US3908628A

US3908628A - Jet impingement recuperator

Info

Publication number: US3908628A
Application number: US407027A
Authority: US
Inventors: Lazaros J Lazaridis; Maurice Nunes; Sander E Nydick
Original assignee: Thermo Electron Corp
Current assignee: Thermo Fisher Scientific Inc
Priority date: 1973-10-16
Filing date: 1973-10-16
Publication date: 1975-09-30
Anticipated expiration: 1992-09-30

Abstract

A recuperative heat exchanger employs a jet impingement technique for transferring heat from hot combustion products to cool combustion air. An interior conduit of the heat exchanger contains baffles for producing this jet impingement effect. The baffles are secured in place during operation of the heat exchanger by a thermally expansible lip on the open end of each baffle. When the heat exchanger is not in operation the baffles may be removed for cleaning or replacement. Certain embodiments of this heat exchanger are specifically suited for use with a radiant tube furnace requiring a low pressure drop through the recuperator.

Description

United States Patent Lazaridis et al.

1 51 Sept. 30, 1975 Primary Exuminer-Charles J. Myhre Assistant E.\-aminer-Theophil W. Streule. Jr. Attorney. Agent, or Firm-James Lt Neal {57] ABSTRACT A recuperative heat exchanger employs a jet impingement technique for transferring heat from hot combustion products to cool combustion air. An interior conduit of the heat exchanger contains baffles for producing this jet impingement effect. The baffles are secured in place during operation of the heat exchanger [52] US. Cl 126/91 A; 432/223; 165/l54 51 1m. 01. F24c 3/00; F28d 7/10 by a thermally cxpanslblc 11p the end of each [58] Field of Search 126/116 R 91 A 116 A; baffle. When the heat exchanger is not in operation 432/222, 223; 165/76 the baffles may be removed for cleaning or replace- 431/350 ment. Certain embodiments of this heat exchanger are specifically suited for use with a radiant tube furnace [56] References Cited requiring a low pressure drop through the recuperator.

UNITED STATES PATENTS 1.734.262 11/1929 Lutschen 165/154 16 laims. 3 Drawing Figures 2.620.167 12/1952 Hobb v. 165/76 1416.011 12/1968 Lyczko 165/154 18 122 120 so 44 5e 42 65 77 16 E 38 l 1 l 74 e:c:a:o:e:a:o:o o o c o o o o o o o o c o c 72 46 D g/503 1 75 :-:-I-:-:-:-:-: :;Z;: k Whig 54 I l l 62 58 34 54 7 e 68 s ak 2 is 5 3o\ 60 a e 4 26 20 48 e2 e2 1 US. Patent Sept. 30,1975 Sheet2of2 3,90,628

J ET IMPINGEMENT RECUPERATOR BACKGROUND OF THE INVENTION 7 through more efficient combustion. A practical means of producing more efficient burning of fuels is through the use of a heat exchanger to preheat the fuelsor air by placing them in proximity with the hot exhaust gases.

ln radiant tube furnaces, 60 percent of the input energy is usually lost to the atmosphere if recuperation is not used, but many heat exchange devices that are now available are too expensive and too inefficient to justify, from an economic point of view, whatever savings in fuel that may result from their use. One way to obtain efficient preheating of the fuel in a multiple tube radiant tube furnace is to employ one large heat exchanger for all of the radiant tubes and channel all exhaust gases into it and all pre-combustion air through and out of it into each radiant tube. However, this method involves great installation costs, complex apparatus and resulting maintenance problems, a large space requirement because of the size of the system, and heat loss in the connecting pipes. I

Another way to preheat fuel or air is to build a heat exchanger to fit each radiant tube so that the operation of each tube is independent of any other. However, prior art devices adapted for use with radiant tubes are typically expensive and characterized by a low heat transfer coefficient. Thus, there is a need for an inexpensive recuperative heat exchanger that is small enough to be fitted onto one radiant tube yet efficient enough to produce high heat transfer coefficients;

One problem with any recuperative heat exchanger is that components in contact with hot flue gases tend to wear out rather quickly and must be replaced. If

these components are not easily removable, the entire heat exchanger has a short life and replacement costs become high.

Another problem with radiant tube heat exchangers is that they tend to develop deposits in the hot flue gas conduit during the period that the radiant tube is warming up. This phenomenon occurs because the gases are not yet hot and are easily cooled by the relatively cool air flowing through the outer conduit. This condensation process can drastically reduce the operating life of the heat exchanger.

Within many radiant tube furnaces the composition of the atmosphere surrounding the radiant tubes is closely controlled and it is highly undesirable for flue gases to leak from the radiant tube and to contaminate the furnaces controlled atmosphere. Pin holes and other small fissures tend to develop in a radiant tube while it is in use. Leakage resulting therefrom can be eliminated by minimizing the pressure differential between the interior of the radiant tube and the furnaces atmosphere filled volume. This low pressure differential although easy to be achieved in the case of a nonrecuperative radiant tube can only be achieved whenever a recuperator is being used, if the pressure drop through the recuperative heat exchanger is also minimized. Too great a reduction in the flue-gas-side pressure drop tends however to result in an unacceptably low heat transfer coefficient for the heat exchanger if conventional heat transfer techniques are to be em ployed.

SUMMARY OF THE INVENTION This invention relates to heat exchangers or recuperators generally and to jet impingement recuperators specifically. A general object of this invention is to reduce fuel consumption by providing an efficient and economical means of heating combustion air prior to entry into a burner. Another object is to provide small and economical recuperators which may be individually associated with a radiant tube furnace. Another object of the invention is to construct a recuperator so that vital and sensitive parts may be easily replaced or removed for servicing and cleaning.

This recuperator includes two concentric cylindrical tubes which define two separate flow conduits for two gases. One conduit is through the inner tube and the other conduit is through the annular space between the two tubes. These gases are confined on either side of the wall of the inner tube which acts as a heat exchange surface. Each conduit contains a set of cylindrical perforated baffles through which gases are forced. These gases then impinge upon the heat exchange surface in a plurality of small jets to produce efficient heat exchange between the two gases. In the preferred embodiment, the combustion products flow through the central conduit while the relatively cooler combustion air destined for the burner passes through the outer conduit defined by the annular space between the two cylinders in an opposed flow mode. When the recuperator is mounted upon a radiant tube heater, the inner tube of the recuperator may be merely an extension of the radiant tube itself.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a preferred embodiment of the heat exchanger;

FIG. 2 is a cut-away view illustrating the baffies in the interior and exterior conduits and the flow of gases through them; and

FIG. 3 is a schematic diagram showing the heat exchanger of FIG. 1 in conjunction with a radiant tube furnace.

DETAILED DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 illustrate a preferred embodiment of a heat exchanger 106. The heat exchanger consists of two concentric tubes, an inner tube 10 and an outer tube 12. These

tubes

10 and 12 may be of any convenient shape, but the preferred shape is cylindrical. Tubes 10 and I2 define two conduits, an exterior conduit 16 in the annular space between the

tubes

10 and 12 and an interior conduit 14 within the inner tube 10. The interior conduit 14 is adapted for a flow of relatively hot gases from an inlet end 36 of the interior conduit 14 to an outlet end 38 thereof, while the exterior conduit 16 is adapted for a flow of relatively cool gases from an inlet port 22 to an outlet port 24. A portion of the inner tube 10 forms a heat exchange wall 56 with heat exchange surfaces along each side thereof. The outer tube 12 is attached and sealed to the inner tube 10 by members 18 and 20 and a gasket 28. The only permitted flow of relatively cool gases is in the inlet port 22 and out theoutlet port 24.

The interior conduit 14 contains a longitudinally stacked series of

perforated baffles

30, 32 and 34, concentric with and wholly contained within the inner tube 10. The cross sectional shape of the

baffles

30, 32 and 34 should be the same as that of the tubes and 12. The shape preferably is cylindrical. The total number of baffles used in series depends on the requirements of the heat exchanger, since the number is a function of the allowable pressure drop from the inlet end 36 to the outlet end 38 of the inner conduit 14. The

baffles

30, 32 and 34 have closed

ends

40, 42 and 44, respectively, facing the outlet end 38 of the interior conduit 14, and

open ends

46, 48 and 49, respectively, facing the inlet end 36. The open end of each baffle includes a thermally expansible protruding lip 54 which expands to engage the heat exchange wall 56 during operation of the heat exchanger 106 in response to the resulting temperature increase. This lip 54 prevents gases from entering the space 58 between the baffle and heat exchange wall 56 except through openings 60 on side walls 62 of the

baffles

30, 32 and 34. The openings 60 may be of any suitable shape but are preferably substantially circular. Each such wall 62 is so aligned that gases flowing from the inlet end 30 to the outlet end 38 must pass through the openings 60 and impinge on the heat exchange wall 56 to form a plurality ofjets. The side walls 62 are separated from the heat exchange wall 56 by a distance 64 the value of which depends upon the requirements of the particular use to which the heat exchanger is intended.

The

baffles

30, 32 and 34 are stacked within the interior conduit 14 so that the separation between the closed end of one baffle and the adjacent open end of the next baffle is sufficiently large to permit the unimpeded flow of gases between the baffles without any discernible loss of pressure. Baffles 30 and 32 are secured to a rod 68 which passes through a hole in their closed ends 40 and 42 and extends through the baffles along their longitudinal axes. The rod ends at the closed end 44 of the baffle 34 where it is securely attached by a washer and nut assembly 78. The attachment of the closed end 44 to the rod 68 may also be effected by a weld.

The

baffles

30, 32 and 34 may be held in place in several ways. They may be welded to the rod 68. Also, they may rest on

cylindrical sleeves

75, 76 and 77 which are only slightly larger in diameter than the rod 68 and surround it. One end of the sleeve 76 abuts the top of the baffle 34 and the other rests against the baffle 32. In alike manner, sleeve 77 separates baffles 32 and 30 while sleeve 75 separates the baffle 30 and a spider arrangement 72.

The end of the rod 68 which faces the outlet end 38 of the interior conduit 14 is attached to the inner tube 10 by means of a pronged spider 72. The rod 68 is either welded or bolted, in a manner similar to that used to secure the baffle 34 to the other end of the rod 68. The prongs of the spider 72 are bolted to the wall of the inner tube 10 at ends 74.

The entire baffle

assembly including elements

30, 32, 34 and 68, hereafter designated assembly 65, may be removed for cleaning, repair or replacement. When the heat exchanger is not in operation, the protruding lips 54 contract and the assembly 65 is supported only by the spider 72. Thus, if the bolt 70 or the bolts at the end of each prong 74 are removed, the entire baffle assembly 65 can be easily extracted. The individual baffles can then be separated and the various parts cleaned or replaced.

The exterior conduit 16 also contains a stacked series of baffles 80 which contain openings 82 and which are intermediate and concentric with the

tubes

10 and 12. The baffles have the cross sectional shape of the

tubes

10 and 12. The openings may be of any suitable shape but are preferably substantially circular. This spacing of the baffles 80 from the heat transfer surface depends on the requirements of the recuperator. These baffles 80 may be welded to the outer tube 12 along one rim 88 while the other rim 90 rests against the wall of the inner tube 10 where an airtight seal is formed by thermal expansion and an interference fit of the rim 90 when the heat exchanger 106 is functioning. The baffles 80 are aligned so that the welded rim 88 is closer to the outlet port 24 than is the free rim 90 and so that gases flowing from the inlet port 22 to the outlet port 24 will be forced to pass through the openings 82 and impinge upon the heat exchange wall 56 in a plurality of jets. The positioning of these baffles 80 results in the formation of a series of annular plenum chambers 120 adjacent to the outer tube 12 and another series of annular plenum chambers 122 adjacent to the inner tube.

FIG. 3 depicts a preferred embodiment of the heat exchanger 106 as a recuperator in a radiant tube furnace 101. The radiant tube furnace 101 comprises a furnace enclosure 102 and a radiant tube means 108. The radiant tube means 108 comprises a fuel burner means 110, a valve means 113 for controlling the flow of combustion air into the burner means 110, a tube for conducting combustion products from the furnace 101 to the heat exchanger 106, an inlet 119 for introducing fuel into the burner, a blower means 104 for drawing combustion products out of the radiant tube 108, and a heat exchanger 106 for heating combustion air prior to its entry into the burner means 110. The heat exchanger 106 is small enough to be easily attached to a single radiant tube means 108. In fact, diameter of the radiant tube means 108 may equal the diameter of the inner tube 10.

In a controlled atmosphere radiant tube furnace it is desirable to maintain a pressure differential from the interior of the radiant tube 108 to an atmosphere of a furnace enclosure 102 which approximates zero. The creation of the negligible pressure differential is necessary to minimize leakage of combustion products from within the radiant tube 108 into the atmosphere of the furnace enclosure 102 through cracks and holes that may develop. A suction blower means 104 may be used to evacuate the combustion products from the interior of the radiant tube 108 and to lower the pressure therein. Primarily, the heat exchanger is so designed that there is a very low pressure drop through the inte rior conduit 14, a necessary condition for the creation of a low pressure differential from the interior of the radiant tube 108 to an atmosphere of a furnace enclosure 102. It is desirable to establish a pressure drop across the interior conduit 14 of the recuperator 106 not greater than 1 inch of [-1 0 if the low pressure differential across the wall of the radiant tube 108 is to be maintained. A pressure drop of 0.1 inch of H 0 is preferred. This must be obtained with a typical radiant tube heat input of 100,000-800,000 BTU/hr. Such a low pressure drop produces a relatively low velocity flow of gases through the openings 60 which may be laminar in nature throughout the interior conduit 14. The number of baffles in the interior conduit 14 is preferably from one to three. The resulting compact recuperator may be 36 inches or less in length.

The openings 60 in the

baffles

30, 32, 34 preferably are substantially circular, but they may be of other shapes. Each opening 60 should be separated from the heat exchange surface 56 by a distance 64 having a ratio to the hydraulic diameter of the opening not less than one or greater than four. Openings 60 are prefera bly spaced in a diamond-shaped configuration. The ratio of the distance between opening centers to the hydraulic diameters of the openings should not be less than two or greater than seven. An optimum hydraulic opening diameter lies between one-eighth and one-half inch.

In the exterior conduit 16, although the magnitude of the pressure drop from the injection port 22 to the extraction port 24 is not as critical as in the interior conduit 14, the pressure drop preferably does not exceed inches of water. Thus, the flow of gases through the exterior conduit 16 is also of relatively low velocity and may be liminar in nature. The openings 82 in the baffles 80 should be substantially circular, but they may also be of other shapes. The openings 82 should be separated from the heat exchange surface 56 by a distance 84 whose ratio to a hydraulic diameter of an opening 82 is not less than one or greater than three. The openings 82 are preferably spaced in a diamond-shaped configuration, and the ratio of the distance between opening centers to the hydraulic diameter of the openings should be not less than two or greater than six. An optimum hydraulic diameter is between one-sixteenth and one-fourth inch.

The configuration and special arrangement of the

openings

60 and 80 are defined in terms of hydraulic diameter. This term, employed in fluid dynamic technology, is used here since it is applicable both to circular and non-circular openings. The hydraulic diameter of an opening is defined as 4 times its cross-sectional area divided by its perimeter. The hydraulic diameter of a circle equals its diameter. It is important that each jet formed by the gases passing through an opening in a baffle have a minimum of interference with any other jet formed by that baffle, and that each such jet impinge directly upon the heat exchange wall 56. Maximum heat exchange coefficients will be achieved when interference is minimized and the gaseous flow maxi mized.

The operation of the heat exchanger 106 of FIGS. 1 and 2 in conjunction with a radiant tube means 108 il lustrated in FIG. 3 will now be discussed. Combustion air enters the system through a pipe 116 and passes to a valve means 1 13 including a first valve 1 12 and a second valve 114. It is either directed to the burner through the second valve 114 or to the exterior conduit of the heat exchanger 106 through the first valve 112. Thereafter, it is introduced into the burner.

The valve means 113 serves three functions. First, during start up, the combustion products in the inner conduit 14 are not yet hot, and if cooler combustion air is allowed to circulate in the exterior conduit 16, the combustion products are cooled to a point where condensation of certain of the combustion products occurs. This is highly undesirable, so during these periods, the cooler combustion air is not allowed to enter into the exterior conduit 16 but is diverted directly to the burner means 110 through the second valve 114 while the first valve is closed. Secondly, the ratio of cool to preheated combustion air entering the burner means 40 may be varied to regulate the temperature in the radiant tube 108. In many conditions, after an initial startup period, all air will be directed through the exterior conduit 16. Thirdly, the amount of air entering the exterior conduit 16 through the first valve 112 may be regulated to influence the temperature of the heat exchanger 106.

The combustion air that passes through the valve means 113 enters the exterior conduit 16 at its injection port 22 and proceeds into an annular plenum chamber 120. The combustion air is forced through the openings 82 in the baffie 80, thus forming a plurality of jets which impinge upon the heat exchange wall 56. Thereafter, the air is forced from the plenum chamber 122 adjacent the heat exchange wall into the next plenum chamber 120 in the series. The combustion air is again forced through the openings 82 as jets and this flow pattern continues until the combustion air reaches the exhaust port 24 of the exterior conduit 16. The combustion air then passes through a pipe 118 to the burner means 110. The resulting combustion products then pass through the length of the radiant tube 108 heating it. Thereafter, they reach the heat exchanger 106.

The inner tube 10 of the heat exchanger 106 is an extension of the radiant tube 108. The inner tube 10, tube and radiant tube 108 may actually be integral. Combustion products enter the inlet end 36 of the interior conduit 14 and are forced through the

baffles

30, 32, 34, thus forming a plurality of jets which impinge upon the heat exchange wall 56. Heat is'transferred from these jets to the heat exchange wall 56 and thence to the cool jets in exterior conduit 16 impinging on the other side of the heat exchange wall. Subsequently combustion products are exhausted through the outlet end 38. The

baffles

30, 32, 34 become heated and radiate energy to the heat exchange wall 56 in significant quantities. This total radiation transfer may contribute up to 40 percent of the total heat transfer in the heat exchanger 116.

The protruding lip 54 on the open ends 46, 48, 49 of each baffle ex ands in response to an increase in temperature caused by contact with the hot combustion products. The lip 54 then sealingly engages the heat exchange wall 56. Thus combustion products are prevented from entering the space 61 between the baffles and the heat exchange wall without passing through the openings 60.

The direction of flow of the combustion products in the interior conduit 14 should be counter to the flow of the combustion air in the exterior conduit 16 to promote the most efficient exchange of heat between the combustion air and the combustion products. However, the flows may be parallel and still permit heat exchange.

In operation, the heat exchanger 106 has produced heat transfer coefficients of l3.5 BTU/F. This effectiveness makes possible the small recuperative heat exchanger capable of fitting directly into the radiant tube 108. Installation is relatively simple and inexpensive. The result is a low equipment cost. In addition, the removeability of the

baffles

30, 32, 34 cuts the replacement and maintenance costs significantly. Finally, this heat exchanger provides substantially reduced fuel consumption.

While this invention has been described with respect to the details of various specific embodiments, many changes and variations which can be made without departing from the scope of the invention will be apparent to those skilled in the art.

I claim:

1. A controlled atmosphere radiant tube heating device comprising a fuel burner means, air intake means for injection of combustion air into the burner means, a radiant tube heated by combustion products from the burner means, an exhaust tube for conducting combustion products from the radiant tube, a length of the wall of the exhaust tube forming a heat exchange surface on each side thereof an outer tube concentric with and surrounding the exhaust tube forming an exterior conduit for confining a flow of combustion air adjacent the heat exchange surface, means for directing the flow of combustion air into the exterior conduit at an inlet end thereof, means for directing the flow of combustion air from the exterior conduit at an outlet end thereof to the burner means, baffle means interior of the exhaust tube, the baffle means having openings therein for directing jets of combustion products onto said length of the wall, the openings having hydraulic diameters not less than one-eighth inch and not greater than one-half inch, the ratio of separation of centers of mass of the openings to the hydraulic diameter of the openings being not less than two or more than seven.

2. A radiant tube heat exchanger as defined in claim 1 wherein the exterior conduit contains second baffle means having openings therein for directing jets of the combustion air onto the heat exchange surface.

3. A radiant tube heat exchanger as defined in claim 1 wherein the ratio of the separation of an opening in the baffle means from the heat exchange surface to the hydraulic diameter of the opening is not less than one or greater than four.

4. A radiant tube heat exchanger as defined in claim 1 wherein the spatial pattern of the openings in the baffie means is diamond shaped.

5. A radiant tube heat exchanger as defined in claim 2 wherein the second baffle means comprises a plurality of sections aligned such that one end of each section is closer than the other end to the inlet end of the exterior conduit, and wherein for each section one end is fixedly attached to either the exhaust tube or the outer tube while the other end of each section comprises thermally expansible means for sealingly engaging the wall of the other tube means.

6. A radiant tube heat exchanger as defined in claim 5 adapted for laminar flow in the exterior conduit and a low pressure drop from the inlet end to the outlet end thereof wherein the second baffle means in the exterior conduit comprises not more than eight annular baffle plates dividing the exterior conduit into a series of annular plenum chambers, the plenum chambers and the openings in the baffle plates constituting the sole passage for the flow of combustion air through the exterior conduit.

7. A radiant tube heat exchanger as defined in claim 6 wherein an optimal ratio of the separation of an opening in the second baffle means from the heat exchange surface to the hydraulic diameter of such an opening is not less than one or greater than three.

8. A radiant tube heat exchanger as defined in claim 6 wherein an optimal hydraulic diameter of the openings in the second baffle means is not less than onesixteenth inch or greater than one-fourth inch.

9. A radiant tube heat exchanger as defined in claim 6 wherein an optimal ratio of a distance between centers of the openings in the second baffle means to the hydraulic diameter of such an opening in the second baffle means is not less than two or greater than six.

10. A radiant tube heat exchanger as defined in claim 6 wherein an optimal spatial pattern of the openings in fles; further comprising a rod extending centrally' through each baffle in the series and attached thereto for enabling the extraction as a unit of the series of baf fles from the exhaust tube, thermally expansible sealing means on each baffle in the series of baffles along the portion thereof facing the injection end of the exhaust tube for sealingly engaging the heat exchange surface only in response to temperature above a predetermined temperature.

14. A controlled atmosphere radiant tube heating device comprising a fuel burner means having a heat output between 100,000 and 800,000 BTU/hr., air intake means for injection of combustion air into the burner means, an enclosure adapted to confine a controlled atmosphere, a radiant tube within the enclosure, the radiant tube confining and being heated by combustion products from the burner means, an exhaust tube for conducting combustion products from the radiant tube, a length of the wall of the exhaust tube forming a heat exchange surface on each side thereof, an outer tube concentric with and surrounding the exhaust tube forming an exterior conduit for confining a flow of combustion air adjacent the heat exchange surface, means for directing the flow of combustion air into the exterior conduit at an inlet end thereof, means for directing the flow of combustion air from the exterior conduit at an outlet end thereof to the burner means, baffle means interior of the exhaust tube for producing a pressure drop not substantially greater than 0.1 inch of water across the exhaust tube, the baffle means having openings therein for directing jets of combustion products onto said length of the wall, the openings hav ing hydraulic diameters not less than one-eighth inch and not greater than one-half inch, the ratio of separation of centers of mass of the openings to the hydraulic diameter of the openings being not less than two or more than seven, each baffle opening being separated from the adjacent heat exchange surface by a distance not less than the hydraulic diameter of the opening or more than 4 times the hydraulic diameter of the opening.

15. A radiant tube heat exchanger as defined in claim 14 wherein the exterior conduit contains second baffle means having openings therein for directing jets of the combustion air onto the heat exchange surface.

16. A radiant tube heat exchanger as defined in claim 15 wherein the second baffle means comprises a plurality of sections aligned such that one end of each section is closer than the other end to the inlet end of the exterior conduit, and wherein for each section one end is fixedly attached to either the exhaust tube or the outer tube while the other end of each section comprises thermally expansible means for sealingly engaging the wall of the other tube means.

Claims

1. A controlled atmosphere radiant tube heating device comprising a fuel burner means, air intake means for injection of combustion air into the burner means, a radiant tube heated by combustion products from the burner means, an exhaust tube for conducting combustion products from the radiant tube, a length of the wall of the exhaust tube forming a heat exchange surface on each side thereof, an outer tube concentric with and surrounding the exhaust tube forming an exterior conduit for confining a flow of combustion air adjacent the heat exchange surface, means for directing the flow of combustion air into the exterior conduit at an inlet end thereof, means for directing the flow of combustion air from the exterior conduit at an outlet end thereof to the burner means, baffle means interior of the exhaust tube, the baffle means having openings therein for directing jets of combustion products onto said length of the wall, the openings having hydraulic diameters not less than one-eighth inch and not greater than one-half inch, the ratio of separation of centers of mass of the openings to the hydraulic diameter of the openings being not less than two or more than seven.

4. A radiant tube heat exchanger as defined in claim 1 wherein the spatial pattern of the openings in the baffle means is diamond shaped.

8. A radiant tube heat exchanger as defined in claim 6 wherein an optimal hydraulic diameter of the openings in the second baffle means is not less than one-sixteenth inch or greater than one-fourth inch.

10. A radiant tube heat exchanger as defined in claim 6 wherein an optimal spatial pattern of the openings in the second baffle means is diamond shaped.

11. A radiant tube heat exchanger as defined in claim 6 wherein the openings in the first said baffle means are substantially circular in shape.

12. A radiant tube heat exchanger as defined in claim 6 wherein the openings in the second baffle means are substantially circular in shape.

13. A radiant tube heat exchanger as defined in claim 1 wherein said baffle means comprises a series of baffles; further comprising a rod extending centrally through each baffle in the series and attached thereto for enabling the extraction as a unit of the series of baffles from the exhaust tube, thermally expansible sealing means on each baffle in the series of baffles along the portion thereof facing the injection end of the exhaust tube for sealingly engaging the heat exchange surface only in response to temperature above a predetermined temperature.

14. A controlled atmosphere radiant tube heating device comprising a fuel burner means having a heat output between 100, 000 and 800,000 BTU/hr., air intake means for injection of combustion air into the burner means, an enclosure adapted to confine a controlled atmosphere, a radiant tube within the enclosure, the radiant tube confining and being heated by combustion products from the burner means, an exhaust tube for conducting combustion products from the radiant tube, a length of the wall of the exhaust tube forming a heat exchange surface on each side thereof, an outer tube concentric with and surrounding the exhaust tube forming an exterior conduit for confining a flow of combustion air adjacent the heat exchange surface, means for directing the flow of combustion air into the exterior conduit at an inlet end thereof, means for directing the flow of combustion air from the exterior conduit at an outlet end thereof to the burner means, baffle means interior of the exhaust tube for producing a pressure drop not substantially greater than 0.1 inch of water across the exhaust tube, the baffle means having openings therein for directing jets of combustion products onto said length of the wall, the openings having hydraulic diameters not less than one-eighth inch and not greater than one-half inch, the ratio of separation of centers of mass of the openings to the hydraulic diameter of the openings being not less than two or more than seven, each baffle opening being separated from the adjacent heat exchange surface by a distance not less than the hydraulic diameter of the opening or more than 4 times the hydraulic diameter of the opening.