Types of Boilers
The boilers used in low-pressure steam and hot-water space heating systems can be classified in a number of different ways. Some of the criteria used in classifying them are:
1. Construction material
2. Construction design
3. Boiler position
4. Number of passes of the hot gases
5. Length of travel of the hot gases
6. Type of heating surface
7. Type of fuel used
Most boilers are constructed of either cast iron or steel. A few are constructed from nonferrous materials such as aluminum. Cast- iron boilers generally display a greater resistance to the corrosive effects of water than steel ones do, but the degree of corrosion in steel boilers can be significantly reduced by chemically treating the water.
The heating core of many boilers is formed by joining together a series of cast-iron sections either horizontally (so-called pancake construction) or vertically (Figure 15-4). In the horizontal cast-iron section design, the heating surface of each cast-iron section is exposed at right angles to the rising flue gases (Figure 15-5). The water travels in a zigzag path from section to section in a manner similar to the flow of water in a steel tube boiler (Figure 15-6).
Steel boilers may be classified with respect to the relative position of water and hot gases in the tubular heating surface. In fire- tube boilers, for example, the hot gases pass within the boiler tubes while the water being heated circulates around them. In water-tube boilers, the reverse is true. Flexible steel tubes are used in some boilers for the circulation of the water around the heat rising from the fire (Figure 15-7).
A hot-water (hydronic) copper-fin tube operates on a different principle from the cast-iron and steel boilers. It is designed to transfer heat almost instantly to the water (Figure 15-8). Water flows across the boiler heat exchanger, picks up heat, and then moves through the pipes to the heat convectors, radiators, or panels.
Note
If the water stops flowing while the burner is still running, heat will build up until the water flashes into steam and damages the boiler. This condition is similar to dry firing in cast- iron and steel boilers. It can be avoided by installing a flow switch in the path of the water. The switch turns off the burner when the water stops flowing.
Boilers can also be classified according to the number of passes made by the hot gases (e.g., one pass, two passes, three passes) The length of travel of the hot gases is another method used for classifying boilers. The efficiency of a boiler heating surface depends, in
part, upon the ratio of the cross-sectional area of the passage to its length.
Among the various fuels used to fire boilers are oil, gas (natural and propane), coal, and coke. Conversion kits for converting a boiler from one gas to another are available from some manufacturers. Changing from coal (or coke) to oil or gas can be accomplished by using conversion chambers and making certain other modifications. See Chapter 16, “Boiler and Furnace Conversions.”
Electricity can also be used to fire boilers. One advantage in using electric-fired boilers is that the draft provisions required by boilers using combustible fuels is not necessary.
Note
Unlike the boilers fired by fossil fuels (oil, gas, coal, etc.), electric boilers do not have an AFUE efficiency rating. They operate at almost 100 percent efficiency.
The classification criteria described above are selective and limited to the more common types in use. Considering the multiplicity of boiler types and designs available, it is extremely difficult to establish a classification system suitable for all of them.