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Bisector air preheaters
Power boiler applications require highly efficient and reliable air preheaters to ensure optimum boiler performance. An air preheater with a gas sector and one air sector is called a bi-sector design.


Gas and oil fired boilers require only bi-sector design air preheaters.  Coal fired boilers also require an air-stream to dry the coal and to transfer the pulverised coal from the coal pulveriser to the furnace.  This air is called primary air and it has to be at a higher pressure than the secondary air to overcome the pressure drop through the coal mill, the pulverised fuel pipework and the coal burner.

In coal fired boilers there are two air supplies :-
    Secondary Air (SA) which is supplied direct to the burners and is the bulk of the combustion air supply. 
    Primary Air (PA) which is used to dry the coal and transport the pulverized coal from the coal mills to the burners.

    Key benefits

    Low capital investment
    .  Rotary regenerative air preheaters have a much reduced size and cost relative to recuperative tubular or plate heat exchangers.  

    Low leakage level without drift.  Most new air preheaters can achieve leakage levels that are close to their design values after initial commissioning or when set up correctly after an outage.  However, after a period of operation of up to 2 or 3 years, the leakage can increase dramatically, typically from 5% - 12% on secondary preheaters and to between 12% - 25% (and frequently higher) on high pressure mill preheaters.  

    Extensive choice of heat transfer elements.  Howden have designed and developed an extensive range of heat transfer elements over the years.  Combinations of elements are selected to optimise performance, pressure drop and cleanability for the given fuels

    Low maintenance.  Conceptual and detailed design targets the achievement of simplicity and minimal maintenance. Fixed sealing surfaces and centre drive greatly reduces maintenance by removing actuators and pin racks. 
    High availability.  The major cause of reduced availability in air preheaters is due to element fouling.  Careful element selection to suit the application, cold end temperature control and the application of the correct cleaning system maintains very high availability.

    Centre column or bottom girder support.  Custom designed and optimised to suit arrangement and client preference.

    Standard Features

    All air preheater types can have a vertical shaft and horizontal rotor arrangement and are termed vertical air preheaters as the rotor shaft is vertical.

    Detailed specifications

    Sealing system.  The sealing system has evolved to have sealing surfaces formed and set to achieve minimal gaps with seals at operating conditions combined with labyrinth multiple seals.

    Centre drive system with inverter speed control.  The shaft mounted centre drive employs a high ratio gearbox with the option of multiple electric motors and an air motor.  The drive system is completely removed from inside the air preheater and doesn’t require the time consuming installation and outage replacement of pin racks.  Pin racks also have the disadvantage of making the axial seal ineffective.  The inverter provides a very soft controlled start of the largest rotors and enables speed reduction for water washing. 

    Standard spherical roller bearings with bath lubrication.  Standard proprietary rolling element bearings with bath lubrication using extremely high viscosity oil has been used by Howden in this application for over fifty years.  Water cooling systems are employed at the hot end bearing. 

    Cleaning Systems.  Unlike tubular or plate recuperative heat exchangers, fouling does not cause deterioration in heat transfer in a rotary regenerative heat exchanger.  Fouling increases pressure differentials and consequently fan power and leakage.  Several proven cleaning systems are available to suit the degree of fouling and space restrictions, including:                        
                      •    Semi retractable steam sootblowers.
            •    Fully retractable multi fluid (air/steam and LP & HP water) sootblowers.
               Semi retractable HP water washing (HP water washing being carried out on load from the cold end only)

    Fire detection and fire fighting systems.  The fire detection system monitors the rotor for a higher temperature hot spot and also a high rate of change of temperature.  The fire fighting system is manually activated once the operator has confirmed that there is a fire and that it is spreading.  The fire fighting system complies with the requirements of NFPA 850.

    Intelligent monitoring system.  To assess flow imbalance between preheaters in a twin stream arrangement, which preheater is fouling and specific location of fouling enabling targeted cleaning. 

    Primary Air

    A primary air fan can be fitted at the air preheater air outlet to increase the pressure of the air to the required value.  This is called a hot PA fan application.  The air preheater is a bi-sector design because both the primary and secondary air pass through the air side of the heater together.

    However, a cold PA fan absorbs less energy to increase the air pressure and does not require high temperature materials.  This is the preferred option in most new power stations.  A separate primary air system with cold PA fan is used. 

    Secondary Air

    One option is to have a main air preheater for the secondary air and a separate mill air preheater.  Many boilers have two main (secondary air) and two mill (primary air) air preheaters.  These were both bi-sector designs.

    The design philosophy has changed and now the normal system is to combine the main and mill air preheater into a single unit.

    See tri-sector and quad sector air preheater sections.

    Selective Catalytic Reduction

    This is where the SCR plant is positioned between the boiler economiser and the air preheater.

    As the plant is positioned after the boiler outlet the flue gas is at a high enough temperature for the reaction to take place. For new built boiler plan,  the SCR plant is planned as part of the boiler layout and therefore positioning it in the high dust position should present no difficulty

    Spare Parts

    Howden now only supply the Ljungström type although we service and provide spares for the Ljungström and Rothemühle types.

    Click for more information about the services and spares we provide.

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