General and Dilution Ventilation - University of Washington
嚜澶eneral and Dilution
Ventilation
General Ventilation - Purpose
? General ventilation
每 Provide heating or cooling
每 Provide make-up air
每 Provide dilution and reduction of contaminants
such as CO2 and body odor
? Dilution ventilation
每
每
每
每
Provide dilution of contaminants to safe levels ( LEV cost
Page 1
Dilution Ventilation - applications
? Toxicity of contaminant is low to moderate (High TLV)
? Velocity and generation rate of contaminant low to
moderate 每 must consider periodic generation too
? Sources are not well localized or identifiable
? Mobile sources or variable work process
? Energy costs are not a significant concern
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 too many
sources to vent them all?
? To have effective DV we need to:
每 Mix contaminated air with large volume of fresh air
每 Have sufficient air changes/hour to prevent build-up
每 Create air movement and mixing at all required locations
Page 2
※Laminar§ Flow Dilution System
Supply Plenum
Exhaust Plenum
Distance from the source and concentration gradients
When No Cross-Drafts Exists
10
50
500 200
See Nicas: Am Ind Hyg Assoc J. 1996 Jun;57(6):542-50
Page 3
Mixing by entrainment
Vo
22∼
0.10 Vo
30 Outlet diameter
High velocity air entrains as much as 20 times
its original volume.
Velocity falls to 10% at 30 diameters.
Interactive Effects of Cross-Drafts
and Proximity to Exposure
40
10
50
350 200
20
Page 4
Effects of Cross-Drafts
and Worker＊s Proximity to the Source
40
10
50
350 200
20
Wake zone downstream of body draws
contaminant from source within hand＊s reach
(※campfire§ phenomenon)
POISON
POISON
Page 5
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