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GAS
FIRED
RADIANT
TUBE
HEATING
SYSTEMS
The high cost of energy has driven the
need for more efficient radiant tube
heating systems, which has led to new
radiant tube designs. With only minor
changes to furnace design, larger
diameter alloy and ceramic radiant tubes
can be used, and additional costs
for flame safety and heat recovery
are reasonable.
Joachim G. Wuenning
WS Thermal Processing Technology, Inc.
Elyria, OH
76
N
atural gas prices, after an extended period of stability,
have become extremely volatile in the last few years.
This is only one of the challenges that companies operating heat treating furnaces are now facing. Worldwide
competition is forcing companies to produce more product
with fewer people. There is pressure to increase furnace
uptime, while furnace operators and maintenance staff are
reduced. In addition, there are tightening emission requirements and as yet unknown future carbon dioxide tariffs.
These challenges are related to radiant tube heating systems. Heat treating furnaces with outdated technology
will not be competitive in the future. However, much effort has been put into the development of new radiant tube
designs, and there is also new technology on the market
that can be adapted for heat treating furnaces.
Radiant Tube Designs
Radiant tube systems can be categorized as recirculating
and non-recirculating. Although non-recirculating tubes
are still widely used, recirculating systems have become
increasingly popular because of their temperature uniformity, NOx performance, integration of waste heat recovery,
and easier sealing (see Figure 1).
Non recirculating tubes are often proportionally controlled. The burner design aims for a flame that is stretched
over the first tube leg. Heat exchangers can be plug-in
recuperators or attached types. The tube temperature uniformity of these designs is rather poor.
Recirculating tubes should be operated in an on/off
mode and with high velocity burners. P-tubes and doubleP-tubes provide good temperature uniformity, but they
also depend on even temperatures to keep the thermal
stress between the center and return leg low. The combination of a recirculating tube design and regenerative air
preheating design leads to an A-tube design. The regenerators are arranged in both legs of the tube, with flow directions changing every 10 seconds. Besides minimizing
exhaust gas losses, this design offers good temperature
uniformity and extremely low NOx-emissions.
Low NOx combustion
Low NOx burners are now in every burner manufacturer’s portfolio. Especially when the combustion air is
preheated, NOx emissions can be extremely high if no
measures are taken. There are several techniques to reduce
thermal NOx formation. These include:
• high velocity combustion
• air staging
• fuel staging
• external recirculation
• internal recirculation
Most low NOx burners incorporate one or more of these
techniques. Other methods, like re-burning and steam injection, are applied in large capacity firings like boilers and
gas turbines only.
Recirculation of exhaust gases has proven to be very effective. External recirculation can be retrofitted to U- and
W-tube systems. A drawback of external recirculation is
that the recirculated gases are passing through the heat
exchanger, thereby lowering efficiency or creating the need
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for a larger one.
Internal recirculation characterizes
recirculating radiant tube designs. A
special (proprietary) form of internal
recirculation leads to a special form
of combustion, called flameless oxidation, which we call Flox. In contrast
to the 20-40 percent recirculation rates
of some systems, recirculation rates
of well over 100 percent lead to drastic
reduction of NOx emissions, even for
very high air preheat. Flameless oxidation technology is a key in keeping
NOx in check, especially in regenerative systems.
Temperature Uniformity
Temperature uniformity is one of
the main goals in radiant tube development. This is not just for good
thermal uniformity in the furnace, but
also because it enhances tube life. Hot
spots will cause tube failures by
burning holes into the tube or by
causing high thermal stresses in localized areas. A radiant tube with
even temperature distribution can
provide more heat to the furnace and
has greater longevity in service.
Energy Efficiency
Waste gas heat is the largest single
loss in most heat treating furnaces.
Without waste heat recovery, more
than half of the energy provided to
the furnace is leaving it. Recuperative
or regenerative waste heat systems
can recover at least a part of the lost
heat. The most common form of
waste heat recovery is using recuperative heat exchangers for preheating
the combustion air.
U- and W-tubes can be equipped
or retrofitted with external, or plugin, recuperators. High NOx emissions
can arise from high air preheat temperatures, as can problems at sealing
surfaces and flanges. Hot air piping
requires thorough insulation to prevent heat losses and unpleasantly
high temperatures around the furnace. The integration of a counter
flow heat exchanger into the radiant
tube eliminates these problems. One
example of an integrated design is the
self recuperative burner. Recuperative burners are used in single ended,
P- and Double-P tubes.
Regenerative systems are expected
to become increasingly important, as
Non-recirculating
straight through
Recirculating
single ended
single ended
P-tube
U-tube
double-P
A-tube
W-tube
Figure 1 — Radiant tube concepts.
energy costs remain high (or increase), and as the systems mature in
design with operational experience.
The additional complexity of regenerative systems is counterbalanced
by outstanding fuel efficiency, temperature uniformity, and almost cold
exhaust gases.
Control Concepts
Figure 2 illustrates two control concepts. One system is proportionally
controlled and has a common gas and
air valve for the whole zone. Gas and
air are cross connected to keep the
fuel/air ratio constant. Dividing the
input to a larger number of tubes does
not add cost to the controls. The
system is simple, but performance is
often poor.
The other system shows single
ended radiant tubes. Each tube is
equipped with air and gas shut off
valves, ignition, and flame safety. The
burners are individually on/off controlled, often in combination with a
pulse firing system. The performance
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main gas shut-off valves
pressure regulator
exhaust
blower
of these systems is very high, but the
costs for controls increase considerably with a large number of tubes.
radiant tubes
gas shut-off
valve
ratio
regulator
radiant tubes
blower air control valve
Figure 2 — Radiant tube control concepts.
50”
50”
40”
side view
4-in. tube arrangement
front view
50”
55”
40”
side view
6-in. tube arrangement
front view
50”
60”
40”
side view
8-in. tube arrangement
Figure 3 — Model furnace cross-sections.
78
front view
Radiant Tube System Comparison
A model furnace is used to compare different radiant tube designs.
The chamber furnace has the inside
dimensions of 40 in. x 50 in. and a
width of 50 in. to 60 in. (see Figure 3).
The comparison includes nine different tube arrangements (Figure 4-1
to 4-9), which all provide the same
tube surface area, but with different
tube diameters. Allowing a net heat
release of 50 BTU/hr.-in.2 would result a total net heat input in to the furnace of 350,000 BTU/hr. Using ceramic radiant tubes with 100
BTU/hr.-in.2 would double that to
700,000 BTU/hr.
The arrangement in Figure 4-1 includes twelve straight four inch diameter through tubes. This configuration was popular in the past
because of its low cost and simplicity.
However, in most cases there is no
heat recovery, no flame safety, and
temperature uniformity and sealing
is often very poor. A similar system
consists of six U-tubes (Figure 4-2).
Using single ended radiant tubes
(Figure 4-3) adds heat recovery to the
system, but when flame safety must
be added, costs get high since control
valves and burner control units have
to be added to every tube.
Using a larger tube diameter and
other designs, the number of tubes
can be reduced. Using 6 in. tubes reduces the number of tubes to 4 (Figures 4-4 and 4-6) or 8 single ended radiant tubes (Figure 4-5).
The arrangements in Fig. 4-8 and
4-9 using double-P-tubes (also see
Figure 5) provide the same heating
surface with just two radiant tubes,
as compared to the arrangement in
Figure 4-1, which incorporates
twelve. This brings the cost of controls and heat recovery down and
makes flame safety very affordable.
The increase in furnace width is only
minor.
The use of ceramic radiant tubes is
becoming increasingly popular. The
most common ceramic tube material
is reaction bonded silicon carbide SiC.
This material has overcome the
thermal shock problems experienced
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4-in. tube arrangements
1)
6-in. tube arrangements
2)
3)
4)
8-in. tube arrangements
5)
6)
7)
8)
9)
Fig. 4 — Model furnace tube arrangements.
Table 1. — Alloy radiant tube configurations (available heat 350,000 BTU/hr.).
No.
1
2
qty.
Type
Eta %
tube dia.
inch
12
I
40
4
gross heatfuel cost based on $6/MMBTU
BTU/hr.
$/hr.
$/5,000hr.
$/MMBTU net
875,000
5.25
26,250
15.00
6
U
45
4
777,778
4.67
23,333
13.33
6
U
60
4
583,333
3.50
17,500
10.00
plug in recuperator
3
12
SER
75
4
466,667
2.80
14,000
8.00
self recuperative
burner
4
4
U
45
6
777,778
4.67
23,333
13.33
4
U
60
6
583,333
3.50
17,500
10.00
plug in recuperator
5
8
SER
75
6
466,667
2.80
14,000
8.00
self recuperative
burner
6
4
P
75
6
466,667
2.80
14,000
8.00
self recuperative
burner
7
6
SER
75
8
466,667
2.80
14,000
8.00
self recuperative
burner
8
2
PP
75
8
466,667
2.80
14,000
8.00
self recuperative
burner
9
2
PP
75
8
466,667
2.80
14,000
8.00
self recuperative
burner
Table 2. — Ceramic radiant tube configurations (available heat 700,000 BTU/hr.).
No.
1
fuel cost based on $6/MMBTU
qty.
Type
Eta %
tube dia.
inch
BTU/hr.
$/hr.
$/5,000hr.
$/MMBTU net
12
I
35
4
200,000
12.00
60,000
17.14
4
175,000
10.50
52,500
15.00
100,000
6.00
30,000
8.57
2
6
U
40
3
12
SER
70
4
self recuperative
burner
4
4
U
40
6
175,000
10.50
52,500
15.00
5
8
SER
70
6
100,000
6.00
30,000
8.57
self recuperative
burner
6
6
SER
70
8
100,000
6.00
30,000
8.57
self recuperative
burner
7
6
SER
70
8
100,000
6.00
30,000
8.57
self recuperative
burner
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ramic tubes. Fuel costs are calculated
per hour, per year (assuming 5,000
hrs. of operation) and per MMBTU/
hr. of net heat, provided to the furnace. Please consider that the data are
exemplary and could vary in certain
installations.
Figure 5 — A double-P radiant tube.
by ceramic materials in the past. In
the last ten to fifteen years, many
thousands of ceramic single ended radiant tubes have been installed
worldwide. The tubes can be operated in higher furnace temperatures
and, in many cases, with doubled
heat release.
The effect of fuel efficiency is very
substantial and summarized in Table
1 for alloy tubes and Table 2 for ce-
Conclusions
The need for more efficient, reliable radiant tube heating systems
has led to new radiant tube designs.
With only minor changes to furnace
design, larger diameter alloy and
ceramic radiant tubes could be used
and additional costs for flame safety
and heat recovery are reasonable.
These additional costs often can be
recovered by lower energy cost and
higher productivity in a very
short period.
Author Joachim G. Wuenning is president of WS Thermal Process Technology
Inc., Elyria, OH. For additional information on this article contact Lee Rabe, technical sales manager, WS Thermal at (440)
365-8029 or visit www.FLOX.com.
Visit us at the ASM HTS Expo Booth 1129
80
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