Variable area flowmeters are the best option when our application is simple and we need an economical solution. Know why it is so.
“Variable area flowmeters,
because sometimes the most complex
it is not the most useful”
Imagine that we have an application in which we need to obtain only an indication of flow, where a mechanical equipment will be enough, we do not need any type of electronical transmitter or indicator (although if we needed we could incorporate it) and also we look for an economic solution.
Here we have the variable area flowmeter!
Variable area flowmeters are also called “rotameters”. They are simple flowmeters but effective for measuring the flow of liquids, gases or steam.
This type of flowmeters consists of a float inside a conical tube. The fluid flows up through the tapered tube forcing the float to a position with sufficient free area to enable the flow to pass. This free area is related to the flow rate, the weight of the float and the density and viscosity of the fluid.
The pressure drop across the flowmeter remains constant over the entire flow range, since it is related to the fluid density and the float weight and dimensions. Flow indication is achieved since the area of flow increases as the flow rate increases.
This is why this type of measuring principle is known as “Variable Area”.
Working principle in the models of
plastic tube and glass tube
Af = Float area
Ac = Flow measuring tube area
Pf = Float weight
E = Fluid thrust
Free area of flow : Al = Ac-Af
Each position of the float corresponds to a flow rate indicated on the scale printed on measuring tube.
The area changes due to the conical shape of the measuring tube.
Now, there is a small difference when the measuring tube is metallic (we explain a little later this subject of the measuring tube materials):
In this case the measuring system consists on a calibrated orifice and it is the float which is conical. The force from the fluid, as it flows from the bottom to the top, displaces the float until it reaches an equilibrium point:
Working principle in the
metal tube models
The equilibrium point in which the float is suspended depends on several factors:
E = Force of the fluid flow
Pf = Weight of the float
Free area of flow : Al = Ao – Af
Ao = Calibrated orifice area
Af = Float area
In this case the area changes due to the conical shape of the float.
Each float position represents an area between the float and the orifice. This area corresponds to a particular flow rate that is shown by the equivalent scales directly engraved in the measuring tube.
Example of different scales and floats for different flows and fluids in series 2000
Example of different plastic scales and tubes of series PS
Example of scales and glass tubes of series 6000
Measuring tubes of different materials
The measuring tube of variable area flowmeters can be made of different materials:
The material of the rest of components; wetted parts, float, gaskets … is also manufactured in different materials depending on the chemical compatibility with the fluid to be measured and the process conditions.
When to use one or the other?
Variable area flowmeters series PS
Variable area flowmeters series 2000 and series 6000
They allow the reading of flow of turbid and opaque liquids. They are more chemically and mechanically resistant in terms of pressure and temperature and also safer against incidents. They are those that have a range of measurement of larger flow rates and better electronic options than the other options. The SC250 series can be made entirely of plastic for special applications. On the contrary, they are the less economical option.
Variable area flowmeter series SC250 and series M21
Practically all Tecfluid series have some option of adjustable switches (reed, inductive, micro-switch) and transmitters depending on the series that we choose. We talk about them in this new article.
You can also see how glass tubes for variable area flowmeters with this type of tube are engraved, we include a video so you can get a good idea.
Advantages and disadvantages of variable area flowmeters
Because of their low cost, variable area flowmeters are one of the most widely used types in the industry, often proving to be one of the most valid solutions for low flow applications.
They are also widely used as simple flow regulators by adding a manual regulating valve integrated in the body of the flowmeter. In this way it is quite simple to regulate non-automated processes.
Series 2000 with regulating valve
Series M21 with regulating valve
So different sectors such as treatment and purification of water, gas processes, control and measurement in machinery, control and research laboratories, industrial and process refrigeration, chemical pharmaceutical and cosmetic industries… all of them use variable area flowmeters even as emergency control in case of power failures.
“Variable area flowmeters are one of the most widely used types in the industry”
… And so far we have arrived today. We hope to clarify some of your doubts, but you already know that you can contact us through your usual Tecfluid sales-engineer or directly to our email or telephone.