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SECTION 14. Industrial Ventilation
General Features of Ventilation Systems Definition Ventilation
refers to the continuous supply and removal of air with respect to
a space. In
industry this is done to control chemical and some physical
hazards, as well as to maintain conditions of temperature and
relative humidity which are compatible with human habitation and
industrial operations.
Air motion To move air requires creating a pressure difference
between two points. Air will then
move from the region of higher to the region of lower pressure,
at a rate that depends on the magnitude of the pressure difference
and on the impedance to air flow offered by ducts, objects and
friction. The pressure difference is created with a fan, or blower,
or by inducing a density difference through differential heating.
The latter mechanism is the source of wind outdoors, but is also
used in workplaces where heating sources exist, as in metal
refining, melting and casting.
System performance requirements The desired performance of the
ventilation system will impose requirements on the
magnitude of the pressure difference to be created by the fan or
heat source.
Capture In many systems the moving air must capture the airborne
contaminant by entraining the
surrounding air and conducting it in the desired direction. This
leads to requirements for air velocity (speed and direction) and
volumetric flow rate.
Transport
Once captured the contaminant must be carried away from the
inhabited space to a
suitable collector or disposal point. This is generally done by
forcing the air to flow through ducts where impedance to air flow
must be overcome.
Dilution
In some cases the principal goal of the system is to dilute the
contaminant to safe levels
by supplying a sufficient volumetric flow rate. Again, there is
often a requirement that the direction of air motion be controlled
to achieve the desired dilution and worker protection.
Behavior of contaminants
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As pointed out earlier, nearly all airborne contaminants of
hygienic importance will follow air motion, so that controlling the
movement of air will cause contaminants to move in the same
direction. The only important exceptions to this occur when
particles are emitted at high velocity from industrial operations
such as grinding, where metal particles leave the work in the
direction of rotation of the grinding wheel, independent of the
movement of air. This effect is important for distances of not more
than about two feet, beyond which the particles are forced to
decelerate and again follow air currents. Further, contaminants
which are in the particle size range that is important for
inhalation are not influenced to significant degree by gravitation
and inertial forces, so they will remain airborne for extended
periods in the workroom. Even dense gases such as chlorinated
solvents will not settle appreciably due to gravity unless they are
present at extremely high concentrations, generally above 1000
parts per million. This means in turn that the physical properties
of air which are important in causing it to move, density and
viscosity, are not affected by the presence of contaminants in the
concentrations important to human health. It should be noted here,
however, that when one or more contaminants are present at volume
fractions in the range of 1 to 10 percent (10,000 to 100,000 ppm)
which is the explosive range for flammable contaminants, the
density and viscosity of air will be affected.
General Ventilation This type of system provides air flow to the
entire volume of a space, either by natural or
mechanical (forced) means. It is commonly used in office and
public buildings, but also is used in some industrial applications.
These are often referred to as heating, ventilating and air
conditioning (HVAC) systems, although they may often be designed
for other purposes as well.
Dilution This form of general ventilation is intended to dilute
the contaminant generated in the
workplace with uncontaminated air so that the concentration
never approaches a hazardous level.
Volume requirements
The volumetric flow rate of dilution air depends primarily on
the evolution rate of the
contaminant to be controlled, but also on the efficiency with
which fresh air mixes with workroom air. In order to keep the
concentration of contaminant in the mixed air at or below the
Threshold Limit Value (TLV), the flow of dilution air supplied is
given by:
Q = ER x K 14.1 TLV where Q = dilution air flow rate, m3/min ER
= contaminant evolution rate, mg/min TLV = Threshold Limit Value,
mg/m3 K = mixing factor
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Industrial Ventilation page 14.3
The factor K accounts for imperfect mixing in the workroom,
which means that more
dilution air is required than in the case of complete mixing,
where K = 1.0. Values chosen by the designer for K generally range
from 3 to 10, but are based on guesswork and experience rather than
hard experimental data. Because of this uncertainty, dilution
ventilation is not recommended for control of contaminants of high
toxicity, or for situations where the rate of evolution is not
steady over time. It may be used, however, in unoccupied spaces
where the main hazard is explosion due to a contaminant present in
high concentration. In this case the TLV in the equation is
replaced with the Lower Explosive Limit (LEL), defined as the
lowest contaminant concentration in air at which an external
ignition source could produce an explosion. For all flammable
vapors, the LEL is 1% or more, so that dilution volumes required to
control this type of hazard are much lower than those required to
control toxic exposures. Conversely, if the concentration of a
flammable contaminant is kept below the TLV by any form of control,
there cannot be an explosion hazard.
Note that equation 14.1 indicates that the flow rate of dilution
air does not depend on the volume of the space to be ventilated.
This is at variance with the common practice of specifying
ventilation requirements in terms of "number of room air changes
per minute or per hour" which directly involves room volume. The
fact is that the rule of thumb based on room air changes per
minute, though in widespread use over many years, has been used
improperly in the control of workplace hazards.
Air flow pattern
Apart from the volumetric flow rate, the other requirement of a
dilution system is that the
direction of air flow in the ventilated space be controlled.
This is necessary to achieve the most uniform mixing, but also to
carry contaminants away from the breathing zones of the workers.
Figure 14.1 shows several examples of dilution ventilation systems
in which the air flow patterns differ depending on the location and
configuration of the air inlet and outlet. The best arrangements
conduct the contaminant in the proper direction, and also include a
fan and distributor for inlet air giving good mixing, as well as a
fan for the outlet. Such dual fan systems are also referred to as
"push-pull" designs. On occasion dilution systems incorporate
provision for recirculation of the outlet air, and still others
permit inadvertent recirculation when the outlet is placed too
close to the dilution air inlet. When toxic contaminants are
evolved in the workroom, recirculation must be avoided.
Applications
As an example of the design of a dilution ventilation, assume
that methyl ethyl ketone is
used in a solvent cleaning operation, and on the basis of
consumption of MEK over time it is estimated that 1 gallon of
solvent is evaporated per hour of operation. This is the same as
3.8 liters/hr, and using the density of liquid MEK the mass
evaporated is 55.1 g/min. The TLV for MEK is 590 mg/m3. If good
mixing is assumed (K = 3), then the flow rate of dilution air
required to control at the TLV is
Q = ER x K TLV
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Industrial Ventilation page 14.4
= 280 m3/min The Lower Explosive Limit for MEK is 2% by volume,
or 20,000 ppm, one hundred
times higher than the TLV. Therefore under similar assumptions,
the dilution flow rate required to control only the explosion
hazard is 2.8 m3/min.
Energy requirements
All forced (fan-driven) ventilation systems require energy to
create air flow and
overcome friction. This is estimated using techniques described
below for local exhaust systems. There is a large additional energy
requirement for dilution systems associated with the volume of
outdoor air usually used as the source of clean dilution air. When
the outdoor air temperature and relative humidity are not suitable
for indoor human habitation, air conditioning is necessary. For the
example worked above with MEK, if outside air is used for dilution
and its average annual temperature is 55 F while the desired indoor
temperature is 70 F, the heating costs for tempering the air can be
estimated. Using oil heat at $1.25 per gallon of fuel, the annual
cost of air tempering for the high flow, TLV target case is
$162,000, which would be considerably higher than the cost of the
energy required to move the air into and out of the workroom. For
the explosive hazard case, the air tempering cost would be 1/100
this figure.
Comfort /Heat Control This form of general ventilation is used
to maintain the temperature and relative humidity
of the air in a workplace within the usual limits for the
comfort of the occupants. At the rate of airflow usually necessary
to do this, odors, carbon dioxide and other man-associated
contaminants are readily diluted to acceptable limits. In addition,
this flow of air will deliver far more oxygen than is required for
respiration even at the highest physiological work rates.
Although carbon dioxide produced by human metabolism does not
reach hazardous levels in office buildings, it is often a useful
indicator of the adequacy of building ventilation. Concentrations
of CO2 above about 1,000 ppm indicate low exchange with outside
air, and are associated with excess complaints from building
occupants.
Comfort ventilation systems do not, however, employ air flow
rates adequate to control health and safety hazards presented by
other chemicals, and are not recommended for that purpose. In the
design and construction of modern office and public buildings,
there has been and understandable but unfortunate trend to
reduction of the flow rate of fresh, outdoor air used in
ventilation systems. Energy costs are the main factor driving this
trend, but the consequence of this is that such systems often fail
to dilute some of the chemicals commonly evolved in office and
light industrial operations. The result is complaints from
occupants of non-specific health problems which are probably
related to the accumulation of airborne residues of cleaning
solvents, copy machine fumes, carpet and drapery emissions and a
variety of other unidentified chemicals emitted by many materials
of construction.
Local Exhaust Ventilation
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Industrial Ventilation page 14.5
A local exhaust system is used to control air contaminant by
trapping it at or near the source, in contrast to dilution
ventilation which lets the contaminant spread throughout the
workroom, later to be diluted. Local exhaust is generally a far
more effective way of controlling highly toxic contaminants before
they reach the workers' breathing zones. This type of system is
usually the proper control method if:
air sampling shows that the contaminant is a serious health
hazard. emission sources are large in magnitude, few in number,
and/or widely dispersed. emission sources are near the workers'
breathing zones. emission rates vary widely with time. there are
several contaminants emitted by the process having different levels
of
toxicity. components The four major components of a local
exhaust system are shown in figure 14.2.
hoods
The hood is the point where air containing the captured
contaminants enters the system.
Its purpose is to direct the air flow so that its direction and
distribution are appropriate for the conditions at the site of
contaminant generation. Capturing hoods create a directed air
current with sufficient velocity to draw contaminants from outside
the hood itself. An example is the downdraft hood often
incorporated into modern kitchen range units which captures cooking
fumes by drawing them sideways and downward before they can escape
into the general room atmosphere. Enclosing hoods surround the
contaminant source as completely as possible, and include
laboratory fume hoods and glove boxes. Receiving hoods are placed
and shaped to receive the contaminant as it is thrown out by a
source, as in the canopy hood over hot work (not recommended) and
in the hood around a grinding wheel.
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Industrial Ventilation page 14.6
ductwork The ductwork is a network of piping which carries the
captured contaminant out of the
workroom to its final disposition. The primary goals in
designing ductwork are to maintain sufficient air velocity in the
piping, and to minimize the resistance to flow created by bends,
junctions and changes in cross-sectional area. Maintenance of air
velocity is important for transport of particulate contaminants,
since if the air velocity falls below a critical (empirically
determined) value, the dust will deposit on the inner surfaces of
the ducts and impede flow. Resistance can be the major part of the
energy required to operate the system, and a poorly designed duct
system may cause so much resistance that the fan is unable to move
the required volume of air.
air cleaner
The air cleaning device removes the captured contaminant before
the exhausted air is
discharged, either outdoors or into the recirculation pathway if
provided. Collectors may range from simple centrifugal collectors
much like the cyclone used to sample respirable dust, to elaborate
filters and gas/vapor adsorbers specially designed for the
contaminant present. All air cleaners add to the total resistance
of the system; generally, the more efficient the collector, the
greater the resistance added.
fan
The final component provides the energy to accelerate the air as
it enters the hood and to
overcome friction and dynamic losses (eg, from elbows,
junctions) between the moving air and the surfaces of the ductwork
and cleaning device. If possible the fan is almost always placed
downstream of the air cleaner, to prevent deposition of contaminant
on the fan blades, or damage due to contact with corrosive
contaminants.
capture velocity For a contaminant to be captured effectively at
a capturing hood, before it is released into
the workroom air, the local exhaust system must create a pattern
of air flow of adequate speed and direction. The minimum air
velocity necessary to capture the contaminant, or capture velocity,
depends on the conditions at and near the point of release, and on
the nature of the contaminant. The range of recommended capture
velocities, in feet per minute (fpm), for common local exhaust
applications is given in Table 14.1. Laboratory fume hoods, as an
example, are generally designed to operate with a minimum capture
velocity of 100 fpm at the face of the hood opening. The influence
of air currents in the room, and especially in the area near the
hood entrance, should be noted. Cross drafts due to other
operations in the room can impair the performance of a local
exhaust system to a severe degree.
flow pattern The direction of air flow near the hood entrance is
also important in effecting capture. In
all suction systems the influence of air moving into the hood
reaches out only a short distance from the entry point. This is in
marked contrast to flow of air out of a hood or duct, where
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Industrial Ventilation page 14.7
the velocity of the air in the jet is still measurable several
duct diameters away, as indicated in figure 14.3. The short reach
for suction systems is shown in further detail in figure 14.4a and
14.4b. For a plain duct opening the velocity of air near the duct
is less than 7.5% of that at the duct opening at a distance of one
duct diameter from the entrance. Adding a flange to the duct
opening, making it a crude hood as in figure 14.4b, provides slight
improvement, but the point to be made is that capture is only
possible at very short distances from the hood opening, even in the
best designed systems. Generally speaking, local exhaust systems
will only be effective in capturing material within one foot of the
hood opening.
energy losses As suggested earlier, energy is required to move
air into and through the local exhaust
system, and this is provided by the fan. The two components of
energy use are the acceleration of the (nearly) still air in the
workroom to the velocity of travel through the hood and ductwork,
and the friction created by air rubbing against the sides of the
ductwork, and the dynamic losses caused by sudden changes in
direction or separation from surfaces such as elbows and junctions.
Both components of energy are "lost" in that they ultimately appear
in the environment as heat and cannot be recovered, so the energy
requirement is referred to as energy loss. Knowledge of the energy
loss components of a system is critical in determining the power
requirements for the fan, which dictates in turn the size of the
fan motor and the amount of electric power needed. The components
of energy loss can be determined by measuring two types of pressure
within the system, as described below.
relation of velocity to flow rate Thus far we have focused on
the velocity of air entering and passing through the system
as the major factor in performance of the capture function. A
second, related factor is the volumetric flow rate of air necessary
for capture, which depends on the physical dimensions of the source
of contaminant, and on the evolution rate. Unfortunately, the
determination of adequate flow rate for capture is almost entirely
empirical, based on accumulated experience with a variety of
applications in industry. The best single source of guidance on
necessary air flow rates for capture is "Industrial Ventilation - A
Manual of Recommended Practice" published by the ACGIH. For our
purposes it will be important only to recognize that air volumetric
flow rate and air velocity are related by a simple expression:
Q = A x V 14.2 where Q is the volumetric air flow rate, in cubic
meters/min or cubic feet/min A is the cross sectional area of the
duct or hood, in square meters or
square feet V is the air velocity in meters/min or feet/min Thus
knowledge of the velocity of air flowing in a portion of the
ductwork, or in the
hood, and of the corresponding cross sectional area will permit
calculation of the air flow rate. This will prove useful in
providing for make-up air, as described in the earlier portion of
this section on general ventilation systems.
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Industrial Ventilation page 14.8
measurement of performance Performance of a local exhaust system
is evaluated somewhat indirectly, by measuring
pressures at various points near the hood and in the hood and
ductwork. From these measurements the velocities and volumetric
flow rates are calculated.
pressures
Two types of pressure can be measured in exhaust ventilation
systems, using the simple
U-tube water-filled manometer, and a specially made piece of
tubing called a Pitot tube. Figure 14.5 shows the types of
pressures and their relationship to energy loss. The "static"
pressure is measured with the manometer connected as shown in part
one of the figure. In each of the two configurations shown the
manometer is exposed to the air via an orifice whose plane is
oriented parallel to the air flow direction. In most cases the
static pressure is the driving force causing air to move from one
point to another. Thus in most parts of the ductwork, the static
pressure must be lower than the pressure in the workroom. The
second type of pressure is called the "velocity" pressure; its
measurement is shown in part two of the figure. The second arm of
the manometer is now oriented with the plane of its orifice
perpendicular to the flow direction, and the velocity pressure is
given as the difference in height of water in the two arms of the
manometer. The velocity pressure is proportional to the square of
the air velocity:
VP = k x (Velocity)2 14.3 where k depends only on physical
properties of the air at existing temperature
and barometric pressure The two pressures are commonly measured
by drilling small holes in the ductwork and
inserting the Pitot-static tube as shown. Part three of the
figure shows the general relationships among the pressures at two
points in a section of ductwork. The velocity pressure, which
depends only on the air velocity, will be the same at the two
points if the duct cross section is fixed. The sum of static
pressure and velocity pressure at each point, sometimes called the
"total" pressure, is also equal to the energy of the flowing air at
each point. Because of friction, there is a loss of energy between
the two points, and this loss is equal to the difference in total
pressures. Total pressure must decrease in value in the direction
of flow.
The relationships are also shown in figure 14.6, a diagram of a
simple local exhaust system including the fan. The principle of
energy conservation requires that the energy delivered to the fan
be accounted for as an increase in the energy content of the air
moved through the system, plus energy dissipated as heat. Starting
with still air in the room outside the hood, the total energy is
proportional to the sum of the static pressure (which is zero
relative to barometric pressure) and the velocity pressure, also
zero since the air here is not in motion. Once inside the duct, the
energy content of the air is again proportional to the sum of the
two pressures: static pressure is now subatmospheric or negative
when measured with the manometer, but the velocity pressure will be
positive because the air is in motion. However, the total energy
content of the air must decrease as it passes along the duct
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Industrial Ventilation page 14.9
approaching the fan because of energy lost to friction. Thus the
algebraic sum of static pressure and velocity pressure at point two
in the figure must be less than the corresponding sum at any point
upstream, including point one outside the hood. As energy losses in
the system increase due to rough edges, sharp turns or deposited
dust, the static pressure must be made more negative to achieve a
given velocity, and the fan must deliver more energy.
Bibliography McDermott HJ. Handbook of Ventilation for
Contaminant Control. 2nd Ed. Boston:
Butterworth Publishers. 1985 ACGIH. Industrial Ventilation. A
Manual of Recommended Practice. 21st Ed.
Cincinnati, OH: American Conference of Governmental Industrial
Hygienists. 1992. McQuiston FC, Parker JD. Heating, Ventilating,
and Air Conditioning. Analysis and
Design. 4th Ed. New York: John Wiley & Sons. 1994.
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