Industrial Utility Efficiency    

Demand Versus Supply, Parts l and ll

Part I

One of the problems I have witnessed over the 38 years that I have audited air systems is the lack of understanding of the relationship between supply and demand. Many an estimate of final results after an aggressive action plan either does not make ROI or the solution will be short lived. The purpose of this article is to investigate the cause and effect that can occur when you reduce demand with no supply changes and the alternative which will produce positive, long term results which you can take to the bank.


A. A 25% Demand Reduction – With No Supply Adjustment

Let’s say that you have discovered that you have 24.4% of the total 1000 scfm demand in the system in leaks and another 20% in open blowing applications. You estimate that this equals 45% in waste of all of the air usage. Your action plan says that you will tag all of the leaks and fix 60% of them. You will also replace the open blowing applications with high efficiency, low volume hand held and stationary blowing devices. The intent is to reduce open blowing by 50%. This sounds like a no brainer. Reduce the total usage by 25%. If you have five 282 scfm compressors with five on and no standby, it’s easy picking to get one unit off-line…..right?! Lets first look at a process flow diagram in Illustration #1.


The Existing Installation

Account for Artificial Demand of Unregulated Uses

We missed a couple of other usages that are important to measure to determine how the system will react. We have listed all of the categories and net volumes of usage to total the 1000 scfm. One of these categories we missed is artificial demand. This is added volume that the demand uses which is created by operating in the system with either unregulated usage or with no regulators at higher pressure than is actually required and no demand expander or master systems density control devices in the system. In a study we did for the American Council for an Energy Efficient Economy some years ago, we determined that in most cases, 80% of all usage, in the 27 systems we audited for the test period, were unregulated or had regulators which were manually jacked wide open by the operators.

Next, we consider that the supply is at 110 psig and the maximum required use pressure is 100 psig @ 70 degrees F. Assuming that the total air volume being consumed is 1000 scfm @ 110 psig @ 70 F, we would ratio the proposed demand control pressure @ .4824 lb/scf or 80 psig @70F divided by the current supply weight of one scf of air @ 110 psig @ 70 F @ .635 lbs/scf, You would get a ratio of .7597. Now multiply 80% or the total percentage of open unregulated usage times the total air usage or .80 X 1000 = 800 scfm. We now know that the difference between the current 1000 scfm and the resulting volume at 80 psig @ 70F is the ratio of the weight of the gas at the two different densities @ 70F or (1 - .7597) X (800- 186) scfm = 148 scfm artificial demand.

You must account for the reduced artificial demand from the other categories which are unregulated. Open drainage was determined during the no load test and deducted from the no load total @ 186 scfm @ 118 psig. This volume is not included in the artificial demand because it is located upstream of the proposed demand expander. The total leaks, which we tested in a no load test, were @ 243 X .7597 = net leaks@ 185 scfm at 80 psig.. Open blowing is 200 scfm X .7597 = 152 scfm @ 80 psig. The balance of the usage is an application which consumes 56.5 scf once every 30 seconds for a duration of 30 seconds at a rate of flow of 113 scfm. The balance of all usage is 200 scfm regulated @ or <80 psig.

Let’s look at Illustration #2 which shows us how the existing system’s equipment profile is set up. We have a detailed look at the signal locations, set points on all of the compressors, and differentials in the current supply system. By simply reducing demand by 25% and doing nothing else in supply besides shutting off the #5 compressor, you would reduce the power usage by 9.8% or 27 kwh. The pressure would continue to operate above 110 psig..This provides a poor return on investment.



A 25% Demand Reduction Only Equated to a 10% Power Reduction


B. A 25% Demand Reduction Coupled with Automation

Let’s assume that we take the same “low hanging fruit” approach to the demand side of the system and then decide to automate the compressors. If you install an “enabling” type of automation and change nothing in terms of the profile, the system will load 4 units and sit there operating at >111 psig.

Enabling systems do not control the compressors nor their motors. They operate based on a pressure signal and when it is too low, will activate the next compressor to do whatever that particular unit is set up to do. When the pressure transducer reads a pre set higher value, it will provide a signal to disengage the last “on “ unit.

Another approach towards automation is installing parallel pressure switches in a control panel and then spreading the switches across the existing settings on the compressor. If the units are in load/no-load, you can reduce some energy, but will also risk high power shut-downs, especially when the compressors are set up as high as these are. Remember, it is the sump pressure that determines the amount of power used and you are starting off with the units operating 10% into the service factor of the motor. In both of the above examples, the system will still maintain 4 units on at all times with the demand the way it is.

One of the problems with load/no-load as an operating format is that as the pressure increases in this system, approximately 80% of the demand volume will increase - making it difficult (at best) to reach the unload pressure for a unit.

I was told by a wise man once that when you automate a problem, you wind up with an automated problem. If you want a proper solution to work, you must determine the right course of actions and the correct order of those actions. In other words, you must come up with a detailed and costed action plan -including a new process flow diagram with a detailed constituents of demand profile and a new signals/differentials/set points profile. When I was twelve, my Dad told me “If you don’t know where you are going, there are thousands of ways to get there. Auditing a system correctly is very effortful, yet also very rewarding.


Part II


A. Let’s Take a Proper Approach to the Problem & Solution

Let’s go back to the process flow diagram and see what should be done to maximize the opportunity. We must first control the density of demand air usage @ or < 80 psig for all users and eliminate artificial demand. We would accomplish this by installing an expander or isentropic regulating device at a central location downstream of the clean up equipment and upstream of the demand piping distribution.

We obviously had to determine not only the constituents of demand, but also the highest pressures for each category of demand. Generally speaking, 80 psig is a good starting point at which to look at demand requirements. If you have a much higher pressure required, for a particular application, you can chose to consider the alternatives to providing a dedicated compressor for the application or getting the required test equipment and testing to determine why such a high pressure is required. For more information on this, please refer to my article, “Pneumatics: Sizing Demand Users” published by Compressed Air Best Practices® Magazine.

Although this may seem simplistic, you can install an expander rated for a higher pressure and volume and then gradually reduce the system’s psig at 1 psig, per period of time, until someone in the plant complains about pressure. You then go to the application, with your test gear, and determine why you need this higher pressure and correct the problem for a P4 initial pressure of 80 psig. You would then return to dropping the expander output value until you achieve your desired system pressure. This particular exercise requires discipline and cooperation on the part of production. This is the predominant reason for having production management involved in the audit from the first day.

It turns out that in this system, the last unidentified application rated @ 113 scfm @ 110 psig is the system’s culprit. In the past, whenever the system’s pressure dropped below 100 psig, this application would drop below the required article pressure and someone would demand a higher system pressure. This is, in fact, why a fifth compressor was added and why the system’s P3 pressure was jacked up to 110 psig. They must of thought that if 100 psig was safe, then 110 psig would be much safer. Let’s look at the proposed process flow diagram.



Demand Reduction Coupled With Automation


B. Required Demand Changes

Let’s first get the demand volume under control. It is common knowledge that leaks have an 80/20 rule. In most systems, 80% of the leak volume is represented in 20% of the leaks by count. What you should do is identify the leaks by volume using an ultrasonic leak detector. Generally 20-25% of the largest leaks will easily reduce 80% of the leak volume. We have also installed a flow meter in the main header immediately downstream of the expander. This not only allows us to monitor flow for leak benchmarking, but also allows us to benchmark total systems flow. It is also much more friendly specifying a flow meter which will be applied to a constant density flow.

The open blowing application is our next group to attack. You can use specialty nozzles, transvectors, ejectors, or low pressure blowers to replace open blowing applications. A 50% reduction is a no-brainer. Please not this well: The higher velocity, lower-mass nozzles cannot be applied in the same manner as the high-mass, low velocity open-blowing nozzles. Generally, they are applied a little further away and at a tangent to the surface you are blowing - as opposed to perpendicular to the surface being blown.

The open drainage is from aftercooler separator drain traps, stuck in the open position. The drain traps need to be repaired or replaced and put back into service. We would suggest pneumatic no-air-loss drains (PNLD’s). Open drainage is identified as an unregulated, cyclical application. If the off-time is equal to or more than the on time, there is an excellent opportunity to reduce demand.

In this case, the application requires a rate of flow of 113 scfm for 30 seconds every minute. That indicates that the actual requirement is 30/60 X the rate-of-flow of 130 scfm = 56.5 scf per minute. Please see Illustration #3 to follow along with the application description. At a rate of flow of 56.5 scfm (½ of the current rate-of-flow), we can store 28.75 scf in a vessel in the 30 seconds that the application is off.

During the on-time, we would continue to flow the other half of the 28.75 scf of air plus the stored volume to satisfy the application. The tank is sized for 28.75 scf in a delta pressure of 80 psig – the required article pressure of 50 psig = 30 psid. To determine the tank size you must multiply the storage volume times 7.48 gallons/ scf times the atmospheric pressure (14.696 psia), divided by the useful differential or 30 psid in this case. The tank size would be >105 gallons or a standard tank size of 120 gallons rated at 100 psig. Because the tank is 120/105 gallons larger than the 105 gallons or 43 scf of storage. At best this makes it difficult to control the application. A simpler approach is the regulate the in flow to a 105/120X30 psid = 26.5 psid + 50 psig article pressure or 76.5 psig initial input pressure to the 120 gallon tank. The net results will be a much lower applications pressure and a reduction of rate-of-flow of 50%. After all demand side actions including fixing the two drain traps on the supply side, the demand requirement has dropped to 369.5 scfm @ 80 psig @ 70 F. Please take a look at Illustration #4 to see the proposed system’s profile.



Open Drainage and Storage

C. Required Supply Changes

By changing the signal location to control storage, changing the set points on the compressors, changing the operation of the compressors to load/no-load, and increasing control storage to handle the failure of an on-line compressor (without the supply psig running into the demand controller), we can easily operate the supply system with 1.33 compressors. The 67% reduction in demand volume will have an appreciable effect on the differential pressure of the air treatment equipment. Depending on the control storage sizing, we could get the supply pressure down appreciably. That will increase the motor efficiency, reduce the stress on the motors, and through proper equipment rotation allow for a more reliable and more cost effective system with less than half of the maintenance cost. The cost of the system, when using the more disciplined approach, will reduce the total cost by $102,540 - compared to $11,977 with the more effortless approach.

Perhaps the greatest benefit to all concerned is the fact that you will have a statistically accurate demand system 100% of the time providing excellent quality control and repeatability for production.

In the ensuing articles, we will discuss such subjects as rate of flow, rate of change, measurement, sizing demand at the point of use equipment, and flat lining high rate of flow and change applications using storage of potential energy to reduce the system's supply pressure, improve the energy used as well as controlling the quality of the application.

Mr. Foss has been auditing compressed air systems since 1969. He has written two books on the subject of compressed air systems, published more than 75 articles, and conducted more than 650 one to three day seminars. He has audited more than 1700 complete systems - generally medium to larger systems and cross audited >2500 added systems.

He can be contacted at Air's a Gas Inc, 3728 Berrenstain Drive, St. Augustine, FL, 32092. His phone numbers are 904-940-6940 office, 904-826-7222 cell. His email address is