Document GKdXv48Km559EBLLegEa6q0Rr

r Heating Ventilating Air Conditioning Guide 1938 TYPES OF AUTOMATIC CONTROL Operating Media or Source oi Power Supply Automatic control systems may be classified in three broad groups based upon their primary operating media or source of power, as follows: 1. Electric Control Systems. In such control systems the primary medium utilized to provide for the operation is electricity, and the basic function of these controls consists of switching or otherwise adjusting electric circuits to govern electric motors, relays or solenoids. The individual units of this type of system are interconnected by line voltage or low voltage wiring, and this wiring serves to complete the circuits carrying the commands of the controllers to the controlled valves or damper motors. 2. Pneumatic Control Systems. In the pneumatic control systems, the primary source of operation is obtained through a medium of compressed air, the pressure of which is varied by the controlling devices. In these systems one or more centrally located air compressors furnish a supply of compressed air which is distributed in special piping to the various con trolling and controlled devices. By means of leak ports or orifices, the pressure of the air is varied in the branch lines and the changing pressures are utilized in air operated damper motors or valves to obtain the move ment necessary to the operation of valves and dampers. 3. Self-Contained, Control Systems. Self-contained control systems have, in general, been restricted to such operations as could be effectively handled by a power unit with integrally mounted or direct-connected controller. Such applications consist of valves utilized to admit steam or other media into coils to regulate the temperature of tanks or to regulate the admission of steam into heating coils or radiators as determined by the controller element. Motion of Controlled Equipment Automatic control: equipment can also be classified into two general types with respect to the characteristics of the motion imparted by the controls to the controlled equipment, such as two position or positive acting control and modulating or graduated action control. In any control system it is necessary to'choose the type of equipment whose characteristics permit the type of control operation desired and in many cases both types of control are used in the same system to best meet various requirements. . 1. Two Position or Positive:-acting Control. This type of control operates positively between two positions such as on and off or open and closed with no intermediate positions or degrees of motion between the two extremes of operation. A simple thermostat which starts and stops an oil burner or a unit heater motor is an example of this type. As applied to a valve or a damper, the action of the controlling device would serve to fully open or fully close the valve or damper. In some applications of this type of control, artificial heat is applied to the sensitive element of the room thermostat at the same time that heat is being added to the space under the control of the thermostat in order to 686 "r" Chapter 37. Automatic Control increase its sensitivity. This usually results in more accurate control and more frequent operation of the heat source. 2. Modulating or Graduated-acting Control. This type of control causes motion in the controlled device in proportion to motion caused in the controller by fractional degree variations in the medium to which the controller is responsive. After a fractional change has been measured at the controller and has effected a new position of the valve or damper in proportion to the amount of such change, the' system stands by awaiting further change at the controller before any additional motion occurs. The extent of the motion is limited only by the limits of the controller and by the intensity of the change of conditions as measured. With this type of control, the damper or control valve may be operated in intermediate positions between its extreme limits in order to properly modulate or proportion the flow of air, steam or water, reacting with changes of con ditions at the controller. Various modifications of this type of control are available, designed to meet special requirements and conditions, all based on operation of the controlled equipment in intermediate positions. This type of control motion cannot be used on valves of one-pipe steam , systems as the partial opening of the valves will not permit the condensate to escape against the flow of incoming steam. This type of control should not be used to control, the flow of steam to a heater coil of a fan system which is in the direct path of untempered outdoor air at temperatures below freezing, because of the possibilities of freezing condensate in the bottom of the coil. Division oi Space under Control Control systems vary considerably with the type and size of the building, occupancy of the building, and with the heating or cooling system, humidity supplying equipment and ventilating means available for control. In the following paragraphs the general requirements of various phases of these different buildings will be discussed. 1. Individual Room Control. The most accurate and flexible form of control for any structure is that calling for the regulation of each indi vidual room by control equipment reacting to conditions in that room only. Such control necessitates a thermostat in each room, located to properly measure the conditions of the room, controlling the radiator, unit heater, unit ventilator or other heating source supplying heat to that room only in which the thermostat is located. This arrangement permits the maintenance of any desired conditions in any room, entirely inde pendent of any other room. In the case of large rooms, where one ther mostat location will not serve to properly measure the conditions through out the room, and where two or more sources of heat supply are provided in the room, additional thermostats may be used, each controlling its respective section of the heating source. This form of control, due primarily to the number of control devices required over the entire building, normally is the most expensive type of control system. How ever, where maximum flexibility and the most accurate control is desired, individual room control can be depended on to furnish the desired results. 2. Single Thermostat Control. Probably more widely used than any other form of control is the type of automatic system regulated entirely 687