Document 6G6Ja4LQqMK4oYMr4VoRGkd3

558 CHAPTER 24 1952 Guide /^ -since both increase with increase of air velocity. Velocities and blow dis tances for the various types of unit heaters illustrated in Figs. 1, 2, 3 and 4 are given in Table 1. In the selection of unit heaters it is.important to ascertain that the blow is sufficient. The blow-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 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 horn- above an entering air temperature 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 tem- Table 1. Outlet. Velocities and Distance of Blow fob Various Types of Unit Heaters Ttpb or Unit Hbateb Outlet Velocities FPM Distances or Blow--Ft* 1500-2500 400-1000 1200-2200 20-200 30-100 70 * Refer to manufacturers' tables-for limits of blow. 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. . Steam. Rating of steam unit heaters has been standardized by a code2 in which the following items are the basis of rating: dry saturated steam at 2 psig pressure at the heater coil; air at 60 F (29.92 in. Hg barometric pressure) entering the heater; and heater operating free of .external resist ance 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 draw-through types. .. Hot Water. . A standard for the rating of hot water type unit heaters has also been established by code' in which the following items are the basis of rating: entering water at 200 F; entering air at 60 F (29.92 in. Hg baro metric pressure); and heater operating free of external resistance to air flow. This code also prescribes a method of translating the output in Btu and the temperature rise as obtained under test conditions to stand ard conditions of air and water temperature. Electric. Electric type unit heaters are available in sizes up to at least 60 kw capacity. They consist of resistance type heating elements combined with fan and motor, together with a suitable casing. Electric unit heaters Unit Heaters and Unit Ventilators 559 Table 2.- Constants fob.Determining.the Capacity of Unit. Heaters.for; . Various Steam Pressures and Temperatures of Entering Air " (Based on. Steam Pressure of % psig and Entering Air Temperature of 60 F) Steam- Pbes- BT7BE PSIO -10* 0 Tempebatube or Entering Aib 10" . . 20 30" 40* 50* .60* 70* 80* 90* 100* W ' c>u* B O o04 H Se s ' .o 2 5 10 15 1.54 1.59 1.64 1.73 1.80 1.45 1.50 1.55 1.64 1.71 1.37 1.41 1.46 1.55 1.61 1.27 1.32 1.37 1.46 1.53 1.19 1.24 1.29 1.38 1.44 1.11 1.16 1.21 1.29 1.34- 1.03 1.08 1:13 1.21 1.28 0.96 1.00 1.05 1.13 1.19 0.88 0.93 0.97 1.06 1.12 0.81 0.85 0.90 0.98 1.04 0.74 0.78 0.831 0.91 0.97 0.67., 0.71 0.76 0.84 0.90 20 1.86 1.77 1.68 1.58: 1.50 1.42 1.33 1.25 i.17 1.10 1.02 0.95 30 1.97 1.87 1.78 1.68 1.60 1.51 1.43 1.35 1.27 1.19 1.12 1.04 -40 . 2.06 1.96 1.86 1.77' 1.68 1.60 1.51 1.43 1.85 1.27 1.19 1.12 50 2.13 2.04 1.94 1.85 1.76 1.67 1.58 1.50 1.42 1.34 1.26 1.19 - 60 2.20 2.09 2.00 1.90 1.81 1.73 1.64 1.56 1.47 1.39 1.31 1.24 70 . 75 80 00 100 2.26 2.28 2.31 2.36 2.41 2.16 2.18 2.21 2.26 2.31 2.06 2.09 2.11 2.16 2.20 1.96 1.99 2.02 2<06 2.11 1.87 1.90 1.93 1.97 2.02 1.78 1.81 1.84 1.88 1.93 1.70 1.72 1.75 1.79 1.84 1.61 1.64 1.66 i:71. 1.75 1.53 1.45 1.37 1.55 1.47: 1.40 1.58. 1.50. 1.42 1.62 1.54 1.46 1.66 1.58 i:50 1.29 1.32 1.34 1.38 1.42 0 1.48 1.41 1.33 1.25 1.18 1.11 1.03 0.96 0.89 0.82,, 0.75 0.69 2 1.52 1.44 1.36 1.29 1.22 1.14 1.07 1.00 0.93 0.86 0.79 0.73 5 1.57 1.49 1.41 1.33 1.26 1.19 1.11 1.05 0.98 0.91 0.84 0.77 w 10 1.64 1.56 1.48 1.40 .1.33 1.25 1.18 1.11 1.04 0.97. 0.90 0.84 >* 15 1.69 1.61 1.53 1.46 1.38 1.31 1.24 1.17 1.10 1.03 0.96 0.90 o-S3 20 1.73 1.65 1.57 1.50 1.42 1.35 1.28 1.21 1.14 1.07 1:00 0.94 30 1.80 1.73 1.65 1.57 1.50 1.42 1.35. 1.28 1.21 1.15 1.08 1.01 o 40 1.86 1.79 1.71 1.64 1.56 1.49 1.42 1.35 1.28 1.22 1.15 1.08 04 50 1.93 1.85 1.77 1.70 1.63 1.55 1.48 1.42 1.35' 1.28 1.21 1.15 H 60 1.97 1.90 1.82 1.75 1.67 1.60 1.53 1.46 1.40 1.33 1.26 1.19 t 70 2.02 1.94 1.87 1.80 1.72 1.65 1.58 1.51 1.44 1.38 1.31 1.2A O . 75 2.04 1.97 1.90 1.82 1.75 1.68 .1.61 1.54 1.47 1.40 1.33 1.27 80 2.06 1.99 1.91 1.84 1:77 1.70 1.63 1.56 1.49 1.42 1135 1.29 80 2.10 2.03 1.95 1.88 1.80 1.73 1.66 1.59 1.52- 1.46 1.39 1.32 100 2.15 2.07 1.99 1.92 1.85 1.77 1.70 1.63 1.66 1.49 1.43 1.36 Note: To determine capacity at any steam pressure and entering temperature, multiply constant from table by rated capacity at GO F entering air and 2 peig. When increasing steam pressure it is important to determine whether the heater is suitable for the' .Increased pressure application, and whether the resulting increased outlet temperature is satis factory. are made in the built-in-wall model, suspension model, and free-standing or portable model. Electric unit heaters are rated on the energy input to the heater, expressed in terms of kilowatts, Btu or EDR. Quite often all three ratings are given in parallel columns in the catalogs. Gas-Fired. Gas-fired unit heaters are built in both suspended and floor models, with either propeller or centrifugal type fans. They are available in a wide range of sizes from about 24,000 to over 4,000,000 Btu per hr ca pacity, and are usually rated in terms of both, input and output according to the approval requirements of the American Standards Association. Any gas-fired unit which is thermostatically controlled and has a pilot must