NGUYÊN LÝ HOẠT ĐỘNG CỦA ROTAMETER ( LƯU LƯỢNG KẾ)

Rotameter là một thiết bị đo lưu lượng dòng chảy của chất lỏng hay chất khí trong một đường ống kín.

It belongs to a class of meters called variable area meters, which measure flow rate by allowing the cross-sectional area the fluid travels through, to vary, causing a measurable effect.

A rotameter consists of a tapered tube, typically made of glass with a ‘float’, made either of anodized aluminum or a ceramic, actually a shaped weight, inside that is pushed up by the drag force of the flow and pulled down by gravity. The drag force for a given fluid and float cross section is a function of flow speed squared only.
A higher volumetric flow rate through a given area increases flow speed and drag force, so the float will be pushed upwards. However, as the inside of the rotameter is cone shaped (widens), the area around the float through which the medium flows increases, the flow speed and drag force decrease until there is mechanical equilibrium with the float’s weight.
Floats are made in many different shapes, with spheres and ellipsoids being the most common. The float may be diagonally grooved and partially colored so that it rotates axially as the fluid passes. This shows if the float is stuck since it will only rotate if it is free. Readings are usually taken at the top of the widest part of the float; the center for an ellipsoid, or the top for a cylinder. Some manufacturers use a different standard.
The “float” must not float in the fluid: it has to have a higher density than the fluid, otherwise it will float to the top even if there is no flow.

ROTAMETERS
The rotameter is an industrial flowmeter used to measure the flowrate of liquids and gases. Its operation is based on the variable area principle: fluid flow raises a float in a tapered tube, increasing the area for passage of the fluid. The greater the flow, the higher the float is raised. The height of the float is directly proportional to the flowrate. With liquids, the float is raised by a combination of the buoyancy of the liquid and the velocity head of the fluid. With gases, buoyancy is negligible, and the float responds to the velocity head alone. The float moves up or down in the tube in proportion to the fluid flowrate and the annular area between the float and the tube wall.

The float reaches a stable position in the tube when the upward force exerted by the flowing fluid equals the downward gravitational force exerted by the weight of the float. A change in flowrate upsets this balance of forces. The float then moves up or down, changing the annular area until it again reaches a position where the forces are in equilibrium.
To satisfy the force equation, the rotameter float assumes a distinct position for every constant flowrate. However, it is important to note that because the float position is gravity dependent, rotameters must be vertically oriented and mounted.

The tapered tube’s gradually increasing diameter provides a related increase in the annular area around the float, and is designed in accordance with the basic equation for volumetric flow rate:

where:k
Q = độ lớn của lưu lượng dòng chảy 
k = một hằng số
A = annular area between the float and the tube wall
g = force of gravity
h = pressure drop (head) across the float
With h being constant in a VA meter, we have A as a direct function of flow rate Q. Thus, the rotameter designer can determine the tube taper so that the height of the float in the tube is a measure of flow rate.

Variable area flowmeters are used primarily to set flowrates. The operator observes the meter, and adjusts the valve to bring the process flow to the proper flowrate.The meter’s ability to repeat or reproduce this flowrate is of primary importance. Rotameters are repeatable up to ±1 ⁄4% of the instantaneous flowrate. This feature enables the operator to reset or adjust the flow with confidence.