US3574497A

US3574497A - Heat distribution system for heat treatment furnaces

Info

Publication number: US3574497A
Application number: US794670*A
Authority: US
Inventors: Cyril Henry Rann; Elizabeth Margaret Hall
Original assignee: North Western Gas Board
Current assignee: North Western Gas Board; North Eastern Gas Board; British Gas PLC
Priority date: 1969-01-28
Filing date: 1969-01-28
Publication date: 1971-04-13
Anticipated expiration: 1988-04-13

Abstract

Apparatus for high convective heat transfer effect at comparatively low terminal head above the temperature in a heat treating furnace embodying a high velocity tunnel burner. Within the furnace distribution duct into which gases are projected from the burner, there is provided an inner mixing duct open at both ends and one or more distribution spaces between the two ducts. A plurality of discharge ports are spaced along the length of the distribution duct. Hot gas Ports permit a net flow back to the burner with part of the flow reinjected into the furnace by the injector action of the burner and part passing into the flue system.

Description

United States Patent Appl. No. Filed Patented Assign ee [54] HEAT DISTRIBUTION SYSTEM FOR HEAT TREATMENT FURNACES 3 Claims, 3 Drawing Figs.

[52] US. Cl 431/116 [51] Int. Cl F231 7/00 [50] Field ofSeai-ch 431/352, 353, 115, 116; 263/19 [56] References Cited UNITED STATES PATENTS 2,651,514 9/1953 Sherman 263/19 3,131,749 5/1964 Davis 431/353 Primary Examiner-Edward G. Favors Attorney-Abraham A. Saffitz ABSTRACT: Apparatus for high convective heat transfer effect at comparatively low terminal head above the temperature in a heat treating furnace embodying a high velocity tunnel burner. Within the furnace distribution duct into which gases are projected from the burner, there is provided an inner mixing duct open at both ends and one or more distribution spaces between the two ducts. A plurality of discharge ports are spaced along the length of the distribution duct. Hot gas Ports permit a net flow back to the burner with part of the flow reinjccted into the furnace by the injector action of the burner and part passing into the flue system.

HEAT DISTRIBUTION SYSTEM FOR HEAT TREATMENT FURNACES This invention relates to heat treatment fumaces and aims at improving the approach to uniformity of heat distribution therein.

One of the prime requirements of heat treatment processes is that heating should be uniform, i.e. ideally all parts of the workpiece to be heat treated should achieve the same temperature in the same time and this uniformity should be further maintained throughout any soaking period which is given. Most furnace designs intended for heat treatment attempt to achieve such uniformity with varying degrees of success, sometimes by using a multiplicity of burners. It is particularly difficult at low heat treatment temperatures, for instance in the range of 400 C.700 C. where radiation can play little part in assisting temperature uniformity. Modern industrys requirements for the heat treatment of large workpieces have added to this problem.

For a number of years efforts have been made largely by the North Eastern Gas Board to achieve uniform temperature in large furnace structures by the use of high velocity techniques and much information concerning this is available in the published literature. Development work along these lines has led to the use of a distribution duct system in the furnace which, combined with a high velocity tunnel burner, could achieve large circulation of hot gases within the furnace structure and thus assist in obtaining uniform temperatures. One object of the present invention is to provide an improved distribution duct system which will produce a greater degree of uniformity in the distribution of hot gases projected into the furnace.

A further object is to achieve a high convective heat transfer effect at a comparatively low thermal head above the temperature of a workpiece in the furnace.

These and other objects and advantages of the invention will be explained in the following description referring to the accompanying diagrammatic drawings, wherein:

FIG. 1 illustrates a usual known arrangement of distribution duct system;

FIG. 2 illustrates by way of example one form of modification according to this invention; and

FIG. 3 represents a cross section through one of the discharge ports shown in Figure 2 but incorporating an adjustable damper.

The usual arrangement, illustrated diagrammatically in Figure 1 of the accompanying drawings, has been for the gases to emerge into the furnace from a system of discharge ports P connected to the duct D into which the gases of combustion are projected from the burner B, generally as shown in FIG. 1. This system had a number of limitations, notably that it requires a length of duct immediately after the burner B where no ports can be placed. If such ports were inserted (as shown at PX in Figure 1) then the net flow through these was into and not out of the duct-due to the injector effect of the gases leaving the burner nozzle.

The system of the present invention is a further development of this idea and is illustrated in FIG. 2 of the accompanying drawings. Within the duct D is a central or inner mixing duct d open at both ends and leaving between it and the duct D an end space d1 communicating with an annular distribution space d2 (or two or more distribution spaces) from which all the ports P lead into the furnace. The inlet end of the mixing duct d is sealed in the inlet end of the duct D. The nozzle of the burner B is so located relative to the inlet end of the mixing duct d, and the latter is so located relative to the furnace interior, as to cause the gases of combustion projected from the burner to reinject into the only slightly above the furnace temperature. These gases pass in one direction along the mixing duct and divide into the distribution duct, flowing back through the distribution space or spaces to emerge from the duct system via the ports. The gases circulate round the furnace from the ports, there being a net flow back to the burner where the path of the gases divided, part being reinjected via the ducts into the furnace and part passing into the flue system. The ports also, due to the high velocity of the emerging gases, act as injectors and thereby create further local circulation in the furnace. The cross-sectional area of each of the ports is arranged to give, so far as possible, an effective equal flow from each to produce a very even heat distribution using only a single burner. To this end the area of each port can be conveniently adjusted for practice by adjustable dampers Pd in the manner indicated in Figure 3 of the accompanying drawings.

The distribution duct assembly according to this invention may be located in the furnace floor or in a trolley mounted floor, and the ports may be directed upwards and/or at any desired angle according to the requirements of each particular installation.

When the duct assembly of this invention is fitted in a furnace floor, it enables the heat to be transferred mainly by conduction to the base of the workpiece which is usually the most difficult part of it to heat.

By means of this integral circulation system according to the invention, it is possible to achieve a high convective heat transfer effect at a comparatively low thermal head above the temperature of the workpiece.

We claim:

1. A heat distribution system for producing a high convective heat transfer effect at a comparatively low thermal head above the temperature of a workpiece in a heat treatment furnace embodying a high velocity tunnel burner, said system using a distribution duct having spaced along its length a plurality of discharge ports arranged to discharge the gases of combustion over a large area of the furnace interior and to circulate them therein; said distribution duct having associated with it a mixing duct arranged to provide a distribution space between the two ducts and to cause the combustion products projected by said burner into one end of the mixing duct and therealong in one direction to then pass in the opposite direction along said space before issuing from said discharge ports, the nozzle of said burner being so located relative to the inlet end of said mixing duct and said inlet end being so located relative to the furnace interior as to cause the burner to reinject into said distribution assembly a part of the gases leaving the furnace interior, thereby recirculating such part through the furnace interior.

2. A heat distribution system according to claim 1, wherein said mixing duct has its inlet end sealed in the inlet end of said distribution duct and its outlet end located near to the closed other end of said distribution duct, said mixing duct being centrally located longitudinally within said distribution duct to enable said distribution space to completely surround said mixing duct.

3. A heat distribution system according to claim 1, wherein adjustable damper means are provided for at least some of said discharge ports for individually varying their respective cross-sectional areas.