Document 3bkoopw8gwpeegZ5N3xrmmja

American Society of Heating and Ventilating Engineers Guide, 1936 equipped with fans which cause the air to pass over the heating surfaces at a fairly high velocity and then direct the warm air in to the space to-be heated. As is the case with the steam-fed unit heaters, the gas-fired appliances may be used for heating stores, shops, and warehouses. They usually are suspended in the space, to .be heated and in most instances 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 arid that they operate in conjunction with ventilators or fans-for exhausting the moisture-laden air. (See discussion of condensation in Chapter 7.) Information on the control of unit heaters will be found iri Chapter 14. ,r V ' UNIT VENTILATORS3 Unit veritilators while designed primarily for ventilation must incor porate controlled heating. Since these units are generally located in the occupied space, they must be pleasing in: design and must harmonize with the furniture or with the decorative scheme. A typical unit venti lator .is illustrated in Fig. 6, They usually consist of a semi-decorative cabinet containing the following necessary or optional parts: - 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). ' '*A. roof ventilator, is'sometimes termed a-unit ventilator. - For .information on roof'ventilators,'see Chapter4.; C- J'.'3 .'ii ' - 228 Chapter 12--Unit Heaters, Ventilators, Coolers, and Air Conditioners Fig. 6. Typical Unit Ventilator Showing One of Many Arrangements of Dampers and Heating Coils ... 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. Device for ozonizing air (optional). 9. Discharge grille or diffuser. 10. Temperature control arrangement. [.. . 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. . . 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 cate 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. 4. To deliver air to the room in such a manner that proper distribution is. obtained without drafts. 5. -To recirculate room air for the purpose of heating or promoting comfort when ventilation is unnecessary. 6. To perform all its functions without objectionable noise. In addition to these functions, unit ventilators frequeritly are arranged so that the air supplied may be cleaned by riieans of filters of either the dry or viscous type. If filters are used, the proper allowance must be made for the increased resistance offered to the air flow. Humidifiers iri unit veritilatbre are rather difficult to control and are only furnished upon special order. . V .^ir Supply for Ventilation. - The outdoor air supply for ventilation is delivered by motor-driven fans operated at comparatively low-speeds; the back of the cabinet being connected to the outside through .rust-proof 229 y