GB1602812A - Industrial oven - Google Patents

Description

PATENT SPECIFICATION

Application No 34614/80 ( 22) Filed 23 Feb 1978 Divided out of No 1602811 Convention Application No 773495 Filed 2 March 1977 in United States of America (US) Complete Specification published 18 Nov 1981

INT CL 3 F 28 D 19/00 BOID 53/36 ( 11) 1 602 812 ( 1 ' ( 52) Index at acceptance F 4 K 23 B 2 23 C 2 2 U Bl W AX ( 72) Inventor RONALD L KONCZALSKI ( 54) INDUSTRIAL OVEN ( 71) We, MICHIGAN OVEN COMPANY, a Corporation organised and existing under the laws of the State of Michigan, United States of America, of 1st Oven Place, Romulus, Michigan 48174, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:-

This invention relates to industrial ovens.

Recovery of heat from the exhaust gases of industrial processes has frequently been effected, using various forms of heat exchangers A rotary heat exchanger has been used for this purpose; examples are provided in Ljungstrom U S Patent No.

1,586,816, Karlsson U S Patent No.

2,680,008, and Dravnieks U S Patent No.

3,733,791 Whilst such rotary heat exchangers can be quite effective in removing and recovering heat from industrial exhaust gas there is a need to develop an industrial oven of improved heat-recovery efficiency.

In accordance with the present invention there is provided an industrial oven oriented on an elongated vertical axis and equipped with a stack for emitting exhaust gas, the oven comprising:

a burner at a predetermined level inside the oven chamber, a thermal regenerator moveable continuously about a closed path which alternately traverses both a first duct carrying a stream of hot exhaust gas and a second duct carrying a stream of fresher, cooler entrant air thereby to exchange heat between the two gas streams so that the temperature of the outgoing fresh air leaving the regenerator is raised and that of the exhaust gas is lowered; drive means to so move the regenerator; a sensor, for sensing the temperature of one of the streams, combined with control means for adjusting the rate of movement of the regenerator to vary the rate of heat exchange as a function of the sensed temperature; and a duct for directing into the oven the outgoing stream of fresh air leaving the regenerator.

By means of the present invention, it is possible to provide an industrial oven of improved efficiency characterized by a new and improved cyclically operable thermal regeneration apparatus which provides for the controlled recovery of heat from the exhaust gas stream in an efficient manner, and yet which is simple and economical in construction.

In accordnace with a specific embodiment of the invention the thermal regenerator may comprise an air-permeable reactor medium of a large surface area and high thermal capacity incorporating an oxidation catalyst By this expedient, oxidizable contaminants present in the hot exhaust gas may be catalytically oxidized to innocuous byproducts, which not only has the effect of purifying the exhaust gas before it is exhausted through the stack but in addition substantial amounts of heat are released, thereby improving the heatrecovery efficiency of the oven.

The invention will now be described further with reference to the accompanying drawings in which:

Fig 1 is a simplified plan view of a preferred embodiment of a thermal regenerator suitable for incorporation in an industrial oven of this invention, illustrating the flow of exhaust gas and fresh air streams through the regenerator.

Fig 2 is an elevation view of the regenerator taken approximately along line 2-2 in Fig I; Fig 3 is a sectional elevation view taken approximately along line 3-3 in Fig 2; Fig 4 is a detail sectional view taken approximately along line 4-4 in Fig 2; ( 21) ( 62) ( 31) ( 32) ( 33) ( 44) ( 51) 00 P.4 1,602,812 Fig 5 is a simplified schematic plan view of a second embodiment of a thermal regenerator suitable for incorporation in an industrial oven of this invention; Fig 6 is a simplified view taken approximately along fine 6-6 in Fig 5; Fig 7 is a graph of operation; Fig 8 is an isometric view of an industrial oven in accordance with one embodiment of this invention; Fig 9 is a side elevation of the top portion of the oven of Fig 8; Fig 10 is a view on the line 10-10 of Fig.

9.

Figs I-4 illustrate the construction and principles of operation of a thermal regenerator 10 constructed in accordance with one preferred embodiment of the present invention and suitable for incorporation in an industrial oven.

Regenerator 10 extends across a first duct 11 comprising an exhaust duct for the oven that produces a highly contaminated hot exhaust gas laden with oxidizable contaminants Typically, the oxidizable contaminants are solvent vapors such as toluene, xylene, etc, though they may constitute other types of contaminants as well The contaminated hot exhaust gas stream enters regenerator 1 U through one section IIA of the exhaust duct 11 and leaves regenerator 10 through a continuing duct section l IB.

Regenerator 10 also intersects a second duct 12 carrying a fresh, cooler, air supply.

The stream of fresh air enters regenerator through a duct section 12 A and leaves through a duct section 12 B The fresh air stream that passes through regenerator 10 is employed as a hot fresh air make-up supply for the oven, as described more fully below in connection with Figs 8-10.

The internal construction of thermal regenerator 10 is illustrated in Figs 2, 3 and 4 As shown therein, regenerator 10 comprises a cylinder 14 mounted upon a shaft 15 Cylinder 14 is packed with an airpermeable reactor medium 16 of high thermal capacity, having a large surface area that is exposed to the two air streams that pass through regenerator 10 The reactor medium 16 may constitute a metallic honeycomb structure or a monolithic structure The reactor medium should be of light weight construction to assure rapid thermal response Materials of this general kind are available in a variety of cell configurations and sizes affording a substantial range of design capabilities.

The air-permeable reactor medium 16, forming the working portion of cylinder 14, is preferably coated throughout with a precious metal catalyst to enable cylinder 14 to function as a catalytic converter The preferred catalysts are precious metal catalysts, particularly platinum and palladium Use of these particular metal catalysts allows for effective catalytic operation with inlet temperatures as low as 4500 F for some solvent laden gases Other catalysts, such as base metal oxides, can be used; however, these are less advantageous in that they require higher operating temperatures and longer contact times within the reactor medium 16.

Shaft 15 is supported in suitable bearings 17 and 18 mounted on the frame of regenerator 10, and one end of shaft 15 is disposed in a pillow block 19 (Figs 3 and 4).

A sprocket 21 affixed to shaft 15 is connected by a drive chain 22 to a sprocket 23 The drive sprocket 23 is mounted upon the output shaft 24 of a gear unit 25 driven by a variable speed electric motor 26 The operational speed of motor 26 is determined by a speed controller 27 connected to a thermal sensor 28 that senses the output temperature of the heated fresh air leaving regenerator 10 through duct section 12 B (Figs 1 and 3).

A central barrier 31 in regenerator 10 maintains two separate passages through cylinder 14 That is, the central barrier 31 affords a cutoff zone that prevents any substantial flow of air between the two ducts 11 and 12 as the air carried by those two ducts traverses regenerator 10.

Insulation is provided, on barrier 31 and in other appropriate locations in regenerator 10.

In operation, a contaminated hot exhaust airstream from the oven chamber enters regenerator 10 through section 1 l A of duct 11; typically this exhaust air is laden with solvents and other oxidizable contaminants 105 Furthermore, the exhaust gas temperature is usually too high to permit direct release to the atmosphere As the hot exhaust gas enters the rotating reactor medium 16 of cylinder 14, the high activity catalyst 110 coating on the reactor medium surfaces causes immediate oxidation of the contaminants That is, the hydrocarbons organic compounds, and other oxidizable contaminants in the exhaust gas stream react 115 chemically with the oxygen in the stream to form innocuous byproducts constituting water vapor and carbon dioxide Assuming that the principal contaminants are ordinary industrial solvents, the chemical reaction 120 releases approximately 100,000 BTU per hour per gallon of solvent, further increasing the temperature of the exhaust gas However, most of this heat and the heat present in the entering exhaust stream is 125 transferred to the rotating reactor medium 16.

As cylinder 14 is rotated by drive motor 26, acting through gear box 25, chain 22, and shaft 15, the two adjacent streams of 130 1,602,812 fresh air from duct 12 and exhaust gas from duct 11 pass through the reactor medium 16 in the cylinder A transfer of sensible heat is caused by the substantial difference in dry bulb temperature between the two air streams, aided by the large area heat transfer surface afforded by the reactor medium 16 and by laminar flow of the two air streams Thus, as each segment of cylinder 14 rotates through the exhaust air stream (duct 11), heat is picked up by the literally thousands of square feet of heat transfer surface of the reactor medium 16 in the cylinder The heated segment then rotates through a cutoff zone defined by the central barrier 31, which precludes any substantial crossflow from one air stream to the other As the heated reactor medium segment enters the fresh air stream (duct 12) its heat is given up to the fresh air Finally, the same segment of the reactor medium passes through another cutoff zone defined by the lower portion of barrier 31 and reenters the exhaust air stream to again pick up heat It should be noted that there are two sources of heat to be transferred to the fresh air stream; one heat source is the heat of the exhaust gas entering regenerator 10 and the other heat source is the heat generated by oxidation of the contaminants in the exhaust gas.

The fresh air and hot exhaust streams could pass through regenerator 10 in the same direction However, the counterflow arrangement illustrated in Fig I is preferred It provides a heat transfer with an efficiency of up to 80 % A second benefit of the illustrated counterflow air stream arrangement is that the flow reversal, which takes place in every half cycle of cylinder 14, tends to keep the passages through the reactor medium 16 clean.

In many oven processes, it is important that the temperature of the heated fresh air leaving regenerator 10 through duct section 12 B (Fig I) for passing into the oven should be effectively controlled Control of the temperature of the heated fresh air is effected by a conventional proportional speed controller 27 connected to the thermal sensor 28 Controller 27 operates to vary the speed of motor 26, which may be a conventional DC motor, adjusting the rotational speed of cylinder 14 in response to changes in the heated fresh air output temperature for regenerator 10 Unlike many heat recovery systems, which rely on dampers and mixture of air volumes to control final temperature, the illustrated apparatus employs a constant air flow volume for the fresh air A fixed volume system that is not affected by damper or metering malfunctions is particularly advantageous in those instances in which system balance is critical.

Figs 5 and 6 illustrate, in somewhat schematic form, a modified thermal regenerator 110 which may be employed in an industrial oven of this invention In this construction, the air permeable reactor 70 medium is constructed in the form of a plurality of rectangular segments 116 which are continuously moved around a closed rectangular path generally indicated by the arrows A For this system, there are four 75 ducts 111 A, Ili B, 112 A and 112 B Ducts 111 A and 111 B are exhaust ducts carrying heated air laden with oxidizable contaminants, like duct 11 in the embodiment of Figs 1-4 Ducts 112 A and 80 112 B are fresh air ducts It can be seen that the operation of the arrangement shown schematically in Figs 5 and 6 is essentially the same as for the first described embodiment except for the path of 85 movement for the reactor medium As before, the speed of movement of the reactor medium can be varied to adjust the output temperature for the fresh air ducts.

Fig 7 is a graph showing how the speed of 90 rotation of cycling of the regenerative catalytic mass supported by cylinder 14 is changed to get a desired temperature of output fresh air, that is, an operating example of the relation between cylinder 95 rotation speed (RPM) and operating temperature for a steady rate of flow (constant volume) of air streams having given specific heats.

For the purpose of Fig 7 it is assumed 100 ambient fresh air enters at duct 12 A at T,= 900 F, and leaves through duct 12 B at a (variable) outlet temperature T 2; whilst the incoming hot exhaust at 1 l A is at T 3 of 7640 F, and leaves (li B) at 5000 F 105 Thus, Ti= 900 F T 2 and RPM are dependent on one another T 3 = 7640 F T 4 = 5000 F Fig 7 shows the interdependency between the outlet temperature (T 2) and RPM; there is direct proportionality between T 2 and RPM (as would be expected 115 since the greater the RPM the faster the exchange for given flow rates) and of course as T 2 increases or decreases there is a converse relation for T 4.

These relationships also enable T 4 to be 120 varied, if desired, by varying RPM for given values of T, and T 3.

The form of the speed controller 27 and the thermal sensor 28 are known Together they compare for any difference, the 125 delivered fresh air temperature (T 2) to a set value A potentiometer (not shown) is varied in accordance with the analog 4 1,602,812 4 representing this difference and the output of the potentiometer is used to vary voltage in turn used to vary the speed of motor 26, which is a known D C motor having a speed which is variable dependent on the applied voltage.

Fig 8 depicts isometrically a tower oven of the present invention for processing small diameter wires coated with insulation, such as wires employed for motor armatures, coils and the like herein referred to as strands After the wire is coated with a thin insulation, dispersed in a solvent, the strand is then introduced into the oven bottom shown in Fig 8 and progresses upwardly therethrough while being exposed to the oven heat to remove the solvent and result in a baked coating of insulation surrounding the strand A known oven of the general form shown in Fig 8 has heretofore been equipped with two burners for establishing the necessary oven heat; under the present invention the oven may be considerably modified so that only one burner is necessary This is accomplished by employing thermal regeneration apparatus of the character described above in a unique manner.

Not only does the present invention enable fuel to be conserved as a result of elimination of the second burner, the temperature of fresh air leaving the rotary heat exchanger may be controlled by altering the RPM of the heat exchanger as already described Unlike prevailing recuperator systems, which rely on dampers and a mixture of air volumes to control final temperature, an oven constructed in accordance with the present invention can depend upon a constant volume of an ambient air stream without utilizing an air mixing system The advantage of a fixed volume which will not be altered by damper malfunction or mis-metering, will be readily apparent to those skilled in the art when considering the heat balancing required for a tower oven Thus, heated, controlled fresh air may be directed into the bottom zone of the oven supplying heat that previously would have been furnished by a second burner.

For the purposes of a better understanding and recognition of the general size of certain components, the tower structure illustrated in Fig 8 in actual practice may be approximately twenty-four feet high.

The thermal regeneration and decontamination apparatus identified by reference character 10 in Fig I may also be referred to as the regenerative thermal process apparatus (RTP) and the RTP equipment thus identified is identified in Fig 8 by reference character RTP positioned near the top of the oven 200 The oven is further characterized by a pair of spaced, insulated side walls 201 and 202 which extend upwardly A narrow chamber 203 is afforded inside the oven, between the side walls 201 and 202, and this narrow chamber serves as a vertical passageway for 70 the coated strand to be processed The wire exits from the top of the oven at a narrow slot 204 representing a known construction according to Windsor U S Patent No.

3,448,969 where internal oven processing of 75 such a strand is also disclosed.

A top zone muffle burner 205 is located in the top zone of the oven chamber and when the oven is operating a stream of hot exhaust gases is rising upwardly inside the 80 oven in surrounding relation to the muffle burner 205, travelling upward therepast and into a hot gas exhaust duct 206 This stream of hot exhaust gases exits from duct 206, moving transversely across and then 85 downward past the exit slot 204 for reasons explained in the Windsor patent As denoted in Fig 8 this stream of hot exhaust gas may be viewed as a constantly criculating stream inside the oven Duct 206 90 is located inside a housing 207 at the top of the oven.

An exhaust stack or pipe 208 is located on the outside of the oven and serves as an exit for the hot effluent exiting into the ambient 95 atmosphere as will be described below.

The recirculating stream of hot gas inside duct 206 is tapped by a duct 211 A, Figs 8 and 10, and this duct corresponds to duct IIA identified in Fig 1 The contaminated 100 hot gas inside duct 211 A enters a blower 209 representing a continuation of duct 211 A and the blower 209 directs the exhaust gases into the RTP unit precisely in the counter-flow (counter-flow to fresh air as will be 105 described) relationship described above in connection with Fig 1 The blower 209 is supported on a platform 210.

The RTP apparatus is, of course, positioned outside the oven chambers A 110 fresh air entrance duct 212 A opens at the outside of the oven chamber and this duct corresponds to duct 12 A for fresh air identified in Fig 1 Thus, inside the RTP unit there is an exchange of heat between 115 the stream of fresh air and the counterflowing stream contaminated hot exhaust.

The heated fresh air leaves the RTP unit through a duct 212 B which corresponds to duct 12 B identified above in connection 120 with Fig 1 This duct is also located on the outside of the oven and at the lower end is configured to allow the heated fresh air to enter the interior of the oven at what may be termed a bottom zone heat chamber 215 125 By so directing the stream of heated fresh air into the bottom of the oven it is possible to eliminate the bottom zone burner heretofore employed in an oven of the character shown in Fig 8 Thus the hot stream of 130 1,602,812 1,602,812 heated fresh air is allowed to circulate downward past a baffle plate 216 into a bottom zone duct 217 and then laterally and upwardly into chamber 203 to impart heat to the wire which is entering the bottom of the heat creating chamber 203.

Returning now to the RTP unit, the contaminated gas in duct 21 1 A was not only subjected to a heat exchange relationship with the fresh air but it was also decontaminated by virtue of the catalytic honey-comb inside the rotating reactor medium or wheel The exchanged stream of exhaust gas leaves the RTP unit through duct 211 B which corresponds to duct li B, Fig I and this duct communcates with the stack 208.

An additional blower 220 is employed to circulate the heated fresh air delivered to the bottom zone of the oven; in the area of the burner 205 the rising stream of heated fresh air mixes with the downcoming stream of exhaust gas as will be evident in Fig 8.

In some chemical processes in which the present oven may be employed, the noxious exhaust gases may be sufficiently corrosive as to destroy any catalyst incorporated in the RTP unit In such cases, the RTP unit will not incorporate a catalyst but will merely serve the heat exchanger role.

Accordingly, and referring to Fig 9, a preburner 230 may be located in duct 211 A, upstream of the RTP unit, to heat the incoming stream of exhaust gas to a predetermined high temperature where entrained chemicals, inimical to the catalyst, may be oxidized to an innocuous state As a further precaution, in the event a catalyst is needed, a stationary or static catalyst chamber 232 may be located in duct 21 1 A downstream of the pre-burner and upstream of the RTP unit.

The thermal regeneration and decontamination apparatus preferably employed in the oven of the invention utilizes a common reactor medium as both a heat exchanger and a catalytic converter, suitable for employment in any drying, curing or other process conducted in the oven which emits exhaust fumes containing oxidizable contaminants In a single stage, the apparatus is capable of cleaning the exhaust gas and recovering latent energy, including both heat present in the exhaust gas entering the apparatus and heat generated in the course of oxidation of any contaminants which may be entrained in the exhaust gas.

The noxious fumes and other contaminants in the exhaust gas can be effectively and rapidly converted into clean, innocuous byproducts, such as water vapor and carbon dioxide The fresh air to which the latent energy is transferred can be used as makeup air for the oven for the process producing the exhaust gas The counterflow arrangement makes it essentially selfcleaning with the reactor medium being purged in every cycle of operation In the preferred construction, a constant volume is maintained for the fresh air side of the 70 apparatus, with thermal control exercised by adjustment of the rate of movement of the reactor medium.

Ihe reader's attention is drawn to our copending Application No 7359/78 (Serial No 75 1602811) from which the present application is divided and which describes and claims thermal regeneration and decontamination apparatus comprising:

an air-permeable reactor medium of large 80 surface area and high thermal capacity incorporating an oxidation catalyst; drive means for continuously moving the reactor medium around a closed path traversing a first duct carrying an air stream 85 laden with oxidizable contaminants and a second duct carrying a stream of relatively clean air, whereby the contaminants in the first duct are catalytically oxidized to innocuous byproducts with release of 90 substantial heat, which heat is exchanged to the air in the second duct, there being means in said path to preclude any substantial cross-flow from one air stream to the other; and 95 a sensor for sensing the temperature of one of the air streams and combined with control means for adjusting the rate of movement of the reactor medium to vary the rate of heat exchanger as a function of 100 the sensed temperature.

Claims (11)

WHAT WE CLAIM IS:-

1 A industrial oven oriented on an elongated vertical axis and equipped with a stack for emitting exhaust gas, the oven 105 comprising:

a burner at a predetermined level inside an oven chamber; a thermal regenerator moveable continuously about a closed path which 110 alternately traverses both a first duct carrying a stream of hot exhaust gas and a second duct carrying a stream of fresher, cooler entrant air thereby to exchange heat between the two gas streams so that the 115 temperature of the outgoing fresh air leaving the regenerator is raised and that of the exhaust gas is lowered; drive means to so move the regenerator; a sensor, for sensing the temperature of 120 one of the streams, combined with control means for adjusting the rate of movement of the regenerator to vary the rate of heat exchanger as a function of the sensed temperature; 125 and a duct for directing into the oven the outgoing stream of fresh air leaving the regenerator.

2 An oven according to Claim 1 in which 1,602,812 the stream of entrant air is ambient fresh air.

3 An oven according to Claim 1 or Claim 2, in which the outgoing stream of air is directed into the oven at a level beneath the burner.

4 An oven according to any preceding claim, in which the streams move through the regenerator in opposite directions.

5 An oven according to any preceding claim, including a duct for directing to the stack the stream of exhaust gas leaving the regenerator and in which the entrant stream of air is ambient air.

6 An oven according to any preceding claim, in which the regenerator is rotary and in which the drive means is a variable speed electric-drive motor, said sensor being located to sense the temperature of the air leaving the regenerator.

7 An oven according to any preceding claim, wherein said thermal regenerator comprises an air-permeable reactor medium of large surface area and high thermal capacity incorporating an oxidation catalyst.

8 An oven according to any one of Claims 1-6, in which a pre-burner is located in the first duct upstream of the thermal regenerator to oxidize entrained chemicals inimical to a catalyst.

9 An oven according to Claim 8, in which a stationary catalytic chamber is positioned in the first duct between the pre-burner and the thermal regenerator.

An oven according to Claim 9, in which a blower is located in the first duct upstream of the pre-burner.

11 An industrial oven, according to Claim I and substantially as hereinbefore described with reference to Figs 8-10 of the accompanying drawings.

LLOYD WISE, TREGEAR & CO, Norman House, 105-109 Strand, London, WC 2 R OAE Printed for Her Majesty's Stationery Office, by the Courier Press, Leamington Spa, 1981 Published by The Patent Office, 25 Southampton Buildings, London, WC 2 A l AY, from which copies may be obtained.

6