Boiler-system design often addresses energy efficiency and emissions, but seldom boiler footprint as well.
A recently formalized partnership between a Europe-based provider of heating and ventilation technology and a U.S.-based boiler manufacturer has resulted in an extended-heat-transfer-surface boiler tube for use in both hot-water and steam boilers. The tube increases heat transfer over a bare boiler tube by a factor of five, allowing a dramatic reduction in both tube count and footprint.
The tube is installed in more than 10,000 hot-water-boiler systems throughout Europe. Several years ago, it was introduced in the United States with the release of a condensing-boiler product line. Recently, it was introduced into several steam-boiler designs.
Inside the carbon or stainless-steel tube is a finned aluminum-alloy insert (Figure 1). Efficient heat transfer is the result of three factors:
The high heat conductivity of the aluminum alloy.
The finned, ridged design, which greatly increases heat-exchange surface area.
The division of the tube into eight flow channels, which maximizes turbulence and heat exchange.
For condensing boilers, the tubes are constructed vertically, which allows condensate to run down to a collection reservoir and drain, rather than remain on the heat-exchange surface.
Despite the tube's small size, the energy-saving potential is great. In fact, two-pass firetube steam boilers offering the same energy efficiency as traditional three- or four-pass boilers are available.
For hot-water systems, single-pass condensing boilers that reduce 2,000°F combustion gas to less than 200°F in only 32 in. are available.
These boilers have footprints much smaller than those of their traditional counterparts. Footprint reductions of 40 to 50 percent are typical when using the tubes.
HOW IT WORKS
In conventional systems, hot gas from the combustion process moves through the heat exchanger, typically in a three- or four-pass boiler configuration. With each pass, gas temperature declines until the gas exits through the stack.
In a typical system, substantial heat-transfer surface is required because a bare boiler tube provides very limited heat transfer. Additionally, because of boundary-layer dynamics, heat transfer at the tube surface is not maximized. A large portion of the gases passes through the center of the tube and misses out on the heat transfer available on the tube's surface.
In an extended-heat-transfer-surface-tube boiler, hot combustion gases pass through the tubes, which are five times more effective at transferring heat than are bare tubes. Therefore, boilers made with extended-heat-transfer-surface tubes feature a dramatically smaller number of tubes or dramatically shorter tubes while achieving the same efficiency. The tubes break up the boundary layer, maximizing heat transfer through multiple channels (Figure 2).
In the United States, the tubes are available in:
10- to 60-hp single-pass steam firetube boilers.
10- to 40-hp single-pass hot-water and steam firetube boilers.
A 300-hp firetube boiler.
A firetube-boiler system featuring greater than 94-percent energy efficiency and less than 9-ppm NOx emissions.
With 20 years of experience in industrial engineering, Dan Willems has extensive knowledge of design engineering, project management, and research and development. As vice president of product development for Cleaver-Brooks, he leads the company's efforts to create solutions that increase fuel efficiency and decrease harmful emissions in boiler rooms. He can be contacted at email@example.com.
For past HPAC Engineering feature articles, visit www.hpac.com.