## Absolute Pressure Ratio

Compressed air driven devices are always given a specification for the compressed air flow at a certain pressure.  For example, an EXAIR model 1101 Super Air Nozzle has a specified flow of 14 SCFM at 80 PSIG.  This means that when this nozzle is operated at 80 PSIG, it will require 14 SCFM of compressed air flow.  But what if the force from the nozzle is too high when operated at 80 PSIG and a lower operating pressure is needed?

Thankfully, we can calculate the compressed air flow at a different pressure using the absolute pressure ratio.  The absolute pressure ratio says that for any given change in absolute operating pressure, there will be a proportional change in the air consumption of a device.  So, what is an absolute pressure?

Put simply, an absolute pressure is the value which you would measure on pressure gauge plus the atmospheric pressure (PSIA, or Pounds per Square Inch Atmospheric).  So, our 80 PSIG operating pressure mentioned above is an absolute pressure of 94.5 PSI (80PSIG + 14.5 PSIA).  Similarly, if we wanted to determine the compressed air flow at an operating pressure of 60 PSIG, our absolute pressure would be 74.5 PSI (60 PSIG + 14.5 PSIA).

The absolute pressure ratio is a ratio of the new absolute operating pressure (new PSIG + PSIA) compared to the known absolute operating pressure (known PSIG + PSIA).  For example, when comparing an operating pressure of 60 PSIG to an operating pressure of 80 PSIG, we will end up with the following ratio:

This means that our absolute pressure ratio in this case is 0.7884.  To determine the compressed air flow for the model 1101 Super Air Nozzle at 60 PSIG, we will take this ratio value and multiply it by the known flow value at 80 PSIG.  This will yield the following:

Utilizing this formula allows us to truly compare a compressed air powered device at different operating pressures.  If we did not use the absolute pressures when comparing compressed air devices at differing pressures, our values would be erroneously low, which could yield to improper compressed air system planning and performance.  And, using the absolute pressure ratio allows anyone to make a true comparison of compressed air device performance.  If specifications are given at different pressures, performance data can be misleading.  But, by using the absolute pressure ratio we can make a more exact evaluation of device operation.

If you have a question about your compressed air device and/or how a change in pressure will impact compressed air flow, contact our Application Engineers.  We’ll be happy to help.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

## Compressed Air Nozzles – Engineered vs. Commercial vs. Homemade

In the world of compressed air blow off solutions, there are a number of options which customers must consider.  Should the plant maintenance personnel configure something on-site?  Is there a low-cost option available from a catalog warehouse?  Or, is there an engineered solution available – and if there is, what does this even mean?

Ultimately, the exercise in comparing these options will help select the option best for the application and best for the company.  In order to make these comparisons, we will consider each option based on the following attributes:  Force, sound level, safety, efficiency, repeatability, and cost.  These are the factors which impact the ability to perform as needed in the application, and effect the bottom line of the company

Force

Blow off applications require a certain amount of force in order to perform the desired task.  If the blow off is in a bottling line, for example, we will aim for a lesser force than if blowing off an engine block.  But, no matter the application need, we will want to consider the ability of the solution to provide a high force, high impact blow off.  Homemade and commercially mass-produced nozzles produce low-to-mid level forces, which translates to a need for more compressed air to complete a task.  Engineered nozzles produce high forces, minimizing compressed air use.

Sound level

Have you ever been to a concert and felt your hearing reduced when you left?  This can be the case for personnel in industrial environments with unregulated noise levels.  Homemade or non-engineered blow off solutions carry the risk of increased sound levels which are outside of the acceptable noise level limits.  EXAIR engineered nozzles, however, are designed to minimize sound level for quiet operation and continual use.

Safety

Workplace safety is a serious matter for everyone from shop floor personnel to executive management.  Whether you’re working with or near a compressed air operated device, or your making decisions for your company which have to do with the compressed air system, safety is undoubtedly a priority.  Unfortunately, homemade and commercially available nozzles normally fail to meet OSHA standards for dead-end pressure requirements (OSHA Standard 29 CFR 1910.242(b)).  This means that these solutions can pose a risk of forcing compressed air through the skin, resulting in an embolism which can cause severe harm or even death.

EXAIR nozzles, however, are designed to NEVER exceed dead-end pressure limits and to provide an escape path for airflow in the even the nozzle is blocked.  This safety aspect is inherent in ALL EXAIR designs, thereby adding safety to an application when an EXAIR product is installed.

Efficiency

Compressed air is the most expensive utility in any facility.  Energy enters as an electrical source and is converted into compressed air through a compressor where up to 2/3 of this energy is lost as heat.  The resulting 1/3 of converted energy is then piped throughout a facility as compressed air, where up to 1/3 of the air is lost to leaks.  With this in mind, maximizing the efficiency of a nozzle solution becomes imperative.  A homemade solution or commercial nozzle does not maximize the use of the compressed air.  The result is a need to increase flow or increase pressure, both of which result in higher energy costs.

EXAIR nozzles are designed for maximum force per CFM.  This means that any of our nozzles will produce the highest force at the lowest possible compressed air consumption.  This, in turn, reduces demand on the compressed air system and allows for a lower energy requirement.  Less energy demand means less energy costs, which goes straight to the bottom line of your company.

Repeatability

When installing a nozzle solution, it is important to have the same force and flow from each unit.  If a solution needs to be replicated, balanced, or adjusted in any way, having various forces and flows from a homemade setup will induce difficulty and could make changes impossible.  Line speed or volume increases may not be possible due to variance in the output flow and forces from homemade setups, but an engineered solution will produce the same output every time.  This means you can adjust the nozzles as needed to achieve the perfect solution in your application.

Cost

For many customers and businesses, the most important aspect of any solution comes down to cost.  Will the solution work?  And, how much does it cost?  When it comes to a homemade or commercial blow off solution, it may or may not work, and it will have a low purchase cost.  But, the purchase price isn’t the whole story when working with compressed air.  The real cost of an item is in the operation and use.  So, while a homemade solution will be cheap to make and install, it will be EXPOENTIALLY more expensive to operate when compared to an engineered solution.  An excellent example is shown above.  An open copper tube is compared to an EXAIR model 1102 Mini Super Air Nozzle.  The copper tube cost only a few dollars to install, many times less than the EXAIR nozzle, but it costs almost two THOUSAND dollars more to operate in a year.  Translation:  Install a cheap blow off solution and pay for it in utility costs.

EXAIR nozzles and blow off solutions are engineered for maximum force, lowest possible noise level, OSHA safety compliance, maximum efficiency, and maximum repeatability.  These factors allow for options which not only solve application problems, but also do so with the lowest total cost possible.  If you have an application in need of a blow off solution, feel free to contact our Application Engineers.  We’ll be happy to help.  And, if your curious about the benefit of our products in your application, consider our Efficiency Lab.  We will test your existing setup next to our recommended EXAIR solution and provide the impact to your bottom line.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

## Receiver Tank Principle and Calculations

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.

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

## Replacing Liquid Nozzles with Engineered Air Nozzles

I wrote a blog a few weeks ago about increasing efficiency with EXAIR Super Air Nozzles.  In the application for that blog we used engineered nozzles to place open pipes, resulting in an efficiency increased of ~65%.  This week’s installment of efficiency improvements boasts similar figures, but through the replacement of misused liquid nozzles rather than open pipe.

The image above shows a compressed air manifold with a number of nozzles.  BUT, the nozzles in this manifold are not compressed air nozzles, nor do they have any engineering for the maximization of compressed air consumption.  These are liquid nozzles, usually used for water rinsing.

In this application, the need was to blow off parts as they exit a shot blasting machine.  When the parts exit the shot blasting process they are covered in a light dust and the dust needs to be blown away.  So, the technicians on site constructed the manifold, finding the liquid nozzles on hand during the process.  They installed these nozzles, ramped up the system pressure to maintain adequate blow off, and considered it finished.

And, it was.  At least until one of our distributors was walking through the plant and noticed the setup.  They asked about compressed air consumption and confirmed the flow rate of 550 m³/hr. (~324 SCFM) at 5 BARG (~73 PSIG).

The end user was happy with the performance, but mentioned difficulty keeping the system pressure maintained when these nozzles were turned on.  So, our distributor helped them implement a solution of 1101SS Super Air Nozzles to replace these inappropriately installed liquid nozzles.

By implementing this solution, performance was maintained and system pressure was stabilized.  The system stabilization was achieved through a 61% reduction in compressed air consumption, which lessened the load on the compressed air system and allowed all components to operate at constant pressure.  Calculations for this solution are shown below.

Existing compressed air consumption:  550 m³/hr. (324 SCFM) @ 6 BARG (87 PSIG)

Compressed air consumption of (9) model 1101SS @ 5.5 BARG (80 PSIG):  214 m³/hr. (126 SCFM)

Total compressed air consumption of 1101SS Super Air Nozzles:

Air consumption of 1101SS nozzles compared to previous nozzles:

Engineered air nozzles saved this customer 61% of their compressed air, stabilized system pressure, improved performance of other devices tied to the compressed air system, and maintained the needed performance of the previous solution.  If you have a similar application or would like to know more about engineered compressed air solutions, contact an EXAIR Application Engineer.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE