The Control Device

In most cases the correcting device to which the controller outputs will be a control valve. Small valves can be pneumatic or electric, larger valves are usually operated by air pressure. This may introduce additional dynamics for the correcting element, which could be approximated by a first-order time constant. 

There are essentially two types of control valve a fail open and fail closed type. They are 

Which type of valve one chooses is a safety issue, if upon a major failure the valve should go to the open position (for example cooling water) or closed position (for example reactants) should be judged by the engineer. If, for example, the valve is used at the outlet of a tank, we may want to close the valve to prevent all the liquid in the tank from being drained. 

Differential pressure measurement Differential pressure measurement  

Orifice plates Flow nozzle Hydrostatic head 

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Introduction An accurate quantitative analysis of the discharge of pollutants from a process must be determined prior to the design and/or selection of control equipment. If the unit is properly engineered by utilizing the emission data as input to the control device and the code requirements as maximum-effluent limitations, most pollutants can be successfully controlled. 

Provide an explanation of the method(s) used by the site to verify that the control device(s) are operating as designed and m compliance with all regulatory requirements. 

Input and output interface This is the interface between the controlling devices and the processor. The input/output (I/O) unit receives signals from the input devices and transmits output action signals to the controlling devices. 

In many cases, heating or cooling of the gaseous effluent will be required before if enters the control device. The engineer must be thoroughly aware of the gas laws, thermodynamic properties, and reactions involved to secure a satisfactory design. For example, if a gas is cooled there may be condensation if the temperature drops below the dewpoint. If water is sprayed into... 

One of the methods of controlling air pollution mentioned in the previous chapter was pollution removal. For pollution removal to be accomplished, the polluted carrier gas must pass through a control device or system, which collects or destroys the pollutant and releases the cleaned carrier gas to the atmosphere. The control device or system selected must be specific for the pollutant of concern. If the pollutant is an aerosol, the device used will, in most cases, be different from the one used for a gaseous pollutant. If the aerosol is a dry solid, a different device must be used than for liquid droplets. 

Control of stationary sources of air pollution requires the application of either the control concepts mentioned in Chapter 28 of the control devices mentioned in Chapter 29. In some cases, more than one system or device must be used to achieve satisfactory control. The three general methods of control are (1) process change to a less polluting process or to a lowered emission from the existing process through modification of the operation,... 

A monitoring system is selected to meet specific needs and is tailored to the unique properties of the emissions from a particular process. It is necessary to take into account the specific process, the nature of the control devices, the peculiarities of the source, and the use of the data obtained (8). 

Add-on Control Device an air pollution control device such as carbon absorber or incinerator that reduces the pollution in an exhaust gas. The control device usually does not affect the process being controlled and thus is "add-on" technology, as opposed to a scheme to control pollution though altering the basic... 

Instrumentation normally is denoted by a circle in which the variable being measured or controlled is denoted by an appropriate letter symbol inside the circle. When the control device is to be located remotely, the circle is divided in half with a horizontal line. Table 1.3 gives various instrumentation symbols and corresponding letter codes. The specific operating details and selection criteria for various process instrumentation are not discussed in this book. The reader is referred to earlier works by Cheremisinoff [1,2] for discussions on essential control and measurement instrumentation. 

In instances where a capacitor is connected directly across the terminal of the motor, the capacitor can act as a source of excitation current after the control device is opened. In order to prevent this the capacitor rating should not exceed 90 per cent of the motor no-load magnetizing current. 

The pressurized air goes to an air receiver for storage and then is processed for use by passing through filters, dryers and, in some cases, lubricators. This pressurized air is normally classified as instrument air when it is used in control systems. This air must be moisture and oil free to prevent the control devices from clogging up. 

As in the case of power stations, where there is known to be considerable variation in operating conditions due to tidal changes, or in estuary waters variations in salinity, automatic control systems may be desirable. For such systems the current output of the transformer-rectifier is controlled by thyristor or transductors. Sensing electrodes are permanently installed on selected piles and transmit the electrode potential of the steel back to the controlling device. This type of system enables the most economic amount of current to be provided under all operating conditions. 

These last two produce an electric signal which must be measured and amplified to operate the controlled device. 

Many of these devices are direct acting on the controlled device and do not require a controller to process the signal. 

Proportional detectors measure the process condition, which can then he compared hy the controller with the required value. They are not direct acting, and need a controller to convert the signal to a working instruction to the controlled device. Proportional detectors include ... 

If a controller is used with an on-off detector, it functions only as an amplifier to transmit the detector signal to the controlled device. It can modify the speed of this action hy a hias or hy a slow-speed operating motor, as in the floating control. 

Some proportional detectors are combined in the same instrument with a suitable transducer which can perform some of the functions of a controller. For example, for pneumatic systems the primary sensing element actuates a variable air jet, thus modulating an air pressure which is transmitted to a further controller or direct to the controlled device. Electric and electronic detectors such as the infrared detector include the sensing and amplifying circuits of the instrument. 

Like any sequence of events, an FFC experiment can be intended as a sequence of elementary intervals during each of which all system control lines maintain constant values. One needs to keep in mind, however, that while a control line transition is always very fast (settling times of the order of Ins), the controlled device/parameter may require a much longer... 

Pressure monitors at the return end of the global loop, coupled with active feedback to an adjustable valve have been used to compensate for fluctuations in line pressure. Unfortunately, the valves used have been diaphragm or needle valves, which have all of the shear challenges. While pressure consistency may be improved for the polishes nearest the control device, the shear challenge has not been addressed. Furthermore, the pressure fluctuation in the middle section of a global loop, as polishers turn on and off, can not be corrected by a device at the end. 

Partial pressure measurement devices which are In common use comprise the measurement system proper (the sensor) and the control device required for Its operation. The sensor contains the Ion source, the separation system and the Ion trap. The separation of Ions differing In masses and charges Is often effected by utilizing phenomena which cause the Ions to resonate In electrical and magnetic fields. 

Control Devices In many installations the use of gas is intermittent, and some means of controlling the output of the compressor is therefore necessary. In other cases constant output is required despite variations in discharge pressure, and the control device must operate to maintain a constant compressor speed. Compressor capacity, speed, or pressure may be varied in accordance with requirements. The nature of the control device will depend on the function to be regulated. Regulation of pressure, volume, temperature, or some other factor determines the type of regulation required and the type of the compressor driver. 

Servo Mechanism A mechanism which responds to the action of a controlling device and operates as if it were directly actuated by the controlling device. However, it is capable of supplying power output much greater than the original controlling device. The power is derived from an external and independent source. Both mechanical and electrical servo units exist. 

The electrical control system is powered by means of 24-V batteries which are charged automatically from a rectified a.c. supply. All the control devices are located beyond the safety wall. The separator is monitored by a closed circuit television system. 

The USEPA is responsible for creating and enforcing the NESHAPs for all hazardous air pollutant sources. The CAA states that new or existing major sources must have emission standards based on the maximum available control technology (M ACT) to reduce hazardous air pollutant emissions. The MACT standards are based on the performance of the best 12% of the control devices in the same source category. These MACT emissions requirements were extended in 1997 to cover wastewater biosolid incinerators at publicly owned treatment works (POTWs) that have the potential to discharge cadmium, lead, and mercury (Richman, 1997). 

Mathematically expressed, NT = Pf, where NT is the number of particulate transfer units achieved and PT is the total energy expended within the collection device, including gas and liquid pressure drop and thermal and mechanical energy added in atomizers. NT is further defined as NT = In [1/(1 -1 )], where Tj is the overall fractional collection efficiency. This was intended as a universal principle, but the constants and y have been found to be functions of the chemical nature of the system and the design of the control device. Others have pointed out that the principle is applicable only when the primary collection mechanism is impaction and direct interception. Calvert (R-10, R-12) has found that plotting particle cut size versus pressure drop (or power expended) as in Fig. 14-129 is a more suitable way to develop a generalized energy-requirement curve for impaction... 

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