Just like in the case of friction between moving solids, viscosity will determine the energy required to make a fluid flow. Viscosity can be measured using various methods from do-it-yourself methods to viscometers that are available in the market.
Watch our webinar on the different types of viscosity! You can think in terms of different fluids taking different times to, for example, pour out of a cup. Water will pour out of a glass in a fraction of a second and will take the shape of the receiving container in just as short of a time.
On the other hand, a high viscosity fluid like honey would take minutes to pour out and the interface between honey and air would take just as long to settle. You might have notice that I have been talking about time when trying to establish which of those two fluids is more viscous. That's what we refer to as a measurement of kinematic viscosity or how fast does a fluid flow for a given force applied to it.
In this example, the applied force is gravity. Generally, a centrifugal pump is suitable for low viscosity fluids — as the pump is generated high liquid shear. If the viscosity increases the pump performance must be adjusted to account for the additional resistance to shear. As this occurs, there is a small reduction in flow, a significant reduction in head or pressure, and a considerable increase in power draw.
Positive displacement pumps are the recommended pump when handling viscous fluids. They operate at lower speeds and transfer lower amounts of shear energy to the fluid in comparison to a centrifugal pump. Based on your application, there are a wide variety of positive displacement pumps that are appropriate. Viscosity is a crucial factor in determining the friction loss, due to the shear energy in a fluid process system.
It is also important to understand as you select the size of valves, filters, instrumentation and piping. Global Pumps have put together a typical list of viscosities. These will help you determine exactly what the viscosity is of the fluid you are pumping. It is also important to note that the viscosity of liquids and gases are affected by temperature but in opposite ways meaning that upon heating, the viscosity of a liquid decreases rapidly, whereas gases flow more sluggishly.
Why is this the case? As temperature increases, the average speed of molecules in a liquid also increases and as a result, they spend less time with their "neighbors. The viscosity of a gas, however, increases as temperature increases because there is an increase in frequency of intermolecular collisions at higher temperatures. Since the molecules are flying around in the void most of the time, any increase in the contact they have with one another will increase the intermolecular force which will ultimately lead to a disability for the whole substance to move.
There are numerous ways to measure viscosity. Shear rate is the measure of the change in speed at which intermediate layers move with respect to one another. Isaac Newton, the man to discover this formula, thought that, at a given temperature and shear stress, the viscosity of a fluid would remain constant regardless of changes to the shear rate. He was only partly right. A few fluids, such as water and honey, do behave this way. We call these fluids Newtonian fluids. Most fluids, however, have viscosities that fluctuate depending on the shear rate.
These are called Non-Newtonian fluids. There are five types of non-Newtonian fluids: thixotropic, rheopectic, pseudoplastic, dilatant, and plastic. Different considerations are required when measuring each of these fluid types.
Viscosity measurements are used in the food industry to maximize production efficiency and cost effectiveness.
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