GB719378A - Improvements in heat exchangers - Google Patents

Abstract

719,378. Heating air for combustion. BABCOCK & WILCOX, Ltd. Oct. 26, 1950 [Oct. 29,1949], No. 26104/50. Class 64(1) In a heat-exchanger in which a cool fluid (e.g. combustion air) is heated by furnace gases, heat-exchange elements are disposed in groups along the cool fluid flow path leading from a cool fluid inlet, and means are provided whereby the gas flow through the group of elements adjacent the cool fluid inlet may be increased to maintain the elements of that group at a sufficiently high temperature to inhibit the condensation thereon of vapour in the furnace gases. The air heater shown comprises two units A, B, Fig. 2, connected in parallel to a common gas inlet 8 and leading to an inlet chamber 23 for each unit and thence through the heat-exchanger elements to individual gas outlet chambers 27 each 'having an outlet duct 26. Each unit A, B comprises successive banks C, D, E, F, Fig. 1, of tubes 16, the tubes in bank C adjacent the inlet 12 for air to be heated being fewer in number and more widely spaced than are the tubes of bank F, while the tubes of bank D are more closely spaced than those of bank C but are fewer and more widely spaced than those of banks E and F. The banks C, F are connected in series by a passage 31 and the banks D, E are similarly connected in series by a passage 32, Fig. 1. Dampers 41, 42 are provided in passages 31, 32 respectively. Combustion air is forced by a fan 13 through the duct 12 or individual ducts corresponding with the sections A, B, over the banks C, D, E, F in succession to the outlet 15 leading to the burners of a furnace. Hot gas is directed through the banks E, F in parallel and thereafter some of the gas is directed by way of passage 32 to bank D, the remainder being directed by way of passage 31 to bank C, which is adjacent the air inlet. By directing relatively hot gas to the bank C, the gas is not cooled below the dew point and corrosion of the tube walls is prevented. The proportion of gas directed to the bank C is controlled according to the temperature of the tubes by means comprising a thermocouple 121 adjacent the tubes in bank C which regulates the opening of the dampers 42. The maintenance of a suitable minimum gas temperature in the tube bank C is aided by the disposition of the tubes therein as compared with the other banks. In bank C the relatively wide tube spacing reduces the speed of air flow and therefore of heat flow from the tubes, and also the reduced number of tubes in bank C as compared with the other banks reduces the gas flow area and therefore increases the relative speed of the gas and therefore the rate of heat absorption from the gas by the tubes C is also increased. The cooled gas is finally discharged through the ducts 26. Additional dampers 11 and 14 are provided for controlling gas and air flow respectively. Provision is made for washing the tubes.