An analytical solution to predict flow conditions in a leaking out/in ventilation column is presented. The solution predicts the pressure and the airflow for fans delivering/removing a required amount of air to or from mine workings located at various depths or at high altitudes. The solution can be used to calculate pressure loss due to friction, the amount of air leaking out or in and electrical motor input power of all fans involved. It can also be used to examine the performance of existing ventilation systems and to classify the operating ducts as excellent, good, fair, or poor. The quality of a column is defined by a friction factor and an effective area of leaking spots. A leaky ventilation column requires more electrical power than a non-leaking one. The additional power increases almost proportionally with the length in systems less than 1,000D long. In a longer duct this power raises as dimensionless length to the power of two or three. New and well-installed ventilation systems have the effective leaking area of 5 to 10 mm~2/m~2 (representing an excellent or a very good duct). However, the leaking area for ducts in service may increase gradually with time to 20 to 60 mm~2/m~2 (making it a fair or a poor duct). A lack of proper maintenance can already increase the electrical power consumption by 10% on systems only 100D long. Auxiliary ventilation systems with L/D > 1,000 should be served by two or more fans. Multiple fans should never be installed in one location, at one end of a column or in the middle of a column. They should be initially distributed along a column so that each fan draws the same amount of electrical power regardless of different pressure-volume duties. However, as duct qualities undergo changes with time equal power criterion will be destroyed.
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