Venturi Flowmeter

Figure 2-16.—Equipment-flow switch. Venturi Flowmeter: As the coolant approaches the contracted portion (throat) of the venturi flow- meter, the velocity of the coolant must increase as it flows through the contracted zone (throat). The angle of approach is such that no turbulence is introduced into the stream. A pressure tap is located at the side wall in the pipe ahead of the meter, and another one is located at the throat. The increase in velocity of the coolant water through the throat results in a lower pressure at the throat. The flow rate is proportional to the difference in pressure between the two taps. The gradual tapering of the meter walls back to pipe size downstream of the throat allows the coolant water to slow down with a minimum of lost energy. This allows a recovery of nearly 99 percent of the pressure on the approach side. To monitor the amount of flow through the venturi flowmeter, a differential pressure gage is used to monitor the pressure difference be- tween the two pressure taps. A calibration chart is usually supplied with the flowmeter to convert the different pressure to gallons per minute (gpm), or the Figure 2-17.—Venturi flowmeter. face of the meter may indicate readings in gpm. Fig- ure 2-17 shows a venturi flowmeter. Orifice Flowmeter: The orifice flowmeter works in the same manner as the venturi flowmeter, but its construction is much simpler and less expen- sive to manufacture. In place of the tapered throat, the orifice flowmeter uses a flat plate with a hole in it, which causes a considerable loss of pressure down- stream. The efficiency of this type of flowrneter can be as low as 65 percent. Rotameter: The rotameter, shown in figure 2-18, is a variable area orifice meter that maintains a constant differential pressure with varying flow. The rotameter has a float positioned inside a tapered, tem- pered glass tube by the action of the distilled water flowing up through the tube. The flow restriction is the space between the float and the tube wall; this area increases as the float rises. The differential pressure is fixed, depending on the weight of the float and the buoyant forces resulting from the combination of float material and the distilled water’s specific gravity. The tapered tube of the rotameter is usually glass, with calibration marks reading directly in gpm. The major advantage of a rotameter over a venturi meter is the visibility of the coolant, as it allows quick determina- tion of excessive entrained air in the coolant. 2-18