Selecting the right steam boiler for an application can be daunting experience. Manufacturers have creative marketing folks that tout features that can mislead anyone who does not fully understand what attributes make a good steam boiler. However, there is no one perfect boiler; instead, each boiler design has features that make it a good choice - or a bad choice - depending on how it will be used.
The ideal boiler only exists in the minds of sales people. The following list includes 10 features to consider when selecting the optimal boiler for your application. One can see some of the features are contradictory; therefore, a boiler that encompasses all 10 features cannot exist. Instead, the specifying engineer must align a boiler type with certain features with each unique application. Failure to do so likely will result in poor performance and an unhappy customer. Here are 10 features you wish every boiler had.
1. High Pressure Vessel MassThe mass of the boiler will be a good indication of the amount of metal used to manufacture the pressure vessel. Mass is related to thickness and thickness is related to durability. Inherently, low mass boilers will not last as long as high mass boilers.
2. High Water VolumeWater under pressure will store steam. Higher water volume means higher stored steam. Applications where you have rapid swing loads will require a high water volume boiler design.
3. High EfficiencyWe all would like to have the most efficient boiler. Efficiency, however, must be defined and compared on equal terms. Often, savvy marketing people publish efficiency based on certain unrealistic conditions such as using a low temperature feedwater. Ensure boiler pressure, feed water temperature, firing rate, fuel BTU value and stack CO2levels are all defined on the same playing field.
4. Quick Startup TimesSimply put, you want steam in a reasonable time frame. Boilers that can make steam quickly likely will have low water and low mass, which may be non-desirable features. One must also realize a quick startup will facilitate a quicker cool down. The boilers on the market today typically require 5 to 20 minutes startup time.
5. Low FootprintBoiler room real estate is quite valuable these days and the less room a boiler occupies, the better. Vertical type boilers have an advantage in floor space demands, but they are limited somewhat by the height of the boiler. Therefore, they are not practical above a certain output. Be sure you take into consideration the amount of room required to perform maintenance on the boiler as part of the overall footprint.
6. Low Heat Flux and High Heating Surface AreaHeat transfer is represented by the equation
Q =m x Cp x ΔT
where m is the heating surface area.
Most heating surface area listings are for the fire-side surface area, with higher fire-side surface area values indicating higher the heat transfer efficiency. However, one also must consider the wetted heating surface area. The wetted heating surface area determines heat flux. Consequently, low wetted heating surface area can have very high heat flux, which can create higher metal temperature and increase scaling potential.
Boilers with high fire-side and high wetted heating surface areas will be the most efficient and have low heat flux.
7. Ease of Clean OutThis is a measure of effectiveness of blow down and ability to keep the pressure vessel clean. Vertical boilers may have the advantage with this parameter because the pressure vessel orientation lends itself to easier clean out. Boilers with lots of small tubes or dead spots will not fair well in this category.
8. Effective TurndownThis feature has a lot to do with the burner performance. Effective turndown is a measure of how low the fuel input and air can be adjusted to maintain good combustion efficiency. Some burners will turn down the fuel but leave high excess air on low fire. This will create high turndown but poor overall combustion efficiency. Look at the highest turndown level while still maintaining good combustion efficiency.
9. Ease of MaintenanceHardly ever will a specifying engineer think about maintenance when selecting a boiler. They should. Blow down valves, stack location, feed water inlet, control panel location, tube removal, burner removal and gas piping maintenance should all be considered as criteria of boiler selection.
10. Large Steam Disengagement AreaLike heating surface area, the more steam disengagement area, the better. Higher steam disengagement area produces higher steam quality, especially for low pressure steam applications. Consequently, horizontal boilers generally will produce higher steam quality than a vertical boiler of the same output.
All specifying engineers should consider these 10 features when specifying what boiler will be the basis of design. For example, if floor space is an issue, footprint will be the dominant feature. Likewise, if swing loads are expected, then a boiler with high water content will be the best choice. Some applications will require certain features not be compromised while other will be more forgiving. A successful installation will match features to the application.