What are the main physical quantities that operationally define a fan? How fans are chosen? Discover it with us!
In the previous article we saw some very important balance equations for the motion of fluids: the material balance and energy balance. In this article, in the light of developed awareness, we will examine more specifically the industrial fans and the quantities that characterize them.
We have seen that the law above is of fundamental importance in the analysis of fluid mechanics. In it, the following parameters appear:
The Bernoulli trinomial tends to preserve it in the different sections taken into account, considering the motion of an ideal fluid. Well, we live in a real world, where often the ideality approximation can not be applied. In the motion of a real fluid there is a further contribution in the equation: the dissipative effect, quantified by the load losses. This is related to viscous forces, which create phenomena of friction.
To carry out the analysis of the equipment of interest to us (the fans) are introduced a new nomenclature.
The unit of measurement used in S.I. for measuring the pressure is the Pascal (Pa). In the ventilation it is frequently used in another unit of measure, namely the millimeters of water column (mmH2O). For the conversion of units the following conversion factor can be used:
In the light of these considerations the initial equation can be written as follows:
If we consider the situation in which the geodetic variations are small (as often happens in the ventilation design) usually geodetic contributions are simplified. So, we get:
We note, therefore, that the effect of the pressure losses is to decrease the total pressure of the system. In particular, we ask ourselves: which contribution of the total pressure decrease? The One linked to the static pressure or the one linked to the dynamic pressure?
We had conducted a similar analysis in the previous article, and we have seen how the term which decreases due to the pressure loss is linked to the static pressure. This is caused by the fact that the velocity of the fluid (which appears in dynamic pressure) is governed by the material balance.
To ensure the conservation of the massive flow, the speed will vary only if there are the corresponding variations of density or of the passage section.
To ensure the conservation of the massive flow, the speed will vary only if there are the corresponding variations of density or of the passage section. Therefore, if we consider a constant cross-section pipe and a situation with scarce density variations, also the fluid velocity will be the same, and therefore also the dynamic pressure of the fluid will remain unchanged.
Let’s dive into the world of fans.
Let us examine the structure together with a fan.
The pipes are connected to the fan through the suction and / or the discharge mouth. Via a specific internal blading, which thanks to an electric motor performs a certain number of revolutions per minute (rpm), the industrial fan is able to create:
So, as you can choose a suitably industrial fan? It is appropriate to clarify the benefits to be obtained.
Especially must be known:
Based on these parameters (Qv and Ps) it can be made a first selection of potential fans, who can work in the required conditions. Each potential choice of the fan is characterized by its own characteristic curves, which describe their operation in different operating conditions. These characteristic curves are shown in a diagram, having:
As can be seen in the image, inside of the graph appear a series of characteristic curves, each associated with a different number of revolutions of the fan. To the point of operation, using other parametric curves shown in the graph, you can associate:
The fan chosen is optimal if in operating point ensures good performance, so if it is close to the maximum efficiency points, which are typically found in the central areas of the same curves.
If you were to deviate too much from the point of maximum efficiency it is good to choose a different type of fan. In particular, if you are too right than the maximum functionality points should choose a bigger fan of the same series. If you are rather too left it is convenient to choose a smaller fan.
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What are the main physical quantities that operationally define a fan? How fans are chosen? Discover it with us!
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