World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 28
TECHNICAL. MANUAL
Cs = pollution at the source.
Cr = pollution in the room.
t = elapsed time.
T = time constant
R = rate of air introduction
(cu.m./min.)
V = volume of room (cu.m.)
E = efficiency of filter.
A = X-sect. area of duct (sq.m.)
L = length of duct. (m.)
td = transport delay time. (mins)
For the case when pollution is increasing in the
room:-
Cr = Cs 2 ( 1 - ˆ
-t / T
)
where T is determined by the volume of the room
and the rate at which air is introduced:
T = V/R (mins)
For the case when pollution is decreasing in the
room:-
Cr = Cs 2 ˆ
-t/T
For the transport delay time:
td = L / ( A 2 R )
Example:
A room has a volume of 5,000 cu.m. and the air conditioning is such that it provides
5 changes of air per hour and the fresh air make-up is 10%. Air enters the building via
an air filter which is 30% efficient at removing pollution and travels 25 metres down
an air duct (2m2 area) to the room. A reference detector is placed after the air filter to
provide reference for a standard detector which samples air leaving the room. The
detector will indicate an alarm if pollution rises to 0.1% obs/m.
If a diesel lorry draws up outside and emits a pollution level of 5% obs/m at the reference
detector; what is the period for which the detector will alarm at more than 0.2% obs/
m and how long will it remain in an alarm condition when the lorry moves away.
5 changes per hour = 5 x 5000 cu.m. per hour
= 416 cu.m per minute
10% make up = 41.6 cu.m. per minute
Transport delay in 2 sq.m. duct = L / ( A 2 R )
= 25 / (41.6 x 2)
= 0.30 minutes
Effective pollution = (1 - E ) 2 [absolute pollution]
= (1 - 0.3) x 5 % obs/m
= 3.5 % obs/m
The alarm will be activated by 0.2% added pollution when the pollution at the outlet
has risen to (3.5 - 0.1) % obs/m = 3.4 % obs/m. This allows for 0.1% added to reach
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