Combustion gas flow control in boiler flues

Claim 1

A HEAT EXCHANGER FOR TRANSFERRING HEAT FROM HOT COMBUSTION GASES TO HEATING SURFACES, COMPRISING: A FIRST FLUE HAVING AN INLET AND AN OUTLET, FIRST HEATING SURFACES PLACED IN SAID FIRST FLUE, A SECOND FLUE HAVING AN INLET CONNECTED TO SAID OUTLET FOR RECEIVING HEATING GAS THEREFROM, AN UPTAKE FOR DISCHARGING COMBUSTION GASES FROM THE HEAT EXCHANGER, SAID SECOND FLUE HAVING AN OUTLET CONNECTED TO SAID UPTAKE FOR DISCHARGING COMBUSTION GASES, SECOND HEATING SURFACES PLACED IN SAID SECOND FLUE, A GAS DUCT HAVING A FIRST END PORTION CONNECTED TO SAID UPTAKE A FIRST PASSAGE CONNECTING THE INLET OF SAID FIRST FLUE AND SAID FIRST END PORTION OF SAID GAS DUCT, SAID GAS DUCT HAVING A SECOND END PORTION FORMING A SECOND PASSGE CONNECTING SAID GAS DUCT AND THE INLET OF SAID SECOND FLUE, DAMPER MEANS PLACED ADJACENT TO SAID SECOND GAS PASSAGE FOR CONTROLLING THE FLOW OF COMBUSTION GASES THROUGH THE OUTLET OF SAID FIRST FLUE, THROUGH SIAD SECOND PASSAGE, AND THROUGH THE INLET OF SAID SECOND FLUE, AND GAS FLOW CONTROL MEANS PLACED ADJACENT TO SAID FIRST PASSAGE AND CONSTRUCTED AND ARRANGED SO AS TO SIMULTANEOUSLY CONTROL THE GAS FLOW FROM THE INLET OF SAID FIRST FLUE THROUGH SAID FIRST PASSAGE AND THROUGH SAID GAS DUCT.