Eclipse TFB030 Manual page 8

Figure 3.4.
1. The required heat release per tube:
total required heat release
number of tubes
500,000 / 4 = 125,000 BTU/hr
2. Tube surface area for each tube:
OD x  x n x L = Tube Surface Area
4.5 x 3.142 x 2 x 75 = 2120.85 in
(n = 2 because it is a U-tube which has two legs)
3. From chart "Maximum Heat Transfer Rate", find the
maximum heat transfer rate:
• 60 BTU/in 2 /hr
4. The maximum permissible heat release (per tube) is:
tube surface area x maximum heat transfer rate
= Maximum heat release
2120.85 x 60 = 127,251 BTU/hr
5. This is sufficient, because only 125,000 BTU/hr is
required.
6. From Table 3.1 "Efficiency", find the efficiency with a
recuperator at 1650°F:
• 64%
7. The gross burner input (per tube) is:
required heat release
efficiency
(125,000 / 64) x 100 = 195,312 BTU/hr
Size the system for 200,000 BTU/hr per burner.
8. Compare the result from step 7 to the required
maximum inputs in Table 3.2. Gross input is less than
300,000 BTU/hr, therefore, the 4" w.c. tube can be
used.
Air Tube Length
The air tube length varies based on the location of the hot
face of the furnace relative to the mounting flange of the
burner.
8
75"
4.5"
= Required heat release per tube
2
x 100 = Gross burner input
Figure 3.5. Air Tube Length
The end of the air tube  must be within ± 0.5" of the face
of the furnace wall .
You choose the length closest to your requirements. You
can find the air tube lengths (dimension B) that are
available in the appropriate Datasheet 310-1 (TFB030),
310-2 (TFB075), or 310-3 (TFB200).
Step 1b: Immersion Tube Burner Application
Determine the net heat release required to the tank
The net heat release to the tank is derived from heat
balance calculations. These calculations are based on the
heat-up and steady-state requirements of the process,
and take into account surface losses, tank wall losses and
tank heat storage. Detailed guidelines for heat balance
calculations are in the Eclipse Combustion Engineering
Guide (EFE 825).
Determine the efficiency
The efficiency of the tube is directly linked to the effective
tube length. The diameter of the tube has no influence on
the efficiency. The efficiency of the tube is the factor
between the burner input to the tube and net output to the
tank. At a given burner input, the net output to the tank is
higher for a longer tube than for a relatively short tube.
NOTE: A commonly used efficiency is 70%. Efficiencies
greater than 85% will produce condensation in the tube
which may shorten tube life or disrupt the system.
Figure 3.6 below shows the relationship between the tube
length and the efficiency.
Eclipse TFB, V2, Design Guide 310, 11/14/2014
L
B=L ± 0.5"