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Assessment of Heat Stress and Heat Strain

determined. Air or any other fluid will always flow from a region of higher total pressure to a region of lower total pressure in the absence of work addition (a fan). There are three different but mathematically related pressures associated with a moving air stream. Static pressure (SP) is defined as the pressure in the duct that tends to burst or collapse the duct and is expressed in inches of water gage (“wg). It is usually measured with a water manometer, hence the units. SP can be positive or negative with respect to the local atmospheric pressure but must be measured perpendicular to the air flow. The holes in the side of a Pitot tube (see Figure 9-9) or a small hole carefu Ily drilled to avoid internal burrs that disturb the air flow (never punched) into the side of a duct will yield SP. Velocity pressure (VP) is defined as that pressure required to accelerate air from zero velocity to some velocity (V) and is proportional to the kinetic energy of the air stream. The relationship between V and VP is given by V= 1096fZf or Vp- V ( ) 2 – P 1096 [1.4] where: v = velocity, fpm VP = velocity pressure, “wg If standard air is assumed to exist in the duct with a density of 0.075 1 bm/ftl, this equation reduces to V=4005JVP or ( ) 2 VP- V 4005 [1.5] VP will only be exerted in the direction of air flow and is always positive. Figure 1-1 shows graphically the difference between SP and VP.