Intelligent Compressed Air: How to Develop a Pressure Profile

An important part of operating and maintaining a compressed air system is taking accurate pressure measurements at various points in the compressed air distribution system, and establishing a baseline and monitoring with data logging.  A Pressure Profile is a useful tool to understand and analyze the compressed air system and how it is functioning.

Pressure Profile 1

Sample Pressure Profile

The profile is generated by taking pressure measurements at the various key locations in the system.  The graph begins with the compressor and its range of operating pressures, and continues through the system down to the regulated points of use, such as Air Knives or Safety Air Guns.  It is important to take the measurements simultaneously to get the most accurate data, and typically, the most valuable data is collected during peak usage periods.

By reviewing the Pressure Profile, the areas of greatest drop can be determined and the impact on any potential low pressure issues at the point of use.  As the above example shows, to get a reliable 75 PSIG supply pressure for a device or tool, 105-115 PSIG must be generated, (30-40 PSIG above the required point of use pressure.)  As a rule of thumb, for every 10 PSIG of compressed air generation increase the energy costs increase 5-7.5%

By developing a total understanding of the compressed air system, including the use of tools such as the Pressure Profile, steps to best maximize the performance while reducing costs can be performed.

If you have questions about getting the most from your compressed air system, 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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Pneumatic Capacitance

Brian Farno and I attended a compressed air training seminar years ago that highlighted best practices, pitfalls, calculations, efficiency, and a variety of other things facing the compressed air industry.  At the same seminar we also discussed pneumatic capacitance.

As it was laid out, pneumatic capacitance is the stored air within a compressed air system – OK, simple enough.  And, in order for there to be any stored energy, there has to be a pressure differential across the storage device – THIS was an AHA moment for me.

I guess I had never really thought about the need for a pressure differential across the storage device in order for there to really be any air stored.  I’m sure if you go back through the tests and exams I took in college there’s some question about it, and I may have known it somewhere in my studies – but the concept really clicked for me in that seminar and at that moment.

I thought about this when visiting a customer’s facility and hearing them complain of dropping line pressure during compressed air operations.  We went to their compressor room and I saw the compressors and tanks in the photos below.

IMG_1439

(3) 75HP Atlas Copco compressors putting out 300 SCFM each. Two of these provide air to the storage tanks below. The third is for operations unrelated to this blog.

IMG_1440

(3) 2200 gallon receiver tanks

Wow!  All this horsepower and air storage and the line pressure is still dropping?  That seems odd.

So, we checked the input and output pressure of the tanks – less than 2 PSI ΔP, effectively limiting the real ability of the tanks.  At this ΔP the tanks were little more than just an addition to the compressed air plumbing of the facility.

We checked output from the compressor and found they had been deliberately decreased to between 80 and 85 PSI.  So, I recommended to leave the output pressure of the compressors (which feed into the tanks) up to 120 PSIG, and to leave the output pressure of the tanks untouched at 80 PSIG.

This change would allow 3 minutes of steady line pressure for the existing compressed air demand (with compressors still loaded) – per tank!  (Calculations at the bottom of this blog.)

This change, while significant, was only part of the solution for this end user.  The bulk of their solution was the installation of EXAIR Super Air Knives at the point of use, which reduce cooling time, improve throughput, and lower compressed air use.

If you think your application may benefit from an EXAIR solution, contact an EXAIR Application Engineer.

 

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

Air calculations:

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)

 

In this application, the values are as follows:

V = 294 cubic feet (per tank)

T = ?

C = 857 CFM (The application required just under 3,000 cubic feet over a duration of 3.5 minutes.  3000 CF/3.5 min = 857 CFM)

Cap = 600 SCFM

Pa = 14.7 PSI

P1 = 120 PSIG

P2 = 80 PSIG

 

So if we manipulate the volume equation just a bit, considering that we know all the values except T, we come up with the following:

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

Therefore,

T = ( (294(120-80)/((857-600)(14.7)) )  —  (units omitted for sanity)

T = 11760 / 3778

T = 3.11 minutes

Compressed Air Calculations, Optimization, and Tips

EXAIR uses our blog platform to communicate everything from new product announcements to personal interests to safe and efficient use of compressed air. We have recently passed our 5 year anniversary of posting blogs (hard for us to believe) and I thought it appropriate to share a few of the entries which explain some more of the technical aspects of compressed air.

Here is a good blog explaining EXAIR’s 6 steps to optimization, a useful process for improving your compressed air efficiency:


One of the Above 6 steps is to provide secondary storage, a receiver tank, to eliminate pressure drops from high use intermittent applications. This blog entry addresses how to size a receiver tank properly:

Here are 5 things everyone should know about compressed air, including how to calculate the cost of compressed air:

These next few entries address a common issue we regularly assist customers with, compressed air plumbing:

In a recent blog post we discuss how to improve the efficiency of your point of use applications:

Thanks for supporting our blog over the past 5 years, we appreciate it. If you need any support with your sustainability or safety initiatives, or with your compressed air applications please contact us.  

Have a great day,
Kirk Edwards
@EXAIR_KE

Advanced Management of Compressed Air – Storage and Capacitance

Receiver Tank Drawing

Last week I attended the Advanced Management of Compressed Air Systems seminar put on by the Compressed Air Challenge.  For those unfamiliar with the Compressed Air Challenge, it’s an organization focused on delivering reliable and sustainable compressed air that has maximized efficiency.  Many of the industry’s best practices are preached, if not mandated, and the ultimate goal is to reduce compressed air use as much as possible.  This fits in line with EXAIR products, their design for maximum efficiency, and the recurring ability of our customers to reduce their compressed air use by using our products.

The “advanced” seminar dives into compressed air system profiles, explores the math and theory behind system design, explains the various types of system controls, and shows how to balance compressed air supply and demand.  These things are great not only on their inherent value, but also because when Brian Farno, Russ Bowman, and I attended the Fundamentals of Compressed Air Systems seminar, we kept raising our hands asking questions that were “too advanced”.  The material presented here answered many of those questions, and sparked a few new ones.

One of the questions that came to me during the training had to do with the capacitance of a compressed air system.  When storing the energy of a compressed air system in a receiver tank, there has to be a pressure gradient in order for there to be energy storage.  If a receiver tank has the same inlet and outlet pressure, it is merely part of the system plumbing and provides no benefit to the system when demand peaks.  So I thought to myself, “if a pressure drop is needed across a receiver tank to achieve system capacitance, and the capacitance of the system is related to the value of that differential, a system could theoretically be supplied enough compressed air volume with the right pressure specs”.

So, I looked to the formula used for sizing a receiver tank.

V = (T x (C – R) x Pa)/P1-P2

Where:

V = Receiver volume in cubic feet

T = Time of the event in minutes (amount of time for which the receiver tank must be able to provide compressed air at the needed rate)

C = Intermittent demand amount (how much flow or “Q”) in CFM

R = Flow into tank during event (through needle valve, spare air in system, etc.) in CFM

Pa = Absolute atmospheric pressure (14.7 PSIA)

P1 = Initial receiver tank pressure (in PSI)

P2 = Final receiver tank pressure (in PSI)

Ok, nothing new there.  First grade stuff.  Plugging in some theoretical values we could say:

T = 1 minute

C = 50 cubic feet per minute

R = 0 cubic feet per minute.  In this example we’ll assume there is no residual compressed air flow and that the receiver tank must deliver all the airflow for the duration of the event.

Pa = 14.7

P1 = 100 PSIG

P2 = 90 PSIG

Using these values, the volume calculates to be 73.5 cubic feet.  But, most receiver tanks are sized in gallons so we can multiply by 7.48 to get the figure in gallons.  (7.48 gallons = 1 cubic foot)  This yields an approximate value of 550 gallons.  In plain terms, for the application above, we would need a 550 gallon receiver tank with an inlet pressure of 100 PSIG and an outlet pressure of 90 PSIG to provide compressed airflow over the needed (1) minute duration.

That’s a big tank.

Now, back to my thought on pressure differentials – if we increase the ΔP, we can decrease the size of the receiver tank.  Let’s say the inlet pressure to the receiver tank can be as high as 130 PSIG (a wet tank, in line before any filters or dryers).  This will quadruple the pressure differential and reduce the size of the tank by 75% to 138 gallons.  Great!

Well, great for a new system, but what about one already in place?  What if the application needs 50 CFM of compressed air flow for 1 minute, and the shop already has a 175 gallon tank.  We can work the equation in reverse to determine the necessary pressure differential that will ensure the system has enough capacitance to sustain the event (approximately 32 PSI).  It’s good to know the math.

As a whole, the seminar was a great success and the presenters proved why they’re experts in the field of compressed air.  We’re not too shabby here at EXAIR either.  If you have an application need, give us a call.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

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