Ventilation Flashcards
Purpose of ventilation
Control odors, temperature and humidity, contaminant concentration
Local exhaust ventilation
Capture contaminant at source and remove from work environment
Dilution ventilation
Dilute concentration by adding clean air
General ventilation
Climate control, comfort of building occupants
Dilution vs LEV
Dilution: moderate to low tox, disperse source, intermittent exposure
LEV: high tox, single source, direct exposure
What affects the density of air?
Temperature, elevation, humidity
Density correction factor
Accounts for non-standard conditions which affect air density
Major assumptions about movement of air
- Incompressible
2. Constant density
Flow profile difference between laminar and turbulent flows
Turbulent: flatter
Laminar: parabolic, center line has higher velocity than sides
How does air move through a duct?
Difference in pressure
Static Pressure
Pressure against the sides of a duct, acts equally in all directions, differences in SP cause air to move, is like potential energy
Velocity Pressure
Pressure of air motion, always positive and represents the pressure required to move air, is like kinetic energy
Total Pressure
SP + VP
Losses
SP in = SP out + losses
Losses are due to friciton and turbulence
Volume flow rate
Q = VA
3 types of hoods
Enclosing, capture, receiving
Static hood pressure
SP[hood] = VP [duct] * (1 + K)
K values can be found in tables
Coefficient of entry
Q[actual]/Q[ideal]
Hood entry loss
he = F[h] x VP, where F[h] = loss factor (found in tables)
How does SP losses occur in ducts?
Due to friction, affected by VP. Factors include smoothness of duct interior and presence of settled dust.
Purpose of a fan?
To create SP
Axial flow fans
- Flow moves along axis of rotation
- General/dilution ventilation
- Not good for LEV, insufficient SP
Centrifugal fans
Used for LEV, can run backwards with correct airflow direction and reduced efficiency
Centrifugal radial fans
Blades look like paddle wheel, self cleaning, work horse
Centrifugal backward-curved fans
Wing shaped blades, higher performance, can’t be used in dusty environments, noisy
Centrifugal forward-curved fans
Not good in dusty environments, moves large Q with little SP reqs, quiet
Fan static pressure
Fan[SP] = SP[out] - SP[in] - VP[in]
Six in three out
Allow 6D of duct into fan, allow 3D out of fan
Air hp
min power needed to move air
Brake hp
actual power needed to operate fan
Motor hp
Bhp plus power for drive losses
Fan Laws
Q ∝ RPM
SP ∝ RPM^2
HP ∝ RPM^3
Volume of air needed to dilute
Q = (403 x 10^6 x SG x ER x k)
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(MW x C)
k = safety factor (0-10) SG = specific gravity ER = Evaporation rate MW = Molecular weight C = Concentration
What are non-equilibrium conditions?
Build up and decay
