Patent GB-892288-A (GB)
Title: Improvements in and relating to the automatic control and operation of chemical or physical processes
Inventor: MARTIN ERNEST NORMAN
IPC: G05B13/02
Filed: 1957-10-02
Abstract: 892,288. Automatic control systems. IMPERIAL CHEMICAL INDUSTRIES Ltd. Sept. 29, 1958 [Oct. 2, 1957], No. 30808/57. Class 38 (4). In an electric automatic control system for maintaining a process at maximum efficiency, measurements of a variable of the process are made at timed intervals, each measurement being compared with the previous measurement, which is stored, so that each difference serves to adjust the process by a fixed amount towards maximum efficiency. In an ammonia synthesis plant, synthesis gas, including a proportion of recycled exit gas from which ammonia has been condensed out, is supplied to a converter in two streams one of which is fed through one heat interchanger and the other of which is fed through another heat interchanger in the catalyst bed. The system described automatically maintains the difference between the outlet temperature from the converter and the gas inlet temperature to the interchangers at a maximum for greatest output. A voltage corresponding to this difference is obtained from thermocouples or resistance thermometers and applied to terminals T1, T2, Fig. 4, so that it is compared with a previous measurement of the temperature difference stored by the setting of a potentiometer P. If the temperature difference is nearer the maximum than at the previous measurement, the polarity of the resultant voltage applied through a modulatoramplifier A operates a motor M2 when contacts CLK3 of a timing relay CLK close so that a switch Hg2 is closed to energize a solenoid DMA of a stepping motor which, by moving a cam C and thereby increasing pressure in a pneumatic transmitter P<SP>1</SP>, makes an incremental adjustment of a remote valve (not shown), by-passing the heat interchanger so as to cause further progress towards the maximum. After a short time interval determined by a resistance-condenser network a relay B operates for a period determined by a further resistance-condenser network to change over contacts B1 so that a motor M1 adjusts potentiometer P to correspond with the latest measurement of the temperature difference in readiness for the next comparison. When relay B operates, self-holding relay CLK is energized to interrupt the circuit to motor M2 and de-energize a clutch CC so that switch Hg2 re-opens under the action of a spring. This condition persists until a timing motor M3 completes its cycle and de-energizes relay CLK to start the next operation of the control system. If the maximum is overshot there is a reversal of polarity applied to amplifier A and motor M2 operates in reverse to close switches Hg1, Hg3 so that a condenser C1 is charged and a relay D energized to change over contacts D1. When, later, clutch CC is de-energized by the timer to re-set switches Hg1, Hg3, a relay CO is momentarily energized to close contacts CO1 and energize stepping solenoid DMB so that a valve adjustment is made in the opposite sense. The change over of contacts D1 ensures that when switch Hg2 closes as the result of approach to the maximum again in the reverse sense, the adjustment of the valve takes place in the appropriate direction. When switch Hg3 opens after the maximum is first overshot a relay E is energized to change over contacts E1 so that when the maximum is again overshot in the reverse sense relay D will be released by energization of an opposing winding. The system is protected against surges and sparking. Modifications involving pneumatic or electric motor operation of cam C are described. Application to the control of pressure, flow, concentration or electric power, or to maintaining a variable at a minimum value is referred to. Specification 758,286 is referred to.