The Relationship Between Pressure and Flow in a Compressed Air System

After more than 25 years in the compressed air industry, it still amazes me that many plant personnel and even those who sell compressed air products for a living don’t fully understand the relationship between flow, or volume (cfm), and pressure (psig). Walk into many body shops or small manufacturing plants, and you will find the compressor operating at an elevated pressure to satisfy the “demand.” If a plant has low air pressure on the production floor, what is the first thing that the maintenance professional does? You guessed it: He or she “jacks” up the pressure on the compressor, not realizing that he or she made the problem worse. Furthermore, the majority of production personnel do not have a clue that compressed air isn’t free. There have been many articles written on this subject over the years, but many of them have been so technical that it was difficult to follow for a person outside of the industry. In this article, I try to address the subject in a way that non-engineering types can understand.

Compressed Air Isn’t Free

I remember an experience that occurred many years ago when I worked for a major U.S. compressor distributor. After thoroughly reviewing a compressed air system, I sold a 200-hp rotary screw compressor to a granite mining facility in east Georgia. One way granite is mined is by using a high-temperature torch that melts the rock to enable a slab to be removed. This process is performed many times vertically down several hundred feet beneath the ground. An air compressor is needed in this process to project the flame. As you can imagine, this is a hot environment for the employee who is working the torch.

A couple of weeks after the compressor was installed, I received a call from the owner, who was complaining that the compressor wouldn’t hold the desired pressure as the ambient temperature increased throughout the day. Being a degreed engineer, I knew that what he was describing wasn’t possible. I also knew that the only way for me to solve this phenomenon was to visit the site to gather data. I arrived early the next morning, and the compressor setting was at 115 psig discharge pressure, just where it was supposed to be. However, as the day got hotter I witnessed just what the owner had said — the discharge pressure began to fall. I knew that this had to be a “system” problem and not a “compressor” problem, so I began to walk around the quarry to see what I could find. After several minutes of investigation I found the culprit, a quarry worker had opened two 1/8-inch ball valves to direct some of the compressed air on his face in an effort to cool himself in this sweltering environment. I instructed the owner to have those two valves closed. When we arrived back at the compressor, the machine was at full pressure.

The moral to the story is that the quarry worker didn’t know that by opening two 1/8-inch valves under 100 psig discharge pressure the system would lose approximately 100 cfm, which equates to a 25-hp rotary screw air compressor (rule of thumb is that a rotary screw compressor delivers 4 to 5 cfm per 1 hp). Therefore, out of these two relatively small leaks, the 200-hp compressor was losing essentially one tenth of its overall capacity. A handy rule of thumb to remember is that a 1/4-inch opening/orifice will leak approximately 100 cfm at 100 psig. Remember, compressed air is “stupid” (i.e. it will follow the path of least resistance). A comical fact when conducting a full plant air audit (both supply side and demand side) is that in many cases the largest event that spikes the system is a shift change. Why you ask? Because production workers typically blow off their workstations between shift changes. Funny but true!

What is the Relationship Between Flow and Pressure?

Another little known fact about compressed air from an end user’s point of view is that discharge pressure has a direct impact on flow. In fact, we know from Boyle’s Law that:

P1 x V1 = P2 x V2

Where P1 is the initial pressure, V1 is the initial volume, P2 is the final pressure, and V2 is the final volume.

Let’s take a look at an example of how to use Boyle’s Law in a real-world scenario:

• A plant has a 25-hp rotary screw compressor rated at 100 acfm at 100 psig.
• However, they can only maintain 80 psig in the production area.
• How much more compressor hp does the plant need to maintain the required 100 psig in the plant header?

Using Boyle’s Law:

100 acfm x 100 psig = (X) acfm x 80 psig

Using a little Algebra:

100 acfm x 100 psig / 80 psig = 125 acfm

125 – 100 = 25 acfm

25/4.5 = 5.5 hp (I assumed 4.5 cfm per 1 hp)

In this simple example, I would recommend adding a 7.5-hp compressor to the system to add volume, which will stabilize the discharge pressure to the desired level.

What is the Cost of Over-Pressurizing the System?

An industry term that has been used in recent years to describe supplying more pressure to the system than is necessary is “artificial demand.” For example, if the production floor only needs 75 psig to maintain steady production, why maintain the compressed air header at 100 psig? The artificial demand in this case is 25 psig. A rule of thumb to remember is that for every 2-psi increase in discharge pressure, the energy (measured at the compressor) goes up by 1 percent.

To determine the “critical pressure” in a production process, you have to leave the comforts of the compressor room and venture out onto the production floor. It’s common to see pressure regulators installed on production equipment — sometimes set for pressures well below the system/header pressure. I’ve personally been in plants where the vast majority of the production equipment only needed 75 psig to function properly, but the plant air header was being maintained at 100 psig. Obviously, there are pressure drops across the clean-up equipment (dryers and filters), which could equate to 10 to 15 psig or more. But if the system was properly sized and maintained, this should be easily factored into what to set the discharge pressure of the compressor. Assuming that the clean-up equipment and piping distribution system were poorly sized and maintained, which equates to a 20-psig pressure drop, you could still cut the pressure on the compressor operating at 125 psig to 100 psig, and still maintain the desired 75 psig on the production floor. Dialing the pressure down by 25 psig, the plant could save 12.5 percent energy on an annual basis. To calculate the energy savings, use the energy equation below:

BHP x .746 x # hours/year x \$/kWh

Motor Efficiency

Note that compressors are rated in hp, not kW. To calculate kW, multiply BHP by the constant (.746). Also note that rotary screw air compressors pull more hp than the motor’s nameplate rating. Typically compressor manufacturers utilize a 1.15 service factor motor, meaning that the motor can safely operate 10 to 15 percent beyond its nameplate rating. Therefore, a 200-hp rotary screw air compressor actually pulls on average 220 BHP at full load, depending on the manufacturer. The exact BHP can be derived from the manufacturer’s technical datasheet, or by contacting a representative of that brand. Let’s go through a quick example:

• 200-hp compressor operating at 125 psig (220 BHP)
• 8,000 hours per year operation
• $0.10 per kWh (the energy cost can be calculated in most cases by obtaining a copy of the end user’s power bill. For this exercise we can calculate the “blended” rate by dividing the total dollar amount of the bill by the total kW usage). For this example, I use \$0.01/kWh
• Motor efficiency of 95%
• How much electrical energy can be saved by lowering the discharge pressure by 25 psig?

220 BHP x .746 x 8000 hours x \$0.1 kWh = \$138,206.00

.95

$131,924 x 12.5% = $17,276.00 per year every year in energy savings!

Keep in mind that this example is for a modulating or load/no load controlled compressor, possibly without adequate storage. If the compressed air system was properly audited on the supply and demand side, which resulted in adding the proper amount of storage, pressure flow controller, etc., then these savings could be much larger. A little hint on determining the critical pressure if it isn’t readily known — simply turn down the pressure switch on the compressor(s) by 2 psig, then wait to see if anyone complains about inadequate plant air pressure. Keep doing this over a period of time, and you will eventually have someone scream at you. Then turn it up 2 psig, and leave it alone.

Another major benefit to dialing down the system/header pressure is that any leaks in the distribution of the compressed air will be reduced. Another way to reduce leaks is to have a leak audit performed and implemented. However, remember that you can effectively reduce leaks, but you can never completely eliminate them.

Education is Key

Compressed air isn’t rocket science, but you do have to know some basics before you can optimize the system. My philosophy to sales has always been to be a consultant and to educate the end user, not to merely be a salesman. If you help an end user solve a problem, that person is much more apt to buy something from you in the future. In addition, an educated customer is a quality customer.

For more information, please contact Chris Downs, Tel. (251) 510-2333.

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