Document x5axr2eyeMXQ0or9QJqzo6XLb

174 CHAPTER 13 o. Direct-acting controUcrs increase the cutout air pressure as the controlled variable increases. For example, a direct-acting thermostat increases output pressure when the sensing element detects a temperature rise. b. Reverse-acting controllers increase the output air pressure as the controlled variable decreases. A reverse-acting thermostat will increase output pressure when the temperature drops. Indicating and Recording Features Controllers may be' of the indicating or recording' type. The nonindicating controller is most common in heat* mg,1 ventilating, ana air-conditioning work,' and includes" all types tn which the weeing element does not provide a visual indication of the value of the controlled variable. If an indication Is desired, a separate thermometert relative humidity indicator, pressure gage, etc., is'required. With room thermostats, for ex ample, a separate thermometer often is attached to the cover.' An indicating controller has a pointer added to the sensing ele ment or attached to it by a linkage, so that the value of the controlled'variable is indicated on a suitable scale. A recording controller is similar to an indicating controller, except that the indicating pointer is replaced by a recording pen-which provides a permanent record on a chart driven by a clock motor. Proportional band is a term used in connection with in dicating ftnri recording controllers and has the same meaning as throttling range. It is usually expressed in percent of the scaie.or chart range of the controller. . CONTROLS FOR HEATING, VENTILATING, AND AIR-CONDITIONING SYSTEMS ' For convenience, the controls for heating, ventilating, and air-conditioning systems are sub-divided into, three groups: (1) controllers, (2) controlled devices, and (3) auxiliary con-, trol equipment. Types of Controllers Controllers are mainly (1) thermostats,-(2) humidistats, and (3) pressure types. - -. 1. Many thermostats are of the simple, single-purpose type,' but there are many others for various special purposes. Var ious types are indicated in paragraphs (a) to (A), which follow.] . a. The 'roam type b designed for mounting on a wall and responds to room temperature. b. The insertion type is designed for mounting on. a duct, with its Bgmring element extending into the duct.. . - c. The immersion type is designed for mounting on a pipe or tank It has a fluid-tight connection to allow the measuring element to extend, into the fluid. A separable-socket or well b often used with immersion thermostats, to avoid the, need for draining the system when removal of. the thermostat-b required. Since the separable socket will reduce the response rate, of the. thermostat, the rocket; should be no heavier-than necessary and should fit snugly around the element. .. 1965 Guide And Data Book <L The remote-bulb type b designed for applications in which the point of temperature measurement.b at some distance frotn the desired thermostat location. It b often used (or centralized panel mounting of the controller. The remoto*bulb element may be of either the insertion or immersion type. e. The surface type is designed for mounting on mid sensing the temperature of a pipe or similar surface. /. The dap-night, or dual room thermostat controls at a reduced temperature at night. It may be indexed (changed from day to night operation)' individually or in groups frotn a remote point by a manual or time switch. Some electric types have an individual clock and switch built into' the thermostat. . ............. . g. The pneumatic dap-might - thermostat uses - a two-pres sure air-supply system, the two pressures often being 13 and 17 peig, or 15 and 20 prig. Changing the presure at a central point from one value to the other actuates switching devices in the thermostat and indexes them-from day to night or vice versa. .Supply-air mains are often divided into two or more dreuita so that switching can be accomplished in various areas of the building at different times. For example, a school building may have separate, circuits far classrooms, offices and administrative areas, auditorium, and gymnasium and locker rooms. h. The heating-coding or summer-winter thermostat - can have its action reversed and, if desired, can have its set point changed by the indexing means. It b used to actuate controlled devices, such as valves or dampers, that regulate a heating source at one time and a cooling source at another time. It is often manually indexed in groups by a switch or automatically, by a thermostat sensing the temperature of the control agent, the outdoor temperature, or other suitable variable. : i- The pneumatic . heating-cooling thermostat uses . a two- pressure air-supply system as described for day-night ther mostats. ................... j. A two-stags thermostat b arranged to operate in two successive steps. A. A submaster thermostat has its set point raised or lowered over a predetermined range, in accordance with- variations in-output from a master controller. The master controller can be a thermostat, manual switch, pressure controller, or aimilar device. For example, a master thermostat measuring outdoor air temperature :can be used to readjust a submaster ther mostat controlling the water temperature in a heating system.- Master-submaster -combinations are sometimes .designated aa single-cascade action.; When such-action b accomplished by a single thermostat having more than one measuring element, it b known as compensated control. 2. Humidislats are of either the room or insertion type. Submaster humidistats of either type are sometimes used in conjunction with an outdoor master thermostat to reduce the humidity in cold weather and prevent condensation on win dows. A wet-bulb .thermostat b often -used for humidity -control in conjunction with proper control of the dry-bulb temperature. A wick,' or other means for. keeping the bulb wet, and rapid air motioo to assure a true wet-bulb measurement are essential.. Automatic Control 175 control applications, a potentiometer for rebalancing the control circuit b also driven by the motor.. . Controlled Devices Controlled devices may be (1) automatic valves, (2) valve operators, (3) automatic dampers,. and (4) damper operators- Other controlled devices include electric heaters, and motors on such equipment as fans, pumps, burners, refrigeration compressors, and similar apparatus. 1. An automatic valve b designed to control the flow of steam, water, gas, and other fluids, and may be considered as a variable orince which b positioned by an electric or pneu matic operator in response to impulses from the controller. It may be equipped with a throttling plug or v-port of special design to provide the desired flow characteristics. Kroewnnle composition discs are common. .They are made of materials beat suited to the media handled by the valve, and the operating temperature and pressure. For high pressures or for superheated steam, metal discs are often employed. Internal parts of valves, such as the seat ring, throttling plug or v- port skirt, disc holder, and stem, are sometimes mane of stain less steel or other hard and corrosion-resistant for use in severe service- Automatic valves should be properly selected and sized for specific applications. See the section on in this chaoter for further details. c. A pneumatic operator, consists of a spring-opposed, flexible diaphragm or bellows attached to the valve stem in such a way that an increase in air pressure moves the valve stem and simultaneously compresses the spring. Springs of various ranges, in terms of air pressure, can be employed to provide eequrace operation of two or more devices by proper selection or adjust ment of the springs. Springless pneumatic operators, using two opposed diaphragms or two sides of a tingle diaphragm, are also tried; but are generally limited to special applications involving lai^e valves or high pressures. Pneumatic operators are designed primarily for proportional control. Two-position control u accomplished by use of a two-position controller, or a two-position pneumatic relay to apply either full air pressure or do pressure to the valve operator. - Pneumatic valves and valves - with spring-return electric operators can be classified as normally open or normally closed. 1 A normally open valve will mumnn an open position when all operating force b removed. " A normally closed valve will assume a closed position when all operating force b removed.-1 - 3. Automatic dampers are designed to control the flow of air-or gases, and function -like valves in thb respect. Steel blades and frames are most common, but other materials are used for special applications, such as those involving corrosive fumes. 'Large dampers are often made in -two or more sections for strength and convenience in handling, the sections being interconnected so as to operate as a .unit. The damper types commonly used are: touows: a. A single-seated voice (A, Fig. 6) is designed for tight shut-off. Appropriate disc materials for various pressure ranges and media a. Single-blade, which b generally restricted to "mall sizes because of the difficulty - of securing proper operation with high-vclocity air. are used. b. Multiblade or louver damper (Fig. 8) which has two or b. A pilot piston valve utilizes the pressure of the control - more blades linked together, -it may be arranged for parallel agent as an aid in operating the valve. It usually is singleseated and b used where large forces are required for valve operation. c. .A double-seated or balanced valve (B, Fig. 6) b designed 80 that the media pressure acting against the valve disc b essen tially halanrArt, thereby reducing the force required of the operator. It b widely used where the fluid pressure is too high.to permit a single-seated .valve to close. It cannot be used-where tight shut-on is required. d. A three-way mixing valve (A, Fig. 7) has two inlet connec tions and one outlet connection, and a double-faced disc operating between two seats. It b used to mix, as required, two fluids entering through the inlet connections and leaving through the the common outlet. . A three-way diverting value (B, Fig. 7) has one inlet connec operation, in which all blades rotate in the same direction; or .opposed operation, in which adjacent blades rotate in opposite directions. See later section, Control of Air Flow, for difference in air flow characteristics. c. Mixing dampers, composed of two sections interlinked so that one section opens as the other doses. 4. Damper operators of the electric type may be unidirec tional; spring-return, or reversible,' similar to electric-motor valve operators. Pneumatic damper' operators are similar to pneumatic valve operators, except that they usually have a longer stroke or the stroke b increased by means of a mul tiplying-lever. Damper operators are mounted on the damper frame and are connected with linkage directly to -a-damper blade, or-are mounted outside the-duct and connected, to a crank -arm attached to a shaft extention of- one of the blades. tion and two outlet connections, and two separate discs and seats. It b used to divert the. flow to either of the outlets or to propor tion the flow to both outlets. /. A butterfly valve consists of a heavy ring enclosing a disc which rotates on an axis at or near its center, and, In principle, is similar to a-round single-blade damper. The'disc usually seats against a ring machined within the body. 2. Valve operators are of three general types: a. A solenoid consists of a magnetic coil operating a mov able plunger. It ts used for two-position operation. The most common use of a solenoid operated valve b to control the flow of gas, water, air, and refrigerants, and-it b generally limited to relatively small sizes. h. An electric-motor-operates the valve stem through a geir train and linkage. Electric motor operators are classified in three types: unidirectional, spring-return, or reversible. (1.) Unidirectional, for two-position operation. The valve opens during one-half revolution of the output shaft and closes during the other one-half revolution. Once started, it continues until the half revolution b completed, regardless of subsequent action by the controller. Limit switches built into the operator stop the motor at the end of each stroke. If the controller has ' been satisfied during thb interval, the operator will continue to the other position. (2.) Spriny-return, for two-position operation. Electric energy drives the valve to one position and holds it there. When the cir cuit b broken, the spring returns the valve to its normal 'position. (3.) Reversible, for floating and proportional operation. The motor can run in either direction and cam stop in any position. It b sometimes equipped with a return spring. For proportional PARALLEL OPERATION . OPPOSED,. OPERATION.. . Fig. 8 ... . Typical Multiblade Dampers .