scope and reliability of records, but drew attention to the importance of the time factor-i. e., to the rate of change in those conditions which affect the over-all results. The realization of the imDortance of the mathematical derivatives with respect to time, rate of change, and acceleration, constituted the most important factor in the development of precise control and formed the basis of successful automatic control. Without appreciating and conforming to the influence exerted by rate of change, a p p r o p r i a t e corrections and smooth control are hardly possible in complicated systems. The effect known as hunting is unavoidable. For instance, LUIS DEFLOREZ with the on-and-off control, correc19 Rector Street, New York, N. Y. tions remain on one direction until the condition-sensitive element has returned to zero. When it returns, however, the system has accumulated sufficient momentum to swing past the zero point and HE development and use of instrumentation represent the procedure repeats itself indefinitely. the greatest single factor in the growth of continuous Such systems as cracking-coil temperature control, based processing. Without accurate knowledge of conditions oq outlet temperature, are analogous to the simple pendulum. obtaining within the apparatus, correction and consequent If we wish to control them without oscillation, we must take control of reactions are impossible. into account not only departure but also rate of change and The first instruments to be developed and adopted were of even acceleration-in short, satisfy the conditions basically ilthe indicating type. Early developed to sufficient precision, lustrated in the general equation for the damped pendulum: they adequately conveyed information concerning existing conditions but served no other function. Actual control redoc d d mained the manual operators' responsibility and required a Kia K2 - Ks= 0 dt dta high degree of judgment based on long experience. Operating log sheets constituted the only record of past operations. Yet, where (*I = departure (angular or equivalent) K1, K2, K s = constants even the best of such records could not furnish instantaneous trends so necessary to the proper evaluation of a correcAlthough precise manual control can be approximated with tion. indicating instruments by skilful operators, these men either After indicating instruments came the recording types. consciously or subconsciously gage the rates of change and This was a notable advance. Their use not only increased the
Automatic Control and the
Chemical Industries
T
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The use of automatic control to govern industrial processes undoubtedly had for its initial purpose the saving of labor. The more modern concept of its use, however, is t o carry out procedures with greater accuracy and reliability than is manually possible, with consequent improvement of products and reduction in unit costs. Successful automatic control can come about only by devising a manual control t h a t will suit the process. The automatic operation of control procedure is a secondary step. The control procedure must be devised by experts in the art. The automatic features can then be adapted and the general principles of mechanical control can be utilized. Successful automatic control can be judged only by improved results. The mere feat of maintaining some variable condition constant or t o make it follow some given path is of no service t o the industry unless the control of this variable proves to be a governing factor in improving industrial results. 1210
NOVEMBER, 1937
IXDUSTRIAL AND ENGINEERING CHEMISTRY
act accordingly. To be really useful, the automatic control device must do the same. I n a sense. the development of recording instruments was the first stepin the field df automatic control. If we break up mocess control into its basic elements. we usuallv find that it Lomprises (a) a knowledge of conditions, ( b ) an iLtegration of these conditions with respect to time or some equivalent, and (c) some manipulation which accords with this integration.
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through a master device which will maintain some predetermined relation between operating conditions.
Accuracy of Operations In the past automatic mechanisms were viewed primarily as leading to reduced labor costs. Today, however, Bhere is a constantly growing appreciation that they can be relied upon
D E F L O RCERZA C K I NUNIT G AT THE PORT ARTHURWORKS OF THE TEXAS COMPANY; CONTROL HOUSEAT RIQHT
Complete automatic control might be considered as the necessary coordination of these three factors without human intervention. The workers who in the early days struggled with the on-and-off type of controller for pressure or temperature, retain a clear picture of the inadequacy of this type of device; it could not respond to rate of change and did not modulate corrections. Perhaps it was these crude early attempts a t control which discouraged operators and delayed the adoption of automatic devices. Today, however, most control devices reflect the effect of the rate of change and make proportional corrections which fit them to control problems formerly considered beyond their scope.
Centralization The delay in the adoption of automatic devices for important processing controls is probably somewhat responsible for an important step in the design of equipment. It has given impetus to the centralization of instruments and controls. This trend was brought about by the desire to put the operations as far as possible under the control of a single highly skilled operator who could interpret readings and apply corrections to the best advantage. The advent of individual automatic control instruments combined with remote control devices materially simplified the concentration of control and facilitated carrying out the designed control procedure. I n the light of present advances it seems probable that further progress in centralization of controls and automatic devices will lead to the ultimate concept of automatic controlnamely, the coordination of multiple automatic devices
to carry out many operations far more accurately and reliably than is possible with the most skilled labor. The reduction of those errors caused by fatigue, poor judgment, and other inherent weaknesses in human control is of vital importance in the operation of processing units of great size. Here accuracy is essential to long periods of operation and t o the quality and yield of products. The increase in size of operating units constitutes the major factor in reducing the cost of operating labor. At the same time, increase in size places additional emphasis on the need for precise control. Unless the operating conditions are controlled within narrow limits, the advantage sought in the cosstruction of large units may be lost. I n the petroleum industry, processes such as cracking are now carried out in units with capacities of the order of 20,000 barrels per day, as compared with units of 1000 barrels per day ten or twelve years ago. Fluctuations or shutdowns due to inaccurate operation would seriously increase production costs. I n the early days of cracking, units of small size were designed and constructed in batteries; fluctuations and shutdowns of individual units were expected and provision made for such eventualities. Today, however, many refineries carry out their cracking operations with one or two units having a capacity equal to whole batteries of former units of the standard size. Such units cannot be as readily shut down and put back on stream as the small unit of the past. I n fact, their size and complexity are such that any unscheduled shutdown will seriously affect the whole refinery operation and increase over-all costs. The hazard of shutting down a large unit is such that we find many processes being carried out be-
