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Superheat Boiler Design and Performance: A Brief Overview

In the boiler industry, there are three main types of watertube boilers: D-type, O-type, and A-type (see the graphic). Boilers can be designed for either saturated or superheated steam, and they can be packaged or field-erected. Package watertube boilers are designed to be small enough for highway transportation and are factory assembled. Field-erected boilers are just as they sound; multiple components are assembled and installed in the field.

For each boiler type, there are differences in superheater design and performance. Before diving into those differences, let’s review each of the main types of watertube boilers. bw boiler types

D-type boilers have one steam drum and one lower drum, centered over each other with the furnace offset to one side. These boilers are designed with only one generating bank. 

O-type boilers also have one steam drum and one lower drum, however, they are centered over each other with the furnace in the center. These boilers are designed with two generating banks (i.e. the flue gas flow splits into the two sides at the end of the furnace). 

A-type boilers are similar to O-type and include two generating banks. The major difference is the number of drums and placement. A-type boilers include one steam drum at the top center, and two lower drums; one placed on each side of the furnace.

Saturated steam means that the steam temperature is purely dependent on the steam pressure and follows the pressure-temperature curve. For example, if the upper drum is at 150 psig, the saturated temperature would be 366°F. If the upper drum is at 300 psig, the saturated temperature would be 422°F. Saturated steam boilers will have a steam nozzle directly on the upper drum regardless of the boiler type. All of the steam produced is also collected in the upper drum.

When the design calls for superheated steam, which would be higher than the saturation temperature, an extra set of steam tubes or coils are provided and added to the heat transfer circuit in the boiler. The superheater tube bank is piped downstream of the steam drum to add heat to the saturated steam and make it superheated. 

ConvectiveOn D-Type boilers, this extra tube bank is added to the convection section (or the generating bank). This is called the convective design. The benefit of this design is the steam coils are out of the radiant heat zone of the burner (the extra heat transferred uses convection only). However, performance is not as good as radiant style superheaters which means at reduced loads, the steam temperature drops off quickly.Radiant

On O-type and A-type boilers, the extra tube bank is added at the end of the furnace. This is called the radiant design. The benefit of this design is that the performance is much better as compared to the convective style throughout the load range. However, since the heat transferred is primarily radiant heat from the burner, the superheater typically doesn’t last as long and requires more maintenance.

In today’s market, D-Type boilers are the most common. The superheat performance issues can be compensated by designing the steam for a higher temperature and then using an attemperator to control steam temperature more closely over the boiler load range. In the rental boiler industry, O-type boilers are the most common because the weight is more evenly distributed making it ideal for trailer-mounting and road transportation. Stay tuned for our next blog where we will touch on a fourth, less commonly known watertube design; the Ds-type. Ds style boilers are patented and manufactured by Superior Boiler.

As a rental boiler supplier, manufacturer’s representative and stocking distributor, Nationwide Boiler has not only sold but actually operated and maintained package watertube boilers for over 50 years. Give us a call today to learn more about what type of boiler is best for your unique application.

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Innovation in the Rental Boiler Industry

Nationwide Boiler has been on the forefront of innovation in our industry since 1967. Our founder, Dick Bliss, pioneered the mounting of a 20,000 lb/hr package watertube boiler onto a highway-legal trailer signaling the start of an entirely new business on the West Coast.

The rising of truly mobile rental boilers provided time and cost savings for those in need of temporary steam, as well as a reliable, purpose-built option for rent. Prior to this development, temporary steam supply was sourced from steam locomotives or “jury-rigging” obsolete boilers that had been scrapped. None of these early systems were efficient or easy to get to a job site.

Throughout the years we have continually upgraded and enhanced our offerings and have been a leading innovator in our industry. We were the first rental boiler company to convert our entire fleet to low NOx levels of 30 ppm back in 1995, and in 1999 we demonstrated the first SCR system for ultra-low NOx performance on a package boiler. Just two years later, we began offering our CataStak™ SCR system which has now been installed in over 200 applications.

Today, our product offerings include many one-of-a-kind systems, including the world’s largest mobile saturated and superheated steam boilers. Take a look at the list below for a complete overview of the products unique to our company.

40-75,000 lb/hr Trailer-Mounted, High-Pressure Saturated Steam Boilers
110,000 lb/hr Trailer-Mounted Superheated Steam Boiler (World’s Largest)
125,000 lb/hr Trailer-Mounted Saturated Steam Boiler (World’s Largest)
225,000 lb/hr Skid-Mounted, High-Pressure Rental Deaerators
Complete Mobile Feedwater Vans
Urea- and Ammonia-Based CataStak™ SCR Rental Units
DataStak™ Emissions & Efficiency Monitoring System

Nationwide Boiler is a well-established company that provides reliable, purpose-built rental boiler equipment and a great deal more. As a manufacturer’s representative, we supply new and reconditioned fuel-fired boilers, electric boilers, boiler burners and economizers for sale, and we have boiler systems in-stock and available for immediate requirements. In addition, our proven CataStak™ SCR system is available for ultra-low NOx solutions, and we build custom, PLC-based control systems and fuel skids at our UL-certified fabrication facility in Washougal, WA.

Give us a call today to learn how we can support your next boiler related need with, “Integrity, Dependability, and Real Customer Service”. 1-800-227-1966

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Meeting Peak Season Demand with Rental Boiler Equipment

Tomato season is here, and we already have a 70,000 lb/hr, 400 psi design trailer-mounted rental boiler on location at one of our customer sites to support peak season steam demand. This boiler is rented annually in the Central Valley of California for three to four months during the tomato processor’s short but critical production season. To meet the requirement of 5 ppm NOx, the boiler is supplied with an ultra-low NOx CataStak SCR System. This solution allows the facility to operate their own boilers without overworking them, while complying with local air regulations and satisfying seasonal demand.

Some years ago, the company identified a bottleneck in its evaporation system that hindered peak production capacities. The bottleneck was easily resolved; however, it would take more steam to fully realize the increase in production. Although the existing boilers were capable of running the newly found evaporation capacity, the increased production required the existing boilers to operate at 100% output continuously, leaving no room for margin and no buffer against any process anomalies. After evaluating the cost of investing in a new boiler, the detrimental effects of operating the existing boilers at 100 percent output, and the process vulnerabilities of having no excess steam capacity, the decision was made to rent a supplementary steam boiler on an annual basis.  

Many industries face periods of increased process steam requirements and to satisfy demand, steam can be supplemented with a temporary rental boiler. This practical solution allows facilities to maximize the resources needed to meet current, short-term and long-term steam needs. A short-term or long-term rental solution can save time, conserve capital, minimize risk and maximize available resources.

Check out our past article from Today’s Boiler Magazine to learn more about how temporary boilers can support seasonal steam demand, and contact us today if a rental boiler may be the solution to your peak season needs!

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Reduce, Reuse, Recycle – Rental Boiler Edition

At Nationwide Boiler, we maintain a purpose-built fleet of over 100 rental boiler systems from as small as 47.5 hp to as large as 200,000 lb/hr. We have multiple configurations including mobile, trailer-mounted boilers, complete mobile boiler rooms and mobile steam plants, skid-mounted firetube and watertube boilers, factory-integrated skid packages, and even electric boiler systems. With a rental fleet as versatile as ours, we can always find a solution when a customer comes calling.

It is not uncommon for a rental boiler to return and need repairs. A unit might need parts to be replaced, an acid cleaning or even a complete retube, but the worst-case scenario would be the need to scrap a boiler entirely due to damage beyond repair. But if a boiler is properly maintained, as we have said before, it can last a lifetime. A boiler retube can transform a system and bring it back to like-new condition. And, with so many different types of boilers in our rental fleet, we have found other ways to transform, reuse, and recycle when our equipment still has years of life left. 

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