Consulting-Specifying Engineer - April 2008 - (Page 72) Equipment Lifecycles Boiling it all down Points to consider when specifying boilers and boiler systems. TIMOTHY M. SCRUBY, PE, Senior Project Manager, Facility Dynamics Engineering, Afton, Va. W hen building, designing, and specifying boilers and boiler systems, consider not just the monetary lifecycle, but also the environmental lifecycle of the design. Systems should be designed from cradle to grave, or better yet “cradle to cradle.” To summarize some of the concepts in “Cradle to Cradle: Remaking the Way We Make Things” by William McDonough and Michael Braungart1, as applied to boiler systems, consider the design life of the facility and what happens to things that wear out during the life of the facility. If the facility will outlive the system designed into it, what is the replacement plan? What happens to the old worn out system? Premature failures mean reduced life and emergency replacements. Engineers must design to prevent premature failures. Many times, proven boiler system designs can and do stay in service for 30 years or more—if properly designed, operated, and maintained. In boilers, most premature failures come from thermal shock, poor water quality, and misapplication. System designs that hit a hot boiler with a slug of cold water or system designs that don’t maintain a minimum flow through the boiler can cause thermal shock. For applications that require these specific designs, specify boilers designed for reduced susceptibility to thermal shock. Poor water quality in hot water boilers generally is caused by either failure to Mixing valve controls water supply temperature 180 F design Boiler (100 hp) Burner controls to 180 F supply temperature Minimum flow can go to nearly 0 (WRONG) W 72 160 F design 160 F design 70 F summer (WRONG) 70 F summer condition O R 120 F summer reset Variable flow system pump (330 gpm design, Typical no minimum flow variable provisions) flow load connection G N 180 F design 180 F design L o a d c o i l Boiler (100 hp) Two-way control valve Boiler circulating pump ensures the minimum flow IG R Burner controls to 180 F supply temperature T H 160 F design 70 F summer condition 180 F design 120 F summer reset L o a d c o i l Typical variable flow load connection Mixing valve controls to limit return minimum temperature and modulate flow to control distribution system temperature Two-way control valve The wrong configuration (left) is an all-too-common arrangement where design conditions violate rules for minimum flow and temperature, causing premature failure from thermal shock and condensation-induced corrosion. The correct arrangement (right) is one of several using a circulating pump to protect the boiler from low return temperature and low flow. This can be applied to modular systems. Source: Timothy Scruby Consulting-Specifying Engineer • APRIL 2008
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