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546 CHAPTER 26 1949 Guide fan draws air through, arid (6) the blow-through type, in which the fan blows air through the heater. Unit heaters are available in any combination of the three preceding general classifications. For example, the steam or hot water type may be secured with either the propeller or ceritrifiigal 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 the top and bottom respectively or on the same side of the unit. Some units are supported by the piping and soine, have independent supports. The heating surface of. steam or water type units is generally made up of a non-ferrous tube-apd-fin assem bly ; or it may be fabricated of steel, or cast in steel. Application of Unit Heaters . Unit heaters, are used principally for heating commercial 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 thatthey operate in conjunction , with ventilators or fans for exhausting the moisture-laden air. See discussion of condensation, in Chapter 6- There are three major factors to.consider in the application of unit heat-' ers, namely: (1) location of, unit,..(2) air distribution, and (3) heating medium. . ., There are a variety of applications .which- aret favorable to the use of electric unit heaters. For supplemental heat in residence bath rooms, ;for .the heating of, ticket booths,, watchmen's. offices, factory, offices,. locker rooms and other isolated, rooms, scattered over- largeareas, their use is : peculiarly,adaptable-. They are particularly,useful in isolated and untended pumping stations or pits where they-pray 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 used. The installation cost of gas fired uriits is usually less than that of a type requiring that a new boiler be installed. .. Oil fired unit heaters are used in industrial plants, garages and. commercial buildings. . .. . Coal fired unit heaters are of finned, welded steel, or cast-iron construction and equipped with centrifugal blowers.- . They are usually stoker-fired to insure proper firing of fuel. They are used principally in large industrial i .plants such as foundries or assembly plants,-and provide a convenient source of heat, readily installed arid econoiriical in operation since all heat -given off |?y the surface of the heater remains within the space being heated. Outlet Velocities " Outlet velocities of unit heaters vary from about 400 to 2500 fpm depend ing upon the type of unit arid the distance to which'the air is'to be projected. Unit.Hesters, Unit Ventilators, Unit Humidifi.ers 547- Noise and drafts must be considered in the choice of air velocities, since both increase with increase of air velocity. Velocities and decibel ratings for the various types of unit heatefs,illus trated in Figs. 1, 2, 3 and .4 are'given in Table 1. (See Chapter 42 !for a discussion of decibel ratings.) :1 In the selectiori of unit heaters it:is important to ascertain that the throw is sufficient. The throw is dependent to a marked degree on the.temper ature of air leaving the heater as well as upon its velocity. See `discussion under heading of Inlet, Outlet, and Space Temperatures with Unit Heaters. Air Outlets ' In order to direct the air to points desired and to diffuse the air to avoid Table 1. Outlet Velocities, Distance of Blow and Decibel Ratings fob Various Types of Unit Heaters Type op Unit Heater ' Outlet FPMVelocities Centrifugal Fan........................ .......... Horizontal Propeller Fan..............-- Vertical Propeller Fan..... -------------- 1500-2500 400-1000 1200-2200 Distances of Blow--Ft 20-200 30-100 70 ' Decibel Rating 34-90 26-84 50-82 * Refer to manufacturers' tables for limits of'blow and decibel ratings. Values in this table are maxima and do not apply to all makes of units. drafts, unit heaters are commonly equipped with directional outlets, ad justable louvers or fixed types of diffusers. RATINGS OF UNIT HEATERS It is standard practice to rate unit heaters on the basis of the amount of heat delivered by the air in Btu per hour above an entering air temperature of 60 F. This applies to all types of unit heaters, the steain or hot water type, the electric type and the direct fired type; There are, however, other factors whichmust be taken into account, especially when an attempt is made to compare one type of heater with another. These are the tem perature of the heating element and the velocity of air through" it. Con sideration is given to these factors in the discussion of ratings for each type of unit heater in the following paragraphs. Ratings for Steam Type ' The; rating of steam type unit heaters has been standardized by a' code* in which the following items are the basis of rating: dry saturated steam at 2 psig pressure (29.92 in. Hg barometric pressure) at the heater.coil; air at 60 F entering the heater; arid heater operating free of external resistance to air flow.. - The capacity of a heater increases as the steam'pressure increases, and decreases as the entering air temperature increases. The heating capacity for any condition of steam pressure and entering air temperature pther.than standardmay be calculated approximately from-any given rating, by the. use of factors in Table 2 for the blow-through or draw-through types.