Heat Recovery Boilers

Heat recovery boilers are used to produce hot water or steam from the heat of passing exhaust gases. The HRSG is essentially a counterflow heat exchanger composed of a

Recovery Boiler Diagram
Fig. 8-10 Schematic Diagram of Reciprocating Engine Hot Water Heat Recovery System. Source: Fairbanks Morse Engine Division

series of superheater, boiler, and economizer sections positioned from gas inlet to gas outlet to maximize heat recovery and supply the rated steam flow at the proper temperature and pressure. In some cases, exhaust gas heat recovery may also provide for deaeration or feedwater pre-heating. Alternative HRSG configurations include firetube boilers and natural forced circulation or once-through watertube boilers. Unfired heat recovery boilers and conventional fired boilers are similar in many respects. The principal difference is that exhaust gases are generally of much lower temperature, as compared with the combustion gases in a fired boiler, resulting in a far lower rate of transfer by radiant heat.

Representative examples of heat recovery boilers applied to reciprocat-

Fig. 8-11 Heat Recovery Boiler Applied to Reciprocating Engine. Source: Vaporphase by Engineering Controls, Inc.

ing engine and gas turbine systems are provided in Figures 8-11 through 8-14. Figures 8-11 and 8-12 include heat recovery boilers applied to reciprocating engines. Figure 813 shows a two-pressure heat recovery boiler applied in a 45 MW combined-cycle cogeneration plant featuring a gas turbine and steam turbine. Figure 8-14 shows a multiple-pressure heat recovery boiler featured in a 150 MW plant.

Firetube Boilers

Exhaust gas firetube boilers route the exhaust gas through a bank of tubes as water is boiled in the surrounding chamber. Design and operation is much the same as with fuel-fired firetube boilers. Figure 8-15 illustrates the exhaust

Fig. 8-12 Jacket Water and Exhaust Heat Recovery Boiler Applied to a 150 kW Reciprocating Engine-Generator Set. Source: Waukesha Engine Division

Fig. 8-14 Multiple-Pressure HRSG Featured in 150 MW System. Source: Nooter/Eriksen

Fig. 8-12 Jacket Water and Exhaust Heat Recovery Boiler Applied to a 150 kW Reciprocating Engine-Generator Set. Source: Waukesha Engine Division

Fig. 8-14 Multiple-Pressure HRSG Featured in 150 MW System. Source: Nooter/Eriksen

Fig. 8-13 HRSG featured in 45 MW Combined-Cycle Plant. Source: Deltac

flow path through a single-pass firetube heat recovery boiler. Figure 8-16 illustrates a similar unit featuring two passes.

Due to their relatively large size, weight, and other design limitations, firetube boilers are generally limited to pressures below 250 psig (18.3 bar) and outputs of under 50,000 lbm (23,000 kg) per hour and do not offer superheat capabilities. They can be equipped with economizers to improve efficiency. They are generally less expensive and

Pass Diagram Boiler
Fig. 8-16 Two-Pass Firetube Heat Recovery Boiler. Source: Superior Boiler Works

Fig. 8-17 Packaged Heat Recovery Low-Pressure Steam Generator. Source: Vaporphase by Engineering Controls, Inc.

require less headroom than watertube boilers of comparable capacity, especially in smaller capacities.

Firetube boilers produce an additional benefit of sound attenuation, which results from the high-pressure drop through the fire tubes. For this reason, they are sometimes referred to as heat recovery silencers. This provides an economic benefit in reciprocating engine applications, in that muffler requirements for sound attenuation are reduced or, in some cases, eliminated.

Firetube hot water boilers are often included in small packaged reciprocating engine-driven systems. Hot water or steam units are sometimes, but not usually, used for small gas turbine applications. In addition to the disadvantage of size and weight, firetube boilers tend to produce high turbine exhaust back-pressure and usually require an induced draft fan. Figure 8-17 shows a vertical packaged HRSG designed to produce low-pressure steam from reciprocating engine coolant system and exhaust heat. The flow paths of exhaust and jacket water are labeled.

Figure 8-18 shows three packaged jacket water and exhaust heat recovery silencers designed to generate 15 psig (2 bar) steam from 450 kW ebullient-cooled, gas-fired, reciprocating engine-generator sets. At full load, each gas engine has the following recovery:

Jacket Water

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