Typically, fire tube units run up to 1,000–1,500 BHP. Larger capacities require a water tube design.
In water tube boilers, the combustion gas flows around the tubes and water flows in them. Units come in a wide range of configurations such as bent tube, A-style, D-Style and O-style, according to the shape of the tube bundles.
Another major difference is in how the water circulates inside the unit. Most water tube boilers use natural heat circulation to move water between a vessel at the top, called the steam drum, and a collection manifold at the bottom, called the mud drum.
A third type of boiler, technically a forced-circulation water tube boiler but commonly referred to as a steam generator, uses a mechanical water pump to control and circulate the internal water flow (Figure 1). Tubes normally are configured in a coil shape, so such units often are called "coil type" boilers.
When purchasing a boiler you should consider a variety of factors:
Costs. Annual fuel cost to run the unit easily can be five times or more the expense to purchase and install it. So, weigh operating costs when comparing prices. The cheapest unit to purchase and install may end up costing you a lot more over a 10-year period. Short-term savings may translate into an inefficient system with high long-term maintenance and repair costs.
Startup. If you don't need steam 24/7, you'll probably want to shut down your boiler at times. Do you need a unit that can stop and start in short order, or is a slower startup acceptable? Every time you start or idle a boiler just to keep it warm, you're wasting fuel. If quick startups are required, consider a steam generator. These efficient boilers can go from startup to steam generating in about 10 minutes.
Efficiency. By and large today's boilers are tremendously more efficient than ever before, thanks to improving technology and tougher federal and state emissions standards. However, you still must pay attention to a boiler's steam efficiency rating, which can range from about 80% to well above 90%.
Fuel supply. Boilers use oil, coal, wood and natural gas. Each fuel has its own benefits and pitfalls. They also can vary greatly as to their efficiency. Local emissions rules may restrict your options. Natural gas usually is the most convenient to deliver, as it can be piped in directly. However, other fuels may be preferable depending on your location.
Service/maintenance. A boiler usually operates for a long time. It will need servicing or repairs. So, always carefully consider vendors' support capabilities when buying a unit.
Safety. Federal and state regulations have made boilers and boiler operation much safer than ever before. But that doesn't mean accidents can't happen. Routine inspections, maintenance and troubleshooting are important no matter what type of boiler you own or plan to buy. Always check the safety record of any boiler or boiler type before purchase to ensure it meets all safety standards and regulations. Look particularly at whether the unit is certified to the American Society of Mechanical Engineers' Boiler and Pressure Vessel (BPV) Code. To find out if a boiler you are investigating has the BPV certification, log on to www.asme.org
Intangibles. Several items that sometimes are difficult to put an actual dollar figure to in a bid comparison can have significant economic impact. For example, how much is factory skid mounting of equipment worth? It not only can cut installation costs but also can save engineering hours on your side. Likewise, the value of service support, parts availability, warranties, performance guarantees, etc., may be hard to quantify but these are important considerations in choosing a vendor. Buy only from suppliers that can back up the equipment with support.
ANDY WALES is Murrieta, Calif.-based western regional manager for Clayton Industries, Inc. E-mail him at Andy.Wales@claytonindustries.com.