Document yr0Z75rZy0NerpGYq7EZOmyBE

594 CHAPTER 34 1946 Guide standard voltages. The individual units of this type of system are inter connected by wiring circuits which carry, the commands of the controller to the motors operating the controlled valves or dampers and to stop and start electric motors driving the controlled.equipment. Simple make and break electric thermostats are used to stop and start motors and for twoposition control of valves and dampers. Modulating electric control may be accomplished by the unbalancing of some form of Wheatstone bridge which in turn causes the repositioning of a motor element which moves the valve or damper. In some controllers the impulse from more than one Wheatstone bridge can be integrated in one motor element.. Controllers for modulating electric control vary from the contact type in that the electrical mechanism consists of a variable potentiometer.: The potentiometer includes a contact finger which moves across a coil of. resistance wire wrapped on a suitable bobbin.. The contact finger is actuated by a temperature,, pressure, or humidity sensitive element. When the same amount of current is flowing through both coils of a balancing relay the contact blade will be in the center of the space between, tiie two contacts and the motor will be at-rest. Changes in the conditions at the controller will affect the circuit so that the amount of current flowing through the two relay/ coils will not beidentical. Whenever the current through these two coils becomes un balanced, one of them will become stronger and the V shaped armature will be tilted. The movement of the armature will close one of the relay contacts which in turn will start the motor running either clockwise or counter-clockwise, until the circuit is again balanced by the resulting movement of the motor balancing, potentiometer, .whereupon the relay contact will break and the motor will stop. - The development of specialized industrial control equipment 'using. electronic principles has made rapid progress during recent years. Tem peratures may be measured and controlled to fractions of a degree by new) applications of the photo-electric cell. Its accuracy and flexibility make) it applicable to a variety of automatic devices and controllers. Pneumatically Operated Equipment In pneumatically, operated equipment the .primary source of power is compressed air at a. pressure of 15 to 25 lb.per: square inch which should/ be clean, dry and, free from:oil vapor. It is desirable to compress-the air' to a pressure of 70 lb per square inch or more to provide for a precipitation of. moisture due to temperature drop in the high pressure receiver andto reduce the dew-point of the air to the controllers below that, of-the: lowest ambient temperature of the air lines. The compressed air piping from the source of supply to the inlet side of the controller is known as the main. The connections "from the outlet) of the controller to the pneumatic motors are known as branches. '. Throttling type controllers, of the pneumatic type, are basically air pressure regulating valves actuated by .the controlled condition to vary the branch line pressure and thereby reposition the controlled valves or : . dampers as required to maintain the desired condition of the controlled' media. A direct acting controller increases its. branch pressure on an. increase of. the controlled condition. A reverse acting controller increases' its branch pressure on a decrease of the controlled condition. Several different'devices niky be operated from the-branch line of a single con- troller'iH selected sequence by the. proper spring loading of the 'respective .pneumatic diaphragm motors.' Small air pressure gages connected into . Automatic Control 595 the branch at each controller will indicate the controller action and are of great convenience to the operator in adjusting the system. The simplest form of pneumatic thermostat is based on the leak stat principle which consists of a restrictor in the air supply to the instrument; a leak port having a larger port than the restrictor, and a bi-metallic strip, or diaphragm chamber which expands on a rising temperature and con tracts on a falling temperature. This power is used to reposition the lid seat to close or open the leak port. As the leak port is gradually closed the pressure in the branch increases until it approaches the full main pressure in the fully closed position.. As the lid seat moves away from the leak port the pressure in the branch decreases until the branch is fully exhausted when the lid seat restriction of the leak port is entirely removed. The basic leak stat principle of operation has been refined by the addition of supplemental relays in nearly all current types. * TYPES AND FUNCTIONS OF CONTROLLERS Descriptions or functions of various types of controllers follow. A single-duty room thermostat controls the heating or cooling devices to maintain the desired temperature. A dual or two-temperature room thermostat controls the respective heat sources to maintain alternately two selected temperatures. They may be indexed manually or by time switch from central points in selected zones. A high-low room thermostat is similar to the dual thermostat, except that it is indexed individually by the manual operation of a push button or switching device built into the individual instrument. A sub-master or remote readjustable room thermostat is a pneumatic room type instru ment fitted with a readjusting diaphragm to which a separate pilot air line connection is made. As the pressure in the pilot line is raised from 1 psi to 13 psi, or as selected, the operating point of the controller may be increased or decreased within predetermined limits. Indexing may be done by a manual gradual switch or master thermostat. A pneumatic master\ or pilot thermostat varies the pilot pressure to the readjusting diaphragm of one or more sub-master thermostats to vary their operating point in accor dance with a predetermined change in the temperature sensed by the master thermostat. A summer-winter room thermostat operates at any selected temperature as a direct acting controller during the heating season and operates at the same or aiiy other selected temperature as a reverse acting controller during the cooling season. Groups of, these instruments may be indexed from a central point or individually indexed by a manual push button, or a switching lever built into each instrument. They may be used to control volume dampers of a summer-winter air conditioning system to maintain a desired temperature condition in the controlled space, and will operate to close normally open dampers on a rising temperature during the heating season and to open same on a rising temperature during the cooling season. A summer-winter sub-master or summer-winter remote readjustable room thermostat .is similar to the summer-winter thermostat but is arranged to permit resetting of the control point by a master thermostat or by a manual gradual switch or other selected device. A humidity compensated thermostat is a room type gradual acting thermostat combined with, and readjusted by, a hygroscopic element. It may be used to maintain a constant effective temperature. The humidity sensitive element usually functions to raise the operating point of the temperature sensitive element one degree for a predetermined drop in relative humidity and lower the operating point of the temperature sensitive element proportionately for a similar increase in relative humidity. An insertion or duct thermostat of tube type is usually equipped with a rigid bulb, consisting of a' brass tube enclosing an invar rod which usually has a maximum range of 0 F to 250 F, or consisting of a- vapor disc type which operates through a similar range but in which the type of disc must be selected to function at given temperature incre ments throughout that range. It is available in the remotely readjustable or sub master type, which may be reset in the same manner as the sub-master room type ther- mostat. This type of instrument may also serve as a master thermostat. / An immersion thermostat is similar to an insertion thermostat, except that the bulb . fitting is threaded for insertion through the wall of a vessel containing a liquid. A union connection and separable well or socket are available where it is desirable to provide for