The possibility to install supplementary firing to the above designs makes these solutions flexible to achieve required steam demand also when this exceeds the available heat from exhaust gas.

All these proven design solutions have been developed in order to give the following advantages:

Drums
In our usual design, the upper and lower drums are installed crosswise the GT flue gas flow.
The drums are manufactured with high quality steel according to the Code and submerged arc automatically welded. All tubes are expanded with automatic tools into the drums holes, in order to obtain an average value all over the interested part. Internal drum provides high steam purity thanks to the wide steam scrubber and the correct design and dimension of the drum itself. Internal drum also assures free end flow to minimise turbulence and level fluctuation. The drums are provided with manholes at both ends allowing a full inspection and accessibility to all tubes for an easier maintenance. Drums are shop tested according to the Standards in use.

Steam heater
Steam heater is of convection type. It is installed in the first part of the convection section of the boiler shielded by a number of screen tubes in order to avoid creep fatigue failure due to exposure at high temperatures over a long period.

Steam heater consists of  sections of vertical finned tube or smooth type coils with multi-pass steam flow and gas cross-flow. The Steam heater is drainable and every section can be laterally removed. Its tubes can be inspected at the rear side of the boiler. The tubes’ bank consists of a series of tubes properly bent and welded to the external headers. High quality and thickness headers are placed externally and provided with flanged nozzles and thermo valves for instrumentation.

Convection section
The convection section is made of finned tubes properly spaced, cross flow invested by the gas. Fin distances (usually 200 fins/m for natural gas), are always checked in order to avoid problems due to external fouling problems. The tubes bank is easily accessible for inspection and cleaning. The outer tubes forming the external wall are membraned providing a continuous gas enclosure membrane wall. The tangent tube wall construction seals the boiler bank (pressure tight) and protects insulation and outer casing from flue gas contact.

Economiser
HRSG economiser recovers waste energy from the flue gases downstream the convection section and transfers it to the boiler feed water increasing thermal efficiency and reducing thermal emissions. Correct feed water treatment combined with BONO design, material choice and manufacturing leads to an economiser designed for a long and efficient life.
Economiser condensation is a problem that a correct design has to face.

BONO economisers are designed with internal provisions for collecting and draining away leakage water before this can cause serious damage.
Typical design of the economiser consists of a bank of horizontal or vertical finned tubes, tubes supporting plates (allowing free expansion of the tubes), strong steel casing - gas tight (complete with thermal insulation), inspection and cleaning doors, Inlet/outlet vertical headers with flanged inlet/outlet nozzles, local instrumentation, valves and accessories.
In applications where the economiser may be subject to severe corrosion concerns, correct tube and fin materials will be adopted (stainless steel, etc).

Hot Water Exchanger
Depending on special requirements, it is possible to further increase the thermal overall efficiency installing downstream the Economiser, on the flue gas path, a hot water exchanger that recovers the final heat of the flue gases and provides hot water to the process. This exchanger can be used to increase the temperature of return condense and make up water before entering in a deareator. Same considerations on design and materials as what described for the economiser are applied, even though this equipment is more exposed to corrosion problems since the tube wall surfaces in hot water exchanger can fall below acid dew point temperatures of vapours such as hydrochloric acid, nitric acid, sulphuric acid or even water vapour, condensation of these vapours can occur on these surfaces, leading to corrosion and tube failures.