What are the types of HRSG's?Heat Recovery Steam Generators (HRSG's) are a critical part of a Combined Cycle Power Plant. As

with any other product or process, there are different types. Here we take a look at the different

types.

The main application of the (Heat Recovery Steam Generators) HRSG is in Combined Cycle power

plants. In these plants the power generation from the Rankine cycle part, ie. the steam turbine, is

around one third of the total power generated . HRSG's produce the steam for this.

Classification of HRSG’s is on application, design, or operation. Some of the types are described

below.

Fired and UnfiredOne way is to classify it based on the heat input.

Normally HRSG's do not have any additional heat input. The performance and output of the HRSG

is dependent on the exhaust heat of the gas turbine. At part loads, this leads to reduced output

from the HRSG's. In addition, ambient conditions also affect the Gas Turbine performance. This

could affect the downstream process were the steam is used. To avoid such situations

supplemental firing of oil or gas takes place. Even though this may not be an efficient process, it

avoids costly production disturbances.

Supplemental Firing takes place in burners in the gas duct at HRSG inlet. Oil or gas is the

supplementary fuel. Since the flue gas at exhaust of a Gas Turbine is high in Oxygen content,

additional air is not required for combustion. This eliminates the need of Forced draft or Induced

Draft fans.

Vertical and Horizontal TypesAnother classification is on the construction or design of the HRSG. Based on the gas flow it can be

vertical or horizontal.

o Vertical types have gas flow vertically upward with coils placed horizontally.

o Horizontal types have gas flows horizontal with coils placed vertically.

From the performance and cost point of view both are the same. More than the technical issues it

is a proprietary design of individual manufacturers or client preferences. Some of the differences

are :

Horizontal types require a 30 % larger footprint area.

More expansion joints are required in horizontal units.

Structural requirements are higher in vertical types.

Horizontal types are more difficult for maintenance and inspections.

Overall cost may be same in both types.

Multiple Pressure Operation.Yet another classification is on the operation pressure.

Smaller HRSG units operate on single pressure. The water to steam conversion takes place

in one single pressure circuit. This is similar to conventional combustion boilers. 

In larger units, for optimizing the performance of the HRSG, steam generation takes place

in multi pressure circuits. The current optimum is to use three pressure levels. (Details in

the next article)

HRSG (HEAT RECOVERY STEAM GENERATORS)

HRSG's are boilers placed downstream of Gas turbines to absorb the exhaust heat and produce steam. This

equipment makes the combined cycle the most efficient power generation system available today. What are

they? How is it different? This series elaborates the different types and optimization tech