Boiler Classification

Boilers may be classified into several categories depending on the following:

1. By application as utility, marine, or industrial boiler. Utility boilers are the large steam generators used in power plants generating 500–1000 MW of electricity. They are generally fired with solid fuels, pulverized coal, or fluidized bed furnaces. Utility boilers generate high-pressure, high-temperature superheated and reheated steam; typical steam parameters are 16,500 kPa (165 barg), 540°C/540°C.

A few utility boilers generate supercritical steam at 240 barg, 590°C/590°C/590°C with double reheat systems. Industrial boilers used in cogeneration plants generate steam at varying steam pressures of 10–100 barg at steam temperatures ranging

104 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

from saturation to 500°C. They are generally fired on oil and gas. Industrial boilers may import saturated steam from other boilers to be superheated or export saturated steam with the balance being superheated. Hence, their designs are more challenging.

2. By circulation methods: Figure 2.6 illustrates the concepts of natural, forced, or controlled and once-through steam generators (OTSGs). Natural circulation designs are used up to 165 barg steam pressure. Beyond this, a controlled circulation system using circulating pumps is preferred. A small operating cost is involved for moving the steam water mixture through the evaporator tubes. However, the circulation ratio (CR) may be kept small based on the design and experience of the boiler supplier. CR of 4–7 is common. The once-through design concept may be imple- mented at any pressure of steam. However it requires zero solids feed water; else deposits inside tubes while evaporation occurs would increase the evaporator tube wall temperature and damage the tubes particularly in the high heat flux region.

3. By fuel type: Type of fuel-fired influences the boiler size significantly. Solid fuel

requires low heat release rates as combustion is difficult, and more residence time is required in the furnace. Oil- and gas-fired furnaces can be smaller. The furnace absorption with oil firing is more due to the higher intensity of flame radiation, and hence, the furnace for oil fuels is slightly larger. This is discussed in Chapter 2.

Steam pressure also affects the boiler furnace size. As can be seen from Chapter 2, the latent heat of steam is much higher at lower steam pressure compared to higher steam pressure. The liquid heat or the energy pickup from feed water temperature at economizer inlet to the saturated steam point is much higher as the steam pressure increases. Hence, very high-pressure boilers may have a small furnace and no evaporator or boiler bank to speak of and a large superheater and a large economizer, while a low steam pressure boiler will have a large furnace and a boiler bank or evaporator and a small superheater and a small economizer.

4. Type of boiler: package boilers are classified as A, D, or O type depending on their

construction as shown in Figures 3.16 and 3.17. O-type boilers: This boiler has a steam drum and a bottom drum and has a cross section like

the letter O. It is symmetrical in design and hence can be shipped easily, which is why this type of design is favored in rental boiler industry where a boiler may be used at various loca- tions during its lifetime. The gas outlet is at the burner end and at the top. An economizer may be located in the gas outlet duct. If a convective superheater is required, it has to be split up as shown in the A-type boiler. If two-stage superheater with interstage attemperation is required, it adds to the complexity of piping, and two desuperheaters are required. A radiant superheater as shown in Figure 3.16a may be used, but unless the furnace is properly sized, flame can impinge on the superheater tubes, causing it to sag or even overheat and fail.

The A type is similar in features to the O-type boiler and being symmetrical in construction is well suited for frequent movements as a rental boiler. The convective superheater has to be split and located in the two parallel sections of the convection bank, while the radiant superheater is similar to that used in the O type. Using a single-stage radiant section is not advisable as temperature control is difficult. Hence, a combination of convection and radiant superheaters is preferred. This is discussed later.

The D type is well suited for large boiler capacities with high controlled steam temperatures (Figures 3.16d and 3.17a, b). Since all the flue gas goes through the convection bank, there is no question of maldistribution between the two convection passes as in A- or O-type boilers.

Steam Generators 105

When steam temperature control is required, it is desirable to use a screen section followed by a two-stage superheater with a desuperheater in between. The economizer can be located in the horizontal gas flow direction as shown in Figure 3.17a to make the arrangement sim- ple. More space is required for the D-type boiler as the gas exit is at the side and not at the boiler front as in O- and A-type boilers. Figure 3.17b shows the economizer of a D-type boiler in the vertical gas flow direction. The stack can be mounted on top of the economizer.

(a) A-type boiler, (b) O-type boiler, (Continued)

106 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

(c)

(d)

FIGURE 3.16 (Continued)

(c) A-type boiler with superheater, and (d) D-type boiler with superheater. (Courtesy of Cleaver Brooks Inc., Engineered Boiler Systems, Thomasville, GA.)

Steam Generators 107

(a) D-type boiler with horizontal gas flow economizer and (b) D-type boiler with economizer in vertical gas flow direction. (Courtesy of Cleaver Brooks Inc., Engineered Boiler Systems, Thomasville, GA.)

108 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

Boiler Classification