Document Ex3q3mn5pmkpQw9Q1dYBNzLXL

HEATINC VENTILATING AIR CONDITIONING CUIDE 1944 of the smaller dampers are usually* made of stock. Solid stops are usually H in. deep. The free area of the duct enclosing the damper is thereby reduced in. in each dimension through the damper. Where channel frames are used, the restriction is in creased accordingly. Damper louvers should be suspended on non-corrosive trunnions.. Thrust bearings are necessary to properly support large vertical louvers. Where ball or rtyopllee.r bearings are required for special conditions, they should be of the non-corrosive DESIGNATION OF TEMPERATURE CONTROL Temperature may be controlled from three basic conditions: Dry-bulb Control: The controller element senses the dry-bulb temperature. Wet-Bulb Control: The controller element senses a true wet-bulb temperature. Wicking of selected material suspended from the controller element to a water trough directly below the element is one of the mediums frequently used to obtain wet-bulb temperature. True wet-bulb temperatures can only be maintained with clean wicking and air motion of not less than 600 fpm. The use of distilled water in the wet-bulb reservoir is desirable. Since the wet-bulb temperature indicates the heat content, this form of control while requiring additional and intelligent care to maintain true wet-bulb conditions, has many definite uses. tainDeedw. -Point Control: A saturated condition at a predetermined temperature is main AUTOMATIC CONTROL TERMINOLOGY The following is a generally accepted terminology referring to the operations of controls in a central air conditioning system. Control Point: The desired constant condition of the controlled media to be main ttaroinlleedr.acStioomn.e change of the controlled condition must occur to alter the prevailing con Thermometer Lag: The instantaneous temperature differential existing at any time between the true temperature of the medium and the temperature indicated on the thermometer. Controller Lag: The time delay in the controller's ability to reposition the controlled sources to properly compensate for a change in the controlled condition. Process Lag: (Process Time Lag). The time which elapses between the instant the control valve is positioned by the controller and the instant the controller element senses the effect of the change. This depends upon the rate of heat transfer, the specific heat, and the velocity of the fluid movement. Generally a decrease in process time lag simpli fies the control problem. Rate of Load Change: Time element of change in the magnitude of load. Magnitude of Load Change: The per cent load at a specified time. It directly affects the size of the controlling valve. Where, the inlet temperature varies constantly, the control problem becomes more difficult as the thermal difference between the inlet tem perature and the leaving temperature of the controlled media becomes greater. Drift: The deviation from the control point due to change in load conditions from 0 to 100 per cent. Drift increases as the sensitivity adjustment of the controller is lowered, thereby tending to decrease hunting. Drift decreases as the sensitivity adjustment of the con troller is raised, thereby increasing hunting. Hunting or Cycling: The temperature variation between the high and low points of the control cycle due to lag. This balances out at the point where each successive wave of temperature reaching the controller is as large as its predecessor. As the sensitivity hisunlotiwnegr.ed, hunting decreases but drift increases. Oversized control valves increase Throttling Range or Sensitivity Adjustment: The total rise in temperature at the controller bulb, required to reposition the control valve from a fully open to a fully closed position or to operate several devices controlled in sequence throughout the full sequence range. In connection with chart type controllers, throttling range is usually expressed in per cent of the total range of the chart. For other types of instruments, sensitivity adjustment is expressed in pounds variation of branch pressure per degree change in temperature, or per other unit of change of the controlled media. 644 CHAPTER 34. AUTOMATIC CONTROL * Low Sensitivity: A small change in branch pressure for a comparatively large tem perature or pressure change in the controlled medium. This results in a greater deviation from the control point. Low sensitivity tends to reduce hunting but the lower the sen sitivity, the greater will be the drift. High Sensitivity: A large change in branch pressure for a comparatively small tem perature or pressure change of the controlled medium. The extreme of high sensitivity is the on-off or two-position control. As the sensitivity is raised, drift decreases, but the tendency to hunt increases. Automatic Reset: A mechanical addition to a controller which corrects for drift by constantly resetting the instrument to operate on the control point, while repositioning the controlled valve or damper from its fully open to its fully closed position. Reset action is superimposed on throttling range and both function simultaneously. The rate of reset is manually adjustable and must be set to meet the load requirements of the individual system. This supplementary device provides the accuracy of high sensitivity adjustment with a minimum of hunting. Diaphragm hysteresis is the amount by which the stem travel of a given motor fails to assume the identical position on the upward and downward strokes for an identical air pressure in the diaphragm top. It prevails in varying degrees in connection with the operation of all pneumatic diaphragm motors, and is due to spring and diaphragm hysteresis and friction of guide bearing and stuffing box. Assume that a direct acting valve moving to a closed position will travel 50 per cent of its total with 8 lb air pressure on the diaphragm top. If the diaphragm pressure is increased to further close the valve and then reduced to 8 lb, the valve may be repositioned to be only 45 per cent open, indicating 5 per cent hysteresis. Hysteresis is usually expressed in per cent of the full stroke of valve stem. RESIDENTIAL CONTROL SYSTEMS The control installation in a residence may vary from the simple regulation of a coal-fired heating plant to the completely automatic all year air conditioning system. Residential installations with automatic fuel burning appliances, such as oil burners, gas burners or stokers, are normally equipped with single room thermostat, limit and safety controls as outlined under Control of Automatic Fuel Appliances. Coal-Fired Heating Plant Control in the normal coal-fired domestic heating plant consists of regulating the combustion rate in accordance with requirements. This function is accomplished by a spring or electric-driven damper motor which, under the command of a room thermostat and through chain linkage, operates the draft and check dampers of a boiler or warm air furnace. Such installation should be protected against excessive tem perature or pressure by means of a limit control serving to check the fire when conditions at the boiler or furnace reach a predetermined maximum. All Year Domestic Hot Water Supply Hot water or steam heating boilers with automatic fuel burning ap pliances can be used for all year heating of domestic water supply. The fuel burning appliance in this case is controlled from the temperature of water or pressure of steam in the boiler to maintain uniform boiler con ditions and domestic hot water is heated by means of an indirect heater. The heating of the residence is normally governed by means of a ther mostat which operates a control valve in the flow line of a gravity hot water or a steam system. Air Conditioning Systems Residential air conditioning systems normally include a heating source and a motor-driven fan for circulating air. In addition, such installations 645