Boilers Design and Components #2

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Continued From ---> Boilers Design and Components #1

In a coal-fired boiler, the adjustment is achieved by altering the draft of combustion air through the grate. As the air supply increases, the fuel burns faster and hotter, increasing the boiler output.

In general, boiler efficiency drops as the mean temperature of the heated fluid rises. As a result, a hot-water boiler will be more efficient heating water from 150°F to 170°F (mean temperature 160°F) than from 160°F to 180°F (mean temperature 170°F). However, the cooler the mean temperature of the heated fluid, the larger the heat-transfer surfaces must be. Here we have another example of where the designer must consider trading the higher ongoing costs
and use of fuel against initial equipment costs.

Because boiler operation is critical for the facility, it is often valuable to have a two boiler system, so that there is always one available for maintenance back up. The illustration below shows a hot water system with two boilers.

The boilers, which are connected in parallel so that one can be valved off and serviced or replaced while the other continues to operate. Two pumps, so that pump failure does not prevent operation. A pressure tank which maintains system pressure and accommodates the changes in water volume as the system is heated up from cold. The pressure tank often has a membrane in it that separates the water from the air, to prevent absorption of oxygen from the air. If the water level drops too low, more water is pumped into the system; if the pressure needs to be increased, more air is pumped into the top of the tank. A spring-loaded safety valve, which is provided for each boiler. The valve is set to release at some pre-determined pressure. Then if, for example, the burner controls jammed at full fire, the hot water or steam would be released, protecting the system from bursting.

Continued ---> Boilers Design and Components #3