T4409
Issue 2
AN OVERVIEW OF ENDOTHERMIC GENERATORS.
by Theodore P. Berry Technical Data
INTRODUCTION
Endothermic atmosphere, commonly called endo, carrier gas or Rx ® gas, is synthesized in the
catalytic retort(s) of endo generators. This gas, combined with an additive gas such as natural gas
or propane, or air, is used in heat treating furnaces to modify the surface chemistry of work
positioned in the furnace. Other carrier gases such as exot hermic gas, dissociated ammonia and
other nitrogen-based atmospheres are found in many heat treating facilities, but endo is most
common. Because endo can have a profound impact on the quality of processing, it is important
to establish a preventive maintenance program for the generator, and then follow the program
religiously.
The most common source of endo is the reaction product of air and natural gas in ratios between
about 2.5 to 1 to 5.5 to 1. Since the reaction is not spontaneous below ratios of 6 to 1, it is
necessary to supply heat to the generator--- hence the term endothermic, meaning heat absorbing.
Typical endogas generators produce an atmosphere of approximately 20% carbon monoxide,
40% hydrogen and 40% nitrogen, trace amounts of carbon dioxid e and other gases originating in
the natural gas or created by the reaction. The application of heat is not sufficient to create the
desired products rapidly, so the reacting gases must be exposed to a catalytic agent to accelerate
the reaction.
Principal components of an endogas generator are:
a heating chamber to supply heat by combustion or electric heating elements,
one or more cylindrical retorts (usually vertical) in the heating chamber with
numerous small, porous ceramic pieces, impregnated with nickel as a catalyst for the
reaction. Also included as part of the generator is
a cooling heat exchanger to rapidly cool the reaction products to a temperature that will
not allow the reaction to proceed further. One of the most critical parts of the system is
the control system that maintains the reaction temperature and adjusts the gas/air ratio
to provide the desired dew point.
* Rx is a registered trade-mark of Surface Combustion
S U P E R S Y S T E M S & T E C H N I C A L D A T A S H E E T
Printed in U.S.A. SSi Super Systems Inc CINCINNATI, OH 45215
The endothermic generator creates an
atmosphere to provide a positive
pressure in a heat treating furnace,
and a platform on which a carburizing
or decarburizing environment can be
formulated, by addition of enriching
gas or dilution air. Generator
maintenance scheduling, operation,
and control are discussed.
T4409
Note: The maintenance schedule suggested in the
following paragraphs has been culled from numerous
"industry standards" as well as over twenty years of
application experience of SSI staff working with the
equipment. Because of the make and design of your
equipment, and your routine maintenance procedures,
Issue 2
ENDOTHERMIC GENERATORS
Technical Data
MAINTENANCE SCHEDULE
not all of these recommendations may apply.
Daily:
Visually examine all instrumentation to assure that operation is normal...without
incident. Determine that control outputs are within the expected range of operation.
Check temperature of water discharged from heat exchanger.
Weekly:
Regenerate (burn out) carbon in generator using air (preferred method), exothermic gas,
or lea n endothermic gas.
After regeneration and readjustment of generator to proper dew point, check the gas
analysis (including CH4 content) with an infrared analyzer if possible.
Clean the air filter.
Monthly:
Clean air-gas mixing valve (carburetor) thoroughly.
Check calibration of gas analysis and control equipment such as automatic dew point
controllers, manual dew point indicators, CO/CO2 analyzers and oxygen probe carbon
potential control systems. The primary standard for carbon analyzers is shim stock tests.
Alnor or equivalent dew point testing gives a reasonable indication of correct operation.
Inspect thermocouples and protection tubes and replace every 3 to 4 months.
Check natural gas pressure after the regulator to maintenance balance to the carburetor.
Verify correct operation of over temperature controls.
Semi- annually:
Replace heat exchanger with standby, clean and refurbish for next service.
Inspect catalyst in retort and fill to proper level or replace.
Inspect and clean all burners.
Clean endo delivery lines to furnaces.
Inspect cooling water thermostats, solenoids.
Perform complete instrument calibration and service, including safet y controls.
Have oxygen probe (carbon sensor) refurbished, inspected and certified.
S U P E R S Y S T E M S & T E C H N I C A L D A T A S H E E T
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T4409
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ENDOTHERMIC GENERATORS
Technical Data
Annually:
Check compressor blades.
Check motor and compressor bearings.
Additional Troubleshooting Considerations
Problems at the furnace can alter an otherwise correct endo atmosphere due to air or water
incursion. Therefore both the atmosphere manifold and the furnace should be checked carefully,
when a problem is encountered, before attempting any corrective changes in the generator
atmosphere control system.
Air Maintenance
One of the major sources of difficulty, and hence a nagging maintenance problem, is poor quality
air. Contaminants in the air can include dust, fumes from acid cleaning tanks and oil quenching
systems. Airborne dust has been linked to failure of pumps and flowmeters, and poor electrical
contact in relays. Combustible vapors can cause a carbon sensor to read low, resulting in over
carburization.
In order to maintain a good air quality, it is necessary to establish a routine for cleaning filters. In
some cases it may be necessary to install ductwork to bring outside, contaminant free air to the
equipment. Dust-tight electrical housings are sometimes necessary to eliminate dust and fume
problems.
Cooler Maintenance
The design objective of coolers is to cool the generated atmosphere as quickly as possible to
below about 2600F in order to stabilize the composition before being delivered to the furnace.
Two problems impact on operation of the water-cooled heat exchanger. First, dissolved and
suspended solids in the water will deposit and coat the tube walls, thereby reducing the heat
transfer rate. Further, dissolved oxygen in the water can promote oxidation of the tubes, resulting
in premature failure.
Water-cooled heat exchangers should be cleaned and pressure tested for leaks during catalyst
replacement. Air-cooled exchangers should be blown out with air or rodded as necessary also
during catalyst replacement.
S U P E R S Y S T E M S & T E C H N I C A L D A T A S H E E T
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T4409
Issue 2
ENDOTHERMIC GENERATORS
Technical Data
Gas Supply
For the successful operation of any gas generator, an adequate supply of gas of the proper
composition is required at all times.
Dirt, scale and water vapor can cause plugging and failure of regulators and control valves.
Flowmeters are especially susceptible to dirt build-up on the float and walls. Gummy deposits
from a poor gas mixture can also cause sticking of valve parts and carburetors.
One of the most troublesome problems is the inconsistent analysis of the supply fuel. Propane -air
additives to natural gas supplies ('peak shaving') alter effective generator ratios. Unsaturated
hydrocarbons such as ethylene and propylene break down quickly into oily soot or coke.
Unfortunately, there is little choice when using utility-supplied natural gas. In some instances,
propane is selected to avoid these problems, but is not always an economical solution.
Sulfur, both naturally occurring and as additive mercaptans, can cause poisoning of the nickel
catalyst and an ultimate failure of the generator to crack gas properly.
It is therefore essential that the gas supply system be kept under close observation, and that all
critical components of the system, such as those previously mentioned, be inspected and cleaned
as required on a programmed basis. It may be advisable to inform your gas/utility supplier of
your special concerns should they, at their discretion opt to alter your feedstock supply by
"dosing" or "spiking" (i.e. peak shaving) your gas. W e find, in many instances, that the gas utility
companies will not advise you of this practice and are reluctant to discuss it.
Maintenance of the Combustion System
The maintenance of the combustion system of a gas fired endothermic generator is not different
from other combustion systems; that is, it should be kept clean and adjusted. Most such systems
are relatively simple.
Electrical Maintenance
A reliable, uniform supply of regulated power is necessary to operate relays and solenoid valves.
Electrical problems are usually easy to spot, since maintenance personnel are well trained and
have sufficient test equipment and wiring diagrams for trouble shooting.
S U P E R S Y S T E M S & T E C H N I C A L D A T A S H E E T
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T4409
Issue 2
ENDOTHERMIC GENERATORS
Technical Data
Thermocouples and Protection Tubes
In most atmosphere generators, we recommend the use of type S thermocouples. Because of the
relatively high temperatures maintained in atmosphere generators (1850oF to 1950oF),
thermocouples should be inspected every thr ee to four months, and replaced if found faulty.
Protection tubes and seal rings should be checked each time the thermocouples are replaced; a
broken or leaking tube will greatly shorten the life of the thermocouple. If there is a substantial
difference in indicated temperature between the control and overtemp instruments, it can very
well be caused by deterioration of one of the protection tubes, causing early failure of the couple.
If this should happen, check, and replace as necessary. It is often standard practice to replace the
protection tubes when the thermocouple is replaced.
Maintenance of Temperature Controls
In addition to temperature controls, most atmosphere generators have an over temperature (high
limit) instrument to shut the system down in the event of a runaway temperature.
Because of the great variety of recorders, recorder/controllers and controllers, it is difficult to
discuss exact inspection, calibration and repair procedures, but general rules of thumb apply.
Pens, ink and charts should be checked daily. This ensures a quick look at the instrument to
determine if it is operating properly. Manufacturers recommendations for routine maintenance
should be scheduled. Temperature calibration is usually fairly stable, but should be checked
every six months, at the same time as the thermocouples.
The overtemperature instrument requires little service, but it does require a monthly check for
proper function, and semi-annual calibration.
Installation of Catalyst
Catalyst specialists reco mmend a layer of untreated (no nickel) alumina substrate on the diffuser
block extending 2" to 4" into the combustion chamber, to act as a buffer zone. This material can
be recovered and reused when changing catalyst.
Slowly pour catalyst into the retor t, avoiding dust that may have resulted from shipping. A good
rule to follow is to fill the retort to a point just below the top of the combustion chamber. On
most generators, this is about 8" below the top plate. Do not overfill. Catalyst in the cooler area
above the combustion zone will create sooting due to a reversible reaction in this temperature
transition zone. Replace the retort cover plate, using a new gasket.
S U P E R S Y S T E M S & T E C H N I C A L D A T A S H E E T
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T4409
Issue 2
ENDOTHERMIC GENERATORS
Technical Data
Breaking in New Catalyst
Heat the generator slowly to 1500oF and hold for one hour This is intended to avoid too rapid
expansion of the retort and refractory lining of the combustion chamber. Then raise to normal
operating temperature and hold for two hours before attempting to crack gas.
The new catalyst can be broken in by operating at your normal air/gas ratio. Considerable
moisture is formed at this point due to reaction with the air in the retort and the pores of the
catalyst substrate. This moisture will normally collect at the cold end of the heat exchanger, and
should be drained out at the petcocks in the cold gas manifold. Allow sufficient time to dry out
the pipes before taking dew point readings and adjusting the ratio for dew point control.
Once the catalyst is broken in and reduced, it is not necessary to repeat this operation if
"burnout" or reactivation is properly conducted.
Catalyst Maintenance
With any specific gas generator, the manufacturer provides recommendations regarding the
catalyst-- the kind used and its care. Maintenance of the catalyst is vital, and cannot be
overemphasized.
The catalyst will last longer if a dewpoint above 35oF is maintained. Generally speaking,
operation at lower dewpoints will cause a buildup of carbon, which will appreciably shorten
catalyst life. When the catalyst breaks down, it is difficult to achieve maximum gas flow. At that
point, new catalyst must be charged. To avert such breakdown, carbon should be burned out of
the catalyst bed every week. This is achieved by halting gas production, establishing a
temperature of 1600oF, and then establishing the recommended flow of air as indicated here:
Generator Size Recommended Air Flow
500 CFH
750CFH
1000CFH
1500CFH
Multiple Retorts
If carbon is present on the catalyst, a blue flame will appear at the generator burnoff can or pipe.
As soon as the flame disappears, stop the airflow. This is important. Do not continue airflow
after the flame disappears. This indicates that all carbon has been removed. If air is continued,
the nickel catalyst will become oxidized, a nd then must be reduced by the procedure outlined for
breaking in new catalyst.
The generator can be idled during weekends at 1500oF to 1600oF without cracking gas. The
amount of time to burn off soot should not take more than a few minutes if the generator has
been operated properly above 35oF. Operating generators at low dew points will require more
time
S U P E R S Y S T E M S & T E C H N I C A L D A T A S H E E T
6
25CFH
35CFH
50CFH
75CFH
75CFH/per retort
T4409
Issue 2
ENDOTHERMIC GENERATORS
Technical Data
to burn off the carbon accumulation. It is important for good, efficient gas cracking to remove
any carbon. It is also important to obtain the maximum catalyst life. The high refractory catalyst
base resists disintegration by carbon, but even the best of catalysts will deteriorate with time in
the presence of a heavy carbon deposit. Remember that any catalyst loses its efficiency if the
nickel is blanketed by a layer of carbon. Therefore, it is important to check your generator
occasionally to see if it is being operated properly and to remove any carbon deposit.
If, after the catalyst reactivation, the dewpoint or CO2 control cannot be achieved, the catalyst
will require changing.
Application of Dewpoint Controls
Numerous attempts have been made to apply automatic control to endo generators. At best, many
would agree that most of these systems have not performed consistently up to user expectations.
Since the early 80's, however, oxygen probe based control systems have provided a more reliable
and meaningful approach to automatic dewpoint control. Surprisingly, they have been shown to
minimize the impact of peak shaving.
Two of the early attempts involve insertion of the probe directly into the top of the retort.... either
vertically, or at an angle, so that the probe is located 2 to 3" above the s urface of the catalyst bed.
By this siting, the probe is exposed to the endogas flow at a temperature between 1550oF and
1750oF typically. While the primary objective of these techniques is to have " in situ" exposure,
a number of operating and maintenance concerns have become evident.
Some of the most troubling features of the in situ mounting are:
High ambient temperatures on the top of the generator;
Difficult maintenance at this location;
Concerns related to mounting in a single retort of a multi-retort unit -what's happening
in the other retort(s)?
Shortened probe life because of the high temperature exposure; and
shortened probe life because of soot buildup in the probe sheath.
Note that burnoff is not an option in this location due to the pressure levels in the endo manifold
and impact on the product by the burnoff air.
A third, more recent application using O2 probes has shown the most promise compared to
previously attempted methods. The overall system is shown in Fig. 1, with a close-up view of the
sensor installation in Fig.2. We are convinced that the advantages of this system far outweigh the
drawbacks. This method does not require accessing the probe into the retort or its exit piping.
Instead, a small sample of cooled endogas from the generator exit manifold, or an individual
retort in a multi-retort generator, is transported to the short probe which is fitted into a reheat
well in the side wall of the generator heating chamber.
S U P E R S Y S T E M S & T E C H N I C A L D A T A S H E E T
7
AIR
TRIM
GAS
TRIM
SSi
CONTROL SYSTEM
CONDITIONING SIGNAL
BURNOFF AIR
FLOW TO SENSOR
BURNOFF FITTING.
ENDO OUT
SAMPLE
WATER IN
WATER OUT
SOV
REGULATOR
GAS PRESSURE
SSOV
REACTION GAS
SOV
TAILED DRAWING.
SEE FIG. 2 FOR MORE DE-
SENSOR AND THERMOWELL
COMBUSTION AIR BLOWER
MAIN
MAIN
FILTER
INLET AIR
BURNER
GAS
AIR
AIR/GAS MIXING PUMP
CARBURETOR
SOV
SSOV
SOV
FILE-T4409FG1.
(MIXING VALVE)
FIG. 1 ENDOTHERMIC GENERATOR WITH REHEAT WELL SAMPLING SYSTEM
COMBUSTION GAS
DIFFUSER BLOCK
SUBSTRATE.
NON-CATALYTIC ALUMINA
S U P E R S Y S T E M S & T E C H N I C A L D A T A S H E E T
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T4409
Issue 2
ENDOTHERMIC GENERATORS
Technical Data
The reheat well can be inserted to a point in the refractory wall that establishes a probe operating
temperature between 1350oF and 1550oF, preferably 1400oF. The thermal well provides a
thermal shock barrier and a transition buffer between the sample and the combustion atmosphere.
This arrangement also allows for a convenient burnoff of the sensor with absolutely no impact on
the generated atmosphere quality. Trouble shooting is comfortably conducted at ground level
without the extreme discomfort encountered when working with retort mounted probes. High
probe temperature/ short life expectancy concerns are eliminated. The proximity of endogas
sampling ports for both the O2 probe and conventional dew point devices (Alnor or equivalent)
provide greater surety and confidence. There is a valid argument for concerns relating to
sampling problems, equilibrium shifts and removal from the "in situ" environment, but we
believe the advantages of the reheat well/O2 probe measuring system overwhelmingly outweigh
the disadvantages.
Closing the loop for endo generator control is accomplished by connection to a state-of -the-art
microprocessor analyzer/ controller. This device should calculate and display dewpoint, control
output, probe millivolt output and temperature. It should regulate the addition of enriching gas or
dilution air for control. If you choose a programmable controller, you can write a "watchdog"
program that will sound an alarm if the control output is nearing its maximum. An alarm display
might state or infer, for example, that "you are adding over 90% of the maximum trim gas
available......adjust your carburetor (mixing valve) to a richer ratio that allows control gas flow to
approach zero". Because the problem has been immediately alarmed, corrective action can be
taken to prevent serious malfunction. If programmability is not available, frequent (daily) visual
inspection of the control output will determine if you are approaching the limit of control so that
you may take steps to adjust the mixing valve.
Fig. 2 shows the control components of Fig.1 and details of the combination reference air/probe
burnoff system complete with timers, flowmeters and a sample filter to remove carbon
particulates. A probe/ well burnoff cycle is initiated on a regular bas is, usually 12 to 24 hours. As
the burnoff starts, a relay operates to disconnect power from the gas/air additive valves in order
to maintain control while burnoff proceeds. After the burnoff period of 5 to 10 minutes is
completed, power is restored to the additive valves and control operation is resumed. Because of
the quick recovery of this system, virtually no deviation from set point can be noted.
It has become an unfortunate fact that oxygen probe control of generators has been so successful
that routine manual dewpoint analysis becomes less and less frequent, in some cases abandoned
altogether. This is a dangerous precedent. We strongly advise that dewpoint be checked at least
once a shift, both to avoid catastrophic losses in the event of malfunction, and to maintain
operator proficiency in operation of the dewpointer.
S U P E R S Y S T E M S & T E C H N I C A L D A T A S H E E T
9
AIR
HOT FACE,
GENERATOR
COMBUSTION
CHAMBER.
ENLARGED VIEW OF
GENERATOR INSTALLATION.
CHILLED ENDO
FILTERED SAMPLE
OR BURNOFF AIR
BURNOFF SYSTEM
SSi
SUPER SYSTEMS INC.
CINCINNATI, OH
REFERENCE
AIR
REFERENCE AIR
SENSOR CABLE AND
T/C EXTENSION WIRE.
MAIN
AIR
GENERATOR MANIFOLD
BURNOFF
AIR
POWER
MAIN
GAS
SAMPLE
3-WAY VALVE
FILTER
ENDO TO FURNACE
CONTROL SYSTEM
SSi
SUPER SYSTEMS INC.
CINCINNATI, OH
CONTROL OUTPUTS
GAS
TRIM
GAS
CARBURETOR
(MIXING VALVE)
TO RETORT
ENDOTHERMIC GENERATOR REHEAT WELL SAMPLING, CONDITIONING AND CONTROL SYSTEM
FIG.2
AIR/GAS MIXING PUMP
S U P E R S Y S T E M S & T E C H N I C A L D A T A S H E E T
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TRIM
AIR
T4409
Issue 2
ENDOTHERMIC GENERATORS
Technical Data
Dew point versus %C control?
A frequent question from owners of control instruments that do not provide the option of dew
point control is, "why can't I control my generator using carbon potential?" To answer this
question, lets first discuss the merits o f using the oxygen probe signal to calculate and control an
endo generator using dew point as the control variable. Any dew point reading displayed by the
control instrument can be immediately verified by cross checking with a conventional dew point
instrument, such as the Alnor. A similar verification when %C is the controlled variable could
require several hours by a trained technician using shim stock analysis.....just not practical.
Further, most heat treat personnel are familiar with the dew point scale for endo generators and
are comfortable working with this control variable.
Finally, dew point is independent of atmosphere temperature. If we were to feed a 40
point endo into three tight furnaces which were individually controlled at 1500F, 1600F and
1700F, a manual dew pointer would show that each furnace atmosphere was exactly 40
each would display carbon potentials of 0.85%, 0.59% and 0.40% respectively (see Table 1). To
demonstrate why it is not practical to control a generator using %C as the controlled variable,
consider the fact that most endo generators typically supply several furnaces. Because the flow
from a generator can vary significantly, the probe temperature will correspondingly change.
Refer again to Table 1. An instrument set to control at 0.6%C at a sensor temperature of 1600F
o
o
F dew
F. But
would provide 39o F dew point gas. A change of temperature to 1550F at the same set point
would deliver 45o F dew point gas. A probe temperature of 1650F would result in a 34o F dew
point. Therefore, you cannot expect to control a generator using carbon potential as the control
variable and expect the dew point to remain constant.
What is the cost justification for oxygen probe control?
Oxygen probe-based automatic dew point control equipment will typically show a payback in
twelve months or less. Assuming that:
uthe generator is 'on line' around the clock for 24 hours/ day, 5 days / week, 50 weeks/
year.
uoperator salary of $11 per hour.
uaverage catalyst replacement cost of $700 and
uaverage retort replacement cost of $3000.
We can estimate the following annual savings using oxygen probe dew point control:
ratio adjustment using manual dew point analysis ...................................$8250
labor to 'burn out' catalyst .....................................................................$1650
catalyst cost and replacement labor .........................................................$ 750
cost of premature retort replacement ......................................................$1100
Estimated savings $11750/ year
S U P E R S Y S T E M S & T E C H N I C A L D A T A S H E E T
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T4409
Issue 2
ENDOTHERMIC GENERATORS
Technical Data
TABLE 1- DEW POINT VS % CARBON, 20%CO, 40%H2
%C
0.30 73 66 60 54 48 43 38 30 22
0.35 68 61 55 49 44 39 34 26 18
0.40 64 57 51 45 40 35 31 22 15
0.45 60 54 48 42 37 32 27 19 11
0.50 57 50 44 39 34 29 24 16 9
0.55 54 47 42 36 31 26 22 14 6
0.60 51 45 39 34 28 24 19 11 4
0.65 48 42 37 31 26 21 17 9 2
0.70 46 40 34 29 24 19 15 7 0
0.75 44 38 32 27 22 17 13 5 -2
0.80 42 36 30 25 20 15 11 3 -4
0.85 40 34 28 23 18 14 9 2 -5
0.90 38 32 26 21 16 12 8 0 -7
0.95 36 30 25 20 15 10 6 -2 -8
1.00 34 28 23 18 13 9 5 -3 -10
S U P E R S Y S T E M S & T E C H N I C A L D A T A S H E E T
1500
F
o
1550
F
o
1600
F
o
1650
F
o
1700
F
o
1750
F
o
1800
F
o
1900
F
o
2000
F
o
SSi SUPER SYSTEMS INC.
7205 EDINGTON DRIVE l CINCINNATI, OH 45249
(800) 666-4330 l (513) 772-0060 l(513) 772-9466
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