US3868779A

US3868779A - Incineration control

Info

Publication number: US3868779A
Application number: US396668A
Authority: US
Inventors: Jr Charles R Wilt; Floyd L Schauermann
Original assignee: Salem Corp
Current assignee: Salem Corp
Priority date: 1973-09-13
Filing date: 1973-09-13
Publication date: 1975-03-04
Anticipated expiration: 1992-03-04
Also published as: DE2403844B2; FR2244135A1; CA992396A; JPS5072249A; GB1465952A; AU6580074A; ES430032A1; DE2403844A1; FR2244135B3; DE2403844C3

Abstract

The specification discloses and describes a method and means for determining concentration of evaporated solvent in gases emitted from a drying oven and delivered to an incinerator, by simple measurement of the temperature of gases constituting products of combustion exhausted from the incinerator. By maintaining the fuel input to the incinerator constant and also maintaining the temperature of the gases entering the incinerator constant, the variation of the temperature of the products of combustion emitted from the incinerator with respect to a standard value reflects the variation in the concentration of evaporated solvents supplied to the incinerator. A valve means, responsively controlled according to the temperature of the products of combustion emitted from the incinerator, functions to regulate the admission of a diluent, such as air, to the oven, thereby regulating the concentration of evaporated solvent to a substantially constant and safe value and effecting regulation of a uniform temperature of gas products of combustion at the outlet of the incinerator.

Description

Unite States Patent 1 1 Wilt, Jr. et a1.

1 Mar. 4, 1975 1 INCINERATION CONTROL [75] Inventors: Charles R. Wilt, Jr.; Floyd L.

Schauermann, both of Pittsburgh,

[73] Assignee: Salem Corporation, Pittsburgh, Pa. [2 2] Filed: Sept. 13. 1973 [21] Appl. No.: 396,668

3.314159 4/1967 Betz 3.472.498 10/1969 Price et a1 3.601.900 8/1971 Erisman et al. 3,706,445 12/1972 Gentry 34/79 Primary Emminer-Kenncth W. Sprague Assistant Examiner-Larry l. Schwartz Armrnar. Agent, or Firm-Buell, Blenko &

Ziesenheim 1 ABSTRACT The specification discloses and describes a method and means for determining concentration of evaporated solvent in gases emitted from a drying oven and delivered to an incinerator, by simple measurement of the temperature of gases constituting products of combustion exhausted from the incinerator. By maintaining the fuel input to the incinerator constant and also maintaining the temperature of the gases entering the incinerator constant, the variation of the temperature of the products of combustion emitted from the incinerator with respect to a standard value reflects the variation in the concentration of evaporated solvents supplied to the incinerator. A valve means, responsively controlled according to the temperature of the products of combustion emitted from the incinerator.

functions to regulate the admission of a diluent. such as air, to the oven, thereby regulating the concentration of evaporated solvent to a substantially constant and safe value and effecting regulation of a uniform temperature of gas products of combustion at the outlet of the incinerator.

10 Claims, 2 Drawing Figures lncmex Slur) In Solvent In Tl L12 0n Stop 1A Air Pulled In with Strip PATENTEU 5 sum 2 5 2 Strip In Zone I Oven 38 Solvent ln On Strip 1K Air Pulled In With Strip 1 INCINERATION CONTROL This invention relates to apparatus for incinerating combustible solvents evaporated in an oven of a paint drying conveyorsystem to thereby effectively eliminate emission of obnoxious pollutants to the atmosphere, as well as to dilution control apparatus for regulating solvent concentration in the oven to a safe value.

in order to safeguard against explosion in the solvent evaporative zone of an oven, such as are included in paint drying oven system, the National Fire Protective Association recommends dilution of the solvent vapor to 25% of the lower explosive limit (L.E.L.), that is 25% of the volumetric concentration of solvent at which a gaseous mixture will explode.

Usually, solvent dilution in an-oven is achieved by a fan or blower pulling fresh air into the oven. Because of the heat required to heat the fresh air drawn into the oven to oven temperatures, it is preferable to recircu-. late gases discharged from the incinerator back to the oven for dilution purposes partly because these gases are already heated and the fuel requirement with respect to that for heating fresh air is thus reduced, and partly because the recirculated gases contain carbon dioxide, nitrogen and water vapor, in addition to air, and these gases are better diluents for preventing an explosion than is air alone.

An oven is designed for a fixed maximum amount of combustible solvent and in order to limit the solvent concentration, within the oven, to approximately 25% of the L.E.L., a fan or blower of correspondingly appropriate capacity is required to insure that the proper amount of diluent is pulled into the oven. A safe rule is to provide 10,000 standard cubic feet of air for each gallon of solvent, or at an air density of 0,075 lb./ft., 750 lbs. of air per gallon of solvent. Assuming a maximum solvent load of 100 gallons/hr., it follows that 75,000 lbs. of air or diluent an hour will be required. Since solvent weights may be taken as an average of 7.5 lbs/gallon, 100 gallons of solvent will weigh 750 lbs. Since the weight of solvent thus represents only 1% of the total weight of air or diluent per hour, the solvent weight may be neglected without seriously affecting the accuracy of the calculations. Accordingly, under the assumed conditions of maximum solvent load, the appropriate capacity of fan required to maintain a safe percentage concentration of solvent is 75,000 lb./hr. -I 0.075 lb./ft. orl X 10 cu.ft. per hour.

When the oven is operated at a solvent input rate less than the fixed maximum rate, the volume of diluent passing through the oven at the maximum fan capacity is more than that required to maintain the 25% L.E.L. If the diluent intake to the oven corresponding to the maximum fan capacity is maintained, therefore, the fuel requirement to heat this maximum diluent intake to the oven will be increased with reduction in solvent concentration.

If it were possible to directly measure the amount of solvent being evaporated in the oven, conceivably it would be possible to reduce the speed of the fan pulling diluent into the oven, to thereby reduce the volume of diluent passing through the oven, with a consequent saving of fuel to heat the diluent with respect to that which would otherwise be required. With present-day equipment, however, it is not practical to directly measure the amount of solvent being evaporated in an oven. Moreover, present-day devices for measuring concentrations of solvent vapor in a gas or gas mixture are not sufficiently reliable to be used efficiently for control purposes.

We are aware of generally pertinent prior art patents, such as U.S. Pat. No. 3,472,498 issued Oct. 14, 1969 to H. A. Price, et al., and U.S. Pat. No. 3,706,445 issued Dec. 19, 1972 to Charles B. Gentry relating to incinerator control systems. These patents disclose apparatus for recirculation of incinerator exhaust gases to an oven, such as a paint drying oven, to reduce fuel requirements for the oven. However, they do not disclose any means for measuring or controlling the concentration of evaporated solvent in the oven.-

It is difficult to hold the solvent load constant in a paint conveyor drying line because of variation in line speed, load surface area, or coating thickness. The evaporated solvent concentration in a paint-drying oven may thus decrease substantially over a prolonged period of time, thus involving prolonged and increased fuel requirements for heating the excess volume ofdiluent in the oven.

Moreover, if the solvent concentration in the gas mixture exhausted from a drying oven and delivered to an incinerator decreases, it follows that with less heat released in the incinerator, in consequence of the burning ofa reduced weight or volume of solvent, additional fuel is required to be supplied to the incinerator to maintain the appropriate temperature within the incinerator for effective combustion. The evaporated solvent delivered to the incinerator has very high chemical heat content of the order of 100,000 BTU. per gallon. Consequently, the loss of this heat, occasioned by a reduced volume of solvent delivered to the incinerator, must be compensated for by heat furnished by additional fuel supplied directly to the incinerator. The cost of this additional fuel is substantial over a period of time.

If the cost of the additional fuel were to be disregarded, it would be possible to simply regulate the supply of fuel to the incinerator automatically in direct response to variations in the temperature of the gaseous products of combustion emitted at the exhaust outlet of the incinerator. Moreover, if relialble solvent analyzers were available conceivably such an analyzer could be arranged to automatically control the speed of the fan, which draws the diluent into the oven, so as to reduce the volume of diluent drawn into the oven with reduction in the percentage of concentration of the solvent. However, due to the current lack of reliable solvent concentration analyzers, the automatic control of the volume of diluent in this manner is not feasible.

It is the purpose of this invention to provide a novel method and arrangement for automatically regulating the solvent concentration in the gas mixture within a paint drying oven to a safe percentage of the lower explosive limit (L.E.L.) by controlling the weight flow of gas mixture from the oven to the incinerator as a function of the temperature of the gas mixture comprising the products of combustion emanating from the incinerator at an exhaust outlet thereof.

It is moreover a purpose of this invention to provide a novel method and means for controlling the opera tion of an incinerator for combustion of noxious solvent vapors derived from a paint drying oven or other source to insure uniformity of operating temperatures thereof, without variation of the normal supply of fuel the gas mixture delivered to the incinerator from an oven. Conversely, the temperature of the products of combustion emanating from the incinerator may be regulated to a constant value by regulating to a constant value the percentage concentration of solvent in the gas mixture delivered to theincinerator in consequence of a variation of the weight of diluent in the gas mixture delivered to the incinerator. This principle may be demonstrated mathematically as follows: I

The following five variables affect the performance of an incinerator:

1. Chemical heat (B.T.U.s) in solvent of mixed stream (m) delivered to incinerator 2. Weight of diluent in stream (m) 3. Temperature of stream (m) 4. Temperature of products of combustion stream emanating from incinerator 5. Amount of fuel supplied by stream (f) directly to incinerator. Using the following quantities, heat and weight balances may be mathematically expressed:

Q Heat Content B.T.U./hr. above 0 R W Weight flowing lbs/hr. Cp Specific Heat B.T.U./lb. R T Temperature Rankin H Chemical heat content B.T.U./lb. d Diluent s solvent Heat Balance (Subscripts denote stream involved) But Q has two components, namely sensible heat by virtue of the stream temperature and chemical heat provided by the solvent. Rewriting (l) s e m pm m f f e c However, the following are constants, or essentially so, and do not affect the accuracy of calculations. K identifies constant.

pl" ill pc c Substituting and rewriting (Ill) s s m m m WIKI= c c v Weight Balance Substitute (V) in (IV) From (Vl), the remaining variables are:

W Solvent Weight as it effects heat input W Diluent weight (By previous assumption solvent weight is eliminated) T Temperature of mixed stream (m) W, Weight of fuel T, Temperature of products of combustion Now assuming the fuel input to the incinerator to be a constant value, then WI: fuel Also, since the solvent load is fixed by the process, then:

stream Substituting in (VI), then:

K! s m m m luel f m ine!) c Accordingly three variables remain, namely:

W (Weight flowing in mixed stream (m)) T (Temperature of gases in mixed stream (m)) T, (Temperature of products of combustion stream Assuming the temperature T,,, to be regulated to a constant, it follows from equation (Vll) that the temperature T, may be controlled by varying the quantity W,,,, the weight of mixed stream (m). Similarly, if the quantity T,, is permitted to vary between limits, these limits will also determine the variation of T If the value of K (solvent load) changes the quantity W that is the weight flowing in the mixed stream, may be correspondingly varied to maintain the quantity T,

(temperature of products of combustion stream (c)) constant of substantially so.

In carrying out the above objects, we provide means for regulating the rate of fuel input to the incinerator to a constant value, means for regulating the recirculation of gases to maintain a constant temperature of the gas mixture at the inlet to the incinerator, and means for controlling the weight of the gas mixture at the oven outlet according to the temperature of the exhaust gases at the outlet of the incinerator.

A preferred arrangement for regulating the weight of the gas mixture admitted to the incinerator and thereby limiting to a safe value the concentration of solvent in the oven is described hereafter and shown in the accompanying drawings, wherein:

FIG. 1 depicts diagrammatically a paint drying conveyor line with radiant type solvent evaporation zone, and

FIG. 2 shows a modified arrangement with regard to recirculation of the product of combustion gases.

Referring to FIG. 1 of the drawings, the apparatus comprising the paint drying conveyor system' includes a housing enclosing a series of spaced solvent evaporation and curing oven zones, designated Zone 1, Zone 2 and Zone 3. For brevity, additional oven zones are omitted and represented merely by the broken line paralleling Zone 3. Oven Zone 1 has an inlet 10 for a conveyor carrying a painted product to be dried. Also shown, diagrammatically, are an inlet 11 for solvent and an inlet 12 for air. Actually, the air enters the oven through oven inlet 10 with the product and the solvent enters as part of the product coating.

Associated with the oven Zone 1 is an incinerator 13 which is in communication with the oven Zone 1 via ductwork 14 in which is includeda blower or fan 15 for supplying the exhaust gas mixture from the oven solvent evaporation Zone 1 to the incinerator 13. Connected to the incinerator 13 is a fuel line 16 having a valve 17 therein which is automatically controlled to regulate the rate of fuel supply to the incinerator to a constant value. 1

Connected to the incinerator 13 is an exhaust gas outlet duct 18 which divides into two branches. One branch, designated 19 goes to a radiant baffle 20 which physically surrounds the work in Zone 1 and radiates heat to the work. The second branch of duct 18 is designated 21 and provides passage for incinerator exhaust gases to succeeding

oven Zones

2, 3 etc. and via a return duct 22, including a blower fan 23, to the oven Zone 1.

Opening out of duct 19 are three branch ducts, designated 24, 25 and 29. Duct 25 returns or recirculates a portion of the gas products of combustion of the incinerator to the oven Zone 1 under the control of a valve 26 which is controlled responsively to the temperature at the incinerator inlet duct 14, by a suitable thermoresponsive device 27, so as to maintain the temperature of gases constant in duct 14.

The branch duct29 opening out of duct 19 supplies a portion of the gas products of combustion from the incinerator to the radiant baffle 20 from which the flow continues to a duct 30 leading to a heat recuperator 31. A portion of the heat from recuperator 31 may be recovered from the system via a duct 33 or returned to atmosphere via a duct 32.

For regulating the volume or weight of gases, supplied to the'radiant baffle 20, a valve 34 is provided having two inversely

operable valve elements

35 and 36. Valve 34 is controlled according to the temperature in the radiant baffle 20 by a thermally-responsive de vice 37. Valve element 35 is opened to increase the flow of gas mixture through duct 24 to duct 30, with an increase in temperature in the radiant baffle 20, while valve element 36 closes to correspondingly reduce the proportion of gases supplied to the radiant baffle 20.

Conversely, upon a reduction of the temperature in the radiant baffle, valve element 35 is operated to reduce flow therepast so as to increase direct flow through duct 29 to the baffle 20, while valve element 36 is opened to accommodate the increased proportion of gas flow therepast from the radiant baffle 20 to the recuperator 31. The temperature of the radiant baffle 20 is thus regulated to a substantially constant'temperature.

Diagrammatically,

Zones

1 and 2 and

Zones

2 and 3 are shown separated by

ducts

38 and 39 respectively although the usual arrangement is for the zones to abut and be separated by internal partitions if regulated. Portions of the total quantity of gas containing products of combustion from the incinerator 13 flowing through duct 21 are diverted through

branch ducts

40 and 41 to

Zones

2 and 3 respectively, under the influence of the incinerator fan 15. The duct 40 is connected to the inlet of a fan 42 as is a duct 44 leading out of the Zone 2. A return duct 45 connects the outlet of fan 42 back to Zone 2. The proportion of gas recirculated from Zone 2 relative to that supplied from duct 40 is determined by valve '46 which is controlled by a thermallyresponsive device 47 which monitors the temperature of the gas returned to Zone 2 via the duct 45. Thus with an increaseof temperature in return duct 45, valve 46 closes to reduce the flow of gas from duct 40 to the Zone 2. Conversely, with a decrease of temperature of gas in the return duct 45, valve 46 opens to increase the flow of gas from duct 40 to Zone 2.

In a similar manner, blower 43 supplies gas proportionally from a duct 48 connected to Zone 3 and from duct 41 to the Zone 3 via a return duct 49. A valve 50 in the duct 41 is controlled by a thermallyresponsive device 51 which monitors the temperature of gas in re turn duct 49.

The gases recirculated to

Zones

2 and 3, as just described, flow from the several zones via branch ducts 52 and 53, respectively. to return duct 22, where they are returned to Zone 1 by fan 23. Although most of the solvent is evaporated in Zone 1, minor amounts will be evaporated

in'Zones

2 and 3 and these must be returned to the incinerator via Zone 1.

In accordance with the objectives of our invention, we further provide a valve 55 in the duct 14 between fan 15 and the incinerator 13, and a thermallyresponsive device 56 which monitors the temperature in duct 18, at the outlet of incinerator 13, for controlling the valve 55. Thermally-responsive device 56 is effective responsively to an increase in the temperature of gases in duct 18 at the outlet of incinerator 13 above a predetermined temperature, to cause valve 55 to be operated toward the open position, thereby increasing the flow of air drawn into Zone 1 via passage 12. Conversely, thermally-responsive device 56 is effective, responsively to a decrease in temperature of gases in duct 18 at the outlet of incinerator 13 below the predetermined temperature, to cause valve 55 to be operated toward the closed position, thereby reducing the flow of air drawn into Zone 1 via passage 12.

Let it be assumed that the system is in operation with a conveyor bearing painted products moving progressively through

Zones

1, 2 and 3. Also, let it be assumed that valve 17 is operating to regulate to a constant value the rate of fuel supplied to the incinerator and that thermallyresponsive device 27 is functioning to regulate a constant temperature in duct 14. Let it also be understood that the solvent concentration in the gases leaving Zone 1 and entering the incinerator 13 is at a safe percentage of the L.E.L. and that the resulting temperature of gases leaving the incinerator is controlled by thermally-responsive device 56.

If, now, the temperature of the gases in duct 18 at the incinerator outlet rises above the predetermined temperature this is an indication that the concentration of solvent in the Zone 1 is increasing. Accordingly, valve 55 is opened to increase the flow of air into the Zone via passage 12, thereby resulting in a reduction in the temperature of gases in duct 18 to the predetermined temperature.

If the temperature of the gases in duct 18 at the incinerator outlet falls below the predetermined temperature, this is an indication that the concentration of the solvent in Zone 1 is reducing. Accordingly valve 55 is operated toward the closed position, thereby reducing the flow of air into Zone 1 via passage 12. In consequence, the concentration of solvent in Zone 1 increases, with the result that the temperature in duct 18 at the outlet of the incinerator is restored to the predetermined temperature.

Referring to FIG. 2, a modified arrangement is shown wherein corresponding parts are designated by the same reference numerals as in FIG. 1. The arrangement in FIG. 2 differs from FIG. 1 in providing a duct 25', in place of duct 25, which by-passes Zone 1 and is connected to duct 14 adjacent the inlet to fan 15. Also the thermally-responsive device 27 is connected to register ture of the gases immediately adjacent the inlet to the incinerator.

It will be seen that this invention provides a novel method and arrangement for determining and controlling evaporated solvent concentration in a paint drying oven or other source as well as for controlling the operation of an incinerator to insure a uniform temperature of the gas products of combustion at the outlet of the incinerator notwithstanding variations in the solvent load or in the degree of evaporated solvent concentration. It will furthermore be seen that this invention provides an incineration control arrangement which enables economical operation with respect to fuel requirements and which also maintains a substantially uniform and efficient operating temperature notwithstanding variations in the chemical heat released incidental to combustion of solvents.

While this invention has been described in connection with a radiant heat transfer type of drying oven, it will be understood that the invention is equally applicable to an oven using convective heat transfer in the solvent evaporation zone, or to other sources of noxious solvent vapors. Also, modifications may be made in the apparatus specifically described within the terms of the following claims.

We claim:

1. In a drying oven of the type having at least one zone in which solvents are evaporated during the drying process and exhausted with other gases to an incinerator for incineration, the improvement which comprises:

a. means providing a constant rate of supply of fuel to the incinerator, b. means regulating the temperature of the gases emitted from the oven during the drying process to a constant value, and

0. means controlling the quantity of air admitted to the oven as a diluent for the solvents in accordance with the variation of the temperature of the gases constituting the products of combustion of the incinerator exhausted therefrom.

2. In a drying oven according to claim 1, wherein conduit means is provided for recirculating back to the oven a portion of the products of combustion of the incinerator, and wherein the temperature regulating means of clause (b) comprises valve means in said conduit means and thermally-responsive means exposed to the gases emitted from the oven which controls the operation of said valve means to so regulate the quantity of products of combustion of the incinerator admitted as a solvent diluent to the oven as to regulate to a substantially constant value the temperature of the gases emitted from the oven.

3. In adrying oven according to claim 1, wherein said oven comprises a radiant baffle to which gases comprising products of combustion of the incinerator are supplied, and temperature controlled means responsive to the radiant temperature of the said baffle for so controlling the admission of the gases comprising products of combustion of the incinerator to the baffle as to maintain a substantially constant radiant temperature.

4. In a drying oven according to claim 1, wherein the oven has a plurality of drying zones, and wherein a portion of the products of combustion of the incinerator are recirculated through subsequent zones back to an initial zone.

5. In a drying oven according to claim 3, wherein said oven comprises a heat recuperating means, and said temperature controlled means comprises valve means controlling the admission of products of combustion of the incinerator to said heat recuperating means.

6. In a drying oven according to claim 4, wherein temperature controlled valve means is provided for controlling the proportions of products of combustion of the incinerator supplied to the respective subsequent zones.

7. In a drying oven according to claim 4 wherein a blower is provided for each subsequent zone to supply a mixture of recirculated products of combustion of the incinerator and zone gases from the corresponding zone back to the same zone, wherein valve means regulates the flow of products of combustion to the blower. and wherein temperature controlled means responsive to the temperature of the gas mixture returned to the zone by the blower controls said valve means to regulate the proportion of products of combustion relative to zone gases returned to the zone.

8. In a drying oven of the type having at least one zone in which combustible materials are evolved from the product being dried during the drying process and exhausted in a gaseous stream to an incinerator for incineration, the improvement which comprises:

a. means providing a constant rate of supply of fuel to the incinerator,

b. means regulating the temperature of the gases emitted from the oven during the drying process to a constant value, and

' 0. means controlling the quantity of air admitted to the oven as a diluent for the combustible materials in accordance with the variation of the temperature of the gases constituting the products of combustion of the incinerator exhausted thereform.

9. In a drying oven according to claim 8, wherein conduit means is provided for recirculating back to the oven a portion of the products of combustion of the incinerator, and wherein the temperature regulating means of clause (b) comprises valve means in said conduit means and thermally'responsive means exposed to the gaseous stream emitted from the oven which controls the operation of said valve means to so regulate the quantity of products of combustion of the incinerator admitted to the oven as a diluent for the combustible materials therein so as to regulate to a substantially constant value the temperature of the gaseous stream emitted from the oven.

10. In a drying oven according to claim 8, wherein said oven comprises a radiant baffle to which gases comprising products of combustion of the incinerator are supplied, and temperature controlled means responsive to the radiant temperature of the said baffle for so controlling the admission of the gases comprising products of combustion of the incinerator to the baffle as to maintain a substantially constant radiant temperature.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 868,779 DATED 3 March 4, 1975 iNVENTOR(S) I CHARLES R. WILT, JR. and FLOYD L. SCHAUERMANN It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line ll, "system" should be --systems-.

Column 3, under the heading "HEAT BALANCE" line 44, should read (I) Q Q Q Column 6, line 36, "thermallyresponsive" should read -thermallyresponsive-.

Column 8, Claim 8, line 42, "thereform" should read therefrom--.

Signed and sealed this 6th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks

Claims

1. In a drying oven of the type having at least one zone in which solvents are evaporated during the drying process and exhausted with other gases to an incinerator for incineration, the improvement which comprises: a. means providing a constant rate of supply of fuel to the incinerator, b. means regulating the temperature of the gases emitted from the oven during the drying process to a constant value, and c. means controlling the quantity of air admitted to the oven as a diluent for the solvents in accordance with the variation of the temperature of the gases constituting the products of combustion of the incinerator exhausted therefrom.

3. In a drying oven according to claim 1, wherein said oven comprises a radiant baffle to which gases comprising products of combustion of the incinerator are supplied, and temperature controlled means responsive to the radiant temperature of the said baffle for so controlling the admission of the gases comprising products of combustion of the incinerator to the baffle as to maintain a substantially constant radiant temperature.

7. In a drying oven according to claim 4 wherein a blower is provided for each subsequent zone to supply a mixture of recirculated products of combustion of the incinerator and zone gases from the corresponding zone back to the same zone, wherein valve means regulates the flow of products of combustion to the blower, and wherein temperature controlled means responsive to the temperature of the gas mixture returned to the zone by the blower controls said valve means to regulate the proportion of products of combustion relative to zone gases returned to the zone.

8. In a drying oven of the type having at least one zone in which combustible materials are evolved from the product being dried during the drying process and exhausted in a gaseous stream to an incinerator for incineration, the improvement which comprises: a. means providing a constant rate of supply of fuel to the incinerator, b. means regulating the temperature of the gases emitted from the oven during the drying process to a constant value, and c. means controlling the quantity of air admitted to the oven as a diluent for the combustible materials in accordance with the variation of the temperature of the gases constituting the products of combustion of the incinerator exhausted thereform.

9. In a drying oven accoRding to claim 8, wherein conduit means is provided for recirculating back to the oven a portion of the products of combustion of the incinerator, and wherein the temperature regulating means of clause (b) comprises valve means in said conduit means and thermally-responsive means exposed to the gaseous stream emitted from the oven which controls the operation of said valve means to so regulate the quantity of products of combustion of the incinerator admitted to the oven as a diluent for the combustible materials therein so as to regulate to a substantially constant value the temperature of the gaseous stream emitted from the oven.