## Viscosity – Why it Matters

EXAIR has been manufacturing Intelligent Compressed Air Products since 1983.  We work every day with air properties, such as compressed air pressures and flow rates, when evaluating product performance to provide a solution to a customer problem. We also need to be versed on a certain liquid property, viscosity, as it relates to the operation of (2) of the EXAIR product families, the Atomizing Spray Nozzles and the Industrial Housekeeping liquid vacuums (Reversible Drum Vac and Chip Trapper).

Viscosity is often referred to the thickness of a fluid.  A common example would be water and honey.  Water is a low viscosity fluid, while honey is a high viscosity material.

Dynamic, Shear, or Absolute Viscosity is a measure of the resistance to shearing flow.  If you think of 2 plates (see below) with the bottom one stationary and the top one moving horizontally, and with a fluid between them – this illustrates Absolute Viscosity. When the top plate moves to the right, there will be induced multiple horizontal layers of the fluid, each moving at different speeds (the fluid will be at rest at the bottom, and the top layer will move with the top plate.) The friction generated between the layers will give rise to a force resisting this motion.  The force is proportional to the speed and area of the plate and inversely proportional to the distance between the plates.

In simplest terms, viscosity is the ratio of the shear stress, τ,  to the velocity gradient du/dy.  Of concern to us, it takes more force to cause a very viscous (thick like honey) fluid flow.

Centipoise, cP,  is the common unit of measure for viscosity, and 1 cP is equal to 0.01 g/cm/s.

Water has a viscosity of 1 cP at room temperature and honey ranges from 2,000-10,000 cP.

How does all this relate to EXAIR? It relates to EXAIR because we engineer products to specifically move and use liquids. There are limitations to the viscosity each of these products can handle and still perform well.

The EXAIR Atomizing Spray Nozzles can be used with liquids up to 800 cP. The Internal Mix type nozzles can be used on liquids up to 300 cP. The External Mix nozzles can be used with liquids over 300 cP and up to 800 cP.  Lastly, the Siphon Fed models are good to 200 cP, and do not require a pressure source for the liquid. Each nozzle type will atomize the fluid and provide a very small droplet size, which is ideal for washing,  rinsing, coating, cooling and other applications. Exceeding the cP each of these nozzles are designed to use will result in larger droplet sizes and a more inconsistent fluid flow, which may impact your process.

The other EXAIR product family where viscosity is important, is the Industrial Housekeeping Products, especially the High Lift Reversible Drum Vac and High Lift Chip Trapper.  They are capable of moving liquids with viscosities of up to 1400 cP through 20′ of hose.  Simply provide compressed air and turn any closed head drum into a high powered liquid vacuum. Great for coolant, hydraulic oils, waster water, and many other fluids. For these products, as the cP increases, so does the time it will take to move the liquids into or out of the drum.

To discuss your fluid viscosity and how an EXAIR Intelligent Compressed Air Product can make your process better, feel free to contact EXAIR and myself or one of our other Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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