This is a sharp, burr free opening through a duct wall provided with a tubing connection of some sort on the outside. 1-A shows a simple thru-wall static pressure tap. These are connected with tubing to a pressure indicating instrument. To sense static pressure, five types of devices are commonly used. With these, air velocity and volume can be quickly calculated. It can be done automatically with the instrument hook-up.įor most industrial and scientific applications, the only air measurements needed are those of static pressure, total pressure and temperature. This subtraction need not be done mathematically. It is an important and useful concept to us because it is easy to determine and, although velocity pressure is not easy to measure directly, it can be determined easily by subtracting static pressure from total pressure. Total pressure is the combination of static and velocity pressures, and is expressed in the same units. When a fan is installed midway between the inlet and discharge of a duct system, it is normal to have a negative static pressure at the fan inlet and positive static pressure at its discharge. In exhaust systems, a negative static pressure will exit on the inlet side of the fan. In pressure or supply systems, static pressure will be positive on the discharge side of the fan. In air conditioning work, this pressure is also measured in inches w.c. Known as static pressure, it act equally in all directions. It s independent of air velocity or movement. In operating duct systems, a second pressure is always present. It is measured in inches of water column (w.c.) or water gage (w.g.). Because of this, a flag or streamer will stand out in the air stream. When force or pressure from the fan blades causes the air to move, the moving air acquires a force or pressure component in its direction or motion due to its weight and inertia. They work by imparting motion and pressure to the air with either a screw propeller or paddle wheel action. To move air, fans or blowers are usually used. However, in these fields the common units of velocity and volume are sometimes different from those used in air conditioning work. The same principles used to determine velocity are also valuable in working with pneumatic conveying, flue gas flow and process gas systems. Velocity or volume measurements can often be used with engineering handbook or design information to reveal proper or improper performance of an airflow system. Volume flow is usually measured in cubic feet per minute (CFM). By multiplying air velocity by the cross section area of a duct, you can determine the air volume flowing past a point in the duct per unit of time. In this field, air velocity (distance traveled per unit of time) is usually expressed in feet per minute (FPM). In air conditioning, heating and ventilating work, it is helpful to understand the techniques used to determine air velocity.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |