Ventilation
There are several types of ventilation and each has different uses. The
following are some of the reasons for ventilation:
To keep flammable gases and
vapors below the lower flammable limit (LFL)
To keep the air movement at a
certain level so that the heat stress can be reduced
To keep toxic contaminants at
or below certain concentrations
To reduce odors
To dilute cigarette smoke
To control microorganisms,
dusts and other particulates
To limit carbon dioxide
buildup
Types
of Ventilation
Dilution (general) ventilation
Local exhaust ventilation
Dilution Ventilation:
This type of ventilation uses clean air (often outside air) to reduce the
level or concentration of contaminants in a building or space.
Dilution ventilation has the following characteristics:
It reduces the concentration
of flammable or combustible gases and vapors below LFL (Lower Flammable Limit)
so that a flame or combustion will not occur in the presence of an ignition
source.
Cost, effectiveness and risk
must be studied prior to investing in this type of ventilation. If a
contaminant is generated at a higher rate, a large amount of ventilation air is
required. This can be very expensive to move large quantities of air.
Does not always reach local
sites where chemical concentrations may exceed the permissible levels (PEL).
May move
contamination to locations that are not contaminated.
It is not an appropriate
system for the removal of highly toxic or very flammable vapors, fumes and
gases that present non-uniform concentrations throughout the day.
Can be used for systems where
concentration generation is uniform and the rate of generation is low.
Local Exhaust Ventilation:
This ventilation system is used to capture contaminants at their source
before they contaminate an entire room or workstation.
Local exhaust ventilation has
the following characteristics:
It captures almost all
contaminants before they leave the source.
Low volume of air is required.
It requires more complex
design which should include the treatment of the toxic materials that are
exhausted at a certain point.
General (Dilution) Ventilation Principles:
(1) Contaminated air should move away from the occupants. Fresh air
should pass by first the occupied areas, then the contaminated areas.
(2) Supply air should be dispersed as widely as possible.
(3) Air distribution through diffusers, grills, registers should not be
blocked by stacking things in front of them. Otherwise, it may cause
concentration buildup.
Concentration Buildup:
Concentration buildup will occur if the contaminant evaporates at a
certain rate and the dilution air is supplied at a fixed rate.
Concentration after a period of time is given by:
C =
G [ (
1 - e -(Q/K)t/V ) / (Q/K) ]
Where,
C = Concentration of gas at time t , (ppm)
G = Rate of generation of contaminant, (ft3/min)
t = Time to reach a specific concentration C, (min)
K = Safety factor (Ranges from 3 to 10. High values apply to those
contaminants with high toxic or flammable hazards, whereas low values apply to
those contaminants with low toxic or flammable hazards)
V = Volume of the space (or room) , (ft3)
Q = Rate of ventilation, (CFM)
The time it takes to reach a certain concentration is given by:
t =
-(VK/Q) { ln [1 - (Q/K) C
/ G] }
Purging:
Concentration will decrease (diluted) after ventilation starts.
Concentration change by ventilation is given by:
ln(
C2/C1) = - Q (t2-t1)/(VK)
where,
C2 = Final concentration (ppm)
C1 = Initial concentration (ppm)
t1 = Initial time (min)
t2 = Final time (min)
D t = t2-t1 =
Total time it takes to reduce the concentration from C1 to C2
level.
Total time to reach a certain concentration level is given by:
D t = t2-t1
= -(VK/Q) [ ln (C1/C2) ]
Make Up Air:
Make up air replaces air removed by ventilation. The following criteria
should be used for contaminated spaces:
It is desirable in a contaminated space to create a negative pressure,
which will draw air through the cracks of doors, windows, ducts
, pipes, etc. and the contaminants will stay in the contaminated space.
For negative pressure:
V makeup
air < V exhaust air
Where,
V makeup air = Volume of make up air
V exhaust air = Volume of exhaust air
If there are contaminants in adjacent spaces, tight seals between spaces
will prevent transfer of the contaminants between spaces. The presence of
positive pressure in a contaminated space will spread contaminated air to
adjacent spaces.
Cleaning Air:
When air is removed from a space through dilution or local exhaust, the
contaminants are moved elsewhere, usually outdoors. Outdoor air quality
standards limit the dumping of contaminants. As a result, it is common to clean
contaminants from, the exhausted air.
Air Cleaning Devices:
Mechanical Separators
Filtration Devices
Wet Collectors
Electrostatic Precipitators
Gas Collectors
Combustion Incinerators
Criteria for Selection of Air Cleaning Devices:
Volume of air flow
Concentration of contaminants
Type of contaminants
Contaminant properties
Temperature
Pressure drop
Contaminant hazards
National, state and local
pollution control laws and regulations
Mechanical Separators:
Gravity Chamber
Impingement Separators
Cyclone Collectors
Gravity Chambers:
Air moves through an enclosure called gravity chamber, which has a large
cross sectional area. When air enters the chamber it s velocity is slower than
the entrance velocity. Due to the gravity , the heavy
particles fall to the bottom of the chamber. Gravity chambers are low cost. Particles smaller than 40 m
m in diameter pass through the chamber and are not collected.
Impingement Separators:
The air passes through a network of baffles. Because the air changes
direction too quickly, particulates have more momentum and cannot make quick
turns. Consequently they impinge on the baffles and the baffles direct them to
one side of the flow. The clean air separated from the particulates passes out
the less contaminated side of the baffles. Particles less than 20 m m in diameter are uncollected. An advantage
is low cost.
Cyclone Collectors:
These are the most common mechanical collectors. Contaminated air enters
tangentially into a circular chamber. The rotating gas causes particulates fall
to the bottom and exit through a port. These separators have relatively low
cost. Particles less than 5 m m
in diameter are not collected.
Filtration Devices:
There are several forms of filtration devices. Mat filters are very
porous and have low efficiency. Some filters. Like those made of glass fibers,
are disposable; others are washable. Ultra filtration filters, such as high
efficiency particulate filters , remove a wide range
of particles but they require considerable maintenance and have high pressure
drops across them.
The most common filtration devices are fabric filters. Some fabric
filters are in the form of tubes or stockings; others have envelope or pleated
form. Air moves through the fabric bags and dust collects inside them. The more
the material collected , the greater the efficiency,
the smaller the particles collected and the higher the pressure drop across the
filter. Many of the large fabric filters are self-cleaning.
Wet Collectors:
The idea of wet collectors is to put contaminants in contact with a
liquid, usually water. Once trapped in the liquid, the contaminants may
accumulate in it. Wet collectors have advantages, such as constant pressure
drop, capability to handle high temperature and humidity, compact design and
moderate cost. Some wet collectors remove 90 % of the particles that are as
small as 1 m m. Water used in
wet collectors may need treatment before disposal.
Electrostatic Precipitators:
Air containing solid or liquid particle passes through
a bank of discharge electrodes that place a high negative charge on particles.
Collecting electrodes or plates with the opposite charge attract the charged
particles. Electrostatic precipitators have high efficiencies, even for small
particulates, and they have very little pressure drop, but they are expensive
to operate.
Gas Collectors:
Gas passing through a liquid may react with or dissolve in the liquid.
Some materials, like activated carbon and alumina, adsorb certain gases and
vapors at the surface of the material. If the gases have economic value , collecting or condensing them may be desirable. The
efficiency of gas collection varies with concentration of gas or vapor in the
air. Another means of removing gases and vapors from air is cooling and
condensation. The incoming air is cooled to form condensation and the resulting
liquid is removed.
Combustion Incinerators:
Combustion incinerators use oxidation to convert gases and vapors into
less harmful materials. However, not all gases and vapors end up in a harmless
form. Combustion may involve direct flame or catalytic combustion. For some
gases and vapors efficiencies may reach 98 %.
Recirculating Air:
If the contaminated air presents high health hazards, it is not
recommended to recirculate it. For recirculation, the
following design factors should be considered:
(1) Contaminants in a recirculated air should
not exceed the recommended concentrations.
Cr
= 0.50 (PEL-Co) (Qt/Qp)
(1/K)
Where,
Co = Concentration in a worker's breathing zone, when local
exhaust is discharged outdoors,
Qt = Total ventilation air flow rate through the affected
space (CFM)
Qp =
Recirculated air flow rate (CFM)
K = Safety factor
PEL = Permissible level of exposure (ppm)
Cr = Permissible concentration exiting a cleaning device prior
to mixing with air in a workspace.
(2) There must be a primary and secondary cleaning system in series, each
with equal efficiency.
(3) There must be a warning system that indicates problem in the cleaning
systems.
(4) Periodic testing of recirculated air is
necessary to ensure that the system is working properly.
(5) Warning signs must tell the occupants of the potential danger from a
failure of the recirculating system.
Standards:
OSHA has several standards requiring ventilation. They involve ventilation
for abrasive blasting, electrostatic spraying operations, powder coating,
textiles, asbestos and other activities.
OSHA Standard: 29 CFR1910.94
Ventilation standard for abrasive blasting, grinding,
polishing, buffing.
Requirements:
(1) Keep concentration below specified levels (1910.100)
(2) Blast cleaning enclosures should be ventilated in such a way as to
maintain a continuous inward flow of air at all openings in the enclosure.
(3) All exhaust and ventilation systems must conform to the principles
and requirements set forth in ANSI standard.
(4) Adequate respiratory protection must be provided by employer.
(5) Dusts are not permitted to accumulate outside the enclosure.
Confined Space:
(1) When inert gases are used in a confined space, the space should be
well ventilated and re-tested before re-entry.
(2) Flammable gases/vapors around LEL can be diluted by ventilation to
lower concentrations. CAUTION: However, if the gases are above UEL,
ventilation can dilute them until they reach an explosive level.
(3) Exhaust and intake air must be located at a distance which will not
permit the mix of exhaust gases with fan intake air.
(4) Standard ventilation unit must have a blower capacity of 600 CFM
(minimum) and 2 x 10 ft flexible ducts and an explosion proof motor must be
used.
(5) All ventilating equipment should be assembled and tested before use.
After using the equipment, it should be cleaned and stored to protect it from
moisture and dirt.
(6) Rule of thumb: Always ventilate the confined space by supplying air
before entry and for the duration of the job performed in a confined space.
Other Standards:
In addition to OSHA, other ventilation standards are set by :
ACGIH (Industrial ventilation)
ASHRAE (General ventilation)
ANSI (Ventilation equipment)
NFPA (Ventilation of highly flammable materials)
Links:
For OSHA ventilation requirements see the following standards on
ventilation:
OSHA Safety and Health
Topics on Ventilation
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