Furnace over-pressure prevention

Claim 1

A furnace having radiant and convection sections and at a location directly below said convection section normally operating at negative pressure relative to ambient barometric pressure, including an apparatus for preventing over-pressuring of said furnace when said negative pressure exceeds said ambient barometric pressure comprising: a. an open passageway located externally of said furnace connecting the interior of said furnace adjacent and below the convection section with the surrounding atmosphere, said passageway being constructed and arranged to permit continuous substantially unobstructed flow therethrough and of uniform size such that during normal operation ambient air normally flows inwardly to said furnace in sufficiently small quantities so that normal furnace operation is undisturbed and when said furnace is at positive pressure the normal inward flow of ambient air is reversed so that combustion gases flow outward through said passageway; b. flow reversal detecting means operably connected with said passageway for detecting the reversal of flow in said passageway, said detecting means comprising a temperature sensing means for detecting changes in temperature in said passageway including indicating a low temperature when said ambient air flows into said furnace and a high temperature when said combustion gases flow out of said furnace; and c. alarm means operably connected to said flow reversal detecting means and responsive to a detected change in flow direction in said passageway for providing an alarm signal when said detecting means indicates a High temperature resulting from combustion gases passing outward through said passageway.