Document Jr42EnDmv6Zn5jnR1444n0zqK

660 CHAPTER 47 1959 Guide Sajed on wind normal to expotore/ room 25 ft deop/ window* doMohtmg, 5 ft wide, 2 window/ opened at thown. fig. 3___ Typical Air Row in Classrooms with Natural Ventilation duct system, the air being delivered through wall grilles or ceiling diffusers, or by self-contained units within the rooms, such as unit ventilators and unit conditioners located at the outside walls. Grilles are generally located most con veniently at the inside or corridor wall. They must be- of the deflecting type, deflecting upward in consideration of the drop in the air stream when air is introduced at lower than room temperature. Diffusers are generally located in ftftitingH of rooms in a pattern to provide proper air dis tribution: In location of both nail grilles and diffusers, it is important that the air-delivery path be free of obstruc tions, which might be light fixtures and crossbeams, since presence of obstacles (will result in uncontrollable and turbulent currents of air extending into the occupied zone and causing drafts. Grilles and diffusers are generally se lected to provide a terminal velocity of 50 fpm at. ap proximately one-fourth the distance from an opposite wall or other obstacle to air flow. Exhaust Methods Window, when used for exhaust, are opened at the top to take advantage of natural rise in the warm air to be exhausted. Windows are not fully effective for exhaust purposes, except in completely still weather. Typical con ditions are shown in Fig. 3, indicating that, if the'windows are on the windward side, air is introduced rather than exhausted. Gravity exhaust ventilation is obtained through' exhaust grilles in the ceiling (or in a wall away from the windows) Sosts: Cooling requirements shown in Fig. 1. A Net hoot goto wrih ventilation air controlled at room teapcmtore of 75 F. 8, C, and 0 = Nef hoot gains using 525, 1000, and 1500 cfa, rospocNvtfy, controlled to 55 F, minimum. Cooling air temperature will be higher at outdoor air temperature ritet above 55 F. fig. 4... Typical Cooling Capacities, Using Outdoor Air connected to roof ventilators. Air is exhausted by wind effect and chimney effect due to differential pressure re sulting from thermal head. This type of exhaust is used in conjunction with air supply obtained by admitting air at the windows. Satisfactory air changes can be obtained- by this method within limits shown in Fig. 3, if the room ventilators are adequately latge. However, room conditions cannot be fully acceptable, since objectionable cold drafts are certain to occur in cold weather. Control of conditions depends on manual adjustment of window openings, a method that is reasonably satisfactory only in mild climates, but is not recommended where the design temperature is lower than 30 F. Gravity exhaust ventilation may be used with satisfactory results in combination with mechanical air supply systems. In mechanical exhaust systems the roof ventilators de scribed in preceding paragraph on gravity exhaust ventila tion are replaced with motor-driven exhaust fans. If this system is used in conjunction with window supply of air, it should be remembered that when windows are closed, replacement air is drawn from the corridor or the ventila tion rate is greatly reduced. At such times, negative pres sures will prevail in the building, with adverse effects on incinerators, hooded equipment, and other local exhaust and fuel-firing equipment. TYPES OF HEATING AND COOLING SYSTEMS Systems may be classified according to methods of heat ing and cooling. Each may be combined with means for ventilation in various ways. Only a few states have specific requirements with respect to types of systems. Accordingly, except in these states, engineers and administrators of school construction are free to analyse and choose kinds of sys tems. The possible combination of such systems are ex tremely numerous. A discussion of those encountered most often will follow. Direct Radiation This type of heating includes cast-iron radiators, castiron or nonferrous convectors, and continuous finned-pipe heating elements in connection with steam or water heat ing systems (see Chapters 14, 26, and 28). The radiation must extend the full length of the outside or window walls and the full width of high windows used in cross-lighted designs illustrated in Fig. 2. Control is obtained by manual valves, by individual automatic control valves for each room, or by an automatic valve or valves controlling zones of the system according to exposure. Direct radiation may be used with natural ventilation, natural ventilation with gravity exhaust, and natural ven- SchooJ Systems 661 tjlatinn with mechanical exhaust. It is designed for the warm-up load and to provide a warm curtain of rising air to mix with the incoming cold air to temper it. However, it is inadequate for this purpose, because heating require ments are satisfied at relatively low outdoor temperatures as soon as classrooms are occupied and lighted, as illustrated in Fig. 1. The system requires the teacher to adjust open ings for ventilation. Installation costs can be relatively low. Electric Heating This consists of electrical heating elements in baseboard type of radiation extending the full length of the window walls, electric ceiling panels, or panels formed of heating cable buried in the ceiling plaster. Control of heating is automatic by means of individual room thermostats. Any of the ventilating systems discussed previously can be com bined with electrical heating, much as discussed under Direct Radiation. Electrical heating has the same short comings as direct radiation; namely, that when classrooms are occupied and lighted, heat is not available for counter acting downdrafts at windows. The system requires the teacher to adjust openings for ventilation. The cost of energy will depend on the local electrical rates. Panel Healing Floor panel heating is adaptable to the slab-on-ground type and to the open-corridor type of school, particularly in the wanner climates. Design data are given in Chapter 30, Panel Heating, which also describes features of panel heating control systems. (See also Chapter -43, Automatic Control.) The floor system lends itself well to individual room,control, although zone control, including-several rooms of the same exposure in the same zone, is practicable. Floor panel heating systems are adaptable in areas having design temperatures down to 0 F. Since the requirement for heating is materially reduced when the spaces are oc cupied and lighted, as illustrated in Fig. 1, the panels should be constructed to have minimum mnra and thus minimum storage effect. This is accomplished by using a slab, only sufficiently, thick to enclose the tubing or piping, underlaid by a slfth of insulating concrete. Even with light concrete panels, there is enough storage effect to cause overheating after warm-up and to make it difficult to follow rapidly r.hftngmg loads. Any of the types of ventilating systems already discussed may be combined with panel heating systems. However, the panel heating system cannot be effective in combating the natural condition of downdrafts at the windows. The system requires the teacher to adjust openings for ventilation. The panel system is well adapted for use in primary classrooms with slab-on-ground floor construction, where students spend much time on the floor. Central Indirect Air Systems In these systems, air is supplied for ventilation and heat ing. At a central point or points, outdoor ventilation air and recirculation air is mixed and heated by automatically controlled steam or hot water coils. Gravity or m^ehanirAl exhaust is coordinated with the air supply system. The central system can be arranged to recirculate air from window sills to intercept the flow of uncontrolled cold window drafts into the classroom. Such returns are fully effective only if the openings are located in the window sills; if located below the alls, the natural air currents will not be intercepted. There are many variations of the system, which may include features of central systems described in Chapter 19: Air supply may be controlled by zones of similar occupancy and identical orientation. In dividually controlled booster or reheat coils may be located within ducts leading to each of the spaces. Individual room control may also be provided by a double duct system, equipped with miring dampers for each room located within a short distance ahead of the supply grilles or diffusers serving the room. Central air conditioning can be included. Split Systems The central indirect air systems may be combined with direct radiatitin or convection units described in the pre vious section. Direct Radiation. In this combination, air may be supplied by the ventilating system at constant temperature, usually 55 F, when the outdoor temperature is less than 55 F. In this case direct radiation may be de signed to take care of the building heat loss. In order to prevent window drafts, the temperature of the ventilation air should be controlled to require operation of the direct radiation when the outdoor temperature is below 55 F. The direct radiation is controlled by automatic valves operated by a room thermostat for the individual room. This system has wide flexibility. Central Direct-fired Air Systems These systems are forced circulation warm air furnace systems. They may be arranged for single or double duct air distribution, as discussed under Central Indirect Air Systems. This type of. system is further discussed in Chapter 18, Warm Air Heating. They are usually installed with zone controls and arranged to permit use of any desired proportion of outdoor air during periods of space occu pancy. Mechanical cooling may be obtained by installation of cooling coils in the main supply ducts. Unit Ventilators Unit ventilators are used in classrooms to provide heat ing, controlled ventilation, and cooling, using outdoor air as the cooling medium. Auditorium unit ventilators are used in large rooms such as auditoriums, gymnasiums, and large lecture rooms. Classroom unit ventilators are often combined with matching storage cabinets or may be com bined with ducts extending the full length of the windows and arranged either to discharge air' upward along the window sills or withdraw air at the window sills and thereby prevent downdrafts from the windows. In order to effect ively intercept natural downdrafts, the return grille must . be located with the top of the unit at the level of the win dow sill. Classroom unit ventilators may also be combined with extended finned-pipe radiation at? the window sills to combat cold window downdrafts. Electrical unit ven tilators are also available. Unit ventilators using hot water for beating and chilled water for cooling are used for school buildings where year-round use is contemplated or where warm fall or spring seasons warrant the installation of airconditioning equipment. These'units and their controls are described in Chapter 15, Unit Ventilators and Unit Heaters. Hie noise level of the unit ventilators'under the operating conditions of the installation should be considered. These systems have a high degree of flexibility. Unit Conditioners Unit conditioners are essentially the same as unit ven tilators, discussed above, except that the cabinets contain refrigeration equipment, a direct-expansion coil, and a steam or hot water coil piped from the central boiler source. Noise