Boiler Blog Archive | Nationwide Boiler Inc.

Nationwide Boiler's rental fleet consists of many boilers in a variety of steam output capacities and design pressures. Some boilers in  our rental fleet remain rental units throughout their useful life, while others find permanent homes through used boiler sales. One unit in particular has seen the best of both worlds. 

This 70,000 lb/hr trailer-mounted package watertube boiler was originally manufacutred in 1997 and was an active part of our rental fleet for roughly 14 years. In 2011 it was purchased by one of Nationwide Boiler’s valued customers in Newark, CA, less than 5 miles from our headquarters in Fremont. In addition to the boiler package itself, we supplied our CataStak™ SCR system for sub 5 ppm NOx emissions and removed the boiler from the trailer for permanent installation within their facility.

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. 

The 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: 

- 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.

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. 

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.