Industrial Utility Efficiency

# Missed - Demand-Side Opportunities Part 7 - The Importance of System Pressure Control

Whenever we start a compressed-air energy survey there are always two key topics plant personnel feel are paramount – leaks and reducing pressure. In this installment of our series on missed demand-side opportunities we’ll address the importance of compressed air system pressure.

The benefits of controlling system pressure are often obscured by a misconception that controlling system pressure is very important because “for every reduction in system pressure of 1 psig there will be a 1/2% reduction in input energy use.” This is a “rule of thumb” that has been misinterpreted – it ONLY applies to the “air compressor 100 psig class discharge pressure.” There will be different multipliers for other pressure related to the input power/energy.

Actually, effective control of “production system pressure” has a great deal more potential energy savings than the 1/2% reduction of input energy. For example, a 100 horsepower (HP) air compressor delivering 500 cfm at 100 psig will save about 5% power (5 HP) with a reduction in discharge pressure from 100 psig to 90 psig (5 HP). Using this “rule of thumb” the annual potential savings is $1,790 per year ($.06 kW/h @ 8000 hr/yr).

#### The most important rule to remember when evaluating “missed opportunities” in demand reduction with regard to system pressure is “the higher the pressure inlet to almost any process the higher the air flow (cfm) usage.”

There is nothing wrong with this savings and if practical it should be harvested. However, the most important rule to remember when evaluating “missed opportunities” in demand reduction with regard to system pressure is “the higher the pressure inlet to almost any process the higher the air flow (cfm) usage.”

### Are Your Regulators Regulating?

This is the single most significant issue we find in almost all plants. Regulators are often selected by opening size, i.e., 3/4 inch, rather than airflow capacity at the lowest possible inlet pressure and allowable pressure loss.

Double acting air cylinders are often a key place to look for this issue. Often the line from the regulator to the cylinder inlet is too small to hold the volume required to feed the cylinder on its stroke waiting for the regulator to open. This is a function of regulator dynamics and response time and available air supply to the cylinder through its complete stroke at a steady satisfactory operating pressure. Often the stroke is ineffective – the operator responds by opening the regulator to higher or even full line pressure and the air demand increases proportionally.

##### Figure 3.

Figure 3 shows one method to correct this – measure the dynamics and calculate the storage required to complete the stroke, install a small storage vessel to supply the stroke air, which will refill when the regulator opens. Other options that may apply are to increase the size of the feed line to accomplish the same thing, or change regulators to one with a more appropriate operating profile. The net requirement is to deliver the required minimum volume of air at the lowest steady inlet pressure. Again, when implemented properly and monitored, productivity and quality will be improved and the demand flow will be at its optimum volume.

Be sure to look for the regulator that is not regulating.

##### Figure 4.

Figure 4 shows a regulator installed to a process that is taking in 110 psig compressed air and delivering 110 psig compressed air to the system. In all probability the process does need 110 psig of air to run but something has interfered with the timing and operating dynamics, or the regulator selected is not appropriate for the applicator. Whatever the cause, this situation calls for an investigation.

### Summary

These projects when successfully implemented usually deliver high results in compressed air reduction with low investment cost. Generally speaking in a 100-psig-class air system, most processes will have an optimum operating inlet pressure of 70 psig to 90 psig. If the process requires a higher inlet pressure you will probably have higher header pressure.

Future articles will cover some methods to handle such situations, particularly when one process requires significantly higher inlet pressure than the rest of the plant.

Here are key takeaways when it comes to system pressure control:

• Know what the lowest effective pressure to each process is.
• Deliver it in a controlled steady manner.
• Monitor it if feasible.
• Identify your demand reduction and go for the “atta boy.”

We hope you’ve found this interesting and look forward to your comments! Contact Hank van Ormer, email: hankvanormer@aol.com, tel: 614.580.2711

To read similar Compressed Air Pressure System Assessment articles visit www.airbestpractices.com/system-assessments/pressure.

Read all the articles of this series:

Missed Demand-Side Opportunities Part 1 - Flow Restrictions from Pipe Headers

Missed Demand-Side Opportunities Part 2 - Integrating Multiple Air Compressor Controls

Missed Demand-Side Opportunities Part 3 - Controlling Open Blowing with Compressed Air

Missed Demand-Side Opportunities Part 4 - Utilizing Air- Driven Venturi Vacuum Generators Efficiently

Missed Demand-Side Opportunities Part 5 - Think Inside the Box to Achieve Savings with Cooling of Control Enclosures

Missed Demand-Side Opportunities Part 6 - Look to AODD Pumps To Lower Compressed Air Demand

Missed-Demand Side Opportunities Part 8 - Optimizing a PET Blow Mold and Filling Plant For a Balanced System