INDUSTRIAL A N D ENGINEERING CHEMISTRY
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(Guide No. 540-125) as having the same combustion hazards as common newsprint in the same form and quantity.
Th,cknesr 0009.
Figure 12-Physical
Vol. 21, No. 5
Application
Cellophane as a utility wrap is used on a variety of products including baked goods, meats, frozen fish,l*fruits, confections, nuts, coffee, textiles, tobacco, and drugs. Among other articles, Cellophane is used for making artificial flowers, tape for stock market tickers, trimmings for millinery, and bandages for surgical dressings.
Properties of Cellophane
The following table gives a comparison of data from combustion tests of Cellophane and newspaper: (Average of four flammability tests made on strips 36 by MATERIAL Plain Transparent Cellophane Moistureproof Cellophane Newspaper
TIMEOF
IGNITION Second 1 1 1
l 3 / 8 inches) TIMEOF FLAME COMBUSTION Inches Seconds 12 17 20 13 20 15
BIOLOGICAL EXPERIMENTS-Animals may be fed both Plain Transparent and Moistureproof Cellophane without harmful physiological effects. Experiments in feeding guinea pigs with R'Ioistureproof Cellophane showed no evidence of toxic effects.
Figure 13-Comparison of Light-Transmitting Qualities of Cellophane. and Window Glass
Each year the uses for Cellophane are extended into new fields so that the demand for both the Plain Transparent and Moistureproof varieties is constantly increasing. I*
Howe, Nation's Business, p . 43 (February, 1929); see also Birdseye.
p. 414, this issue.
Automatic Control in the Chemical Industries' Ismar Ginsberg 113 WEST 4
2 STREET, ~ ~ NEW YORP, N. Y
ROGRESS in the chemical industries has usually been considered to be due to the establishment and development of the basic chemical and physical principles upon which a manufacturing process is founded, and to the design and construction of machinery for carrying out the operations. Chemical advancement is, therefore, dependent, not only upon the chemist, but upon the engincer as well. It is true that without the service that has been rendered the chemical industries by the manufacturer of equipment and apparatus, these industries could not have attained the positions that they occupy a t the present time. On the other hand, the best designed apparatus is useless unless the chemical principles of the process carried out in it are known and applied under control, so that the best results are obtained a t least cost. It appears that the function which is played by instruments that make possible this required control has not been altogether understood or appreciated by the chemical manufacturer. He cannot avoid the use, in the plant, of instruments that measure temperature, pressure, humidity, speed, etc., but he has not used these instruments to the best advantage. The role that the measuring and recording and also the controlling instrument plays in modern industry, and in the chemical industries particularly, is becoming of greater importance from day to day. This is particularly true of the controlling instrument, for its use involves certain economies and increased efficiencies of operation which are very welcome in these days of intensive competition.
P
1
Received February 21, 1929.
While our theme is the automatic controlling instrument,
it is impossible to consider it without saying something about the other instruments as well. Indicating or simple measuring instruments merely indicate the temperature, pressure, etc., through the movement of a pointer around a circular scale as in the pressure gage and dial thermometer, by the rise of 3 column of liquid as in the glass thermometer, or in other ways. Indicating instruments do not interest us here, and we can leave them with just this word. The usefulness of the dial thermometer, which measures and indicates temperature, has not been entirely appreciated by the chemical industries. The dial part of this instrument is connected by flexible tubing to the bulb, which is permanently fixed in the medium whose temperature it is desired to measure. The dial can therefore be located a t any convenient point and is easily read, much more so than the column of mercury in the glass thermometer. When the temperature of masses contained in open kettles, evolving clouds of steam during the operation, has to be determined, the dial thermometer is a much more convenient and more arcurate instrument than the ordinary long-stemmed glass thermometer. Reading temperatures on such a thermometer over an open kettle with steam condensing on the glass stem is not an easy or a comfortable job, or one that is conducive toward accurate readings. There are many operations in the chemical industries where the dial thermometer can be used to far greater advantage that the ordinary glass thermometer. Controlling or regulating instruments, which automatically keep temperature, pressure, humidity, etc., at certain definite
May, 1929
INDUSTRIAL A N D ENGINEERING CHEMISTRY
and desirable points, merely control the physical conditions, and do not afford any record of the operation of the apparatus on which they are used. The recording instrument, the recording thermometer, recording pressure gage, recording tachometer, and the like, produce an indelible rerord of the entire operation of the apparatus. Such a record is of great imnortance. For it will not onlv tell that the temperature or p r e s s u r e has been maintained at the proper points during the process, hut it will also show bow well the apparatus was used productively. The breaks in the recorded line d l indicate time spent in charging and discharging the apparatus, a n d will thus show how m u c h of t h e o p e r a t i n g period is taken up-with nonproductive work. Furthermore, the recorderwill check the controller, and it has been found that the ideal installation is that in which both a recording i n s t r u ment and a controlling instrument are used. Automatic Temperature and Pressure Controllers
By far the most important automatic controlling i n s t r u m e n t s used in the chemical arid allied industries are the automatic temperature controller and the a u t o m a t i c pressure conAmerican Recision Temperature troller. The automatic ternController Made by T h e Conaolidated Ashcraft Hancock perature controller includes Company both instruments used for controlling low temperatures, and those used at high temperature, the automatic pyrometer controller. Sometimes, for example, in the vulcanization of rubber, it is necessary to control both time and temperature. Time-temperature controllers are used For this purpose. These instruments are designed to shut off the henting medium-steam, For cxampl-Fter the temperature has been mintained for a definite period of time. There are two general systems on which controllers OF temperature and pressure are designed and operated-airactuated and self-contained types. The self-operated type is actuated by the medium that it controls. The advantage of this instrument is that no compressed air or other auxiliary force is required For its operation. The motive power which opens and shuts the valve is supplied directly from the thermostatic system in the case of the temperature controller or by the controlled pressure in the case of the pressure cont~roller. This motive power actuates the motor (diaphragms), which in turn operates the valvc stem by expansion and contraction, thus opening OT closing the valve to the heating or cooling medium. The disadvantage of tlie self-operated type of temperature controller is that it has a narrow range, only as high as 370" to 390" F. as a maximum. It is also not so accurate an instrument as the air-operated type, but where temperatures are low and where it is not required to maintain them within more than a bdegree range, this instrument will work satisfactorily. The same conditions a.pply to the use of the self-operated pressure controller. This instrument is particularly useful in installations where steam is used as
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the heating medium. It is also used for reducing pressures in steam and air lines. The air-operated type of controller both for temperature and pressure is an instrument which represents the finest skill and workmanship of the instrument maker. It can he made to control temperatures within a range of 1' to 2" F. Recently, new types of this instrument have appeared on the market under the name of precision controllers, and these instrnments will he found sat,isfactory where the most stringent conditions of uniiormity of temperature have to be met. There are four general t,ypes of air-operated instruments. The first is known as the expansion stem type. Its principle is the expansion and contraction of two metals of different coefficient of expansion. This instrument does not require flexible tubing. It can, however, he used with temperatures below 100' and above 500" F. Its usefulness is, however, limited, and it is best employed in controlling the temperature OF rooms and enclosures by k i n g suspended therein. The second type of air-operated instrument is known as the vapor tension type. It is actuated by the pressure of the vapor of a volatile liquid which is contained in the actuating system, which pressure has a definite relation t o the temperature OF the medium surrounding the bulb and is affected by slight temperature changes. The third type is actuated by gas, this gas filling the actuating system and the pressure of the fluid, as it changes with temperature, operating tlie instrument. The fourth type i s the mercury-actuated instrument. This is by far the best of all the t.ypes, since mercury is a liquid which is extremely susceptible to temperature changes and
American Pressure Cmhtroller Made by T h e Consolidated Ashccaff Hanuock Company
*
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IA'DUSTRIAL A N D EN(YINEERING CHEilfISTRY
whose expansion and contraction over a wide range of temperature remain constant. Mercury is positive and certain in its action, and is the most accurate of all the temperature controllers. The reason which makes the mercury-actuated controller so valuable an iustrument is the same which makes the mercury thermometer so superior over other types of thermometers. Air-operated controllers are not actuated directly by the change in temperature or pressure, but the pressure opens an air valve which admits compressed air to what is called
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Another very useful instrument is the automatic liquid-level controller which is designed to control the level of liquids in tanks, evaporators, etc. The automatic pyrometer controller is merely a type of temperature controller used a t high temperatures. Mechanism of Controllers The controllers can work in one of two ways. Either they will snap open a valve or snap it shut, or they will gradually open and close it-that is, throttle the valve. The self-contained controllers are of the latter type, while the air-operated are of the former. The selection of the proper controller will depend entirely upon the use to which i t is going to be put, on the accuracy with which the temperature or pressure must be regulated, and upon the condition wherein the heat must be entirely shut off or gradually reduced. The use of the air-operated instrument also necessitates the availability of compressed air a t at least 15 pounds and not more than 50 pounds pressure. This is, however, a condition which is easily sabislied in most plants, but where compressed air is not available the self-contained type of instrument must !x used. Uses ofPressure and Temperature Controlters
Temperature Cuntr'oller for Chromium-Plating Rocesa
the diaphragm motor valve, causing i t to close in part and thus decrease the flow of the heating medium. The heat acts directly upon a diaphragm chamber which accomplishes this effect by expanding, and when it Contracts it has the opposite effect, for then the pressure exerted against the coiled spring of the air valve is decreased because tho space withiii which this pressure is exerted by the compressed air is increased and the result is that the motor diaphragm valve is opened and the flow of heating medium is increased. The instrument acts in the reverse manner when a cooling medium is controlled; rise in temperature causes the flow of more cooling medium and decrease in temperature reduces the flow. There are variations in the design and const,ruction of both the simple temperature and the simple pressure controller. There are instruments which will control both the pressure and the temperature simultaneously. Others will control two temperatures at the same time. Still anofher type of instrument acts as a differential controller, which means that i t simultaneously controls two sources of heating or cooling or one source of heating and one source of cooling. Controllers are also provided with electrical and other connections for operating electromagnets and the like, so that t.hey can open and shut steam valves, gas valves, electric circuits, etr. Pressure controllers are also made in various types. Thus a single-system pressure controller will automatically control the pressure of steam, air, gas, or liquid. The tniii-system controller nil1 do the same for two different media. The differential pressure controller is used for controlling simultaneously tu-o relative pressures. Pressure controllers are also of the vacuum type-that is, they will control a vacuum within an apparatus just as well as pressure. Other Controlling Instruments Among other controlling instruments mention should he made of the automatic hygrometer, which controls humidity. This is a very useful type of instrument and is largely employed in controlling the humidity of air in drying operations.
The pressure controller and the temperature controller find many uses in the chemical and allied industries. These uses, however, are not nearly so well defined in the strictly chemical industries-that is, the manufacture of organic and inorganic cheniicals, dyestuffs, synthetic drugs, perfumes, and the like-as in the allied indiistries-the rubber, textile, petroleum, and food industritx, for example. The reason for this is that the uses to which the instruments are put in
chemical operations are often guarded as secrets. Nevertheless, the principle of the operation of the instrument and the results that have been obtained with them in other industries clearly indicate that they can and probably are being used for certain standard chemical operations. Temperature controllers are used with good results in distilling operations. They may he used directly on the still
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INDUSTRIAL AND ENGINEERING CHEMISTRY
to cont,rol the flow of heating medium, the bulb of the instrument being preferably inserted in the outlet leading the d i p tilled vapors out of the still. The location of this bulb is very important and must be carefully selected if the controller is to regulate the temperature of the apparatus properly. It has been found advisable to locate the bulb in this case in the
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Temperature regulators have been used on all sorts of kettles, both of the open and closed type, for carrying out chemioal reactions, such as nitration, sulfonation, reduction, etc. I n all these cases a heating medium may be required to give a certain temperat.ure beyond which had results will Be obtained. The controller will prevent too high temperatures in the apparatus. Sometimes the operation is such that the materials are first brought itp to a definit.e temporature, when the steam must be shut off and the contents cooled down to a lower temperature or cooled to maintain them a t tlie saine temperature, the cooling being required to absorb the generated heat. A differeiitial temperature controller may be used in this instance, first to control the flow of heat to hring t,he contents of the auboclave up to the desired point, nnd then to regulate the flow of
