Consulting Specifying Engineer April 2015-CSE : Page 69

Boiler types that provides heating and process media to the campus through direct buried or tunnel distribution systems. Care needs to be taken in evaluating central boiler plants versus building-specific boiler plants. Each system type carries pros and cons with redundant capacity, operator availability, capital cost, lifecycle costs, and existing system configuration weigh-ing the new design. In either scenario, the boilers specified to generate steam, high-temperature hot water, and hot water vary in construction and size depending on the amount and temperature of media required. Packaged boiler steam systems vary in pressure from 15 to 1000 psig. Piping for high-, medium-, and low-pressure steam sys-tems in industrial settings is governed by ASME B31.3: Process Piping Design. In addition, ASME B31.9: Building Ser-vices Piping may be followed depending on system pressure and temperature. Boilers are designed to ASME Boiler and Pressure Vessel Code requirements. Piping and boiler system materials are dictated by the design pressures and tem-peratures desired for the system. High-temperature and pressure systems may require the use of alloy steel piping to distribute steam between the boiler and processes. Piping systems of boilers that will be used to generate power will be designed to ASME B31.1: Power Piping. Each of the respective codes will iden-tify the allowable operating and design stresses for materials based on tempera-ture. Steam and hot water distribution at lower temperatures for industrial and process systems can use both carbon and stainless steel piping depending on final service requirements. Boiler types Figure 2: In the top row, different styles of water tube boilers are shown. Shown at the bot-tom left are O-and A-style boiler flue gas flow patterns. The bottom right shows a D-style boiler flue gas flow pattern. Courtesy: Babcock & Wilcox Co. Boilers come in two primary designs: fire tube and water tube. In a fire tube boiler, the boiler gases and heat are with-in the tubes in the boiler; while in water tube boilers, boiler feed water is within the tubes and drums of the boiler. Water tube boilers primarily are used to generate steam, while fire tube boilers are used for both steam and hot water (see Table 1). Fire tube boilers are most commonly found in single building applications where steam is used for small process systems and building heat. These boil-ers are typically about 50% to 60% of the cost of a water tube boiler due to the smaller size and less steel content for a given boiler capacity. Steam generating fire tube boilers have less water in the boiler than a water tube boiler, which limits the amount of load swing that the boiler is able to handle. Yearly cleaning and inspection on the fire tube boiler is easier than on a water tube boiler, and tube repair can be performed from the boiler exterior with adequate tube pull space rather than from within the boiler (see Figure 1). Fire tube boilers come in 2-, 3-, and 4-pass flue gas designs. Increasing the number of passes in the boiler increases boiler efficiency while also increasing cost. Steam capacity for fire tube boil-ers generally ranges between 5,000 and 75,000 lb/h. Hot water boilers are sized in million Btus and range from 2 to 3 MMBtu up to 100 MMBtu. Water tube boilers come in four basic designs: D-style, A-style, O-style, and Modular layouts. D-, A-, and O-style boil-ers each contain upper and lower drums with steam generation tubes between the drums and on the outside walls. There are two areas in a water tube boiler, the con-vection pass and the furnace. The furnace section of the boiler contains the burner flame with boiler tubes on the outer wall (cooling the outer skin of the boiler) and separating walls of the boiler. In the convection (or generation) section of the boiler, tubes run between the upper and lower drums transferring heat from the flue gas into the feed water. Modular boil-ers are typically smaller in capacity with inlet and outlet manifolds connected by generating tubes. The small water capac-ity of a modular boiler allows it to heat up and produce steam much quicker than a water tube boiler. Packaged water tube boilers range in capacity from modular boilers at 3,000 to 300,000 lb/h in D-style boilers (see Figure 2). Water tube boilers generate steam at 15 to 1000 psig and can be fitted with radiant or convective superheat systems to produce superheated steam. Super-heated steam is steam that is heated above saturation temperature to create dry steam. Superheated steam is most often used in steam turbines for gener-ating electricity, but can be used in dis-tribution systems to reduce steam trap load. Convective superheat sections are more desirable than radiant sections because the convective section is not subject to direct flame impingement that 69 Consulting-Specifying Engineer • APRIL 2015

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