Document YjOOzpr5gReM7bE0oQ4mpmbzD

558 CHAPTER 26 1949-Guide cient to warm the air needed to meet the ventilating requirements, a corre sponding reduction may be made in the amount of direct radiation installed. The- greater the amount of excess capacity of the unit, the more efficient will be the temperature regulation of the room. The split system permits the heating of the room during failure of electric current, since the direct radiators will furnish heat, but it permits a careless operator to avoid oper ating the ventilating equipment. ' The combined system employs a unit ventilator with sufficient capacity for both'ventilation and the normal heat transfer losses. In such a case no direct radiation is required/ The necessary operation of the fan When the room is being heated also gives assurance that some ventilation is being provided, especially if automatic dampers are used in the air intake and in the recirculating intake. These dampers are arranged to provide a certain quantity of outside air, depending upon the weather conditions. , The cost of installation of a combined system is usually less than that of a split sys tem and there is less danger of overheating, but if the electric energy, fails there will be available only the heating effect of the units acting, as convectors. Unit Heaters, Unit Ventilators, Unit Humidifiers 559. vent openings of from. 600 to 800 fpm produce the. best diffusion results from thjs system. Many states, however, have regulations .that wilLnot permit velocities as high as 800 fpm. If a vent opening at or near the floor is near a desk or place where a person is seated, a .velocity of:800 fpm in the vent opening will produce an objectionable draft. In such a case the velocity in the vent opening should not exceed 400 to 450-fpm, although duct velocities are maintained at 600 to 800 fpm if codes permit. ' In school buildings provided with wardrobes or cloakrooms the vents may be so located that the air shall pass through these'spaces, ventilating them with air which otherwise would be passed to the outside without being used to the best advantage. Many state codes for ventilation of public buildings make this arrangement mandatory. > . WINDOW VENTILATORS A window ventilator illustrated in Fig. 10 consists of filter and switch controlled motor driven fans enclosed in a cabinet to be mounted on the window sill. Such units accomplish ventilation, air cleaning, and air cir- Fig. 10. Typical Window Ventilator Location of Unit Ventilatqr The location of the unit ventilator in a room is important. Wherever possible it should be placed against an outside wall and on the center line of the room. It is difficult to obtain proper air distribution if the unit is installed either on an inside wall or in a corner^ of the room. Standard units discharge the air stream upward, but for special cases units may be installed to discharge air horizontally. Units may be set away from the wall or partially recessed into the wall to save space without materially affecting the results. The air inlet may enter the cabinet at the back at any point from top to bottom. Air Exhaust Vents and Flues .' ' The: size arid location of the'air exhaust vent* outlet1 is important. In mafiy caseS'ihe sizes for public buildings'are regulated by law. ' ' ", : .In cases where no codes govern, the location and size of vents are left to 'the'rhM.rttiehof'the^^engmeer.;. .. .L '. . .. , . ' JBest results have, been, obtained with a velocity through the vent, open ings $ea,rly equal to that at which the air is introduced into the room', thus rnftint.ftiriing a siight pressure in the room. ' Calculated velocities at the culation, but have no means of heating the air. The direction of air dis charge is manually adjustable for seasonal operation. UNIT HUMIDIFIERS Unit humidifiers fall into four general classifications, depending on-the. method of causing evaporation: These; are as follows: (1) Nozzle Type, (2) Rotary' Type, (3) Cascade Type, and (4) Heater Type. .. , In the nozzle type of humidifier water is sprayed into-the air and evapo ration is effected by adiabatic exchange of energy. ' Units of this type-in simplest form spray a fine mist of water directly into the air-in a space. They are used to a great extent in the textile industry. : To the simple type of nozzle unit may be added the following accessories: a water heater whose function is to increase the vapor pressure, an air heater to heat the air either before or after humidification takes place, a screen or air filter over which the water is sprayed and through which the air is drawn for intimate contact with the water, a fan to create an air stream and to deliver air to the space to be humidified, and a housing, to enclose all the- elements. This type of unit has been made in practically all of the variations mentioned. It has'the disadvantage of clogging, the nozzles and must be serviced continually. Fig.'ll illustrates a nozzle umt having a humidifying capacity sufficient for a residence or small building. These units usually include air filters and in some cases provide ventilation air by means of an outside air duct X