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5 Main Ventilation
P n = P 2 − P 3
(5.2)
So that (Eq. 5.3):
P n = γ (1−2) h d − γ (3−4) h u
(5.3)
Note that specific weights are not constant, that is, they depend on other variables.
In order to take this aspect into account, the universal equation of perfect gases can
be used: P V = n R mole T . This equation can be transformed to operate with mass
instead of moles leading to:
P V = m R mass T
Moreover, in terms of weight, the equation can be written as:
P V = W R w T
Rearranging the equation:
W
V
=
P
R w T
Since
W
V
= γ (specific weight), replacing γ in the equation we have:
P 2 − P 3 = h d
P md
R wd T md
− h u
P mu
R wu T mu
where
• P md : Average pressure in the downcast shaft (Pa),
• P mu : Average pressure in the upcast shaft (Pa),
• R wd : Constant per unit mass of gases in the downcast shaft
J
K kg
, and
• R wu : Constant per unit mass of the gases in the upcast shaft
J
K kg
.
At this point, a certain humidity can be assumed for the intake depending on the
climatology of the place. In the case of the return shaft, a humidity close to 100%
can be admissible. With both parameters set, it is possible to obtain accurate values
for R we and R ws from air data tables. Despite this fact, simplifications can still be
made, specifically:
• To use the same value of the constant R for both the intake and return shafts.