Document pBE1G6mdYYrrZNkna33KLgY97

1132 CHAPTER 51 1955 Guide; Air discharged toward cold surfaces should be designed for a terminal velocity of 5+ fpm at approximately one-quarter the distance from a cold wall. Grille outlet velocities in excess of 600 fpm under 9 and 10 ft ceilings are likely to cause drafts at some points in the occupancy zone. Forced Air Systems (Central Steam) In these systems the air supplied for ventilation is tempered by automatically controlled steam coils and the normal heat loss for each room is supplied by direct radiators or convection. Provision is made for exhaust equal to the amount of out door air supplied. There are many variations of these systems, and they may in-" elude any features of central systems described in Chapter 30. Hot water may also be used as the heating medium. This type of system can also be designed to provide the entire heating require ment without using any direct radiation. This arrangement is quite satisfactory, but must be controlled for zones of similar occupancy which have exactly the same orientation. Where used as a two-duct system as described under Forced Warm Air Systems in preceding section, much more satisfactory results can be expected. Unit Ventilator Systems Unit ventilators are used for supplying heat and ventilation for individual class rooms. These units and their controls are described in Chapters 25 and 39. They can be combined or furnished with storage cabinets, book shelves, extensions for additional direct radiation, and extended grilles to discharge upward along the window sills. As in previous systems described, the excess air must be provided a means of escape. Panel Heating Systems A number of schools have employed panel heating of various designs. It is par ticularly adaptable in the slab-on-ground type of school to overcome the cold floor. It is also quite adaptable to the open corridor type of school using natural or window ventilation in the warmer climates. In the cola climates it should be augmented by a tempered-air ventilating system as previously described. Design data are given in Chapter 24, but the designer must include ample thermo static control which employs some means of varying the water temperature in in verse ratio to the outside temperature. With well-insulated walls and ceilings this feature usually results in an appreciable anticipation of the heating requirements. Several control systems are outlined in Chapter 39. Individual room control is desirable but not mandatory. A separate zone control for each exposure and for each floor is absolutely necessary for satisfactory results. When coils are installed in the floor slab, these systems must be operated continu ously (day and night). Night setback is not desirable, but when used it must be placed on the room thermostats and the setback must not exceed 10 deg. Generally room temperatures, read on wall thermometers placed 60 in. from the floor, can be one to two deg lower than with other types of systems for equal comfort. Floor temperature must not exceed 85 F on design days except that a Peri*??tif band, not exceeding 15 in. from the walls, may have a surface temperature of 109 * Floor panels should not be expected to warm the large volumes of air required for school ventilation, and for this reason a tempered air supply is required in all excep mild climates. MAINTENANCE AND OPERATION Controls, boilers and combustion equipment, fans and mechanical heat ing devices, traps and valves, all need attention at least annually. Every designer must use extreme care in selecting equipment that will require a minimum of attention, and see that it is accessible for service. While school custodial forces may be competent, they frequently have more work than can be handled during normal working hours. "Unusual maintenance problems may therefore result in deterioration of equipm" and systems. While first cost is important, final selection of equiproen should not be made without consideration of the overall cost which ineludes maintenance and service. CHAPTER 52 INSTRUMENTS AND MEASUREMENTS Temperature Measurement, Pressure Measurement, Air Flow Measurement, Air Change Measurement, Humidity Measurement, Heat Transfer Through Building Materials, Evaluation of Thermal Environment. Combustion Analysis, Smoke Density Measurements, Determination of Air Contaminants, Sound and Vibration Measurements HEATING and air conditioning engineers and technicians require instruments for both laboratory and field use and somewhat more pre cision is attainable and essential in the laboratory, where research and de velopment are undertaken, than in the field, where acceptance and adjust ment tests are conducted. Some instruments have attained an adequate state of development, while others fall far short of the desirable. For in stance, temperatures can now be measured readily with ample accuracy for most purposes, while a convenient and precise method for determining or comparing the dustiness of atmospheres is lacking, and improvement in existing hygrometers and humidity controllers is essential. Codes and standards covering different types of heating and air condi tioning devices and apparatus have been promulgated by various authori tative organizations, and instruments essential for performance or com pliance testing are enumerated in the relevant publications.1, 2> 4 The present purpose is to discuss the use and characteristics of the more im portant instruments. TEMPERATURE MEASUREMENT5,6 Thermometers Any device capable of indicating temperature is a thermometer, but in common usage the term thermometer without qualification has come to signify the ordinary mercury-in-glass temperature indicating device. This imn p a use^u' rane from --40 F, the freezing point of mercury, to about 1000 F, at or near which the glass usually softens. Lower temperatures can be measured with alcohol-filled thermometers for which the range is a.^?ut ~94F to +248 F. The better thermometers have their scales, either Fahrenheit or Centigrade, etched with acid into the glass which forms eir steins. The probable error for etched stem thermometers is plus or Th*'-8 ne SCa*6 <^vlslorb and calibration is desirable for much test work, eimometers are calibrated during manufacture at not less than two mperatures--the freezing and boiling points of water--and calibration at accomP^shed with the instrument completely immersed in a bath plieij f known temperature. The intervening scale divisions are then ap- com it i e- s^em' When such a thermometer is used with the stem insarv f ^ immersed, a correction known as the stem correction is necesv,, ' tor accurate measurements, and its magnitude is usually computed y means of the following formula: , where K = 0.00009 D (I, - t,) K = correction to be added, Fahrenheit degrees. (1)