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Heating Ventilating Air Conditioning Guide 1938 leave the entire floor and wall area free for commercial use. Partial or complete automatic control also may be secured on appliances of this type. This type of heater is often used for temporary heat during building construction or where the installation of a steam or hot water plant is for some reason not justified. For permanent installations, it is usually advisable to provide an exhaust duct from the gas-fired unit heaters to remove products of combustion from the occupied space. While this is not necessary in large open industrial plants, in smaller closed rooms, it becomes essential. Turbine Driven Where high pressure steam is available it is sometimes used to drive a steam turbine direct-connected to the unit heater. The exhaust from this turbine, reduced in pressure, is then passed into the heating coil where it is condensed and returned to the boiler. INDUSTRIAL USES In addition to their prime function of heating buildings, unit heaters may be adapted to a number of industrial processes, such as drying and curing, with which the use of heated air in rapid circulation with uniform distribution is of particular advantage. They may be used for moisture absorption, such as fog removal in dye-houses, or for the pre vention of condensation on ceilings or other cold surfaces of buildings in which process moisture is given off. When such conditions are severe, it is necessary that the heaters draw air from outside in enough volume to provide a rapid air change and that they operate in conjunction with ventilators or fans for exhausting the moisture-laden air. (See discussion of condensation in Chapter 7.) UNIT VENTILATORS 5 Unit ventilators while designed primarily for ventilation must incor porate controlled heating. A typical unit ventilator is illustrated in Fig. 6. They usually consist of a semi-decorative cabinet containing the following necessary or optional parts: v 1. Outside air inlet. 2. Inlet damper for closing the opening to the outside air inlet when the unit is not in use. 3. Adhesive or dry type filters for cleaning the air (optional). 4. A heating element usually of special design and intended for low pressure steam. 5. Motor and fan assembly. 6. Mixing chamber where warm and cold air streams are brought together. (No mixing chamber is normally provided where sectional type convectors are used.) 7. Outdoor air inlet and recirculating air mixing damper (optional). 8. Discharge grille or diffuser. 9. Temperature control arrangement. A roof ventilator ia sometimes termed a unit ventilator. For information on roof ventilators, see Chapter 36. ' 444 Chapter 23. Unit Heaters, Ventilators, Air Cqndit10ning, Cooling Units The primary functions of a unit ventilator are: 1, To supply a given quantity of outdoor air for ventilation or to .mix indoor and outdoor air. (See A.S.H.V.E. Ventilation Standards, Chapter 45). 2. To warm the air to approximately the room temperature if the unit is intended for ventilation only, or to a higher temperature if it is intended to take care of all or a part of the heat transmission losses from the room. 3. To control the temperature of the air delivered so as to prevent both cold drafts and overheating. (See Chapter 37). wit4h.ouTto ddrealfitvse. r air to the room in such a manner that proper distribution is obtained 5. To recirculate room air for the purpose of heating or promoting comfort when ventilation is unnecessary. (Ordinances should be consulted). 6. To perform all its functions without objectionable noise. 7. To clean the air properly. Fig. 6. Typical Unit Ventilator Showing One of Many " Arrangements of Dampers and Heating Coils SPLIT AND COMBINED SYSTEMS In a split system the unit is used primarily for ventilation. Air is delivered to the room at very near the room temperature, and enough separate direct heaters are placed in the room to warm.it to the desired temperature, independently of the unit. Their principal advantage lies in offsetting the cooling effect of window and wall surfaces long before these can be heated to room temperature and in retaining heat for this purpose after the ventilation is shut down. Where the unit ventilator selected has a capacity more than sufficient to warm the air needed , to meet the ventilating requirements, a cor responding reduction may be made in the amount of direct heating surface 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, 445