World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 26
T E C H N I C A L
.
M A N U A L
The SOURCE is considered as the point where pollution from an external source enters
the controllable system. In most cases this is the building in which the protected room
is sited.
In Fig. 1. there is some distance between the source and the room. There is thus a delay
in air reaching the room corresponding to the time taken for air to travel the distance.
The graphs in Figs. 3. & 4. label this as TRANSPORT DELAY. For Fig. 2. there is no
distance between the SOURCE and the protected room, therefore the TRANSPORT
DELAY is zero. It is important to note that this situation is not inherent in the forced air
ventilation case because the INLET may be from another part of the same controllable
system.
The INLET is the point where air enters the protected room. For the sake of simplicity,
this is considered to be a single point but in practice this is not necessarily the case.
The OUTLET is the point where air leaves the protected room. Again, for the sake of
simplicity, this is considered to be a single point but in practice this is not necessarily
the case.
Filtering indicated in Fig. 1. may have an effect in reducing the amount of pollution in
the incoming air. Its efficiency is shown as E in Figs. 3 & 4. If it is 100% efficient then
E = 1 and there is no apparent need for referencing.
The rate of change of pollution at the OUTLET is shown to be inversely exponential. The
assumption is that polluted air entering the room is instantly mixed with the existing
air in the room. Since air is entering the room air must also be leaving the room and
a proportion of this air must be polluted air. The proportion of polluted air leaving will
increase as its proportion in the room increases. This is a classic case indicating an
inverse exponential increase. If air is not perfectly mixed and is not mixed at all, a “wave
front” of polluted air will advance across the room and the pollution at the OUTLET will
suddenly rise when the wave front reaches it. This will correspond to a delay which is
the time taken for the “wave front” to traverse the room. This is an unlikely state of
affairs. If the mixing is short of being perfect but exists, then the rise will be similar to
that shown but not follow an exact inverse exponential path. Assuming perfect mixing
is fairly close to what can be expected in practice and allows a mathematical analysis
of the situation.
It will be noted from Fig. 3 that there is a period for the
CALCULATED
POLLUTION
ADDED
when
