Document DvgBGM4o2oEG8Om6VeZkaXqp4

CHAPTER 25 1955 Guide Unit Heaters and Unit Ventilators 589 588 eral classifications. For example, the steam or hot .water type may be secured with either the propeller or centrifugal type of fans, and in either the draw-through or blow-through type. Unit heaters also vary in other minor respects. For example, steam and return inlets and outlets may be located on top and bottom, respectively, or on the same side of the unit. Some units are supported by the piping and some have independent supports. The heating surface of steam or water type units is generally made up of a non-ferrous tube-and-fin assem bly; or it may be fabricated of steel, or cast in steel or iron. Application of Unit Heaters There are three major factors to consider in the application of unit heat ers, namely: (1) heating medium, (2) air distribution, and (3) location of unit. Steam or hot water unit heaters are used principally for heating com mercial and industrial structures such as garages, factories, laboratories, and stores. They may also be used for heating finished rooms, if properly applied and concealed, and if some consideration is given-to the problem of noise. Unit heaters may also be adapted to a number of industrial processes, such as drying and curing, in 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 released. When such conditions are severe, it is necessary that the unit 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 10.) Electric unit healers can be used to advantage for supplemental heat in residences, for the heating of ticket booths, watchmen's offices, factory offices, locker rooms, and other isolated rooms scattered over large areas. They are particularly useful in isolated and untended pumping stations or pits where they may be thermostatically controlled to prevent freezing temperatures. Gas-fired unit heaters find application in industrial plants, offices, stores, garages; in fact, in almost every location where steam-type units are usedThe installation cost of gas-fired units is usually less than that of a type requiring that a new boiler be installed, unless the number to be installed would justify the cost of a boiler and steam or hot-water unit heater system. Oil-fired unit healers are used in industrial plants, garages and commercial buildings. Coal-fired unit heaters are used principally in large industrial plants such as foundries. Outlet Velocities . Outlet velocities of unit heaters vary from about 400 to 2500 fpm, dependmg upon the type of unit and the distance (blow) to which the air to is be Projected. Noise and drafts must be considered in the choice of air velocile> both generally increase with increase of air velocity. in the selection of unit heaters it is important to ascertain that the blow and coverage are sufficient. The blow is dependent to a marked degree on