Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSBOILER SYSTEM CONTROLINTRODUCTIONA boiler is a closed vessel intended to heat water and producehot water or steam through combustion of a fuel or through theaction of electrodes or electric resistance elements. Steam andhot water boilers are available in standard sizes from very smallboilers for apartments and residences to very large boilers forcommercial and industrial uses.BOILER TYPESBoilers are classified by water temperature or steam pressure.They are further classified by type of metal used in construction(cast iron, steel, or copper), by type of fuel (oil, gas, orelectricity), or by relationship of fire or water to the tubes(firetube or watertube).Steel boilers come in a large variety of configurations. Theyare factory assembled and welded and shipped as a unit. Figure38 illustrates a firetube boiler. The fire and flue gases aresubstantially surrounded by water. The products of combustionpass through tubes to the back then to the front and once moreto the back before finally exiting at the front. This makes it afour-pass boiler. Firetube boilers are manufactured in manyother configurations such as:– External firebox, firebox not surrounded by water.– Dry back, firetubes directly available from clean-outdoors at back of boiler.– Scotch-Marine, low water volume and fast response.FIRE TUBESJACKET– Low-pressure boilers are those designed to producesteam up to 15 psig or hot water up to 250F withpressures up to 160 psig.– Medium and high pressure boilers produce steam above15 psig or hot water above 160 psig or 250F or both.CAST IRON AND STEEL BOILERSBoilers are typically constructed of cast iron or welded steel.Cast iron boilers (Fig. 37) are made of individually cast sectionsand are joined using screws or nuts and tie rods or screwednipples to join sections. The number of sections can be variedto provide a range of capacities.JACKETPhoto Courtesy of Cleaver BrooksFig. 38. Typical Firetube Boiler.M11416Watertube boilers are steel body boilers used for high capacityrequirements of more than two million Btuh. Watertube boilersuse a water-cooled firebox which prolongs the life of furnacewalls and refractories.MODULAR BOILERSM11415Photo Courtesy of Cleaver BrooksFig. 37. Typical Cast Iron Boiler (Watertube).Modular boilers are small hot water boilers rated at 200,000Btuh to 900,000 Btuh input. These boilers are available with 85percent or higher gross efficiency. Figure 39 shows features of atypical modular boiler. These boilers are often used in tandem toprovide hot water for space heating and/or domestic hot water.For example, if the design heating load were 2,000,000 Btuh,four 600,000 Btuh (input) modular boilers might be used. If theload were 25 percent or less on a particular day, only one boilerwould fire and cycle on and off to supply the load. The otherthree boilers would remain off with no water flow. This reducesflue and jacket (covering of the boiler) heat losses.ENGINEERING MANUAL OF AUTOMATIC CONTROL304