INDUSTRIAL VENTILATION & Dust Collectiom
Modern industrial processes produce significant quantities of
airborne pollutants in all forms -- particulate, gases, vapors,
fumes and mists. Many are toxic and concentrations often exceed
safe levels of exposure. Reducing the pollutants to acceptable
levels is critical for the safe operation of many industrial processes
and mandatory to meet stringent emission regulations.
Sizing ductwork for standard hvac applications is daunting enough,
but doing it for industrial ventilation and exhaust systems can
be hair raising. At first glance there seems to be great similarity
in the design goals of these different types of duct systems making
one job as easy as the other. But ventilation and exhaust system
duct sizing has quite a few more wrinkles than standard hvac duct
The main goal of designing hvac duct systems is to use the lowest
cost (read smallest) duct sizes that can be used without violating
certain sizing constraints. For example, hvac duct sections are
typically sized so that air velocities don't exceed 400-800 feet
per minute and the pressure loss per 100 feet of duct length does
not exceed 0.1" wg. These constraints effectively limit how
small ducts can be sized.
If high air velocities and a large pressure drop per 100 feet
of duct are allowed, ducts can be sized relatively small. But
excessive noise and a large total pressure drop necessitating
a powerful and noisy fan are almost certain results of downsized
Still, velocity constraints can be varied for individual duct
sections so that duct sizes can be selectively minimized without
adversely affecting noise considerations. Likewise, the maximum
allowable pressure drop per 100 feet of duct can sometimes be
increased when it is known that the resulting greater pressure
loss is still within the capacity of the fan.
Besides meeting the desired sizing constraints, an hvac duct
system should be routed so as to minimize the individual lengths
of the various duct runs. Optimally routing duct work minimizes
material costs and helps to create a naturally balanced system
where the static pressure and air velocities available at each
diffuser are as similar as possible. A computer program can help
to calculate pressure losses and air velocities, but intelligent
placement of the air handler and routing of the ductwork are still
the art of the hvac designer.
Except for differences in desired velocities and pressure drops,
all of the above would seem to apply to industrial ventilation
and exhaust duct systems. This is not so for several reasons.
Industrial ventilation systems routinely utilize components rarely
seen in hvac duct systems such as hoods, dust collectors, blast
gates, and other such items.
These unique components not only require special consideration
in calculating their pressure loss, they also greatly influence
the design of the duct system. For example, a hood usually has
slots through which particulate or gases are drawn through. For
the hood to work properly, the connecting ductwork must allow
sufficient velocity (typically 3,500-4,500 fpm) so that the particulate
stays in suspension of the transporting air.
The dust collector of a ventilation/exhaust system not only contributes
a large pressure loss, it can also vary the density of the air
stream if it is a wet collector where moisture is added. Density
changes at the collector thus affect the pressure loss calculations
through all subsequent duct work.
Whereas the goal of an hvac duct system design is to use the
smallest possible duct sizes that do not violate certain constraints,
the goal of a ventilation/exhaust system design is to use the
largest duct sizes that will still maintain the minimum velocity
needed to keep particulates in suspension.
In ventilation/exhaust system design, maintaining the minimum
velocity to keep particles in suspension is absolutely critical.
However, maintaining high air velocities requires considerable
fan power and electric energy consumption. If air velocities are
maintained above what's necessary, a significant penalty is paid
on operating costs. Not to mention that an already noisy system
gets even louder.
The extra material expense of large ducts is usually insignificant
when compared to the operating costs saved on fan energy. However,
ducts can only be made so large before the air velocity begins
to fall below the minimum required. Hence, good ventilation/exhaust
system design strives for maximum useable duct sizes while optimal
hvac duct design aims for the smallest practical sizes.
Perhaps even more than sizing ductwork, the real problem in ventilation/exhaust
system design is balancing the static pressure at each converging
junction of ductwork. Industrial ventilation and exhaust systems
consist mostly of hoods with ductwork that converges back to a
collector and fan. Weye fittings typically connect the converging
air streams from hoods.
As two air streams converge, it is important for the static pressure
in each duct branch leading to the junction calculate to nearly
equivalent values. If they do not calculate within a 5% difference,
the desired cfm flow quantities will not occur in the actual operating
Collection System Design
Dust Collector Types
A dust explosion is very similar to a gas or vapour cloud explosion,
i.e. when a volume of a flammable mixture is ignited, resulting
in a rapid pressure increase and fire moving through the cloud.
A dust explosion occurs when a combustible material is dispersed
in the air forming a flammable cloud and a flame propagates through
it. This of course also depends on the supply of oxygen to the
fire, and the concentration of the fuel, if either of these are
in too high or low then the explosion will not occur.