Document DMqdXbaaGx6MYomexdpJgn3ea
590
CHAPTER 25
1955 Guide
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.) Recommended coverage and mounting heights should be obtained from the manufacturer.
Air Outlets
In order to obtain the desired air distribution and heat diffusion, 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 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 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 1 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 code3 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 (Equivalent 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 wkl
range of capacities. Ratings are based on heat delivered at the beate outlet in Btu per hour.
Unit Heaters and Unit Ventilators
591
Table I- Constants for Determining the Capacity of Unit Heaters Various Steam Pressures and Temperatures of Entering Air
{Bated on Steam Pressure o} t psig and Entering Air Temperature of 60 F)
Steam Pbbb*
PS1Q
-10*
0*
ATempebatcbe op Entering ib 10 20 30 40 50* 60* 70* 80* 90* 100*
a iii
;* H
0 5
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
6.81 0.85 0.90 0.98 1.04
0.74 0.78 0.83 0.91 0.97
0.67 0.71
0.84 0.90
H
A O
a H o J A
20
1.86 1.97 2.06
2.20
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
1.50 1.60 1.68 1.76 1.81
1.42 1.51 1.60 1.67 1.73
1.33 1.43 1-51 1.58 1.64
1.25 1.35 1.43 1.50 1.56
1.17 1.27 1.35 1.42 1.47
1.10 1.19 1.27 1.34 1.39
1.02 1.12 1.19 1.26 1.31
0.95 1.12 1.24
70
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
l;64
1.66 1.71
1.75
1.53 1.55 1.58 1.62 1.66
1.45 1.47 1.50 1.54 1.58
1.37 1.40 1.42 1.46 1.50
1.29 1.34 1.42
125
2.51 2.60
2.41 2.50
2.31 2.40
2.21 2.30
2.11 2.20
2.02 1.93 2.11 2.02
1.84 1.93
1.76 1.84
1.68 1.76
1.59 1.67
1.59
a CL,
H
O D O os a
2; Q
__
0
20 40 i)U 70 75
- 125 150
1.48 1.52 1.57 1.64 1.69
1.41 1.44 1.49 1.56 1.61
1.33 1.36 1.41 1.48 1.53
1.25 1.29 1.33 1.40 1.46
1.18 1.22 1.26 1.33 1.38
1.11 1.14 1.19 1.25 1.31
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 0.86 0.91 0.97 1.03
0.75 0.79 0.84 0.90 0.96
0.69 0.73 0.77 0.84 0.90
1.73 1.80 1.86 1.93 1.97
1.65 3.73 1.79 1.85 1.90
1.57 1.65 1.71 1.77 3.82
1.50 1.57 1.64 1.70 1.75
1.42 1.50 1.56 1.63 1.67
1.35 1.42 1.49 1.55 1.60
1.28 1.35 1.42 1.48 1.53
1.2* 1.28 1.35 1.42 1.46
1.14 1.21 1.28 1.35 1.40
1.07 1.15 1.22 1.28 1.33
1.00 1.08 1.15 1.21 1.26
1.01 1.19
2.02 2.04 2.06 2.10 2.15
1.94 1.97 1.99 2.03 2.07
1.87 1.90 1.91 1.95 1.99
1.80 1.82 1.84 1.88 1.92
1.72 1.75 1.77 1.80 1.85
1.65 1.68 1.70 1.73 3.77
1.58 1.61 1.63 1.66 1.70
1.51 1.54 1.56 1.59 1.63
1.44 1.47 1.49 1.52 1.56
1.38 1.40 1.42 1.46 1.49
1.31 1.33 1.35 1.39 1.43
1.24 1.36
2.21 2.28
2.14 2.20
2.06 2.13
1.99 2.05
1.91 1.84 1.98 | *"
1.77 1.84
1.70 1.77
1.63 1.70
1.56 1.63
1.49 1.56
1.50
________ _
*Uy Bwaun pressure and entering temperature, multiply rated capacity
at 60 F entering air and 2 psig by constant irons table.
When increasing steam pressure it is important to determine whether the beater is suitahle for the
increased pressure application, and whether the resulting increased outlet temperature is satin' factory.
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 eduction 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