This invention relates to fire detection apparatus in a preheater, such as a rotary regenerative air preheater, comprising a preheater housing with an air duct and an exhaust duct, a mass of heat absorbent material in said housing, means for imparting a relative rotation between the mass of material and the ducts, a plurality of temperature sensitive detectors sensing the temperature of radially adjacent sections of the material in said housing to each produce a signal proportional to the temperature in said respective sections and means actuated by said signals for indicating when a rate of change of the temperature in any of said sections is in excess of a predetermined rate of change.
As hot exhaust gases traverse the heat absorbent material unburned fuel and/or soot is frequently deposited on the material. These deposits may in a short time build up to a point where air or gas flow will be partially restricted. Moreover, heat will frequently begin to be exothermically generated in such deposits to a point where the deposits will glow and cause an incipient fire or a so called "hot-spot" which may rapidly increase in temperature to such an extent that surrounding metal will ignite causing serious damage to the surrounding structure.
Known prior art is represented by e.g. British Pat. Specification No. 1 126 466 showing an air preheater and a fire detection apparatus having a plurality of temperature sensitive detectors each comprising a fast response thermocouple and a slow response thermocouple adapted to signal when a temperature within the preheater changes at a rate of change in excess of a predetermined rate of change.
As a rule fires start connection with starting-up and load variation operations during which operations, however, the known fire detection apparatus indicates a rate of temperature change in excess of the predetermined rate of change, i.e. the known apparatus is inoperative during the most dangerous working operations.
The object of the present invention is to eliminate this serious drawback of the known apparatus and provide an improved fire detecting apparatus which is operative during starting-up periods as well as during normal operation.
Another object of the invention is to provide a simple and uncomplicated apparatus in which each temperature sensitive detector comprises a single thermocouple or similar temperature sensing member.