Consulting Specifying Engineer April 2015-CSE : Page 70

Selecting a boiler the radiant section may see with an out-of-adjustment burner. Water tube boilers hold more water in the upper and lower drums and in the tubes connecting them. This allows the boilers to take larger swings in steam output without creating steam quality issues. Water tube boilers are most often the selection of choice as prime boilers in steam district energy and campus envi-ronments because of their capacity and operability range. Their ability to run at higher pressures and temperatures with superheat makes them ideal as backup boilers to electrical generating systems and as primary steam movers for generat-ing steam in cogeneration systems. Boilers with superheat sections may require steam temperature control between superheat sections in the boiler or at the superheater outlet. Temperature control of this type is called attemperation. Steam attemperators or de-superheaters reduce the steam temperature by spraying water into the steam line. The spray water evapo-rates in the steam line, cooling the steam to the desired temperature (see Figure 3). The best water to use for attempera-tion (regulation) is pure condensate. This does not include boiler feed water as feedwater normally contains make-up water that may contain impurities. If clean condensate is not available, another source of attemperation water is a “sweet water” condenser. A sweet water condenser condenses saturated of construction to combat the acidic con-densate generated as the flue gas is con-densed, capturing the latent heat in the flue gas. Return water temperature and total system design play a large role in the selection of the boiler and final system efficiency goal. Water tube boilers are most often the selection of choice as prime boilers in steam district energy and campus environments. steam out of the boiler, while heating boiler feedwater, to create condensate, which is then metered into the super-heated steam outlet of the boiler through a control valve. Steam boiler efficiencies in the 80% to 84% level are common with the higher-efficiency boilers including feedwater economizers. Condensing boilers use integral feedwater economizers (dis-cussed further below) that take advan-tage of condensing the latent heat out of the flue gas stream to bring boiler effi-ciencies above 90%. These condensing boilers do require higher alloy materials In stand-alone building systems, unless steam is required for some process within the building, hot water boilers often are used. Hot water generated at 130 F and returned to the boiler at 90 F provides ideal conditions for a condensing boiler where efficiencies are above 90%. Added advantages to this system type are safety and reduced energy loss in distribution. The 130 F outlet temperature reduces burn potential and heat loss in the supply side piping. The system still maintains the common 40 F delta T used in most HVAC systems, but will require that the air handling unit heating, dehumidifica-tion coils, and terminal equipment be sized appropriately. One measure used to rate boilers is the heat release rate of the boiler, which is measured in Btu/ft3/hr. The volume of the convective and radiant furnace area of the boiler is used in this standard calculation. The radiant area of a boiler is the area in which the burner flame is in direct prox-imity to the boiler tubes or plenum. As the flue gas turns the corner in a water tube boiler or begins the second pass in a fire tube boiler, the convective section of the boiler begins. Improper flame adjust-ment can allow the flame to enter into the convective section of the boiler, but this is not normal operation and is not advised. The more metal or heat transfer surfaces within the convection and radi-ant sections of the boiler, the higher the heat release rate will be in the boiler. Adding a feedwater economizer will increase boiler efficiency by using boiler exhaust gases to heat boiler feed water D-Type superheater Figure 3: In this graphic, the upper left shows a D-type water tube boiler showing superheater location. The right highlights a D-style boiler superheater section. At lower left, the boiler drum tube connection openings are shown. In the lower center are D-type water tube boiler generation section tubes with opening for superheater section placement. Courtesy: Babcock & Wilcox Co. 70 Consulting-Specifying Engineer • APRIL 2015

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