Laminar Flow vs. Turbulent Flow – Calculations and Examples

Super Air Knife

What is laminar flow and turbulent flow?  Osborne Reynolds popularized this phenomenon with a dimensionless number, Re. This number is the ratio of the inertial forces to the viscous forces.  If the inertial forces are dominant over the viscous forces, the fluid will act in a violent and chaotic manner.  The formula to determine the Reynolds number is as follows:

Equation 1:  Re = V * Dh/u

Re – Reynolds Number (no dimensions)

V – Velocity (Feet/sec or Meters/sec)

Dh – hydraulic diameter (Feet or Meters)

u – Kinematic Viscosity (Feet^2/sec or Meter^2/sec)

The value of Re will determine the state in which the fluid (liquid or gas) will move.  If the Reynolds number, Re, is below 2300, then it is considered laminar (streamline and predictable).  If Re is greater than 4000, then it is considered turbulent (chaotic and disarrayed).  The area between these two numbers is the transitional area where you start to get small eddy currents and velocities in a non-linear direction.  When it comes to effective blowing, cleaning and lower noise levels, laminar flow is optimal.

Let’s do a comparison of Reynolds numbers between the EXAIR Super Air Knife and a blower-type air knife.  Both products are designed to clean and blow off wide areas like conveyor belts.  The EXAIR Super Air Knife is powered by compressed air, and the blower-type air knife is powered by an air blower.  The main difference between the two products is the dimension of the slot opening.  The Super Air Knife has a gap opening of 0.002″ (0.05mm).  It uses the force of the compressed air to “push” it through the small opening to create a strong velocity.  A blower does not generate a high force, so the opening of the blower-type air knife has to be larger to overcome any back pressure the opening creates.  The gap opening is typically 0.5″ (13mm).  From Equation 1 above, the gap opening helps determine the hydraulic diameter, Dh.  The hydraulic diameter is an equivalent tube diameter from a non-circular flow area.  Since both the Super Air Knives and blower-type air knives have rectangular cross sections, the Dh can be calculated as follows:

Equation 2: Dh = 2 * a * b/ (a + b)

Dh – Hydraulic Diameter (feet or meter)

a – Gap Opening (feet or meter)

b – Gap Width (feet or meter)

If we compare for example a standard 12″ wide air knife, we can calculate the hydraulic diameter, Dh, by using Equation 2:

Hydraulic Diameter Calculations

 

The exit velocity of the Super Air Knives can be changed by regulating the air pressure.  The higher the air pressure, the higher the velocity.  The blower type air knives can use a blower with a variable frequency drive (VFD) to change the exit velocity .  A reasonable air pressure for the Super Air Knife is 80 PSIG, and the exit velocity is near 540 ft/sec (164 m/s).  To equate this to a blower system, the size of the blower will determine the maximum velocity.  To do this comparison, I will use the same velocity as the Super Air Knife.  With the kinematic viscosity of air, it has a value of 0.000164 ft^2/sec (0.000015 m^2/sec) at 70 deg. F (21 deg C).  Now we have all the information for the comparison, and we can now find the Reynolds number from Equation 1:

Reynolds Number Calculation

As you can see from the above calculations, the Super Air Knife has a Reynolds number, Re, below 2300.  The flow characteristic is in the region of laminar (predictable and streamline).  The blower air knife has a Reynolds number, Re, above 4000.  The flow dynamic coming out of the blower-type air knife is turbulent (chaotic and disoriented).  To better show the difference in laminar flow and turbulent flow, I have a picture below that shows both states with water as a fluid (being that air is an invisible fluid).   Here is an example of water  coming out of a drain pipe at Cave Run Lake (first picture below).  With the inertial forces much higher than the viscosity of the water, it is in a turbulent state;  loud and disorderly.  Reynolds number is greater than 4000.  The water is traveling in different directions, even upstream.  As the water flows into the mouth of the river after the channel (second picture below), the waves transform from a violent mess into a quiet, calm stream flowing in the same direction.  This is laminar flow (Re is less than 2300).

Turbulent Water from Pipe
Turbulent Water from Pipe

 

From Channel to River
From Channel to River

With the engineered design of the Super Air Knife, the thin slot helps to create that laminar flow.  All the air is moving in the same direction, working together to give a higher force to remove debris.  If you have turbulent flow like that of a blower air knife, the noise level is much higher, and the disoriented forces are less effective in blowing.  Turbulence is useful for mixing, but horrible for trying to clean or wipe a conveyor belt.  If you have any open pipes, drilled pipes or blower-type air knives in your application, you should try an EXAIR product to see the difference.  An Application Engineers can help you take advantage of laminar airflow.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

Receiver Tank Principle and Calculations

 

Visualization of the receiver tank concept

A receiver tank is a form of dry compressed air storage in a compressed air system.  Normally installed after drying and filtration, and before end use devices, receiver tanks help to store compressed air.  The compressed air is created by the supply side, stored by the receiver tank, and released as needed to the demand side of the system.

But how does this work?

The principle behind this concept is rooted in pressure differentials.  Just as we increase pressure when reducing volume of a gas, we can increase volume when reducing pressure.  So, if we have a given volume of compressed air at a certain pressure (P1), we will have a different volume of compressed air when converting this same air to a different pressure (P2).

This is the idea behind a receiver tank.  We store the compressed air at a higher pressure than what is needed by the system, creating a favorable pressure differential to release compressed air when it is needed.  And, in order to properly use a receiver tank, we must be able to properly calculate the required size/volume of the tank.  To do so, we must familiarize ourselves with the receiver tank capacity formula.

An EXAIR 60 gallon receiver tank

Receiver tank capacity formula

V = ( T(C-Cap)(Pa)/(P1-P2) )

 

Where,

V = Volume of receiver tank in cubic feet

T = Time interval in minutes during which compressed air demand will occur

C = Air requirement of demand in cubic feet per minute

Cap = Compressor capacity in cubic feet per minute

Pa = Absolute atmospheric pressure, given in PSIA

P1 = Initial tank pressure (Compressor discharge pressure)

P2 = minimum tank pressure (Pressure required at output of tank to operate compressed air devices)

An example:

Let’s consider an application with an intermittent demand spike of 50 SCFM of compressed air at 80 PSIG.  The system is operating from a 10HP compressor which produces 40 SCFM at 110 PSIG, and the compressed air devices need to operate for (5) minutes at this volume.

We can use a receiver tank and the pressure differential between the output of the compressor and the demand of the system to create a reservoir of compressed air.  This stored air will release into the system to maintain pressure while demand is high and rebuild when the excess demand is gone.

In this application, the values are as follows:

V = ?

T = 5 minutes

C = 50 CFM

Cap = 40 SCFM

Pa = 14.5 PSI

P1 = 110 PSIG

P2 = 80 PSIG

Running these numbers out we end up with:

This means we will need a receiver tank with a volume of 24.2 ft.³ (24.2 cubic feet equates to approximately 180 gallons – most receiver tanks have capacities rated in gallons) to store the required volume of compressed air needed in this system.  Doing so will result in a constant supply of 80 PSIG, even at a demand volume which exceeds the ability of the compressor.  By installing a properly sized receiver tank with proper pressure differential, the reliability of the system can be improved.

This improvement in system reliability translates to a more repeatable result from the compressed air driven devices connected to the system.  If you have questions about improving the reliability of your compressed air system, exactly how it can be improved, or what an engineered solution could provide, contact an EXAIR Application Engineer.  We’re here to help.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

About Rotary Screw Air Compressors

Recently, EXAIR Application Engineers have written blogs about reciprocating type air compressors: Single Acting (by Lee Evans) and Dual Acting (by John Ball.) Today, I would like to introduce you, dear EXAIR blog reader, to another type: the Rotary Screw Air Compressor.

Like a reciprocating compressor, a rotary screw design uses a motor to turn a drive shaft. Where the reciprocating models use cams to move pistons back & forth to draw in air, compress it, and push it out under pressure, a rotary screw compressor’s drive shaft turns a screw (that looks an awful lot like a great big drill bit) whose threads are intermeshed with another counter-rotating screw. It draws air in at one end of the screw, and as it is forced through the decreasing spaces formed by the meshing threads, it’s compressed until it exits into the compressed air system.

Rotary Screw Air Compressor…how it works.

So…what are the pros & cons of rotary screw compressors?

Pros:

*Efficiency.  With no “down-stroke,” all the energy of the shaft rotation is used to compress air.

*Quiet operation.  Obviously, a simple shaft rotating makes a lot less noise than pistons going up & down inside cylinders.

*Higher volume, lower energy cost.  Again, with no “down-stroke,” the moving parts are always compressing air instead of spending half their time returning to the position where they’re ready to compress more air

*Suitable for continuous operation.  The process of compression is one smooth, continuous motion.

*Availability of most efficient control of output via a variable frequency drive motor.

*They operate on the exact same principle as a supercharger on a high performance sports car (not a “pro” strictly speaking from an operation sense, but pretty cool nonetheless.)

Cons:

*Purchase cost.  They tend to run a little more expensive than a similarly rated reciprocating compressor.  Or more than a little, depending on options that can lower operating costs.  Actually, this is only a “con” if you ignore the fact that, if you shop right, you do indeed get what you pay for.

*Not ideal for intermittent loads.  Stopping & starting a rotary screw compressor might be about the worst thing you can do to it.  Except for slacking on maintenance.  And speaking of which:

*Degree of maintenance.  Most maintenance on a reciprocating compressor is fairly straightforward (think “put the new part in the same way the old one came out.”)  Working on a rotary screw compressor often involves reassembly & alignment of internal parts to precision tolerances…something better suited to the professionals, and they don’t work cheap.

Like anything else, there are important factors to take under consideration when deciding which type of air compressor is most suitable for your needs.  At EXAIR, we always recommend consulting a reputable air compressor dealer in your area, helping them fully understand your needs, and selecting the one that fits your operation and budget.

Russ Bowman
Application Engineer
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Super Air Knives vs. Other Alternatives

There are many ways to blowoff, cool, and/or dry materials.  A few of these methods are the drilled pipe, an array of flat nozzles, using a blower driven air knife and the EXAIR Super Air Knife.  We’ll examine each in further detail, for blowoff of water after a bottling cleaning operation.  Testing was done at 60 PSIG of supply pressure.  The blower utilized a 10 hp motor and was a centrifugal type spinning at 18,000 RPM.  Sound levels were taken with product not present to test the sound of each of the blowoff types.

SAK black1 (2)

pipe-black (2)Drilled pipe is a common blowoff because it is very inexpensive and easy to make.  But drilled pipe performs poorly.  The low cost to make the drilled pipe is quickly outpaced by the inefficiency and high compressed air costs.  The holes are easily blocked and the noise level is excessive, both of which violate OSHA requirements.  Also, the air pattern across the length can be very inconsistent, with areas of low flow and areas of turbulent flow.

flatnozzle (2)Flat air nozzles installed along a length of pipe is another inexpensive option, but it can be a poor performer.  The flat nozzles are available in many materials, from many manufacturers.  The flat nozzles do offer some efficiencies, but similar to drilled pipe, the operating costs and noise levels are high. Air pattern across the length can be inconsistent with areas of high and low flows, leading to incomplete drying or cooling. Also, many of these nozzles are made from plastic material which breaks or cracks when it it hit which causes additional expense and maintenance to replace broken nozzles.

blower (2)A blower air knife can prove to be an expensive and noisy option.  Typically, the initial purchase price is high.  Operating costs are lower than the drilled pipe and flat nozzles and in line with the Super Air Knives.  The blowers can be very large and space for two 3″ diameter hoses requires extra mounting space compared to low profile other options. Noise levels are high, at 90 dBA.  Annual costs for bearing and filter maintenance can be significant.

gh_SAK_750x696EXAIR Super Air Knives performed exceptionally well in removing the water in one pass due to the strong, laminar flow of air.  Sound level was low at just 69 dBA, well within OSHA requirements for an hour 8 hour exposure time. Safe operation is assured, as the Super Air Knife design cannot be dead-ended.  Maintenance costs are low, as the Super Air Knife has no moving parts to breakdown or wear out.

Air-Knife-Blowoff-Comparison
** A pair of 12″ Super Air Knives was used for this comparison

Ultimately, the Super Air Knife is a low cost way to blowoff, dry, clean and cool.

If you have questions about Super Air Knives, or would like to talk about any EXAIR Intelligent Compressed Air® Product, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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What Makes A Compressed Air System “Complete”?

It’s a good question.  When do you know that your compressed air system is complete?  And, really, when do you know, with confidence, that it is ready for use?

A typical compressed air system. Image courtesy of Compressed Air Challenge.

Any compressed air system has the basic components shown above.  A compressed air source, a receiver, dryer, filter, and end points of use.   But, what do all these terms mean?

A compressor or compressed air source, is just as it sounds.  It is the device which supplies air (or another gas) at an increased pressure.  This increase in pressure is accomplished through a reduction in volume, and this conversion is achieved through compressing the air.  So, the compressor, well, compresses (the air).

A control receiver (wet receiver) is the storage vessel or tank placed immediately after the compressor.  This tank is referred to as a “wet” receiver because the air has not yet been dried, thus it is “wet”.  This tank helps to cool the compressed air by having a large surface area, and reduces pulsations in the compressed air flow which occur naturally.

The dryer, like the compressor, is just as the name implies.  This device dries the compressed air, removing liquid from the compressed air system.  Prior to this device the air is full of moisture which can damage downstream components and devices.  After drying, the air is almost ready for use.

To be truly ready for use, the compressed air must also be clean.  Dirt and particulates must be removed from the compressed air so that they do not cause damage to the system and the devices which connect to the system.  This task is accomplished through the filter, after which the system is almost ready for use.

To really be ready for use, the system must have a continuous system pressure and flow.  End-use devices are specified to perform with a required compressed air supply, and when this supply is compromised, performance is as well.  This is where the dry receiver comes into play.  The dry receiver is provides pneumatic capacitance for the system, alleviating pressure changes with varying demand loads.  The dry receiver helps to maintain constant pressure and flow.

In addition to this, the diagram above shows an optional device – a pressure/flow control valve.  A flow control valve will regulate the volume (flow) of compressed air in a system in response to changes in flow (or pressure).  These devices further stabilize the compressed air system, providing increased reliability in the supply of compressed air for end user devices.

Now, at long last, the system is ready for use.  But, what will it do?  What are the points of use?

Points of use in a compressed air system are referred to by their end use.  These are the components around which the entire system is built.  This can be a pneumatic drill, an impact wrench, a blow off nozzle, a pneumatic pump, or any other device which requires compressed air to operate.

If your end use devices are for coating, cleaning, cooling, conveying or static elimination, EXAIR Application Engineers can help with engineered solutions to maximize the efficiency and use of your compressed air.  After placing so much effort into creating a proper system, having engineered solutions is a must.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

Air: What is it?

Air Balloons

What is Air? Air is an invisible gas that supports life on earth. Dry air is made from a mixture of 78% Nitrogen, 21% Oxygen, and 1% of remaining gases like carbon dioxide and other inert gases.  Ambient air contains an average of 1% water vapor, and it has a density of 0.0749 Lbs./cubic foot (1.22 Kg/cubic meter) at standard conditions.  Air that surrounds us does not have a smell, color, or taste, but it is considered a fluid as it follows the rules of fluid dynamics. But unlike liquids, gases like air are compressible.  Once we discovered the potential of compressing the surrounding air, we were able to advance many technologies.

Bellows

Guess when the earliest air compressor was used?  Believe it or not, it was when we started to breathe air.  Our diaphragms are like compressors.  It pulls and pushes the air in and out of our lungs.  We can generate up to 1.2 PSI (80 mbar) of air pressure.  During the iron age, hotter fires were required for smelting.  Around 1500 B.C., a new type of air compressor was created, called a bellows.  You probably seen them hanging by the fireplaces.  It is a hand-held device with a flexible bag that you squeeze together to compress the air.  The high stream of air was able to get higher temperature fires to melt metals.

Then we started to move into the industrial era.  Air compressors were used in mining industries to move air into deep caverns and shafts.  Then as the manufacturing technologies advanced, the requirements for higher air pressures were needed.  The stored energy created by compressing the air allowed us to develop better pneumatic systems for manufacturing, automation, and construction.  I do not know what the future holds in compressed air systems, but I am excited to find out.

Since air is a gas, it will follow the basic rules of the ideal gas law;

PV = nRT  (Equation 1)

P – Pressure

V – Volume

n – Amount of gas in moles

R – Universal Gas Constant

T – Temperature

If we express the equation in an isothermal process (same temperature), we can see how the volume and pressure are related.  The equation for two different states of a gas can be written as follows:

P1 * V1 = P2 * V2  (Equation 2)

P1 – Pressure at initial state 1

V1 – Volume at initial state 1

P2 – Pressure at changed state 2

V2 – Volume at changed state 2

If we solve for P2, we have:

P2 = (P1 * V1)/V2  (Equation 3)

In looking at Equation 3, if the volume, V2, gets smaller, the pressure, P2, gets higher.  This is the idea behind how air compressors work.  They decrease the volume inside a chamber to increase the pressure of the air.  Most industrial compressors will compress the air to about 125 PSI (8.5 bar).  A PSI is a pound of force over a square inch.  For metric pressure, a bar is a kg of force over a square centimeter.  So, at 125 PSI, there will be 125 pounds of force over a 1” X 1” square.  This amount of potential energy is very useful to do work for pneumatic equipment.  To simplify the system, the air gets compressed, stored as energy, released as work and is ready to be used again in the cycle.

Air Compressor

Compressed air is a clean utility that is used in many different applications.  It is much safer than electrical or hydraulic systems.  Since air is all around us, it is an abundant commodity for air compressors to use.  But because of the compressibility factor of air, much energy is required to create enough pressure in a typical system.  It takes roughly 1 horsepower (746 watts) of power to compress 4 cubic feet of air (113L) to 125 PSI (8.5 bar) every minute.  With almost every manufacturing plant in the world utilizing compressed air in one form or another, the amount of energy used to compress air is extraordinary.  So, utilizing compressed air as efficiently as possible is mandatory.  Air is free, but making compressed air is expensive

If you have questions about getting the most from your compressed air system, or would like to talk about any EXAIR Intelligent Compressed Air® Products, you can contact an Application Engineer at EXAIR.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

 

Picture: Hot Air Rises by Paul VanDerWerf. Creative Commons Attribution 2.0 Generic.

Picture: Bellows by Joanna Bourne. Creative Commons Attribution 2.0 Generic.

Picture: Air Compressor by Chris Bartle. Creative Commons Attribution 2.0 Generic.

Intelligent Compressed Air: Deliquescent Dryers – What are They and How do They Work?

EXAIR has written blogs about the different types of dryers that are used to remove liquid from compressed air systems. In this blog, I will be discussing the deliquescent dryer. This dryer falls under the desiccant dryer category, and unlike the regenerative cousins, it is the least commonly used type of dryer. The regenerative desiccant dryers use a medium that will adsorb the water vapor, and the deliquescent dryers use a hygroscopic material that will absorb the water vapor. This salt-like medium has a strong affinity for water, and it comes in a tablet or briquette form. Placed inside a single unit pressure vessel, the “wet” compressed air passes through the bed to become dry. The size of the pressure vessel is determined by the compressed air usage which allows for the proper amount of contact time with the hygroscopic bed. Generally, the dew point will be between 20 to 50 deg. F (11 – 28 deg. C) less than the compressed air inlet temperature. Unlike most dryers, the dew point after deliquescent dryers will vary with the inlet air temperatures.

Vessel Design

The design of vessel is very important for the function of a deliquescent dryer. A grate is required to hold the medium off the bottom. The compressed air will flow from the bottom, up through the bed, and out from the top. The predetermined space between the bed and the bottom of the vessel is used for the liquid that is generated. When “wet” compressed air passes through the bed, the hygroscopic material will absorb the water and change the tablets from a solid into a liquid. Deliquescent dryers got the name from the definition of the verb, “deliquesce” which is “becomes liquid by absorbing moisture from the air”. Once the material is turned into a liquid, it cannot be regenerated. The liquid must be discarded periodically from the vessel and new solid material must be added. With the single tower design, the deliquescent dryers are relatively inexpensive.

Some advantages in using the deliquescent dryers are that they do not require any electricity or have any moving parts. So, they can be used in remote locations, rugged areas, or hazardous locations. They are commonly used to reduce the dew point in compressed air, natural gas, landfill gas and biogas systems. Without the ability for regeneration, no additional compressed air will be lost or used. In comparing the power requirement to other compressed air dryers, the deliquescent dryers have the lowest power requirement at 0.2Kw/100 cfm of air. (This energy rating is only due to the additional power required for the air compressor to overcome the pressure drop in the dryer).

Some disadvantages in using the deliquescent dryers is that the hygroscopic material degrades. The deliquesced liquid does have to be drained and disposed, and new material does have to be added. Even though they do not have any moving parts, they still require periodic maintenance. The deliquescent material can be corrosive. So, after-filters are required to capture any liquid or dust material that may carry over and damage downstream piping and pneumatic components. Also, the variation in the dew point suppression can limit locations and areas where it can be used.

If you have questions about getting the most from your compressed air system, or would like to talk about any EXAIR Intelligent Compressed Air® Products, you can contact an Application Engineer at EXAIR. We would be happy to hear from you.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

 

Photos:  used from Compressed Air Challenge Handbook