After laying out the ductwork for an air conditioning project and computing the air flow to each terminal, the designer knows the air flow and length for each supply and return duct segment, and can select a duct height that is compatible with the geometric restraints in the area where the ducts will be routed. A segment can be defined as a run of duct from one branch to the next. A branch may be a major junction or a take-off for a terminal diffuser or return grille. His problem is therefore to select a diameter for the duct segment, find a width if the duct is to be rectangular, and then to estimate the duct friction and fitting dynamic losses for each segment of the critical duct routing path. The sum of losses of all of the supply and return segments will make up the external static pressure requirement for the air handling unit. If the filter is external to the unit, the filter and bracket may be included as a segment, or the filter pressure loss may simply be added if internal to a packaged air handler.

This static pressure happens to be positive relative to the air outside the pipe (the atmospheric pressure), and so any leaks in the pipe would push water out into the air. If, however, this were a length of pipe being used to draw water from a reservoir below into a pump (in suction) the relative static pressure would be negative; and if this simple manometer tube were still attached to the pipe, it would suck air into the system. If a tube is added to the inside of the original pipe facing the direction of the flow, known as a ‘Pitot tube’ (as shown in drawing (b) in Fig 2), then the height of the water in this manometer would be greater as this will now additionally reflect the velocity energy of the flowing water (that is always positive) as well as the static head. The manometer column height, z, will give the sum of the static head + velocity head. Used in conjunction with an adjacent static head reading (as in drawing (a) in Fig 2) the velocity head may be determined by subtracting the static head from the combined total head (static head + velocity head); and this assumes that the potential head is the same for both measurements, and so cancels itself out.