Plant expansions and production increases are clear signals that
a company is healthy and growing. But good news for the company
often translates into some interesting challenges for facilities
engineers. Among them: plant space, new equipment, construction,
schedules, capacities, utilities and more.
As paint systems are expanded, more process-ventilation air is
exhausted and more solvents are discharged. Increasing the capacity
of VOC-abatement systems to handle the additional solvent/air volume
is a necessary part of the growth.
Flex-N-Gate (Ada, OK)
is a major tier one supplier of automotive products, and the company
produces fascias and claddings for General Motors at its Ada plant.
The original plant—purchased by Flex-N-Gate in 2001—was
constructed in 1995. A regenerative thermal oxidizer (RTO) was installed
with the original paint shop to remove solvent from exhaust air
and thus meet emissions regulations. The RTO has been in continuous
use for the past eight years, with no significant maintenance service.
The expansion was constructed in 2003 and was planned for completion
by the end of the year.
As originally installed, the RTO was connected to the exhaust from
spray booths, topcoat ovens and the paint kitchen. The expansion
also required connecting air exhausts from a new prime-spray booth
and prime oven. Altogether, expansion of the paint system increased
the volume of solvent-laden exhaust air from 34,000 cfm to 61,000
cfm.
Flex-N-Gate considered three alternatives to deal with these higher
exhaust rates:
- Force more air volume through existing equipment.
- Add new abatement equipment.
- Increase the capacity of existing equipment.
The existing RTO had been reliable and virtually maintenance free
for many years. It had met the performance and operating cost criteria
originally promised. So Flex-N-Gate decided it was in the company’s
best interest to find a way to continue to use the existing RTO,
modifying it to accommodate the increased volume.
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Figure 1: Design of
Flex-N-Gate RTO prior to expansion of new chamber (left) and
post-construction (right).
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As demonstrated in Figure 1, the design of this RTO consists basically
of an oxidation chamber with burners, regenerative thermal recovery
chambers, a system to periodically change the air flow through the
regenerative chambers and a fan to move the air. Physical size of
the RTO is a direct function of exhaust volume. It is affected by
the heat-recovery media and its optimum face velocity. This in turn
is affected by the pressure across the media as well as the heat
storage and release characteristics. Typically, RTOs are built to
handle a specific air volume.
Other design factors that had to be considered:
- Burner capacity and placement, to assure uniform temperature
distribution at the selected destruction temperature, usually
around 1500°F.
- Size of valves and manifolds for the air volume.
- Residence time is a particularly important factor. It is the
minimum time for the process air to move from the hot side of
one regenerative bed to the hot side of an adjacent regenerative
bed. This is usually 0.5 second, which is determined by making
air at oxidation temperature travel the shortest path between
the chambers.
- Turbulence of the air during oxidation contributes to maximum
destruction efficiency (DE). As VOCs oxidize, oxygen around
the solvent gases is depleted and turbulence continuously mixes
the remaining VOCs with avail- able oxygen in the combustion zone.
Oxidation occurs in two areas: in the oxidation chamber mixing
is caused by the cyclical changes of air flow from one inlet or
outlet to another set of inlet outlet regenerative chambers, and
some oxidation begins within the heat sink beds as the gas temperature
reaches the auto ignition temperature of the VOCs. Mixing in the
heat sink bed is most effec- tive in media that does not have
long straight passages. Saddles and corrugated media work
well for this. An air- mixing type of heat- sink media, such as
saddles, functions well in such an application.
- Number of regenerative chambers. This involves two considerations:
air volume (and its effect on practical construction and shipping
sizes) and the destruction efficiency. DE below 98% usually can
be attained using an even number of chambers. For higher DE, another
chamber is required. The odd number of chambers allows purging
between inlet and outlet cycles.
Exploring the Options
Two practical opportunities exist for moving more process air through
the RTO. The engineer can change the heat-recovery media to one
that has less pressure drop while still functioning effectively
as a heat sink; or he can add another regenerative chamber to increase
the cross sectional area of the regenerative beds. In either case,
the fan would have to be modified to handle the additional volume,
and the burner capacity would have to be increased.
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RTO prior to expansion.
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The original RTO at Flex-N-Gate was constructed to be expanded
in the future to handle up to 72,000 cfm by adding regenerative
chambers, burners and an additional exhaust fan. The heat sink media
in the regenerative chambers was one-inch ceramic saddles at a depth
of 7.5 ft.
After evaluating the RTO volume increase options, Flex-N-Gate
considered removing the existing heat sink saddles and installing
ceramic structured packing. This packing, often called honeycomb,
is an extruded material installed in square blocks. The blocks have
square holes running through them, usually in an array of 40x40
cells in a 6x6-inch block. This material has had fairly good success
in similar applications. Although it is comparatively fragile, it
has less pressure drop across the heat sink bed than saddles and
therefore allows more air to pass through.
Installing the structured packing would involve removing the saddles
from all three regenerative chambers, modifying the supports and
then carefully installing the blocks to a depth of approximately
five feet. Additional burner capacity would be required to maintain
the 1,450°F oxidation temperature needed for complete VOC destruction.
The existing exhaust fan would be modified to deliver the new air
volume.
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RTO during expansion
of new chamber.
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Based on this approach the RTO would be expected to have a pressure
drop across two heat sink beds of approximately 10 -11 inch wc,
and a thermal recovery of 94-95%. This is assuming an inlet air
volume of 61,000 scfm at 100°F and an oxidation chamber temperature
of 1,450°F.
Installation time would be critical. Available RTO shut down periods
were limited and indeterminate. All work would have to be done during
a unit shut down period; remove the saddles, rework the supports,
increase the burner capacity, modify the fan and start up and adjust
the system.
Another Answer
Another option was presented by Salem
Savard, Detroit, MI, whose engineers were involved in the original
RTO design and construction. Using three chambers allows higher
destruction efficiency than constructing with two chambers. A look
at the permit requirements disclosed that the system is acceptable
at 97% DE, and it was determined that this would be achievable in
an RTO that has no “odd” chamber for purging.
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RTO upon completion
of expansion.
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Salem Savard—which was ultimately awarded the project—proposed
the addition of one more chamber, so that there would be two inlet
and two outlet chambers. They also planned to use the saddles as
originally installed, which have proven effective in thermal recovery
and durable over years of operation.
In addition to adding another chamber, Salem Savard extended the
system’s oxidation chamber, added another burner placed over
the addition, modified the fan wheel for the additional capacity
and connected new exhaust ducts.
Because the inlet velocity at the regenerative beds is lower,
due to larger cross sectional area per cfm, the expected pressure
drop would be 10-11 inch w.c., just as in the structured packing
option. Comparative energy costs are essentially a non-issue.
Flex-N-Gate also discovered that installation schedule constraints
were lower than might have been expected. Most construction work
was able to be done while the existing RTO was in operation, requiring
shut down only for the final tie-in to the RTO and re-start and
adjustments.
