general and dilution ventilation
TRANSCRIPT
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General and DilutionVentilation
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General Ventilation - Purpose
General ventilationProvide heating or coolingProvide make-up airProvide dilution and reduction of contaminants
such as CO2 and body odor
Dilution ventilationProvide dilution of contaminants to safe levels (<TLV or
LEL)Constrained by comfort and other factorsUsually initial cost: DV cost << LEV costUsually for operation: DV cost >> LEV cost
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Dilution Ventilation - applications
Toxicity of contaminant is low to moderate(High TLV)
Velocity and generation rate of contaminantlow to moderate – must consider periodicgeneration too
Sources are not well localized or identifiable
Mobile sources or variable work process
Energy costs are not a significant concern
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Dilution Ventilation
The solution to pollution is dilution?
Do you want to move a lot of air?
What happens in the winter?
How do you get a sweeping effect?
Why bother with local exhaust if there are toomany sources to vent them all?
To have effective DV we need to:Mix contaminated air with large volume of fresh air
Have sufficient air changes/hour to prevent build-up
Create air movement and mixing at all required locations
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“Laminar” Flow Dilution System
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WITH ARROWS
POISON
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YES, SIR!
THE IMPORTANCE OF ARROWS
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WITHOUT ARROWS
POISON
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Distance from the source and concentration gradientsWhen No Cross-Drafts Exists
See Nicas: Am Ind Hyg Assoc J. 1996 Jun;57(6):542-50
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Mixing byentrainment
High velocity air entrains as much as 20 timesits original volume.
Velocity falls to 10% at 30 diameters.
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Interactive Effects of Cross-Draftsand Proximity to Exposure
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Effects of Cross-Draftsand Worker’s Proximity to the Source
Wake zone downstream of body drawscontaminant from source within hand’s reach(“campfire” phenomenon)
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POISON POISON
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Reverse the flow?
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Blowing with entrainment produces large scaleeddies & mixing
Elevation
ExhaustSupply
General
Zones of Ventilation
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G =d Volvapor( )
dtG =
Amount Evaporatedt2 − t1
We are concerned about dynamic conditions
Assume constant generation rate
Computing Generation Rate
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Calculating dilution volumes
VolumeCont = MassOfTheLiquidMassForOneMole
∗VolumeForOneMole
VaporVol =MassOfTheLiquid
MW∗ 24.04L ∗ 273.15C + T
293.15
760mmHgOPatm
VaporVol =sp.grav.∗ ρH 2O ∗Volliquid( )
MW∗24.04L ∗ 273.15C + T
293.15
760mmHgOPatm
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Target Concentration
Toxicity
TLV ppm Ct as a % TLV
highly toxic, radioa ctive < 20 local exhaust onlyor carcinogenic
moderately toxic 20–100 25somewhat toxic 100-200 50slightly toxic > 200 75
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Concentration if perfect mixing
C =GQ
Concentration if not perfect mixing
C =G
Q / m
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Values of mi for each work station
mi =Ci
Cexhaust
m due to non-uniformitySupply air mixingContaminant releaseDistance from worker
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Mixing factors
Cavg =1
8hoursCiti
i
n
∑
mi =Ci
Cexhaust
Cavg =Cexhaust8hours miti
i
n
∑
Mpeak =C15min
CexhaustMavg =
18hrs
mitii
n
∑
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Accumulation = Generation – RemovalAccumulation = Generation – Removal
RCt =Gdt +Qms
Cs dt –
Qms
Ct dt
C2 = C1e-Qt/mR +mGQ
+ Cs
1- e-Qt /mR( )
−+= ts Cm
QC
m
QG
dt
dCV
( )mVQts
mVQt eCQ
mGeCC //12 1 −− −
++=
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ROOM VOLUME
Important for transient conditionsIrrelevant for steady state
Ct =mGQ
( )mVQts
mVQt eCQ
mGeCC //12 1 −− −
++=
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Application of EquationsApplication of Equations
Ct =mGQ
( )mVQts
mVQt eCQ
mGeCC //12 1 −− −
++=
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Application of EquationsApplication of Equations
( )mVQts
mVQt eCQ
mGeCC //12 1 −− −
++=
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Cyclic operations
If short cycles, effects “average out”
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When conditions change withtime
Solve for time interval during which all conditions areconstant
If conditions change continuously, make interval oneminute
Use result as initial conditions for next interval
( )mVQts
mVQt eCQ
mGeCC //12 1 −− −
++=
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DESIGNING NEW SYSTEM
Locate sources near exhaust fansLocate supply air outlets to directair away from face and towardsexhaust fansSeparate sources from trafficusing barriersBlock undesirable cross draftsand competing sources of motionusing barriers.
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EXISTING SYSTEM IMPROVEMENTS
Substitute less volatile or toxicchemicals
Install or improve local exhaust hoods
Reduce incidence of spills and leaks
Relocate supply and exhaust points
Relocate workers or the sources — orboth
Increase airflow
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Confined Space Ventilation
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Confined Space Ventilation
G
•Drain before purging
•Lock out
•Purge before entering
•Blow air in; don’t suck itout
•Move nozzle around
•Measure beforeentering
•Purge during work
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Purging fans
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Confined Space Ventilation-Purge Time
G
•Should be computed
•Then test againstmeasurements
•Wait minimum time evenif measurement says okay
•Time less with biggerblower
•Bigger blower morecostly, harder tomanuever
If supply air clean, then:
=
=
2
1
2
1
ln
ln
C
C
t
VmQ
C
C
Q
Vmt
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Example Problem
Initial measurements indicate 10,000 ppm of xylene in aconfined space. Assuming that Ct = 0.25 * TLV, howmuch should Q be to allow entry in 30 minutes if:a.
R=1000 ft3, M=3, Cs = 0
Q =RmΔt
ln G +QCs /m –QCo /m
G +QCs / m – QC2 /m
= 1000 ft 3 * 3
30min
ln 010−2
025 *10−6
= 599 ft3/min
b. R=2000 ft3, M=3, Cs = 0 : Solution: Q = 1198 ft3/minc. R=1000 ft3, M=6, Cs = 0 : Solution: Q = 1198 ft3/mind. R=1000 ft3, M=6, Cs = 15 ppm: Solution: Q = 1381 ft3/min
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Summary
• Estimating G and m is difficult
• Reduce G as much as possible
• Reduce greatest contributors to exposure andperceived exposure first
• Use sweeping, but be realistic about it
• Complement local exhaust systems
• Provide winter and summer
• Purge before and during confined space entry