Air Preheater

The invention relates to an air heater, and seeks to provide a simple heater arrangement which can accommodate substantial variations in the air inlet temperature.

Known air heaters operate on a straightforward heat exchange principle; a hot fluid, normal gas such as combustion gases from a burner gives up heat to the air as it flows through the heater. A typical heat exchanger employs heat pipes to effect the transfer of heat from an hot fluid duct to the air duct. While this arrangement is generally satisfactory, when the air inlet temperature falls, so does the efficiency of the heat exchanger which is adversely affected, resulting in a disproportionately lowered outlet temperature. This can produce problems in the installation to which the heated air is to be fed.

According to the invention an air preheater comprises an heat source coupled to an heat exchanger located in an air flow path between an inlet duct, and an outlet duct therefor; an air preheater in the inlet duct; and means for selectively thermally coupling the preheater with heated air in the outlet duct to heat incoming air prior to passage through the heat exchanger. Normally, the heat source comprises a duct for hot gases located adjacent the air flow path, and the heat exchanger comprises a plurality of heat pipes traversing the hot gas duct and the air flow path.

In normal operation the heated air bypasses the thermal coupling, but when the air inlet temperature is particularly low, the heated air can be directed over the thermal coupling to maintain the preheater temperature and ensure that the air inlet temperature to the main heat exchanger is sustained at a substantially constant level. This enables the heat exchanger to operate under substantially the same temperature conditions at all times and thus at the same optimum efficiency. The preheater may also be of the heat pipe type, and the thermal coupling can be selectively exposed to the heated air by means of a simple damper arrangement.

The invention will now be described by way of example and with reference to the accompanying schematic drawings wherein:

FIG. 1 is a plan view of an heat pipe air heater demonstrating the principle of the invention; and

FIG. 2 is a broken perspective view showing one preferred orientation of the heat pipes.

In the heater of FIG. 1 hot gases from for example a burner or boiler pass along a duct 2, across which is arranged a heat exchanger bank 4 of heat pipes 6. The pipes 6 extend into and across an air duct 8 which receives air to be heated from an inlet 10. From the duct 8 the heated air passes to a discharge chamber 12 and thence to an air outlet 14. Arranged in the duct 8 upstream of the pipes 6 is a preheater 16. The preheater 16 comprises a second bank 18 of heat pipes 20 which extend into the discharge chamber 12. A damper 22 arranged in the discharge chamber 12 is movable between a first position shown, at which the heated air is directed across the extending pipes 20 to a second position indicated in dotted line at which the heated air is directed over the pipes 20 which thus thermally couple the discharge chamber to the air duct 8, upstream of the heat exchanger 4.

With the damper 22 in its first position the heater will operate under normal conditions with typical temperatures being about 370.degree. C. to 150.degree. C. for the gas and about 25.degree. C. to 300.degree. C. for the air. When the air inlet temperature falls substantially to say -18.degree. C., the damper is moved to its second position, boosting the preheater to maintain the air temperature at the entrance to the heat exchanger 4 at about 25.degree. C. The air temperature at the outlet of the heat exchanger 4 will therefore be maintained at about 300.degree., assuming the air flow rates remain substantially constant, although the air will of course be cooled at the thermal coupling with the preheater 16 to produce an outlet temperature of about 260.degree. C. More importantly though, the main heat exchanger 4 will operate under the same temperature conditions and in this way can operate at the same (optimum) efficiency. A temperature sensitive switch may be included to automatically move the damper 22 to its second position when the temperature of the incoming air falls below a predetermined value, and vice versa.

In the heater shown in FIG. 2 the arrangement is essentially the same but the heat pipes 6 of the heat exchanger 4 are arranged laterally, while those 20 of the preheater bank 18 are arranged vertically. This facilitates pipe withdrawal for maintenance and makes a simple practical arrangement. Also, the damper 22 is here located downstream of the heat pipes 20. In both arrangements illustrated, the preheater pipes 20 in the discharge chamber 12 can be maintained at a substantially constant temperature of about 90.degree. C. by the heated air.