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9FIN-PRC004-EN
Effect of Temperature Drop On
Fin-Tube Ratings
The effect of temperature drop on heat
output of a wall fin element can be
readily determined. Select the heating
element using average water
temperature (entering water
temperature minus
1
/2 water
temperature drop) and correct for water
velocity as follows:
Selection
Procedure
By use of the relationship,
GPM =
BTU
*500 x Water Temperature Drop
the waterflow rate for a wall fin unit of
the required Btu and at a definite
temperature drop, is easily calculated.
(Many engineers use gallons per minute
as the basis for selecting pipe sizes, since
charts are set up on this basis in the
ASHRAE Guide. Therefore, this would
not be an extra step.)
The heat output of a given wall fin unit
will vary somewhat with different water
velocities, all other conditions being
equal. From Chart S-1, the water velocity
can be found for a wall fin element of the
particular tube size and gpm waterflow
required. The velocity correction factor
can be determined by using Chart S-1.
Table S-2 — Wall Fin Rating Correction Factors*
Entering Air Temperature
Steam Press. Steam Temp. 45°F55°F65°F70°F75°F80°F85°F90°F 100°F 110°F 120°F 130°F 140°F 150°F
PSIG kPa (F) (c) 7°C13°C 18C 21°C24°C27°C29°C32°C38°C43°C49°C54°C60°C66°C
0 0 212.0 100.0 1.19 1.09 0.97 0.92 0.87 0.82 0.77 0.70 0.63 0.54 0.46 0.38 0.31 0.25
.899 6.2 215.0 101.7 1.22 1.11 1.00 0.95 0.90 0.84 0.80 0.75 0.65 0.57 0.48 0.40 0.33 0.26
5 34.5 227.1 108.4 1.34 1.22 1.11 1.05 1.00 0.95 0.90 0.81 0.75 0.66 0.57 0.49 0.41 0.34
10 69.0 239.4 115.2 1.45 1.33 1.22 1.17 1.11 1.05 1.00 0.91 0.85 0.75 0.66 0.58 0.50 0.42
20 137.9 258.8 126.0 1.63 1.52 1.40 1.33 1.28 1.23 1.17 1.07 1.02 0.92 0.82 0.73 0.64 0.55
30 206.9 274.0 134.4 1.78 1.66 1.54 1.48 1.42 1.37 1.31 1.21 1.15 1.05 0.95 0.85 0.76 0.68
40 275.8 286.7 141.5 1.91 1.79 1.66 1.61 1.54 1.49 1.43 1.32 1.27 1.16 1.06 0.97 0.87 0.78
50 344.8 297.7 147.6 2.02 1.90 1.77 1.71 1.65 1.60 1.54 1.42 1.37 1.26 1.16 1.06 0.96 0.87
100 689.5 337.9 169.9 2.43 2.31 2.18 2.11 2.05 2.00 1.94 1.81 1.77 1.65 1.54 1.44 1.33 1.23
*For steam pressures and air temperatures other than 1 psi and 65°F (6.895 kPa and 18.3°C). For process applications, deduct heating effect after applying above factor to 1 psi, 65°F (6.895 kPa,
18.3°C air) air rating for desired element arrangement.
Table S-1 — Pipe Water Capacities and Quantities Circulated at Velocity of 3* Feet Per Second (.91 m/s)
Nominal Expanded Gals. Per Liters Per Gals./Min. @ 3’ Liters per Sec. Lbs./Hr. @ 3’ Kg/S @ .91 m
Pipe Size ID Linear Ft. Meter Sec. Vel.* @ .91 M/Sec. Vel. Sec. Vel.* Sel. Vel.
3
/4” CA (19 mm) 0.836” (21 mm) .029 .36 5.13 .324 2,555 .322
1” CA (25 mm) 1.073” (27 mm) .047 .58 8.45 .533 4,215 .531
1
1
/4” CA (32 mm) 1.311” (33 mm) .070 .87 12.6 .795 6,285 .792
1
1
/4” ST (32 mm) 1.40” (36 mm) .080 .99 14.39 .908 7,170 .903
*3 Ft./Sec. (.91 m/Sec) Velocity is Basis for Hot Water Rating Factors Shown in Chart S-1.
Velocity Ft./Sec. =
Lbs. Per Hour
(Gals. Per Ft.) (3600) (8.3)
For quiet operation, a maximum of 5 ft/sec (1.5 m/Sec) velocity is recommended.