Choosing the best boiler for your buck

Corporate and government sustainability initiatives to cut energy use and carbon dioxide emissions compel closer attention to how to upgrade facilities efficiently.

By Joshua Summers, RMF Engineering

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The average large industrial boiler can operate normally for more than 20 years, but as a boiler ages, operating costs and reliability concerns can crop up. In light of new efficiency regulations, many more organizations are opting to replace traditional boilers, but this can be a costly and complicated process. Plant and building owners and operators are looking for increased efficiency, reliability, automation, value, and ease of maintenance when considering new boilers.

Watertube boilers

Industrial watertube boilers are used primarily in district energy applications and process industries, including chemical manufacturing, paper manufacturing, and refining. Watertube boilers are generally manufactured in D- and O-style configuration and are used when high steam pressures are required – generally 100 psig pressure and higher. The D and O configuration describes the relationship of the steam mud drums and interconnecting tubes. Thermal efficiencies for watertube boilers without economizers are typically between 75% and 80%. The efficiency gain from adding a conventional flue gas economizer can be 3% to 5% or more.

Watertube boilers have smaller reserves of water but large heating surface areas; this allows for faster initial steaming times and quick responses to changes in steam pressure. However, the smaller water reserve means a more-responsive water level control is required.

Small-diameter (2”-3”) steel tubes allow for significant operating pressures (600 psi+) before special alloys are required. When properly maintained, watertube boilers often have service lives of 30 years or more before rebuilding is necessary. Replacing tubes in these units can be labor-intensive and may require disassembly of the casing as well as a significant amount of workspace. Typically, many tubes have to be removed and reinstalled to make the repair on a particular tube. Packaged watertube boilers often have applications beginning at the 20,000 PPH (600 hp) size, and can be shipped as shop-assembled units or field-erected in place. Their high output vs. small footprint make them especially suitable for the large steam plants that must produce hundreds of thousands or millions of pounds of steam per hour.

The introduction of computational fluid dynamics, or CFD, modeling to the boiler industry is arguably the most significant technological advancement of the past 25 years in watertube boiler design. Once a tool reserved for aerospace engineers, CFD has allowed manufacturers to combine decades of field data with cutting-edge computer modeling to develop larger, more-efficient designs, often in the same footprint as older units. As a result, both watertube and firetube boilers have grown significantly in capacity over the years. Firetubes are now available in sizes nearing 2,500 hp.

In the past, 250,000 PPH of steam flow on a railcar was the upper limit. Now, with modularization and specialized transportation equipment, steam flows from watertube boilers can increase to levels as high as 500,000 PPH or more, depending upon shipping limits.

This progress is significant when one considers the latest units are nearly twice the size of those available a quarter century ago. Boiler furnace geometry can be better matched to burner flame shape. Furnace heat flux rates can be precisely modeled, allowing for accurate calculations of tube wall temperatures and resulting boiler lifespan. Boiler heating surfaces can be optimized to reduce gas-side pressure loss and resulting fan power consumption. Boiler wall construction has also changed. Tangent tube walls have been replaced with tube and membrane seal welded walls to eliminate flue gas bypassing from the furnace to the convection zone. The tube and membrane walls are seal-welded and have no opening between tubes. This newer design has helped with emission compliance issues (specifically carbon monoxide emissions).

CFD modeling has played a large part in another major technological advancement – the elimination of refractory in the boiler vessel, primarily in the furnace. Refractory is a high-temperature material used for decades in boilers as an insulator and gas seal. Eliminating refractory is accomplished by the increased utilization of membrane (water-cooled) boiler wall construction, backed by CFD modeling. The performance benefits of refractory reduction include increased useable heating surface and reduced formation of the atmospheric pollutant nitrogen oxide, which is known for producing smog. Of more importance, the safety benefits of refractory reduction include mitigated risk of fuel re-ignition in the furnace after an emergency shutdown and the reduction of refractory gas seals that are prone to failure. From an operational standpoint, elimination of costly maintenance inherent to refractory materials (resulting from cracking and spalling) reduces downtime. Today, most packaged watertube boiler manufacturers recognize the performance, safety, and cost-reduction benefits of eliminating refractory and their designs, which have evolved to meet the market’s ever-increasing expectations.

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