Document qmQgyxwapmbmk44DJjk9R2xaK

598 CHAPTER 25- 1956 Guide Table 1. Constants fob Determining the Capacity of, Unit Heaters fob Various Steam Pressures and Temperatures of Entering Air (Baud on Steam Pressure of t psig and Entering Air Temperature of 60 F) Steam Preb- 8UHE PBJG -10* 0* Temperature op Entering Aib 10* 20 30 40* 50* 60*- 70" 80* 90* 100* W 'H a o a o a Eh ,. 0 1.54 1.45 1.37 1.27 1.19 1.11 1.03 0.96 6.88 0.81 0.74 0.67 2 1.59 ' 1.50 1.41 1.32 1.24 1.16 1.08 1.00 0.93 0.85 '0.78 0.71 5 1.64 1.55 1.46 1.37 1.29 1.21 1.13 1.05 0.97 0.90 0.83 0.76 10 1.73 1.64 1.55 1.46 1.38 1.29 1.21 1.13 1.06 0.98 0.91 0:84 15 1.80 1.71 1.61 1.53 1.44 1.34 1.28 1.19 1.12 1.04 0.97 0.90 20 1.86 1.77 1.68 1.58 1.50 1.42 1.33 1.25 1.17 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 1.35 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 : 2.26 2.16 2.06 1.96 1.87 1.78 -1.70 1.61 1.53 1.45 1.37 1.29 75 2.28 2.18 2.09 1.99 1.90 1.81 1.72 1.64 1.55 1.47 1.40 1.32 80 2.31 2.21 2.11 2.02 1.93 1.84 . 1.75 1.66 1.58 1.50 1.42 1.34 90 2.36 2.26 2.16 2.06 1.97 1.88 1.79 1.71 1.62 1.54 1.48 1.38 100 2.41 2.31 2.20 2.11 2:02 1.93 1.84 1.75 1.66 1.58 1.50 1.42 125 2.51 2.41 2.31 2.21 2.11 2.02 1.93 1.84 1.76 1.68 1.59 1.51 150 2.60 2.50 2.40 2.30 2.20 2.11 2.02 1.93 1.84 1.76 1.67 1.59 W >* ' H & O w Eh S' Q 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 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 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 i.01 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 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 60 1.97 1.90 1.82 1.75 1.67 1.60 1.53 1.46 1.40 1.38 1.26 K19 70 2.02 1.94 1.87 1.80 1.72 1.65 1.58 1.51 1.44 1.38 1.31 1.24 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 1.35 1.29 90 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 1.56 1.49 1.43 1.36 125 2.21 2.14 2.06 1.99 1.91 1.84 1.77 1.70 1.63 1.56 1.49 1.43 150 2.28 2.20 2.13 2.05 1.98 1.91 1.84 1.77 1.70 1.63 1.56 L50 Note: To determine capacity at any steam pressure and entering temperature, multiply rated capacity at GO F entering air and 2 psig by constant from tabce. 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. 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 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 Unit Heaters and Unit Ventilators 599 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 1 for the blow-through or draw-through types: Hoi Water. A standard for the rating of hot water type unit heaters has also been established by code8 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 are rated on the energy input to the heater, expressed in terms of kilowatts, Btuh or EDR--(EquivaIent Direct Radiation). Gas-Fired. Gas-fired unit heaters are built in both suspended and floor models, with either propeller or centrifugal type fans. They are rated in terms of both input and output in accordance with the approval require ments of the American Gas Association. Oil-Fired. The oil-fired unit heater is usually equipped with a centrif ugal fan or fans and is available in either the floor-mounted or in smaller sizes in the suspended type: Ratings are based on Heat delivered at heater outlet in Btu per hour. Stoker-Fired. The stoker-fired unit heater can be obtained in a wide range of capacities. Ratings are based on Heat delivered at the heater outlet in Btu per hour. Effect of Resistance Upon Capacity ; Unit heaters are customarily rated as free delivery type units. If out side air intakes, air filters, or ducts on the inlet or discharge are used, a reduction in air and heating capacity will result because of this added resistance to air flow. The percentage of this reduction in capacity will depend upon the characteristics of the heater, and on the type, design, and speed of the fans, so that no specific percentage reduction can be assigned for all heaters at a given added resistance. The heat output to be expected under other than free delivery conditions should be secured from the manu facturer. Location of Unit Heaters Care should be taken in the location of unit heaters to insure free air circulation to the intake and proper heat distribution over the working level. Manufacturers' catalogs usually give suggestions for best arrange ments of the various heaters. Types and makes of available heaters are shown in the Catalog Data Section of this volume. , Hot blasts in working zones should have their discharge outlets above the head line, and suspended heaters should be so adjusted that the heated air stream will not be objectionable. In connection with the use of vertical type unit heaters, care must be