TM-5-855-4
3-4. Tunnel intake air tempering.
a. Fresh or outside air needed for ventilation is often introduced to installations through shafts or
nnels with bare walls. For a tunnel in continuous use, heat is transferred from the air to the rock in
summer and from the rock to the air in winter. Savings are possible under both conditions, because the
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delivered air is warmed in winter and cooled and possibly somewhat dehumidified in summer, thus
reducing the heating and cooling loads, respectively.
b. The temperature of the air at the exit TL, like that at the entrance To, oscillates above and below the
mean annual temperature T1, but the temperature change (TL - T1 ) is smaller at the exit. This problem is
subject to analytical treatment if it is assumed that the average outside air temperature departures (To -
8760 hours period) according to the basic harmonic equation 3-22:
where the time t expressed in hours, is started at zero on
on 15 July and a minimum temperature on 15 January.
c. The amplitude (T2 - Tl) is the maximum temperature departure of outside air from the mean
annual air temperature. Because the variation of outdoor temperature is based on a single harmonic and
not on diurnal changes, the amplitude (T2 - T1 ) is assumed to be half of the annual range defined as the
difference between the mean temperature of the warmest and the coldest month.
d. Assuming also that the rock temperature in the vicinity of the tunnel is the mean annual
temperature T1 , it maybe shown that the temperature departure (TL - TI ) at distance L downstream from
the tunnel inlet is also periodic.
where
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