INDUhYTRIAL A N D ENGINEERIdVG CHEMISTRY
July, 1926
703
Tools of the Chemical Engineer' IV-Automatic Temperature Control By D. H. Killeffer 19 EAST2
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4 ST. ~ hTEW ~ YORK,N. Y.
CCURATE control of temperature in chemical op-
erations is second only in importance to the control A of concentration. There are few reactions that do not involve either the absorption or the evolution of heat in such quantities as to make necessary a more or less accurate control of it- flow. In endothermic reactions temperatures are readily controlled by the amount of heat supplied to the reaction mixture, but the evolution of heat from exothermic reactions is a t once more necessary and more difficult to keep within prescribed limits. The development of most chemical processes has been through a laborious period of hand control which has often been profitably followed by the substitution of automatic deviceb possessing a high degree of infallability. Indeed, quantity production of the type in which American industry generally excels has made the substitution of automatic devices for carrying out all manner of processes absolutely essential. Wierever reactions are likely to proceed too rapidly without control, wherever materials handled are in the hast liable to damage by either too much or too little heat, wherever maximum speed of reaction must be maintained, and wherever changes in reaction temperature may result in undesired side reactiony the substitution of automatic for human control of temperature will repay careful investigation by the plant engineer. The development of automatic control has proceeded so far that the problem of heat treatment or control which cannot be handled by means of available devices is rare indeed. Any temperature range in which changes may be converted to linear motion can be covered by existing controlling instruments. The change of dimension of a solid body. the change of volume of a liquid or a gas, the change i n 1
Receired September 26 1924
vapor pressure of a liquid, the change in electrical resistance of a solid body, and the change in voltage produced by a thermocouple under varying temperatures have all been applied to automatic temperature control. Usually the power derived from direct change by any of these devices is insufficient to accomplish the work necessary to the operation of the actual control. Therefore, the force or magnitude of the change is amplified through electric relays or through relay valves controlling the flow of low-pressure air through a line leading to the actual controlling mechanism. Sometimes the power generated in the thermal element can be readily used for the direct control of the supply of fuel or heat i n other forms. Control lMechanism
Occabionally, the change in dimension of solid bodies is utilized directly for the control of fluid fuel supply. Heattreating furnaces operated at high temperatures are sometimes roughly controlled by the expansion of the muffle in which the work is heated, such expansion operating a valve in the fuel-supply line. The differential expansion of two dissimilar bodies is used in two types of controllers. Two bars of dissimilar metals-brass and steel are often usedmay be so riveted together that the composite bar is bent one way or the other by changes in temperature, thus operating an electrical contact; or a bar of one material may be mounted within a tube of another having a different coefficient of thermal expansion in such a way as to control the relative positions of the two elements of a valvc in the fuel supply or auxiliary power line. Changes of volume of liquid> and gases and changes of vapor preswre with temperature changes are utilized by con-
Typical Layout for Temperature Control through Air Relay System
ISDCSTRIAL A S D ESGISEERISG CHEMISTRY
704
fining the liquid or gas in an elastic system whose volume varies with changes of pressure within it. The volume changes thus produced may operate a hand before a dial carrying electric contacts which are made or broken by its movement, or may operate a control valve in the line carrying lowpressure air for the actual operation of the controlled valve. In some cases, particularly with liquid expansion and vapor pressure changes, it is possible to operate the controlled valve directly from the thermal element. Electric thermometers used for control are ordinarily operated by converting the electrical change to the motion of a hand which makes and breaks contact in an electric circuit which, in turn, operates the control of the heat supply either through a relay or a motor-operated valve. The most widely used types of controllers operate by change of liquid volume, by change of vapor pressure, and by change of e . m. f. from a thermocouple. Ordinarily, liquid expansion is used for low temperatures, vapor A Type of Diaphragm Valve pressure change for an interfor Pressure Control. A Similar Valve Is Used to Control mediate range, and thermoTemperature couples for high temperatures.
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JJ’hererer it is necessary to control temperature only a t a definite level, without reference to time, pressure, or other changes, existing devices do the work in a verv satisfactory manner. In d$ing delicate fabrics, or with heat-sensitive dyes, as well as in the preparation of the fibers, the teniperature is readily kept within the limits of maximum efficiency by controllers operating upon the steam-supply line to the kettles whose temperature is being regulated. In the automatic oven of the modern bakery, where time of exposure of the product is controlled by the speed of the machine, the temperature must be carefully regulated in its different parts to produce loaves of uniformly high quality. The dough and proofing rooms of chemically controlled bakeries are similarly controlled for maximum efficiency. The heat-treatment of steel and its alloys, through the necessity of large-quantity production of small articles, particularly for the automobile industry, could not now depend solely on the skilled eye of the workman, who by this means alone controlled the heat-treating of the past. Not only were the errors in judgment of the workman, however slight, fatal to quality production, but it was impossible to extend his vision to include all the separate pieces of work which he must produce. Here the introduction of automatic
Applications of Control
The application of automatic control to low temperatures such as are met in domestic heating has involved chiefly two types of devices, solid expansion and vapor pressure change. The dimensional changes in solids have been utilized in both of these forms. Through an electric relay system the bending of a composite bar controls the heating plant by opening and closing dampers or by regulating fluid fuel supply. The tube-bar type of controller is successfully used to govern the supply of gas or oil direct to the heater. Vapor pressure controllers used for this purpose are most frequently of the direct operating type and work against a spring or a balance arm in such a way as to operate
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.I-Fxpandinp qtem B-Nonexpanding stem C-Indicator
Phantom View of Air Relays i n Clock-Operated Humidity Controller Showing Use of Cams ( A and B ) to Give Desired Temperature Cycle;
D--Adjusting post E--Air connections F-Cushion spring
Typical Solid Expansion Thermostat Operating through Cornpressed Air Relay
distant dampers and drafts by means of chain connections. I n this type the arrangement is such as to give the maximum movement a t the maximum and minimum points of temperature desired.
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devices for the control of all the operations of heat treating has made possible great industries. Ball bearings, for instance, were never produced under the control of the workman’s eye in any satisfactory uniformity or quantity. Automatic temperature controllers have now been installed so successfully that it is unnecessary to provide supervision for the operation beyond that needed in case of failure of some unit to function properly. All manner of small parts are simply fed into a hopper atdoneend of the cycle and collected at the other in final form, having in the meantime gone through all the necessary operations of hardening and tempering on a conveyor system. Casehardening is also accomplished more satisfactorily and uniformly in baths whose temperatures are controlled automatically. The control in this industry is ordinarily obtained by means of thermocouples, as it is difficult to build other types of controllers that are sufficiently accurate a t the temperatures required. Strictly Chemical Possibilities
I n the more strictly chemical industries there seems to be a neglected but fertile field for automatic controllers. Here
as nowliero else reactiurla arc‘ met rvliere tlie heat balance of the system involvx the transfer of comparatively large quanti-
matic cont.rol inay lie readily applied; otlierwise some difliculty iu likely to be encountered.
ties of lieat either into it or out for dissipation. In the nitratioii of liydrocarhons such aiitornatic mcaiis for keeping
Control of Temperature Cycles
Of even greater importance in many industries than control a t a definite point is tlie control uf temperature at a definite point for a definite length of time. I n the rubber industry we have a good illustration of the application of a combination controller which varies temperature according to a predetermined schedule. I n vulcanizing rubber, to obtain a uniform product the time of heating must be eontrolled with as minute care as itsintensity. For thispurposc, to the thermostatic controller is added a clock-operated cam, by means of which the base lime of the thermal element-and hence the temperature that it controls-is varied according to a predetermined time schedule. By changing the shape of this cam or by providing others to replace it, the most coinplcx temperature cycle may he carried through with extreme precision. The temperature of the controlled system may he raised or lowered from m y point to any other in a definite prriod of time, and it may also be held coilstant for any desired ueriod at anv within tire range of the particular ” noint . instrument in use. Humidity Control
B o t h t e m p e r a t u r e and humidity can he contrullrd with the greatest precision by combining a wet and a dry bulb controlling thermometer in the same instrument in i>irwam of Tire Press Equipped with Time Control of such a way that the difference H e a t h ? Condensasion Outlet and Cooling to Carry Out Auroms;ically the Compiete Cu;inS Cycle between the readings of the two governs the admission of tlie reaction wit,tiin due bouiids would be profitable. In this moisture to ttie air in the conas in many other processes the temperature is controlled by trolled chamber. In a similar the additioii of a reagen-here it is mixed acid-and it is way humidity may be iwiiio more difficult t,o remilate automatically the flow of a retrolled with respect to time agent, under w r y slight pressure than to (:ontrol steam flow by the introdiiction of proper in a comparatively high-pressure line. Indeed, any endo- cams and clocks into the ooiitlit~r~nic process and any exothermic one in which the addition trol system. This type of of a reagent causes the evoliition of heat can profitably be liiimidity regiilatnr is moFt often made up of vapor pressure thennal elements. Hiim i d i t y c o n t r o l has l m v i widely and succersfully applied to the kiln dryiilg of lumber, where drying must proceed at a definite rate to get maximum efficiency from kilns and a t the same time p r e v e n t c h e c k i n g of tlic liirn tier. 111tlic procesring of f o d .tuffs, especially those of ipecial delicacy and tliare trmperatiiies a? well as to which must be processed ill its alnorption at high tem- glass containers, the cycle of prratures. Caseow, liquid, operations becoines increasand m e t a l l i c e x p a n s i o n ingly complicated; and here Electric Ilelsy f:mfacfn on mal Indicafin)! Therrnumefer types arc used for this pur- not only innst temperaturc pose. The prime require- he controlled according to a iiient is that there shall be in the system controlled a de- rather cumplex time cycle, cided tendency to change its temperature. If there is hut pressure must follow a roficient difference in temperature between the system and inore or less definitely predeits siirroundings, or if tlie tendency of the
