Industrial Utility Efficiency    

Air System Pressure Influences Compressor Power - Part 3: The Influence of System Pressure on Compressed Air Demand

Energy conservation measures (ECM) associated with compressed air have received a significant amount of attention over the years, mostly due to a reasonably short financial return compared with other energy consuming equipment. Over time many of the corrective actions put forward to reduce compressed air energy consumption have been simplified with the goal of encouraging action. Although this is done with the best of intentions, sometimes simplifications and generalizations do not necessarily lead to positive results. One of the most common energy conservation measures for compressed air that leverages best practice calculations involves reducing system pressure. It is the objective of this series of articles to highlight some of the more common issues associated with estimating energy conservation resulting from changing system pressure.

Part 1 of this series identified issues with common methods used to calculate energy savings associated with the influence of system pressure on air compressor power. Part 2 focused specifically on centrifugal compressors to identify the relationship between pressure, capacity and power. This third and final article will focus on the influence of system pressure on compressed air demand.

 

The Influence of System Pressure on Compressed Air Demand

The relationship between compressed air demand and pressure is reasonably intuitive and can be easily observed by witnessing air escaping a balloon or turning down the pressure on a regulator ahead of an air tool or blow gun. The concept is simple, but accurately quantifying the impact of pressure across a network with hundreds of compressed air consumers can become complex. Simple rule-of-thumb calculations have been used by salespeople in the compressor industry for decades to estimate how much additional compressor supply capacity would be required to operate a system at an increased pressure.

As energy conservation through compressed air gained attention, so did the opportunity to reduce compressed air consumption by reducing pressure. The reduction in compressed air demand associated with reducing pressure is sometimes referred to as artificial demand. Although this term has been used by some to include all forms of compressed air waste, the original intent was to define a segment demand associated with operating at an elevated pressure. For simplicity, the amount (quantity) of compressed air associated with a change in pressure will be referred to as artificial demand for the balance of this article.

 

Calculating Artificial Demand

Artificial demand for most systems is a category of compressed air demand, consisting of several time-weighted values estimated as a function of load conditions or some other segmentation of compressed air demand relative to a change in pressures. Artificial demand is stated as a flow unit with respect to a target pressure. It is important to note that a change in operating pressure is required for artificial demand to exist. Artificial demand is typically estimated by applying one of the following three calculations for volume relative to pressure:

  1. Orifice Table Method: The orifice table is a fundamental reference for many in the compressed air industry, listing volume of air in scfm for a range of orifices and pressures. Using an initial and proposed pressure for the artificial demand calculation, volumes from the table for matching pressures are divided to establish a correction factor that is applied to compressed air system demand.
  2. Air Density Ratio: This method uses a table of compressed air density relative to gauge pressure as a source. The density of compressed air for the proposed pressure is divided by the density for the initial pressures and factored against the initial demand to estimate the proposed demand.
  3. Absolute Pressure Ratio: For this method, initial and proposed absolute pressures are divided and factored against the initial demand to estimate the proposed demand. This method will deliver the same results as the orifice chart provided the same standard conditions are used to correct from gauge to absolute pressure. The density method will also deliver very similar results. This is the most commonly used calculation for correcting volume relative to changes in pressure, because it does not require a table reference and can be used as a simple equation.

Since artificial demand is the difference in flow associated with operating at a different pressure, it has been stated using two equations to simplify interpretation of the calculation. Please note; volume in this context refers to the flow of air expressed as a volume with respect to time with V representing the demand in scfm and P representing gauge pressure in psig.

V final = V initial x [(P final+14.5) / (P initial+14.5)]

V artificial = (V initial – V final) x (% influenced load)

 

Artificial Demand — Potential Calculation Errors

When calculating artificial demand, it is very important to segment load conditions and the associated pressures. For many systems, network pressure is highest when demand for compressed air is lowest. This is normally the case for a few reasons:

  • For systems with multiple compressors using cascaded control set points, as fewer compressors are required the next trim compressor in the cascade will dictate the supply pressure using higher pressure values.
  • With compressors using pneumatic inlet modulation controls, pressure increases as compressor supply is reduced. These machines use a subtractive pilot that operates using proportional logic, modulating the inlet valve as a function of the signal pressure.
  • Pressure losses across filters and dryers are reduced as flow across the components becomes less. This applies to systems with multiple compressors connected to common purification equipment or when compressors have some form of supply reduction controls.
  • Any pressure losses across the pipe network, sub-headers and shared point-of-use components will decrease as flow through the components is reduced. This will elevate pressure to some applications as demand decreases.

Since artificial demand is the additional volume of compressed air consumed relative to an initial operating pressure, it is imperative to use pressure values related to each load condition. This is a common error, and for some systems, can be significant depending on how much pressure and demand vary between load conditions. To assist with the explanation, the sample system from Part 1 of this series published in the July 2013 issue will be referenced as follows:

 

Sample system

This example is based on a simple system with four identical 100 hp compressors operating using loaded/unloaded local controls, and a simple pressure cascade between compressor control settings. Compressors are rated for 400 scfm at site conditions, consuming 100 hp at 115 psig and 70 hp at 50 percent load. Each compressor has a 20-second start-permissive (off to full-load). Total system storage is 660 U.S. gallons. For simplicity, the system has no filters or dryers and total ∆P from compressor package discharge to furthest point in the network is <0.4 psi. After recording pressure, amps and flow for a seven-day period, four distinct load conditions were identified:

  1. Day shift, operating eight hours a day, 40 hours a week with an average pressure of 107 psig, three compressors fully loaded and a fourth unit in trim using an online/offline control constantly cycling between 114 psig and 100 psig at 50 percent load.
  2. Afternoon shift, operating eight hours a day, 40 hours a week with an average pressure of 113 psig, one compressor fully loaded and a second unit in trim constantly cycling between 120 psig and 106 psig at 50 percent load.
  3. Night shift and weekends, operating 88 hours a week with an average pressure of 116 psig, with only one compressor in trim constantly cycling between 123 psig and 109 psig at 50 percent load.
  4. Weekdays at 7 a.m. the day shift starts, with demand transitioning from lowest load to highest load linearly in 60 seconds. Since demand increases faster than compressor supply, pressure falls below 100 psig — at times as low as 86 psig — before recovering back to 100 psig. The total event lasts around 90 seconds, followed by the system returning to the normal day shift condition (6.5 h/y). This occurs every weekday morning and no one at the facility has ever complained about insufficient pressure.

For this system, a recommendation was made to install a compressor system controller that would operate any combination of compressors within a 10 psi control band using a rate of change anticipatory control logic, limiting pressure decay to less than 5 psi during the transition from night to day shift. Proposed system would operate at an average pressure of 96 psig +/- 5 psi.

 

The maximum recorded pressure of 123 psig from the sample system cannot be used to calculate artificial demand for all load conditions. Aside from the low-load period when demand averages 200 scfm, the other load conditions operate at lower pressures. Assuming a proposed operating pressure of 96 psig, a reduction in demand of almost 20 percent would be falsely estimated for all conditions by incorrectly using the maximum pressure value of 123 psig. During the day shift when demand is highest, average pressure is only 107 psig. Using this pressure, artificial demand would represent a potential 9 percent reduction in demand; less than half of the savings compared with the estimate using the maximum pressure.

The most common and significant error when calculating artificial demand is assuming that the relative changes in network or supply pressure will impact all compressed-air-consuming equipment. It is important to understand that compressed air demand consists of compressed air discharging to the atmosphere or some reduced pressure across an interface. Depending on the size of the system, there could be hundreds or thousands of individual points, each contributing to the total system demand. This could be air expanding to drive an assembly tool, air actuating a valve by filling a cylinder or air leaking through a blister in a hose. At some point for each individual consumer, there is an interface where pressure can influence air density, velocity and the volume flow rate. Depending on the system and how compressed air is consumed, a reduction in network pressure could directly impact the entire demand, a percentage of demand or have no impact at all. Some frequent artificial demand considerations are as follows:

  • Point-of-use regulators. The majority of compressed air applications have one or several pressure regulators influencing point-of-use pressure. Depending on the design, size and regulator settings, pressure downstream may not decrease at all based on a reduction in network pressure. If pressure downstream of the regulator does not change, the demand associated with the application will not change. The percentage of compressed air demand that falls into this category will not have artificial demand associated with network pressure and this percentage of the system demand must be excluded from any artificial demand calculations. It is important to acknowledge that having a regulator installed does not necessarily isolate network pressure from the point-of-use. Many low-cost regulators will track with the upstream network pressure, causing pressure to decrease at the point of influence. Depending on the installation, artificial demand for the application relative to the change in network pressure could be a percentage of the estimated value or in some situations greater when regulated pressure is very low.
  • Sonic velocity. Some applications will not be influenced by a reduction in network pressure, regardless of whether they are regulated or not. These are applications where air has reached a maximum internal velocity at some lower pressure. As long as the supplied pressure is above some critical value, demand will not change. It is not uncommon to find an application reach a velocity limit at 30 to 40 psig. The portion of compressed air demand operating in these conditions will not have artificial demand and must be excluded from the calculation.
  • Pressure independent control. Some modern compressed-air-consuming equipment controls compressed air at the application based on a desired outcome. An example would be high-speed weaving machines where compressed air is used to move a thread across a defined path in a required time. Internal pressure requirements are low and the unit will automatically adjust internal pressure to achieve the required speed. If network pressure were reduced to this application, it would internally compensate for a change in network pressure, and compressed air demand would not change. This type of application does not have artificial demand, and the volume of total system demand associated with these applications must be excluded from any artificial demand estimations.
  • Compressed air leaks. Some compressed air auditors assume 30 percent of compressed air demand is associated with leaks without any form of measurement or validation. The second assumption is that all leaks are unregulated and any reduction in system pressure will have an associated reduction in artificial demand. For most industrial systems with more than 200 horsepower of compressor supply, a significant percentage of overhead piping joints are welded and not very susceptible to developing leaks. If every mechanical joint used to connect hose, tube or pipe is considered a potential leak, the majority are typically located downstream of one or more regulators. Consequently it is not correct to assume 30 percent of the system demand is unregulated and directly influenced by a reduction in system pressure.

 

Correcting Artificial Demand at the Point-of-Use — Application Tuning

Many compressed air components are installed with a filter-regulator-lubricator (FRL) without detailed expectations regarding application pressure. Air cylinders are a great example of an application that can potentially have artificial demand. It is not uncommon to find regulators set at 90 to 100 psi for a cylinder that will deliver sufficient force and speed at a significantly lower pressure. During installation and tuning of the installed equipment, the cylinder speed is reduced by increasing back pressure on the exhaust metering valve instead of lowering the pressure. The net result for example, is 95 psig supply with 55 psig back pressure. Unless the 95 psig is required to deliver a specified force after the cylinder is fully extended, this same cylinder could be tuned to operate at close to 40 psig by adjusting the regulator and exhaust valve, reducing the required volume of compressed air 49 percent. The higher regulated pressure may also be required to compensate for undersized components causing excessive pressure drop while the cylinder is extending. This can often be seen by watching the pressure gauge when the cylinder strokes. When friction (pressure drop) ahead of the cylinder or the regulator itself is the issue, the gauge reading will initially drop while the cylinder is extending and then recover after the cylinder has completed the designated task. After the cylinder is fully extended, the required work has been done and air is flowing to the cylinder for no purpose other than to raise the pressure unnecessarily, increasing the consumed volume of air. After the cylinder is fully extended, the rate of flow will decrease, along with the pressure drop as pressure increases in the cylinder until flow has stopped. It is not uncommon to see 30 to 50 psi deflection that can be corrected to reduce demand. Although this action may not be as glamorous as a demand expander with a segmented valve and PID control, it can be implemented with almost no capital investment and can deliver significant results for many systems. A facility could reduce total compressed air consumption 20 to 40 percent by diligently tuning point-of-use applications.

 

Energy Conservation and Artificial Demand

It is important to reaffirm that artificial demand is a reduction in demand, not energy. Assuming the pressure at the discharge of the compressor does not change and network pressure was reduced using some type of pressure-reducing device, the energy reduction will be based on how the installed compressors reduce consumed power relative to the reduction in supply requirements. A best-in-class system will reduce power almost directly proportionate to the change in demand. Other systems will deliver a reduction in power that is a percentage of the demand reduction with the extreme being a system with centrifugal compressors that have no more throttle capability and are discharging excess air to the atmosphere in an effort to control pressure. For this type of system, a reduction in demand will have no impact on power.

 

Closing Comments

Although efforts to reduce compressed air pressure can potentially deliver significant energy savings with an attractive rate of return, topics discussed in all three parts of this series identify issues that can erode some or all of the assumed savings. Simple rule-of-thumb estimates are an easy way to quickly assess an opportunity to determine if more detailed analysis is warranted but more detail is required for investment grade projects. The sample system referenced in this series of articles illustrates how energy savings estimates associated with artificial demand and compressor power could vary from more than 50 percent to less than 3 percent depending on calculation method, compressor design and how compressed air is consumed. For larger systems or projects that require validated results, the experience and capabilities of the individuals assessing the system becomes more significant. Contracting credible resources with a reference list of implemented systems becomes an investment as the costs for corrective actions and risks associated with overestimating savings become substantial.

 

About the Author

Mark Krisa is director of global services solutions at Ingersoll Rand and leads the company’s compressed air audit program. This program is designed to deliver customer value by leveraging engineering and compressed air science to improve system reliability, quality and efficiency.

Krisa graduated from the University of Western Ontario in Canada with a degree in engineering science, and has worked in the compressed air industry for more than 20 years. His experience in the industry is diverse, ranging from compressor service technician to engineering and compressed air system auditor. Krisa has authored several papers and speaks regularly at conferences and training events across the Americas. You can contact Krisa with questions or comments.

 

To read Part 1 of the article, please click here. To read Part 2 of the article, please click here

 

To read more System Assessment articles, visit www.airbestpractices.com/system-assessments

 

 


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

October 2013