Document 48Bv63GQnKOqxz42DGKZKJzx

630 CHAPTER 43 1960 Guide thermostat decreases the air pressure as the temperature in creases. Indicating and Recording Features Controllers may be of the indicating or recording type. The nonindicaling controller is most common in heat ing, ventilating, and air-conditioning work, and includes all those in which the measuring element does not provide a visual indication of the value of the controlled variable. If an indication is desired, a separate thermometer, relative humidity indicator, pressure gage, etc., is required. With room thermostats, for example, a separate thermometer often is attached to the cover. An indicating controller has a pointer added to the measur ing element 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 bond is a term used in connection with in dicating and recording controllers and has the same meaning as throttling range. It is usually expressed in percent of the bt-aIa or chart range of the controller. 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 (k) which follow. a. The room type is designed for mounting on a wall and responds to room temperature. b. The insertion type is designed for mounting on a duct with its measuring element extending into the duct. c. The immersion type is designed for mounting on a pipe or tank. It baa & fluid-tight connection to allow the measuring element to extend into the fluid. A separable socket or well is often used with immersion thermostats to avoid the need for draining the system when removal of the thermostat is required. Smce the separable socket will reduce the response rate of the thermostat, the socket should be no heavier than necessary and should fit snugly around the element. d. The remote-bulb type is designed for applications where the point of temperature measurement is at some distance from the desired thermostat location. It is often used for centralised panel mounting of the controller. The remote-bulb element may be of either the insertion or immersion type. e. The surface type is designed for mounting on and measur ing the temperature of a pipe or similar surface. /. The day-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 from a remote point by a manual or time switch. Some electric types have an individual dock and switch built into the thermostat. g. The pneumatic day-wight thermostat uses a two-pres sure air-supply system, the two pressures often being 13 and 17 psig or IS and 20 psig. Changing the pressure 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 often are divided into two or more circuits so that switching can be accomplished in various areas of the building at different times, in accordance with use. For example, a school building may have separate circuits for classrooms, offices and administrative areas, auditorium, and gymnasium and locker rooms. h. The heating-cooling or summer-winter thermostat can have its action reversed and, if desired, have its set point changed by the indexing means. It is used to actuate con trolled devices, such as valves or dampers, that regulate a heating source at one time and a cooling source at another time. It is often indexed in groups manually by a switch or automatically by a thermostat measuring the temperature of the control agent, the outdoor temperature, or other suitable variable. 1. The pneumatic heating-cooling thermostat uses a two- pressure air-eupply system as described for day-night ther mostats. j. A two-stage thermostat is arranged to operate in two successive steps. k. 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 similar 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 as single-cascade action. When such action is accomplished by a single thermostat having more than one measuring element, it is known as compensated control. 2. HumuUstals 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 is often used for humidity control in conjunction with proper control of the dry-bulb tempera ture. A wick or other means for keeping the bulb wet, and rapid air motion to assure a true wet-bulb measurement are essential. 3. A pressure or static-pressure controller is made for mount ing directly on a pipe or remotely on a panel or wall. A small pipe or tube may be used for transmitting the pressure signals to the instrument. 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. l. An automatic valve is designed to control the flow of steam, water, gas, and other fluids and may be considered as a variable orifice which is 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. Renewable composition discs are common. They are made of materials best suited to the media bandied by the valve, and its 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 vport skirt, disc holder, and stem sometimes are made of stain less steel or other hard and corrosion-resistant metals for use in severe service. Automatic valves should be properly selected and sized for the specific applications. See the section on Design Coordination in this chapter for further details. The functions of various types of automatic valves are as follows: a. A single-seated valve is designed for tight shut-off. Ap propriate din materials for various presure ranges and media are used. b. A pilot piston valve utilizes the pressure of the control agent as an aid in operating the valve. It usually is singleseated and is used where large forces are required for valve operation. c. A double-seated or balanced valve is designed so that the media pressure acting against the valve disc is essentially balanced thereby reducing the force required of the operator. It is widely used where the fluid pressure is too high to permit a single-seated valve to close. It cannot be used where tight shut-off is required. d. A three-way mixing valve has two inlet connections and one outlet connection and a double-faced disc operating be tween two seats. It is used to mix, as required, two fluids entering the inlet connections and leaving through the com mon outlet. Automatic Control 631 e. A three-way diverting valve has one inlet connection and two outlet connections and two separate discs and seats. It is 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 A\bp. 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 is used for two-position operation. The most common use of a solenoid operated valve is to control the flow of gas, water, air, and refrigerants and generally it is limited to relatively small sizes. b. An electric motor operates the valve stem through a gear train and linkage. Electric motor operators are classified in three distinct types: unidirectional, spring return, or revers ible. Unidirectional, for two-position operation. The valve opens during one-half revolution of the output draft and closes dur ing the other one-half revolution. Once started, it continues until the half revolution is 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 this interval the operator will con tinue to the other position. Spring-return, for two-position operation. Electric energy drives the valve to one position and holds it. When the circuit is broken, the spring returns the valve to its normal position. Reversible, for floating and proportional operation. The motor run in either direction can stop in any position. It is sometimes equipped with a return spring. For propor tional control applications, a potentiometer for rebalancing the control circuit is also driven by the motor. c. A pneumatic operator consists of a spring-opposed flex ible diaphragm or bellows attached to the valve stem in such a way that an increase in air pressure moves the valve stem and at the same time compresses the spring. Springs of vari ous ranges in terms of air pressure can be employed to provide sequence operation of two or more devices by proper selection or adjustment of the springs. Springless pneumatic operators, using two opposed diaphragms or two sides of a single dia phragm, also are used but generally are limited to special ap plications involving large valves or high pressures. Pneumatic operators are designed primarily for proportional control. Twoposition control is accomplished by use of a two-postion con troller, or a two-position pneumatic relay to apply either full air pressure Or no pressure to the valve operator. Pneumatic valves and valves with spring-return electric operators can be classified as normally open or normally closed. A normally open valve will assume an open position when all operating force is removed. A normally closed valve will assume a dosed position when all operating force is removed. 3. Automatic dampers are designed to control the flow of air or gases and function like valves in this respect. Steel blades and frames are most common, but other materials are used for spedal applications such as those involving corrosive fumes. Large dampers often are 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: a. Single-blade, which is generally restricted to small because of the difficulty of securing proper operation with high-velocity air. b Multiblade or louver damper which bp* two or more blades linked together. It may be arranged for parallel opera tion, in which all blades' rotate in the same direction or op posed operation, in which adjacent blades rotate in opposite directions and provide more linear air-flow characteristics than obtained with parallel operation. c. Mixing dampers, composed of two sections interlinked so that one section opens as the other clones 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 ramilur to pneumatic valve operators except that they usually, have a longer stroke or the stroke is 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 draft extension of one of the blades. On large dampers two or more operators may be required, and usually are applied at separate points on the damper. Normally open or oormaily closed operation is obtained ac cording to the method of mounting the operator and connecting the linkage. Auxiliary Control Equipment In addition to the conventional controllers and controlled devices described, many control systems require auxiliary devices to perform various functions. Auxiliary controls for electric systems include: 1. Transformers to provide current at the required voltage. 2. Electric relays for control of electric heaters or to start and stop oil burners, refrigeration compressors, fans, pumps, or other apparatus for which the electrical load is too large to be handled directly by the controller. Other uses include time-delay, circuit-interlocking safety applications, and similar functions. 3. Potentiometers for manual positioning of proportional control devices or for remote set-point adjustment of elec tronic controllers. 4. Manual switches for manually performing a variety of operations. These may be of two-position or multiple-position type or may be of single or multiple-pole type. 5. Auxiliary switches on valve and damper operators for providing a selected sequence of operations. Auxiliary control equipment for pneumatic systems in cludes: 1. Air compressors and accessories, to provide & source of air at the required pressure. 2. Electropneumatic relays, which are electrically actuated air valves for operating pneumatic equipment in accordance with variations in electrical input to the relay. 3. Pneumatic-electric relays, which are actuated by air pressure and make or break an electrical circuit. 4. Pneumatic relays, which are actuated by the pressure from a controller to perform numerous functions. They may be divided into two groups: a. Two-position relays, which permit a controller actuating a proportional device to actuate also one or more two-position devices. They also are used in various automatic switching operations. b. Proportional relays, whieh are used to reverse the action of a proportional controller, select the higher or lower of two pressures, average two or more pressures, respond to the difference between two pressures, add or subtract pressures, amplify or retard pressure changes, and perform other similar functions. 5. Positioning relays, which are devices for assuring ac curate positioning of a valve or damper operator in response to changes in pressure from a controller. They are affected by the position of the operator and the pressure from the controller, and whenever the two are out of balance will change the pressure applied to the operator until is restored. 6. Switching relays, which are pneumatically operated air valves for diverting air from one circuit to another or for opening and closing air circuits. 7. Pneumatic sxuitches, which are manually operated devices for diverting air from one circuit to another or foe opening and closing air circuits. They may be of two-position or mul tiple-position type. 8. Gradual switches, which are proportional devices for manually varying the air pressure in a circuit. Auxiliary control devices common to both electric and pneumatic systems include: 1. Sequence controllers for operating a number of electric