Eclipse TFB030 Manual page 7

The maximum heat transfer rate of a tube depends on the
temperature of the chamber and how the tube is mounted
inside the furnace or not enclosed.
An enclosed tube has a lower maximum heat transfer rate
than a tube which is tree to radiate in all directions.
Figure 3.2.
Recommended Flux
70
60
50
40
Tube enclosed
on 3 sides
30
1500 1600
Furnace Temperature, ºF
Figure 3.3.
Calculate the Maximum Heat Release
Multiply the previously calculated tube surface area by the
maximum heat transfer rate:
Maximum heat release = tube surface area x
maximum heat transfer rate
Compare the heat releases
Compare the required heat release with the maximum
heat release.
If the required heat release is greater than the maximum
heat release, then the number or the size of the radiant
tubes must be increased.
■ Exceeding the maximum
significantly shorten the tube life.
Eclipse TFB, V2, Design Guide 310, 11/14/2014
Not enclosed tube
Enclosed tube
Tube free to radiate
on 3 sides
1800
1700
CAUTION
heat release
Determine Efficiency
Decide whether or not you want to use a recuperator. A
recuperator is a heat exchanger which uses heat from the
exhaust to pre-heat the combustion air. The effect of a
recuperator on the efficiency of the system can be
significant, as shown in the table below.
Table 3.1 Estimated Gross Efficiency*
Furnace Chamber
Temperature
1000°F (538°C)
1300°F (704°C)
1550°F (843°C)
1650°F (899°C)
1750°F (954°C)
1850°F (1010°C)
*Actual efficiency will vary depending on gas type,
recuperator, excess air, piping losses, etc.
Calculate the Gross Burner Input
Calculate the gross burner input (BTU/hr) with this
formula:
Gross Burner Input = Required Heat Release / Efficiency
Compare the Gross Burner Input
Compare the gross burner input with the maximum tube
input. If the gross burner input is greater than the
maximum tube input from the table, below, then the size
of the radiant tube must be increased.
1900
Table 3.2 Maximum Tube Input
Tube ID (inches)
4
5
6
8
10
12
Exceeding these inputs may result in burner pulsation or
other operational problems.
Sizing Example
Application parameter
• 4 U-tube - 4.5" OD x 75" effective length
• 500,000 BTU/hr total required heat release -
recuperated
• 1650°F chamber temperature
• open radiate angles (not enclosed tubes)
will
Without With
Recuperator Recuperator
(Ambient Air) (Preheated Air)
57% 71%
51% 68%
47% 65%
44% 64%
41% 63%
39% 62%
Maximum Input (1000 BTU/hr)
300
600
900
1500
2500
3500
7