Document b56Yr3Eeex3aXrNX7VMGJjKGy

534- CHAPTER 24 : " 1950'Guide In the selection 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 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 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, Distance op Blow and Decibel Ratings fob Various. Types op UnIt Heaters Type op Unit Heater Outlet Velocities FPM 1500-2500 400-1000 1200-2200 Distances op Blow--Ft* 20-200 30-100 70 Decibel Rating 34rOO 28-84 50-82 * Refer to manufacturers' tables^or limits of blow and decibel ratings. Values In this table are maxima and do not apply to all makes of units. 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 code* 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 barometric 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 standard 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 heatingelements combined with fan and motor, together with a suitable casing. Electric unit heaters Unit Heaters and Unit Ventilators 535 ...Table 2. Constants fob Determining the Capacitt of UnitHeaters fob Various Steam Pressures and .Temperatures of Entering Air ; (Based on Steam Pressure of t psig and Entering Air Temperature of 60 F) .. .SURE PSI 0 3 ; -s . 04* Jx .H 10 15 W: . 20 80 ' m3 - 40 50 ..H 60 & Q 70- 75 80 90 100 -10" 1.54 1.69 1.64 1.73 1.80 1.86 1.97 2.08 2.13 . 2.20 2.28 2.28 2.31 2.36 2.41 Temperature op Entering Aib 0 10 i -20* 1.45 1.871.50 1.41 1.55 -1.46 1.64 1.55 i:7f. 1.81 1:27 1.32 1.37 1.46 1.53 1.77 1.87' 1.96 2.04 2.09 1.68 1.78 1.86 1.94. 2.00 1:58 1.68 1.77 1.85 1.90 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 30 40*.. 50" . 60* . 70 . 80 90* ' 100* 1.19; - 1.11 1.24 1.16 1.29, 1.21 1.38 1.29 1.44 1:34 1.03 .0.98 1.08 - 1.00 1.13 1.05 t.21. 1.13 1.28 1.19 0.88 0.93 0.97 1.06 1:12' 0.81 0:74: 0.85 0.78. 0.90 0.83'* 0.98, 0.91 ' 1.04 ' 0.97- 0.67;> 0.71., 0.76 0.84 0.90 ; 1.50 1.42 1.33 1.25; 1.17 1.10' 1.08 0.9S l 1.60 1.61 1.43 1.35 1.27 ` T;19- "1.12 i.04.:l 1.68 1.60 1.51 1.43 1.35 1.27. v1.19,, - 1.13-J* 1.76 1.67 1.68 1.50- 1.42 1.34 1.26. 1.19 : 1.81 1.73 1.64. 1.56. 1.47 1.39 1.31 1.24 ^ 1.87 1.78 1.70 1.81 1.90 1.81 1.72 1.64 1.93 1.84 1.75 1.66 1.97 1.88 1.79 1.71 2.02- 1.93 -1.84* 4.75 1.53 1.45 . 1.37 1.55 1.47 1:40 1.32.. 1.58 ; 1.50 1.42 ` 1.34:1 1.62 1.64 1.46 1.38 1.66 1.58 1.50- -1.42 v ' T H W WH 0 .2 5 10 15 20 80 . 40 . 50 60. 1.48 '1.52 1.57 1.64 1.69 1.73 t;so . 1.86 1.93 1.97 < 70 2.02 . P5 75 . 2.04 80 2.06 90 2; 10 100 2.15 1.41 -1.44. 1.49 1.56 1.61 1.65 1.73 1.79 1.85 i.90 1.94 1.97 1.992.03 2.07 1.33 1.361.41 1.48 1.53 1.57 1.65 1.71 1.77 1.82 1.87 1.90 1.91 1.95 1.99 1.25 1.29 1.33 1.40 1.46 1.50 1.57 1.64 1.70. 1.75 1.80 i:82 1.84 1.88 1.92 1.18 1.22 1.28 1.33 1.38 1.11 i 1.14 1.19 1.25 1.31 1.42 1.50 1.56 1.63 1.67 . 1.35. 1.42 1.49 1.55 T.60. 1.72 1.65 1.75 1.68 1.77 1.70 1.80 1.73 1.85 - 1.77 1.03 1.07 1.11 1.18 1.24 0.96 1.00 1.05 1.11 1.17 0.89 0.93 0.98 1.04 1.10 0.82 0.75 0.69 0.86 0:79' 0.73 P;91;. 0.84 * 0.77-: 0.97 0.90 0.84 1.03 ..0,98 - . 0.90 - 1.28 1.21 1.14 1.07 1.00 0.94 1.35 1.28 f.21 1.15 ` 1.08 .1.01 1.42 1:35 1.28> * 1.22 1.15 1.08 > 1.48 1.42 ,1.35; . 1.28 . 1.21 1.15 ^ 1.63- 1.46 1.40 1.33 1-28. 1.58 ,1.51 1.61 1.54 1.63- 1.66 1.66. 1.59 1.70 1.63 .1.44 1.47 -1.49 1.52 .1.66- 1-.88. 1.40' 1.42 1.46 1:49 1.31 1.33. 1.35 1.39 1.45 * 1.24 1.27 1.29 1.32.1.36 Note: To'determine capacity at any steam pressure and entering temperature, multiply constant from ,table,by rated capacity at 60 F entering air and 2 psi. : 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 sat& , ' factory. are made in the built-in-wall model, suspension model, mid 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 have, an element in the pilot flame which will automatically close the gas valve on pilot failure. . ! ` -. '. Oil-Fired. - The oil-fired unit heater is usually equippe&w&h.^.centrifugal fan;.andcanbe obtained in sizes ranging from-125,000 to. 1,650,000 .Btu per hr oUtput capacity- in standard units.'' It is furnished in either tlie floor-mounted or in smaller sizes in the suspended type.