Types of system

There are five types in common use:

1. Self-acting. The pressure, force or displacement produced as a signal by the measuring element is used directly as the power source at the final control element. A good example is the thermostatic expansion valve (Figure 12.1) in which the vapour pressure generated in the bulb by the temperature measured is transmitted along a capillary and acts above the diaphragm of a valve, causing it to move and so regulate the flow of refrigerant. This form of system is simple (no external power is needed) and proportional in nature, but has a fairly wide proportional band because of the need to produce enough power to move the final control element between its extreme positions.

2. Pneumatic. Compressed air is piped to each controller which, by bleeding some to waste reduces the air pressure to a value related to the measured value of the controlled variable. This reduced pressure is then transmitted to the final control element. Economy in the use of compressed air may be achieved by modifying the system shown in Figure 13.4(a) so that air is only bled to waste when the final control element is actually moving. Sizing the
pressure-reducing orifice shown in the inlet pressure line in the figure is critical: if it is too big an excessive amount of air will be bled to waste and if it is too small airflow into the space above the valve diaphragm will be slow and the response of the final control element sluggish. It is therefore important that dirt, moisture or oil are absent from the compressed air supply. It is usual to have two air compressors (one stand-by) and to store the compressed air in a receiver at a suitable pressure up to a maximum of 8.6 bar (gauge). As the air is used by the controls the stored pressure falls through a fairly wide differential, at the lower end of which the duty compressor is started. The receiver is sized to keep compressor starts to less than 12 per hour. Industrial quality compressed air, used for driving tools and machines is not at all suitable for automatic controls, which must have instrument quality compressed air.

Hinge Flap Air ble

Adjustable spring

Temperature-sensitive bi-metallic strip

Compressed air

^ control pressure line

Iy Pressure, reducing orifice

Control Pressure

Inlet Pressure

Diaphragm

Water flow rate

(a) Simple pneumatic system.

Line 230 V Neutral

Transformer

]

Switch operated by a mechanical link to a temperature-sensitive

Element in order to make or break a circuit

Mechanical link to sensor

From 24 V
Solenoid valve giving two-position control
Water flow rate

(b) Simple electrical system.

Electrical load to be switched

(c) Simple electrical holding circuit Fig. 13.4

This must be clean, free of oil and dried to a dew point low enough to satisfy the application: driers should be in duplicate and may be by mechanical refrigeration or chemical means, with an automatic changeover.

3. Hydraulic. Oil or water is used to transmit a signal in a way similar to that for compressed air systems but the application is for much higher power transmission than pneumatic systems can cope with.

4. Electrical. There are two basic functions: switching and resistance variation. If large thermostats of slow response are employed the electric current switched may flow directly through the contacts of the thermostat but continuous switching generates high temperatures locally and the contacts will bum and even weld together. Indirect switching can be achieved (Figure 13.4(6)) by the mechanical displacement of a temperature-sensitive element (for example the cam on the spindle of a step-controller, rotated by a Bourdon tube and filled system). Another technique is to use a holding circuit, as shown in Figure 13.4(c). A rise in temperature makes the contact A, say, and current flows through the holding coil, which pulls in switches 1 and 2, magnetically, making the main power circuit. Only a small current flows through A. When the temperature drops the power is maintained after A is broken until the contact at B is made. This then energises the breaking coil via switch 1 and B, which opens switches 1 and 2 magnetically and turns off the power supply to the load. The other application is the variation of the resistance of one leg of a Wheatstone bridge. Figure 13.5 shows a classical circuit for giving proportional control over a final control element (or a ‘modulating valve’). When in balance no current flows through W1 and W2, normal running and reverse running coils of an electric motor that drives the modulating valve, V. These are used to regulate the flow of hot water, say, to a heater battery. On change in temperature sensed by a thermostat, T, mechanical displacement of the linkage moves the wiper across the potentiometer windings represented by R1 and R2. The resistance on one side of the bridge (R1 + R3) is then different from that on the other (R2 + R4). Current flows through the coils Cl and C2 of the balancing relay which is consequently moved magnetically to make a contact at a or b, energising W1 or W2. The valve motor then runs in its forward or reverse direction, depending on which winding is energised, opening or closing the valve. To prevent the valve merely running to its extreme positions there is a mechanical connection between the valve position and a wiper on the balancing potentiometer of the bridge which is moved in a reverse direction to that of the first wiper, so tending to bring the bridge back into balance by making (R1 + R3) = (R2 + R4) again. The modulating valve thus moves in a series of small increments and provides stable

R1 R2 Balancing potentiometer Fig. 13.5 Classical bridge circuit used for conventional electric modulating valves.

Control. Technological advances have brought improvements in electrical modulating valves, with variable position, solenoid-type valves achieving proportional control over water flow in a simpler fashion than illustrated in Figure 13.5.

5. Electronic. Electronic controls have almost entirely displaced pneumatic controls in commercial applications, since about 1985, because of the technical advances in the subject and the development of digital techniques and computers. Much smaller signal strengths are transmitted from the sensors to amplifiers, which measure phase changes and decide on the strength of output signals for control purposes. Digital controllers convert electronic signals from the sensors into numbers which can then be used for calculations by a microcomputer according to a program. Data can be stored, programs can be changed, set points altered, and many other operations carried out, locally or at a distance. See section 13.16.

Posted in Air Conditioning Engineering