and dust collector could be heated by the sun, temperatures in the
dust collector could rise to exceed normal design limitations. This
could cause sealing compounds (on filters and bolted joints) to be
compromised. As a countermeasure, if high ambient temperatures
are expected, the dust collector can be fitted with heat-resistant
filter elements, sealing compounds, and even special paint.
Most dust collectors for thermal spray applications use
compressed air to clean filters during operation. If moisture is
present in the compressed air supplied to the dust collector, cold
temperatures can cause problems. Small electrical solenoids that
control the cleaning function can sometimes freeze and become
stuck open or closed. This can cause an upset condition in the
dust collector and risk damage to filters, causing an interruption
in the thermal spray process. A solution for cold climates would
be a heated solenoid enclosure to help prevent this problem.
Furthermore, compressed air piping and associated tubing on the
dust collector can be protected with heated insulation (“heattrace”)
— Fig. 1.
Air handling fans used in the dust collection system can
also be significantly affected by temperature, according to Adam
Conley of Systech Design Inc., which provides fan design and
engineering for heavy manufacturing, pharmaceutical, and
aerospace facilities.
“A 15-hp, direct-drive fan designed for 5000 ft3/min of 70°F
air would need an increase to 20-hp if the temperature was 110°F,”
said Conley. This practice, called “derating,” adds mathematical
factors based on temperature to correct for the proper fan power
needed to meet design criteria.
Condensation can be another challenge related to temperature.
At the start of a shift, condensation can sometimes develop in the
dust collection system when warm air is drawn into a cold dust
collector. Wall, of Praxair, explained that a simple operating step
for this particular application can help. “Pulse clean your filters at
the end of the day,” he advised. “Run the system for 30 min before
you start spraying the next morning. The air from the plant will
help to equalize the temperature in the dust collection system
before introducing dust.”
Altitude
A local condition that is sometimes overlooked is altitude.
“Altitude affects air density,” noted Conley. “The higher you go,
the lighter the air.”
This is almost always a local condition that will affect the
design of the dust collection fan. Systems installed in mountain
regions will not work properly if they were based on the sea-level
air density, which is often the default variable used.
“You need more fan, and more motor with the higher
altitude,” said Conley. “For example, that 15-hp, direct-drive fan
designed for 5000 ft3/min at sea-level would need a 20-hp motor
at 2000 ft elevation.”
Colorado, Utah, New Mexico, Wyoming, and Arizona are a
few places where high-altitude fans are often needed. In Conley’s
experience, at altitudes above 3500 ft, even the fan motors need
to be special duty. Fortunately, geographic elevation information
is readily available, and fan builders can usually tailor systems to
meet the requirements.
Wind and Seismic
As with many design considerations, state, county, and municipal
authorities have strict rules about wind and seismic design
criteria. California earthquake risks and Atlantic hurricanes are
common design constraints, and requirements are usually well
understood by local engineering professionals. Dust collectors are
among the equipment that needs to meet local wind and seismic
design requirements. Keep in mind that the actual limits for
wind and seismic loads differ if the collector is located above the
ground, such as on a roof or on the upper floor of a facility.
Guidance can be found in the Industrial Ventilation manual
published by the American Council of Governmental Industrial
Hygienists.
Physical Location at the Facility
The location of the dust collection system at your plant —
indoors or outdoors — has many determining factors, and
weather should be planned into the equation.
Most thermal spray processes require relatively large airflow
volumes to properly exhaust particulate and heat from the booth,
and the associated dust collectors are usually installed outdoors.
There are benefits to this approach. Valuable factory floor space
can be used for other purposes, and there is no increase in noise
on the factory floor. If there are explosion vents on the dust
collector, they can be more easily located to meet regulations.
Along with these advantages to outdoor location, there are
some practicalities to consider, such as protection from debris.
“You have snow, ice, and rainfall coming off the roof of the
building, and sometimes right down onto the dust collector,” Wall
warns. “An appropriate structure to protect the dust collector in
severe weather can be very useful.”
The ideal outdoor location should also allow easy access for
maintenance. For instance, facility managers have seen dust
disposal become an issue.
“With some thermal spray dust, a full 55-gallon drum can
weigh well over 1000 lb. You have to know how you’re going
to deal with that before you select the location for the dust
collector,” Wall advised. “Most people like to use a forklift to move
the waste drums, so make sure you have that figured out before
you install the dust collector.”
Another consideration in dust collector location are the
ducts. It is possible to design a system with ducts over long
distances, but there are short and long-term costs to consider. The
most obvious are the material and installation costs of the ducts.
Longer ducts will require higher static pressure (vacuum) to create
the airflow needed. In some cases, that higher static pressure can
be enough to collapse the duct itself, so heavier ducts might be
needed. Finally, the fan will use additional energy to create the
design airflow, so expect this higher cost to be a permanent part
of the design.
Indoor installation, although less common, is used in some
thermal spray facilities. There might be restrictions to exterior
spaces, or even some strict security regarding the collected dust,
as in classified military applications. In most cases, the filtered air
and gases will have to be exhausted to the atmosphere outdoors.
Environmental health and safety managers will typically guide
industrial ventilation designers to meet noise, maintenance
access, and other important safety requirements.
With either an outdoor or indoor dust collector location,
remember that there will likely be high-voltage electricity
required to power the dust collection fan. The controls for the
dust collector might be installed indoors near the thermal spray
process, and associated wiring between the electrical panel and
the dust collector can represent a significant part of installation
costs — Fig. 2.
thermalspray.org 17 SPRAYTIME | 2018 First Quarter