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O2 Trim for Increased Boiler Efficiency & Emissions Compliance

Given today’s awareness to the advantages of minimizing energy usage and carbon footprint, boiler operators and plant managers are always on the lookout for ways to improve boiler efficiency and ensure emissions compliance. With improved efficiency, fuel usage is minimized which in turn reduces the carbon footprint (i.e. reduces CO2 emissions from the boiler) and reduces issues around emissions compliance. One way to increase boiler efficiency is to use oxygen trimming, or O2 Trim, at the stack.

A typical burner will operate from 3 to 4% O2 at 50% boiler load and higher. This stack O2 concentration corresponds to the amount of excess air at the burner, and excess air is required for burner operation to assure complete combustion of the fuel.  For example, for natural gas firing, 3% O2 corresponds to 15% excess air. During commissioning, the burner service engineer will set the fuel / air ratio so that there is always excess air over the firing range of the burner.  The service engineer must also keep in mind that ambient conditions (mainly air temperature changes) will affect air density which will affect the burner fan air flow output.  On cold days the fan will flow more air due to a higher air density, and on hotter days the flow will be less. Varying air flow conditions can adversely affect burner operation.

Boiler efficiency is affected by the excess air concentration in the flue gas. The rule of thumb is that for every 5% more excess air, boiler efficiency decreases by 0.5%. If not adjusted, the boiler stack can vary by at least 2% O2 (i.e., if normal operation is 3% O2, it can increase to 5% O2 on a cold day). That corresponds to about 1% boiler efficiency loss. Saving 1% efficiency over a year operation can save big on fuel costs. If the normal fuel bill is $10,000,000 per year, you would save $100,000. Adding an O2 Trim system would cost a fraction of that amount (assuming a 150,000 lb/hr steam boiler or smaller), providing a quick and worthwhile ROI. So, what exactly is an O2 Trim System?

Many burners use a control system where the fan air flow does not vary based on air temperature.  As explained above, the air flow can vary based on ambient conditions causing the stack O2 to vary; this can be solved by adding O2 Trim to the control system. O2 Trim is an air flow trimming system where stack O2 is measured (using an O2 probe) and the air flow is adjusted (trimmed) based on the reading. It’s a closed loop control system since changes in air flow will directly affect the stack O2 reading (assuming fuel flow is the same). By maintaining a consistent air flow rate, O2 trim reduces fuel usage in turn increasing boiler efficiency.

In addition to increased boiler efficiency, utilizing O2 Trim will ensure stable and safe O2 levels. On hot days with reduced fan air flow, the stack O2 level can drop to dangerously low levels and boiler emissions can go out of compliance. With O2 monitoring, alarms can be created to alert the boiler operator to either reduce fuel flow or increase air flow, to return to safe operating conditions.  

O2 Trim isn’t ideal for every boiler, though. Due to the residence time in the boiler and ducting, it takes time for the changes in burner fan flow to reach the stack. This causes a time delay with an O2 Trim system, which works well for boilers with slow load changes. However, for systems with rapid boiler load changes, the O2 Trim system typically can’t keep up easily and it is often “hunting” for the optimum air flow.

If you are interested in learning more about whether an O2 Trim System will benefit your operations, reach out to one of our qualified parts specialists or call 800-227-1966. Check out other articles on Nationwide’s Boiler Blog for more tips and tricks for improved boiler efficiency, routine maintenance, and more!

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Watertube Boiler Design: Superior Boiler's DS Type

In our last blog, we reviewed the three common types of watertube boilers: D-Type, O-Type, and A-Type. There is, however, another package watertube boiler on the market today; the Ds-type, providing a fourth option to consider when you are planning for a new boiler. In fact, the Ds-type design is patented and manufactured by Superior Boiler and is one of their most popular watertube boiler models. 

Ds vs DtypeThe configuration for this type of boiler is similar to the D-Type, with one upper steam drum, one lower drum, and one convection bank. One of the main differences lies in the convection bank flow area (the bank of tubes that provide convection heat transfer), which is much larger and utilizes upside-down L shaped tubes. This larger area means a reduced boiler pressure drop on the flue gas side. A smaller fan is required, reducing both cost and energy usage.

With Ds-type boilers, the furnace is more centralized and the cross section is more square compared to the rectangular furnace cross section of a D-type boiler. The square shape improves the heat flux from the burner due to walls being more equidistant from the heat source, and with a more uniform heat flux we get improved heat transfer with lower emissions.

Compared to D-type boilers, the Ds-type offers additional features and advantages: Ds vs Dtype frontview

- Improved shipping and rigging due to a more centralized center of gravity.
- More compact footprint; roughly 1-2ft shorter than typical package boiler. 
- No need for a burner platform since the burner is lower to the ground. 
- The option to be designed with a top exhaust outlet, providing space savings with the ability to mount the economizer in a vertical configuration above the boiler. D-type boilers are limited to a side outlet and horizontal, ground-mounted configuration. 
- An overall lower capital cost caused by fewer transitions and breeching, smaller fan requirement, better heat transfer, and better performance.

The Ds-type boiler is also known as Superior’s “Shawnee” model. With design pressures of up to 1,200 psi, these boilers can be built to supply as little as 10,000 lbs/hr to as much as 250,000 lbs/hr of saturated or superheated steam. The patented design provides an unmatched, compact footprint and is a great option when looking to replace an existing unit or add capacity to your facility.

Contact us today to learn more about how Superior’s Ds-type boiler could be a good match for your next project. 800-227-1966

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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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