Superheater coils are one of the most important components of steam boilers. They are highly used to convert saturated steam or wet steam into superheated steam or dry steam. In turn, this superheated steam is used in steam turbines to generate electricity, in steam engines, and in processes such as steam reforming.
Common types of superheater coils are:
Radiant superheaters
A radiant superheater is the one that’s placed directly in the radiant zone of the combustion chamber near the water wall to absorb heat from radiation.
Convection superheaters
A convection superheater is also called a primary superheater. It’s placed in the path of the hot flue gasses and they absorb heat through convection. They are often located in the flue gas ducts or above the primary boiler section, where gasses have cooled slightly from their peak temperature.
Separately fired superheaters
This type remains external to the boiler, has its own furnace and is used to superheat steam to very high temperatures.
Superheater design and performance
Before diving into the superheat design and performance, we must know the D-type boilers, O-type boilers and A-type boilers.
- D-type boilers have one steam drum and one lower drum. Here, the furnace is present on one side of the boiler and comes with one generating bank.
- O-type is similar to D-type but with the furnace present in the center and comes with two generating banks.
- The A-type has one steam drum at the top center and two lower drums on either side of the furnace and this type also comes with two generating banks.
Superheater design
There are two types of steam: saturated steam, which follows the pressure-temperature curve and superheated steam, which is heated beyond the saturation temperature.
Here, to generate superheated steam, the saturation temperature must be higher and an extra set of steam tubes or coils must be added to the heat transfer circuit in the boiler. So, the superheater tube bank is piped downstream of the steam drum to add heat to the saturated steam and make it superheated.
How does the superheated design help with this?
D-type boilers
Superheater tubes are added in the convection section, also known as the generating bank. This design, called the convective design, places the superheater tubes away from the radiant heat zone of the burner.
Though this setup offers the advantage of reduced exposure to high radiant heat, it may result in lower superheat performance, especially at lower boiler loads.
O-type and A-type boilers
In contrast to D-type boilers, the superheater tubes in O-type and A-type boilers are added at the end of the furnace. This design, known as the radiant design, exposes the superheater tubes to direct radiant heat from the burner.
Though, when compared to the D-type boilers, these two offer better superheater performance, they require higher maintenance as they’re highly exposed to intense radiant heat.
In the end, choosing between convective and radiant superheater designs depends on what you expect from the boiler: how well you want the boiler to heat steam, how much maintenance you can do, and what you are using the boiler for.