Measurement of Flow of Fluids

In a milk product manufacturing process it is often required to measure the proportion of materials introduced in to a process and the amount of materials produced. For this purpose it is essential to measure the flow rate and flow quantities of fluids.Also the flow measurement is required for the purpose of cost accounting for the services like steam and water.

i. Types of meters

The flow of fluids in closed pipes can be measured by many methods, each working on separate principle of operation. These are:

a) Head meter: These meters operate by measuring the pressure differential across a suitable restriction to flow. For example, orifice meters, the venturi tube, weirs etc. These instruments found almost no application in milk processing industries.

b) Area meter: They operate on the principle of variation in area of a flow stream.For example, rotameter. These are very commonly employed in dairy industry.

c) Quantity meter: They measure the time integral of flow rate. That is, the

quantity of fluid passed at a given point. For example domestic water meter.In this section, the description of flow meter shall be restricted to the brief study of construction and working of a rotameter.

ii. Rotameter

Principle of operation: Rotameter is a type of area flow meter. It operates on the principle that the variation in area of the flow stream required to produce a constant pressure differential at a restriction of flow is proportional to the flow rate.

Construction: It consists of a tapered metering tube and a float which is free to move up or down within the tube. The metering tube is
mounted vertically with the smaller end at the bottom. The fluid whose flow rate is to be measured enters the tube at the bottom, passes around the float and moves out of the tube at the top.

When there is no flow through the rotameter, the float rests at the bottom of the tube. At the bottom of the tube the diameter of the float is approximately same as that of the tube. Thus the area of float nearly equals the area of the tube and there is a very small annular opening between the float and the tube. When the fluid enters the tube, the pressure drop across the float increases and it raises the float.This upward movement of the float increases the area between the float and the tube until the upward hydraulic forces acting on the float are balanced by its weight. The metering float now floats in the fluid stream.The float moves up or down in the tube in proportion to the fluid flow rate and the annular area between the float and the tube. It reaches a stable position in the tube .when the forces are in equilibrium. Every float position corresponds to one particular flow rate for a fluid of a given density and viscosity. A calibration scale is provided on the tube and flow rate can be determined by direct observation of the position of the float in the metering tube.

Materials: The tapered tube is made up of Pyrex glass. Metal tapering tubes are used in applications where glass could not be used. In case of metal tubes float position is determined indirectly. This is done by magnetic or electrical techniques.The use of indirect float sensors is better than direct visual indication. The float is made up of dense material such as silver or tantalum. The shape of the float is of bob-shape or inverted cone shape so to provide constant viscous drag at all flow rates.

Installation: For proper functioning and accurate results the float must centreitself in the fluid stream. To achieve this, the tube must be installed vertically. It must be plumb to within about two geometrical degrees.

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