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Guide: Choosing A Boiler System
provided by: Consulting
When 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 toproperly initially treat water, or by leaks that go unrepaired and result in loss of treatment and introduction of corrosive raw water into the system.
Poor water quality in steam boilers comes from a myriad of reasons: failure of treatment systems, failure of manual or automatic blowdown, or excessive make-up. For a boiler system that supplies mostly humidification (a high make-up system), consider point-of-use steam to steam humidifiers, or an unfired steam generator with materials of construction (including pipe valves and fittings) that are designed for the untreated boiler water that is the norm for humidification steam today. This substantially closes the loop on the fired boiler system, greatly increasing its life. Don?t forget the automatic blowdown system for unsupervised steam boilers and steam generators.
Selecting for efficiency
Higher efficiency means lower operating cost, possible higher first cost and also reduced life. The case in point here is condensing gas boilers. The acidic condensate from the products of combustion will react with the metal in the condensing portions of the heat exchanger and the drainage path and may cause them to fail before the rest of the unit. In evaluating a condensing boiler design, ask the manufacturer what parts can be replaced and the cost of the parts. Compare materials of construction and corrosion allowances. Ask manufacturers what spare parts they recommend and specify sets to be included with the boilers. Ask manufacturers about their equipment?s typical service lives of their and find out their warranty policies and lengths. Ask to see boilers of theirs that is five or more years old and applied in condensing applications.
For a new space-heating system with a condensing boiler, remember to maximize the time the system can operate in its condensing mode. Look at designing the system to produce peak output at 140 F or lower water supply temperature. If you have spent your career designing at 180 F water supply temperature and 20 F ? T, you will find yourself looking at radiant floor systems, more rows in the coils, and more surface area in the radiators.
Don?t give up hope for a renovation of an old system that was designed for 180 F or higher water temperatures. Commissioning agents have found that there are quite a few systems out there that are so conservatively designed at 180 or 200 F that operate well at much lower temperatures.
Bells and whistles
In specifying coil tube thickness for hydronic coils, particularly for air handlers, consider a wall thickness greater than the minimum thickness offered to get the pressure rating to extend the life of the coils. This allows for some internal corrosion that can result from poor water quality, and will allow for some metal loss from coil cleaners and external corrosion.
Ask your boiler vendor what options they offer in the areas of controls and trim that can extend the life of the system and improve operating efficiency (e.g. outdoor reset). You probably will find that there are a number of options to improve reliability and overall life. For larger boilers parallel positioning controls, O
Consider that more than one grade of valve is available for a given application. Some valves are designed to be repaired. Others are design to be discarded. Alternate materials of construction for the trim also affect the serviceability of the valve. If your facility is designed for 50 or more years, you might want to specify the better grade. Learn the details.
Determine how the system will be replaced at the end of its life. Ascertain paths in and out of the building. If you are evaluating a replacement project, don?t rule out retubing, new controls, or a new burner to solve problems when the pressure containing parts are fundamentally sound. And to ensure cradle to cradle, evaluate disposal, reuse, and recycling options.
After considering options for your repair, replacement, or new design, compare a few leading contenders and do a lifecycle economic analysis, taking into account first cost, operating cost (including fuel, electricity, and maintenance), and end of life disposal cost. If you are feeling brave, consider a lifecycle assessment (LCA). An LCA is the investigation and valuation of the environmental impacts of a given product or service caused or necessitated by its existence. As years go by, LCA will become more a part of the evaluation of engineered solutions.
Understanding owner?s needs and goals ensures that not only has the equipment lifecycle been examined, but also most importantly the solution provided will meet the owner?s needs.
Scruby performs all elements of design and construction phase commissioning for mechanical, electrical, controls, and process systems as well as managing and mentoring others at Facility Dynamics.
- McDonough, William and Michael Braungart. 2000. Cradle to Cradle: Remaking the Way We Make Things, North Point Press.
author: Timothy M. Scruby, PE, Senior Project Manager, Facility Dynamics Engineering, Afton, Va.
Consulting. Copyright © 2008 Reed Business Information, a division of Reed Elsevier Inc. All rights reserved.