EXPERIMENT PITOT TUBE

Transcription

1 Read the following BEFORE getting started: EXPERIMENT PITOT TUBE Objectives, Pitot tube: a) Understand dynamic pressure, static pressure, and total pressure, and how they can be used to find air velocity. b) Realize the effect that relative humidity, altitude, temperature, and barometric pressure can have on air density and therefore calculated air velocity c) Realize that calculated air velocity is a nonlinear function of hv, Velocity pressure d) Realize the applications that a pitot tube may have in determining air velocity, and therefore, airflow (CFM), in HVAC. e) Determine maximum static pressure EXPERIMENT Air velocity/flowrate Measurements 1. Read the following about duct system static and velocity pressures BEFORE starting: 2. Start experiment with only 6 inch duct (ATTACHED TO FAN). Read the static and velocity pressures from the manometers. (Note: the inclined manometer is graduated in 0.1 inches of water pressure; it is inclined for easier reading. Thu U-tube manometer, for static pressure, is also graduated in 0.1 inches of water pressure, but more difficult to read). Record the static and velocity pressures for this first case. The total pressure (total energy in airflow) is the sum of these two pressures. 3. Now block off all air flow (with book) at end of 6 inch duct and record static pressure only. How does this compare with total pressure in step 2, since now, there is no velocity pressure. Record the pressure. 4. Now, reduce the duct to 4 inch using the 6 inch X 4 inch reducer and 18 inch length of 4 inch pipe. Record the static and velocity pressures. The sum of these is total pressure. How does total pressure for 4 inch duct compare to total pressure for 6 inch duct? Explain why it is a smaller value (see duct system figure below). 5. Now, add the 4 inch Tee and insert into 6 inch X 4 inch reducer. Reduce the outlet branches of the Tee to 3 inch using the 4 inch X 3 inch reducers and 2 short lengths of 3 1

2 inch pipe. Note the 4 inch supply to the Tee must be reduced to two 3 inch outlets. Why? Record the velocity pressure at each 3 inch branch (outlet). 6. Now calculate the air velocities for each step above using velocity pressures (or read directly from inclined manometer, feet/min.) 7. Finally, calculate flowrates for each step above using flowrate calculator or Q=vA. (Q ft 3 /min, v ft/min, A, ft 2 -cross sectional area of duct). 8. Tabulate and discuss each step of the above procedures. 2

3 Experiment 4(A): AIR VELOCITY CALCULATION USING PITOT TUBE 1.Objective Calculation of air velocity using Pitot tube. 2.Apparatus: Pitot tube, manometer, fan. 3.Theory: The apparatus works on the principle of Bernoulli s principle. 3

4 Bernoulli s Principle states that the total pressure is the sum of the static pressure and the dynamic pressure. To move air fans and blowers are usually used. They work by imparting motion and pressure to the air. When the 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. This force is called the velocity pressure. It is measured in inches of water column (w.c). In operating duct system a second pressure is always present. It is independent of air velocity. Know as Static pressure, it acts equally in all directions. Total pressure is the sum of static pressure and velocity pressure. 4

5 Manometers for pitot measurement are offered in a choice of 2 scale types. Some are made specifically for air velocity measurement and are calibrated for ft/min.to correct the air reading for other than standard air conditions, the actual air density must be known. It may be known if relative humidity, barometric pressure and temperature are known. 4.Procedure: 1. The manometer is first checked for water level and adjusted by adding more. When the water is at the reference point the manometer is allowed to settle down. 2. The blower is switched on and the Pitot tube is held at the exits of the tubes. 3. The air pressure causes the water to move in the manometer which is calibrated in inches of water. a. record the air velocity and dynamic pressure off the red-dyed-water scale b. Record the static pressure from the U-Tube c. Measure and the diameter of the outlets in a suitable table. 4. Remove sections of the tubes and repeat step 3 until only 1 section of pipe is left 5. Use the velocity recordings and pipe diameters to calculate flow rate. Dwyer Calculator: 1.Set relative humidity on the scale provided. On scale opposite know dry bulb temperature, read correction factor. 2.Set temperature under barometric pressure scale. Read density of air over correction factor established in #1. 3.On the other side of calculator, set air density reading just obtained on the scale provided. 4.Under Pitot tube reading (velocity pressure, inches of water) read air velocity, feet per minute. 5.Observations: Most manometer scales are calibrated in inches of water. Using readings from such an instrument, the air velocity may be calculated using the basic formula. Reading from the manometer = inches of water V hv d V = Velocity of air in feet per minute 5

6 To determine the dry air density: hv Velocity pressureininchesof water d density of air inpoundsper cubicfoot P d b d density of air in pounds per cubicfeet P b=barometric static pressure in inches of mercury T absolute temperature (degrees F +460) Velocity of air This manometer includes a scale for reading air velocity at room temperature. T 6. Discussion of Results: Draw a figure of the experimental setup and label (or make a legend of) the flow rates, air velocities and pressures at each place recorded. 6