Air enters a downcast shaft at the pressure and temperature of the atmosphere existing at the top of the shaft. As the air falls down the shaft, its pressure increases, just as the pressure on a diver increases as he descends into the ocean. The rise in pressure is caused by the increasing weight of the overlying column of fluid as we plunge deeper into the fluid. However, that simple observation belies the common belief that air always flows from a high pressure region to a connected lower pressure region. In a downcast shaft, exactly the opposite occurs showing how easily we can be misled by simplistic conceptions.
The process of gravitational compression, or autocompression, in the downcast shaft produces an increase in temperature of the air. This is independent of any frictional effects and will be superimposed upon the influence of any heat transfer with the surrounding strata that may occur across the shaft walls. The raten of that heat transfer depends upon the thermal properties of the rock and the difference between the rock temperature and air temperature at any given horizon.
As the air returning from most underground facilities remains at a fairly constant temperature (the ‘thermal flywheel’), upcast shafts are much less susceptible to variations in heat exchange than downcast shafts. It is common to find that an upcast shaft is operating at near adiabatic conditions. The path line 3 to 4 on the Ts diagram indicates a typical situation, descending from P3, T3 to P4, T4 and diverted to the right of an isentrope by the effects of friction.
A thermodynamic analysis of the downcast shaft was given in Section 8.2.2. The corresponding analysis for the upcast shaft follows the same logic and, indeed, the equations derived for the downcast shaft can also be used, with suitable changes of subscripts, for the upcast shaft. For that reason, details of the analysis will not be repeated. The reader may care to carry out a self test by attempting to derive the following results and to prove the annotation of areas shown on the Ts diagram.