Basic Calculations for Occupational Safety and Environmental Health Professionals - Mathematics and Statistics for the Environment - page 411

the source to remove the contaminant prior to its release into the workplace air. The second type
of ventilation system is the supply ventilation system , which (as the name implies) adds air to the
work area, usually to dilute work area contaminants to lower the concentration of these contami-
nants. However, a supplied-air system does much more; it also provides movement to air within
the space (especially when an area is equipped with both an exhaust and supply system—a usual
practice, because it allows movement of air from inlet to outlet and is important in replenishing
exhausted air with fresh air).
Air movement in a ventilation system is a result of differences in pressure. Note that pressures in
a ventilation system are measured in relation to atmospheric pressure. In the workplace, the existing
atmospheric pressure is assumed to be the zero point. In the supply system, the pressure created
by the system is in addition to the atmospheric pressure that exists in the workplace (i.e., a positive
pressure). In an exhaust system, the objective is to lower the pressure in the system below the atmo-
spheric pressure (i.e., a negative pressure).
When we speak of increasing and decreasing pressure levels within a ventilation system, what
we are really talking about is creating small differences in pressure—small when compared to the
atmospheric pressure of the work area. For this reason, these differences are measured in terms of
inches of water or water gauge , which results in the desired sensitivity of measurement. Air can be
assumed to be incompressible, because of the small-scale differences in pressure.
Let's get back to the water gauge or inches of water. Because 1 psi of pressure is equal to 27 in.
of water, 1 in. of water is equal to 0.036 lb pressure, or 0.24% of standard atmospheric pressure.
Remember the potential for error introduced by considering air to be incompressible is very small
at the pressure that exists with a ventilation system. The environmental professional responsible
for ventilation in the workplace must be familiar with the three pressures important in ventilation:
velocity pressure, static pressure, and the total pressure. To understand these three pressures and
their function in ventilation systems, you must first be familiar with pressure itself. In fluid mechan-
ics, the energy of a fluid (air) that is flowing is termed head . Head is measured in terms of unit
weight of the fluid or in foot-pounds/pound of fluid flowing. Note that the usual convention is to
describe head in terms of feet of fluid that is flowing.
So what is pressure? Pressure is the force per unit area exerted by the fluid. In the English system
of measurement, this force is measured in lb/ft 2 . Because we have stated that the fluid in a ventila-
tion system is incompressible, the pressure of the fluid is equal to the head. Velocity pressure (VP)
is created as air travels at a given velocity through a ventilation system. Velocity pressure is only
exerted in the direction of airflow and is always positive (i.e., above atmospheric pressure). When
you think about it, velocity pressure has to be positive, and obviously the force or pressure that
causes it also must be positive. Note that the velocity of the air moving within a ventilation system is
directly related to the velocity pressure of the system. This relationship can be derived into the stan-
dard equation for determining velocity (and clearly demonstrates the relationship between velocity
of moving air and the velocity pressure):

V = 4005 VP (15.13)

Static pressure (SP) is the pressure that is exerted in all directions by the air within the system,
which tends to burst or collapse the duct. It is expressed in inches of water gauge (in. wg). A simple
example may help you grasp the concept of static pressure. Consider the balloon that is inflated at
a given pressure. The pressure within the balloon is exerted equally on all sides of the balloon. No
air velocity exits within the balloon itself. The pressure in the balloon is totally the result of static
pressure. Note that static pressure can be both negative and positive with respect to the local atmo-
spheric pressure.
Total pressure (TP) is defined as the algebraic sum of the static and velocity pressures or

TP = SP + VP (15.14)