In the nirvana of compressed air systems, there is no wasted compressed air power. The air is supplied at just the right pressure, quality and volume for effective production. Compressed air supply always meets demand exactly. In the real world, this never happens — the compressor supply produces as much air as it can at the highest possible pressure and the production system uses all it can get. "When there isn’t enough air, the pressure falls, and we add another compressor." Have you heard this reasoning before? Sure you have! Compressed air energy savings opportunities are the hot topic this year.
Still, much of the focus is on the supply side and how to produce and deliver the air to the system most efficiently. Typical responses include replacing older, less efficient air compressors; installing more efficient, responsive control systems; replacing older, less efficient motors with high efficiency motors and so forth. Do not misunderstand. There are real air power energy savings on the supply side and a review of the supply side is important.
For example, the design efficiency of compressors varies from 4 to 5.5 cfm per hp, 10 to 20% by type and 5 to 10% within the same type. Older motors may have a mechanical efficiency of 85 to 90% whereas newer motors fall in the range of 93 to 96%. This 4 to 8% variance has about the same impact on the energy cost as the 5 to 10% variance within the same type of air compressor.
Correctly applied unloading controls of the proper type have a maximum variance range of 5 to 10%. When the controls are grossly misapplied or improper, this variance can escalate. The most important aspect of unloading controls is that they decrease system usage and power consumption. Without them, there is no significant power savings.
Air dryers and filters have less than a 5% variance with respect to performance and pressure when correctly applied. When misapplied, these are items that can be, and often are, a significant opportunity for energy savings.
As an adjunct to the supply side evaluation, determine the basic energy cost per cfm per psig for your compressor. This number is based on your energy rate, the basic compressor efficiency and the number of operating hours per year. It allows easy calculation of potential energy and dollar recovery for identified system air savings opportunities. This may be a lot greater than you realize.
For example, consider a 100-hp compressor producing 4 cfm per input hp using electricity at $0.06 per kW and operating 8,000 hours a year. The annual cost of power per cfm is $100 and the annual cost of power per psig is $398. Keep these numbers in mind as you identify basic opportunities in the demand or process side of the compressed air system.
The supply side of the air system is important
Opportunities for saving upward of 10 to 15% exist even for a reasonably well applied system. More importantly, the supply side must be in tune with the total system to translate reduced air usage into lower energy costs. However, most of the attention seems to be paid here — and much less on the demand or production process side of the air system — that can account for 20 to 30% of the available opportunities.
Consider that the demand for air actually starts at the process. How much air is required to optimize production? What quality of air? At what pressure? Delivering just the right air to the process at the lowest possible cost is more easily said than done!
Examine the system
Let’s take a journey through your compressed air system. Other than the obvious leaks, what should we be looking for? Pressure loss in piping is one factor often overlooked — we just assume it is acceptable. Measuring pressure loss at full production load is all-important. In a system that is laid out well, the interconnecting piping between the air supply and the distribution piping should exhibit no pressure drop. Let’s review some of the more common piping errors you might find.
A tee connection can have a feed line of compressed air trying to break into a flowing stream of air. This type of connection is common and the turbulence caused by the 90-degree entry often amounts to a pressure drop of 2 or 3 psi. In our 100-hp example, you spend $800 to $1,200 every year to produce the pressure that is lost here.
Using a 30- or 45-degree angle entry instead of a tee eliminates this pressure loss. The tee connection has 15 times the loss of the directional entry. The extra installed cost of the directional entry is negligible.
Using 90-degree elbows instead of long-radius elbows is another source of pressure drop. Standard elbows cause 25% more turbulence than long- or swept-ells. Again, the cost difference is negligible during the initial installation.
The term "dead head" refers to flows causing extreme turbulence by coming together at opposite ends of a tee connection. In the example shown in Figure 1, the pressure loss was almost 10 psi. Replacing the dead head with a long ell and 30-degree directional entry reduces the loss to 0 psi. This represents 300 hp worth of air — about $1,200 per psig or $12,000 annual power cost to produce the 10 psi wasted at the dead head.