ABSTRACT
This chapter begins by explaining both the principles of "conservation of mass" and "conservation of energy" and how these principles apply to ventilation. Two pressures are important to ventilation: the change in static pressure associated with molecular energy within the air and the velocity pressure associated with the kinetic energy of moving air. The air's total pressure at any location is the sum of its molecular and velocity pressures at that location. The concept that the air's total pressure is its total energy underlies the design and operation of all local exhaust ventilation systems. Mercury, as used in barometers, is too dense to be deflected by ventilation pressures. The chapter also explains why a relative pressure rather than the absolute pressure of air is used in ventilation.
