Document DVgEd56kJZZGKKaxXvn0R3KO

496 `CHAPTER,26 1948 Guide fan draws air through, and (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 centrifugal-type of`.fans; arid in either thri'draw-through or blow-through type,. ;:' Unit heaters also-vary in other, minor, respects; , For example, steam and return inlets arid outlets may- be located on the top and bottom respectively or on the same side of the uhit. 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 tubeand-fin assembly; or it may be fabricated of steel,.or,cast in steel. - Application of-Unit .Hesters J ./-.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 circu lation with uniform distribution is of partidular advantage., They may, be used for moisture absorption', such as fog removal in dye houses, or for the prevention of condensation on ceilings or other cold surfaces of buildings in which process moisture is released. When such coriditioris are severe, it is necessary that the unit heaters draw1 air from outside in enough Volume to provide a rapid air change arid that they operate in conjunction with ventilators or fans for exhaustirig the moisture-laden-air. See dis cussion of condensation in Chapter 6. There are three major factors to consider in the application:of unit heaters, namely: (1) location of .-unit, .-(2) air distribution, and (3) heating medium. . -.' There are a variety of applications which are favorable to; the use-of electric unit heaters. For supplemental heat in residence bathrooms, for the heating of ticket booths, watchmen's offices, factory offices, locker rooms and other isolated rooms scattered over]large areas, their.use is peculiarly adaptable. 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 used. The installation cost of gas fired units 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 com mercial buildings. - ...............- . > ; `Coal fired unit heaters are of finned, welded steel, or cast-iron construc tion and equipped with centrifugal blowers: They are usually 'stoker- fired to insure proper firing of fuel. They are used principally in large industrial plants such as foundries or. assembly plants, and provide a very economical source of heat, both from the installation and the operating standpoint. Outlet Velocities . Outlet velocities of unit heaters vary from about'400 to 2500 fpm depending upon the type of unit and the distance to which the air is to be Unit Heaters, Unit Ventilators, Unit Humidifiers 497 projected. 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 heaters illustrated in Figs. 1, 2, 3 and 4 are given in Table 1. (See Chapter 42 for a discussion of decibel ratings). In the selection of unit heaters; it'is important to ascertain that the throw insufficient. Thri throw is dependent to a marked degree .on the temperature 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 -" -r'--':-' 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 foe Various Types of Unit Heaters Type of Unit Heater Outlet Velocities PPM Centrifugal Fan______ _____ - Horizontal Propeller Fan___________ Vertical Propeller Fan....... * 1500-2500 400-1000 1200-2200 Distances op Blow--Fr 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. 'v drafts, unit heaters are commonly equipped with directional, outlets, adjustable 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 tem perature of 60 F. This applies to all. types of unit heaters, the steam or hot water type, the electric type and the direct fired type. There are, however, other, factors, which, must -be taken .into account, especially when an attempt is made to compare one type of heater with another. These are the temperature of the heating element and the velocity of air through it. Consideration 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 striata 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; and 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 other than standard may be calculated approximately from any given rating by the use of factors in Table 2 for the blow-through or drqw-through types of units.