The primary air for drying and transporting the pulverised coal from the coal mill to the furnace is at a much higher pressure. It is very important to minimise the leakage of the air to gas for a number of reasons which will be described later.
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. Similar increases in leakage are experienced on tri-sector preheaters, which are the most common type of air preheater adopted on pulverised coal fired boilers. These increased leakages increase FD, ID &PA fan power and in extreme cases de-rate the Unit. The Howden VN sealing concept maintains low design leakage between outages.
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.
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.
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.