expressed on square feet of equivalent direct radiation (EDR) or thousands of Btu/h. Sometimes a boiler horsepower rating is also given, but this is has proven to be misleading.[16]For steam boilers, 0.09 m2 of equivalent direct radiation (EDR) is equal to the emission of 253.21kJ/h. For a water boiler, 0.09 m2 of EDR is considered equal to the emission of 158.26kJ/h.[17]A boiler horsepower (bhp) os the evaporation of 15.66kg of water onto dry steam from and at 100℃. For rating purposes, 1 bhp is considered as the heat equivalent of 13.02㎡of steamradiation per hour. In some cases bhp ratings are oltained by dividing steam SBI ratings by 140.[18]A boiler is rated according to its operating or heating capacity, but this rating will vary in accordance with the type of load used as the basis for the rating. The three types of connected loads used to determine the rating of a boiler are:(1)Net load(2)Design load(3)Gross load[19]Net load refers to the actual connected load of the heat-emitting units in the steam or hot-water heating system. Design load includes the met-load rating plus an allowance for piping heat loss. Finally, gross load will equal the net load and the piping heat loss, plus an additional allowance for the pickup load.Boiler Heating Surface[20]The boiler heating surface (expressed in square feet ) is that portion of the surface of the heat transfer apparatus in contact with the fluid being heated on one side and the gas or refractory being cooled in the other side. The direct or radiant hearing surface is the surface against which the fire strikes. The surface that cones in contact with the hot gases is called the indirect irconvection surface.[21]The heating capacity of any boiler is influenced by the amount and arrangement of the heating surface and the temperature on either side, the arrangement if the heating surface refers to the ratio of the diameter if each passage to its length, as well as its contour (straight or curved ), cross-sectional shape, number of passes, and other design variables.Boiler Efficiency[22]the boiler efficiency is the ratio of the heat output to the caloric value of the fuel. Boiler efficiency is determined by various factors including the type of fuel used, the method of firing, and the control settings. For example, oil-and gas-fired boilers have boiler efficiencies ranging from 70 to 80 percent. A hand-fired boiler in which anthracite coal is used will have a boiler efficiency of 60 to 75 percent.Boiler energy Efficiency[23]Two government programs have been created within the last 20 years to rate the energy efficiency of different heating appliances such as furnaces, boilers, water heaters, and heat pumps. These two programs are (1) the annual fuel utilization capacity (AFUE) programs and (2) the Energy Star Certificationprogram.[24]Annual Fuel Utilization Capacity (AFUE). The energy efficiency of an oil-, gas-, or coal-fired boiler is measured by its annual fuel utilization capacity (AFUE). The AFUE reatings for boilers manufactured today are listed in the boiler manufacturer’s literature. Look for the EnerGuide emblem for the efficiency rating of that particular model. The higher the rating, the more efficient the boiler. The government has established a minimum rating foe boilers of 78 percent. Mid-efficiency boilers have AFUE ratings ranging from 78 to 80 percent. Higher-efficiency (condensing ) boilers have AFUE ratings ranging from 88 to 97 percent. Conventional ( noncondensing ) steam and hot-water space heating boilers have AFUE ratings of approximately 60 to 65 Percent.[25]Energy Star Certification. Energy Star os an energy performance rating system created in 1992 by U.S. Environmental Protection Agency (EPA) to identify and certify certain energy-efficient appliances. The goal is give special recognition to companies who manufacture products that help reduce greenhouse gas emissions. This voluntary labeling program was expanded by 1995 to include furnaces, boiler, heat pumps, and other HV AC equipment. Both the Energy Star labeland an AFUE rating are used to identify an energy-efficient appliance.Types of Boiler[26]The boiler used in low-pressure steam and hot-water space heating systems can be classified in a number of different ways. Some of the criteria used in classifying them are:(1)Construction material(2)Construction design(3)Boiler position(4)Number of passes of the hot gases(5)Length of travel of the hot gases(6)Type of heating surface(7)Type of fuel used[27]Most boilers are constructed of either cast iron or steel. A few are constructed from nonferrous materials such as aluminum. Cast-iron boilers generally display a greater resistance to the corrosive effects of water than steel ones do, but the degree of corrosion in steel boilers can be significantly reduced by chemically treating the water. [28]The heating core of many boilers is formed by joining together a series of cast-iron sections either horizontally (so-called pancake construction ) or vertically. In the horizontalcast-iron section design, the heating surface of each cast-iron section is exposed at right angles to the rising flue gases. The water travels in a zigzag path from section to section in a manner similar to the flow of water on a steel tube boiler. [29]Steel boilers may be classified with respect to the relative position of water and hot gases in the tubular heating surface. In fire-tube boilers, for example, the hot gases pass within the boiler tube while the water being heated circulates around them. In water-tube boilers, the reverse is true. Flexible steel tubes are used in some boilers for the circulation of the water around the heat rising from the fire.[30]A hot-water (hydronic) copper-fin tube operates on a different principle from the cast-iron and steel boilers. It is designed to transfer heat almost instantly to the water. Water flows across the boiler heat exchanger, picks up heat, and then moves through the pipes to the heat convectors, radiators, or panels.Note If the water stops flowing while the burner is still running, heat will build up until the water flashes into steam and damages the boiler. This condition is similar to dry firing in cast-iron and steek boilers. It can be avoided by installing a flow switch in the path of the water. The switch turns off the burner when the waterstops flowing.[31]Boilers can also be classified according to the number of passes made by the hot gases (e.g. One pass, two passes, and three passes). The length of travel of the hot gases is another method used for classifying boilers. The efficiency of a boiler heating surface depends, in part, upon the ratio of the cross-sectional area of the passage to its length.[32]Among the various fuels used to fire boilers are oil, gas ( natural and propane),coal, and coke. Conversion kits for converting a boiler from one gas to another are available from some manufacturers. Changing from coal (or coke ) to oil or gas can be accomplished by using conversion chambers and making certain other modifications.[33]Electricity can also be used to fire boilers. One advantage in using electric-fired boilers is that the draft provisions required by boilers using combustible fuels is not necessary. Unlike the boilers fired by fossil fuels (oil, gas, coal, etc.), electric boilers do not have an AFUE efficiency rating. They operate at almost 100 percent efficiency.[34]The classification criteria described above are selective and limited to the more common types in use. Considering the multiplicity of boiler types and designs available, it is extremelydifficult to establish a classification system suitable for all of them.。