1456 Velocity and Flow Rate Measurement

In many situations, it is necessary to know the flow rate of some substance, such as air or steam, to determine where energy is being used. For example, a factory with several buildings and a central steam system may not be metered so that the steam consumption of each building can be determined. As another example, it is generally difficult to estimate the amount of air being moved by a fan in an air-conditioning system without making some measurement of air velocity. Three types of measuring instruments used are flow hoods, pitot tubes, and orifice plates.

Flow Hoods

A flow hood resembles an inverted pyramid with the top replaced by a small cube. The inverted pyramid is made of cloth treated to minimize air leaks. The small cube is a turbine that generates current which is measured by an attached meter. In practice, the opening of the hood is placed over the grill emitting the air. Air is forced to the base of the pyramid, turns the turbine, and generates electricity. The meter is calibrated in ft/min (or m/sec). Since the cross-sectional area is known at the point where the velocity is being measured, the number of ft3/min (or m3/sec) can be immediately calculated.

Pitot Tubes

The pitot tube operates on the principle that air flow across the end of an open tube creates a pressure drop, and a measurement of this pressure drop can be converted into a measurement of the air velocity at the end of the tube. Pitot tubes have been used in applications ranging from air flow rates from a duct to determining the air speed of an airplane. They can be used to estimate the air flow velocity across a duct and may be more convenient than flow hoods for some applications. Care must be taken, however, that enough readings are made to give a representative velocity profile—one reading is not enough.

Orifice Plates15

An orifice plate consists of a disk with a hole of known diameter mounted in a pipe or duct with a manometer attached to the pipe upstream and downstream from the orifice plate. Since the hole in the orifice plate is always smaller than the inside diameter of the pipe, the pressure downstream of the orifice plate is smaller than the pressure upstream of the plate. This pressure difference can be used together with the diameter of the orifice and the inside diameter of the pipe and the density of the material to give a value for the flow rate.

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