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402 Chapter 21 1945 Guide fore, a boiler capacity requirement of less than 50 per cent of that required for units taking all their air from the outside. Heat Required for Recirculating Only If all of the air is recirculated, the total heat required is the same as the heat loss of the room, or Ht = H = 0.24 W (iy - 0 (12) In Example 1, if the quantity of air taken in from the outside is reduced to zero, or all of the air handled by the unit is recirculated, the total heat requirements Ht reduced from 99,600 to 24,000 Btu per hour, or to about one-fourth. Applications of Unit Ventilators Items to be considered in the.application of.unit ventilators are: (1) combination with other means of heating, (2) location of units, and (3) method of venting or exhausting. Fig. 10. Typical Window Ventilator In a split system the unit is used primarily for ventilation. Air is delivered to the room at or slightly above room temperature, and enough \ radiation is installed in the room to take care of the normal heat transfer , losses. Where the unit ventilator selected has a capacity more than sufficient to warm the air needed to meet the ventilating requirements, a ;s corresponding reduction may be made in the amount of direct radiation installed. The greater the amount of excess capacity of the unit, the more -.1 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 operating 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. It then becomes necessary for the fanf-. to be running whenever the room is heated, but this also gives assurance.^' of ventilation, especially if automatic dampers are used in the air intake.^, from out-of-doors and in the recirculating intake arranged so as to give;''; a certain quantity of air from the outside (commensurate with weatherjj; conditions) whenever the unit is operating and after the room is heatecfc|f; The cost of installation of a combined system is usually less than that.of; a split system and there is less danger of overheating, but if the elect : Unit Heaters, Unit Ventilators, Unit Humidifiers 403 energy-, fails there will be available only the heating effect of the units acting as, convectors. Location of Unit Ventilator \ , The location of the unit ventilator in, a room is important. Wherever possible it should be placed against an outside wall. It is difficult to obtain proper air distribution if the unit is erected .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 airhorizontally. -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 and location of the air exhaust vent6 outlet is important. In many cases the sizes for public buildings are regulated by law. See table of state codes and standards in Chapter 48. In cases where no codes govern, the location and size of vents is left to the discretion of the engineer. Best results have been obtained with a velocity through the vent openings nearly equal to that at which the air is introduced into the room, thus maintaining a slight pressure in the room. Calculated velocities at the vent openings of from 600. to 800 .fpm produce the best diffusion results from this system. Many states, however, have regulations that will not permit of 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 schoQl 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 filterand motor driven fans enclosed in a cabinet to be mounted on the window sill. These units accomplish ventilation, air cleaning, and air circulation, but have no means of heating the air. The direction of air discharge is manually adjustable for seasonal operation. Their operation is controlled by an electric switch. UNIT HUMIDIFIERS Unit humidifiers which may be procured in the market at the present time, fall into four general classifications, depending on the method of producing the humidity. These are as follows: (1) Nozzle Type, (2) Rotary Type, (3) Cascade Type, and (4) Heater Type. In the nozzle type oi humidifier water is sprayed into the air and Ar <5tu tM*?0r1*, D- w- Nelsronepaonrdt Rw.o.W9.36K--ubInavsetasti(gAa.tSio.nH.oVf.EA.irTOruatnlestascItnio,Cnlsa.ssVRolo. o3m8, V1e9n3t2i.lapti.on4,63b)y. vj& At * * Research Report No. 1017--Air Supply to Classrooms in Relation to Vent Flue Openings, C. Houghten, Carl Gutberlet and M. F. Lichtenfels (A.S.H1V.E. Transactions. Vol. 41,1935, p. 279).