Document BZ2E6YQr4rxNMJg8VpqwRDBm

824 CHAPTER 38 1951 Guide tween maximum and minimum load. The rate of reset is manually adjustable and must be set to meet the load requirements of the individual system. Controllers may also be designated by types as: a rum-indicating con troller, when it does not indicate the controlled condition and performs the control function only; an indicating controller, when fitted with a pointer, thermometer, or gauge which indicates the controlled condition; a recording controller, when it is combined with a clock mechanism and chart which records the controlled condition. ACTUATING DEVICES The starting point of any control system is the thermostat, hygrostat, pressure regulator, or other mechanism which is sensitive to a change and responds in the desired manner. Thermostats are usually of the room, duct, or immersion types. Various types of thermostats found in common use are defined in the following paragraphs. 1. A thermostat is an instrument which is responsive to changes in temperature, and initiates a force that repositions valves, dampers, etc., to maintain selected tempera tures. 2. A room thermostat is usually mounted on the wall of the space to be controlled with the measuring element arranged so that it is affected by the room temperature. 3. A duct thermostat is provided with fittings suitable for installation in duct work. The insertion type is equipped with a rigid bulb and is arranged so that the temperature responsive element or bulb extends through the wall of the duct. The re mote bulb type is arranged so that the bulb and instrument head are connected by means of a flexible tube of the desired length. The bulb is inserted in the duct, and the head is located where it is accessible for adjustment and inspection. 4. An immersion thermostat is provided with fittings suitable for installation in a pipe line or tank where a fluid tight connection is required. Both insertion and re mote bulb types are available. A union connection and separable socket, when used, permit removal of the bulb without draining the line or tank. The sockets may be of copper, stainless steel, or other materials. 5. A day-night or two-temperature-thermostat controls a heating or cooling source to maintain either of two selected temperatures. They may be indexed (set at de sired control temperature) individually or in groups from a remote point by means of a manual or time switch. 6. A summer-winter or heating-cooling thermostat is similar to the day-night type, except that both the temperature setting and action are changed by the indexing means. Such a thermostat could open a volume damper on a rise in temperature in summer, and close the same damper on a rise in temperature in winter. 7. A submaster thermostat has its temperature setting raised or lowered a pre determined amount for a given change in some other variable. For example, the water temperature on a heating system may be raised as the outdoor temperature drops. A master instrument is used to.reset a submaster thermostat and may be a switch, pressure controller, thermostat, or similar device. In the foregoing example the master thermostat would be located where it would respond to outdoor tempera ture, and the submaster thermostat would be located in the pipe line of the heating system. A hygrostat is a controller which is sensitive to changes in relative humid ity, and is available in room and duct types. Where the controlled condi tion is below 20 percent or above 80 percent, or the temperature is above 100 F, selection of a suitable type, and kind of hygroscopic element is essential. A ;pressure regulator is a device which is sensitive to changes in pressure. It may be of the type which controls a single pressure or of the differential type which maintains a predetermined difference between two pressures. For pressures in duct work, sialic pressure regulators are available in the differential type. They are sensitive to changes of 1/100 in. of water. Automatic Control 825.. ACTUATED CONTROLS Thermostats, hygrostats, pressure regulators and other actuating devices obtain control of heating and cooling mediums, fuels, liquids, etc., by actuating various control devices such as control valves, dampers, damper motors, relays, or controllers defined in the following paragraphs. A control valve is designed to control the flow of fluids, and may be con sidered as a variable orifice which is repositioned by a motor operator, as directed by a thermostat or other controlling device. 1. A normally open or direct acting valve will assume an open position when all operating power is removed. 2. A normally closed or reverse acting valve will assume a closed position when all operating power is removed. 3. Single seated valves are designed for tight shut-off using appropriate disc mate rials for various pressure ranges. 4. Pilot piston valves serve a similar function on high pressure installations. 5. Double sealed or balanced valves are designed for applications where tight shut off is not required. They are not affected by varying inlet pressures or pressure dif ferentials, and thus are widely used where these conditions exist. 6. A three-way valve is fitted with a double faced disc, operating between two ports, and functioning to close one port as the other is opened. Depending upon how it is installed, it may be used as: a. A three-way mixing valve to mix as required two fluids entering the two inlet ports and leaving through the common outlet port. b. A three-way diverting valve to divert the flow from the inlet port to either of the outlet ports. Valve discs, poppets, and seats are available in various shapes to meet any desired flow characteristics with various materials as required by serv ice conditions. A damper is designed to control the flow of air or gases, and is similar to a valve in this respect. Single blade dampers are generally restricted in size because of the difficulty of securing proper operation with high velocity air. Multi-blade or louver dampers can be furnished so that adjacent blades move in the same direction or in opposite directions. The opposed blade type gives better directional and flow characteristics than the parallel blade type. For long life and trouble free operation, dampers should be constructed with heavy metal frames, blades of iron adequately braced, and ample bearing surfaces of non-corrosive materials. When fairly tight closing is desired, felt may be glued and riveted on the edges and ends of the blades.. Other materials for blades and frames are also used for special services. A damper motor is repositioned by a controlling instrument, and is connected to the damper blades as required to give the desired movement. It can be mounted on the damper frames or mounted outside the duct and connected to an extended shaft on one or more damper blades. Suitable brackets are available for floor, wall, or duct mounting of the motor. A relay is a device which uses an auxiliary source of energy to amplify or convert the force of a controller into available energy at a valve or damper motor. Various types of relays are designated as follows: 1. An electro-pneumatic relay, when electrically energized, starts or stops the flow of air as required. 2. A pneumatic-electric relay, when affected by different air pressures, starts or stops the flow of electrical energy as required. 3. A twitching relay or pilot valve may be used to switch the operation of & con-