For those willing to look, and use some good old common sense, it is a land of opportunity. Just how expensive is compressed air? It takes about 8 hp of electrical energy to produce 1-hp-worth of work with compressed air. Using compressed air to cool electrical cabinets — a typical application — makes no sense! Here’s some food for thought:
- 1 cubic ft. per minute (cfm) cost - $100 per year in energy cost;
- 2 psig cost — $398 per year in energy cost for every 100-hp;
- 50% of the compressed air generated isn’t used for production (Figure 1).
Figure 1. It is not unusual for 50% of the air to be wasted in plant compressed-air systems.
The values are based on some assumptions: a compressor operates 8,000 hours a year; the cost of electricity is $0.06/kWh and the compressor produces 4.0 cfm at an expenditure of 1 hp (break horsepower) at a discharge temperature of 100°F and 100 psig. Considering the recent rise in energy costs an audit of the compressed air system could yield significant savings. Based on years of plant surveys, we expect that a Level One audit will identify a number of problems with air systems (Level 1: equipment supplying air: compressors, dryers, header, etc.; Level 2: equipment using air.). Here is what we typically see:
- Ignoring the electrical cost of compressors;
- Pressure losses;
- Air leaks;
- Running the header pressure higher than required;
- Pressure regulators operate full open (because they work better that way!);
- Inappropriate uses for compressed air;
- Not recovering heat;
- Inadequate or poor maintenance;
- Not measuring air usage.
Ignoring the cost
There is more to a compressed air system than pipes and a compressor. There is the supply side of the system and the demand side. The supply side may consist of an inlet filter, compressor(s), an after-cooler, a dryer, a receiver, and header piping. Never mind the fixed costs! It takes skilled labor, and electricity. Chilled water may be required for the after-cooler and dryer. Just operating a 100-hp compressor 24/7, 47-weeks/year will cost about $43,000/year ($0.06/kWh, 90% motor efficiency) in electricity alone. The demand-side is every application where air is used. A solenoid valve that stays open most of the time could cost you as much as $9,000/year — this is a demand-side loss. Your system works best when the flow and pressure produced by the supply side match the optimum conditions, i.e., minimum flow at lowest pressure, required by the demand side. Without a complete audit, how will you know what your real costs are?
Pressure losses are costing you money
There are unnecessary losses on the supply side and on the demand side. Causes are: undersized equipment such as dryers and filters, undersized piping, dog-legs, crossing tees, and condensation (oil and water). In a well-designed system, the pressure drops in the piping before the dryer, and after, and to the header, should be zero; the velocity throughout the system should be less than 20 ft. per second (fps). The dryer and filter should have a drop less than 2 psig — together. If you don’t meet any of these conditions, then you are wasting power and disrupting the unloading controls on your compressors. This can be expensive: a reciprocating compressor can run at 0% power in the unloaded state; a screw compressor uses some power in this state. Some common pressure losses are shown in Figure 2.
Figure 2. A poor layout costs lots of money. (Click to enlarge.)
Compressed air leaks
After you have addressed problems with your layout, it is time to look at the next ranking problem: leaks. Thirty percent, or more, of lost air in most facilities is from leaks! It takes a cooperative effort to eliminate this cost. You need a leak management program involving maintenance and production. We suggest a plan with short- term and long-term objectives.
The goal of the short-term program is to determine which areas of the facility have the most leaks. Short-term means a year, with audits every three months. Inspections should be conducted with an ultrasonic leak locator when the plant is running and when it isn’t.
Goals for the long-term program are broader: educating personnel in operations and maintenance to identify and repair leaks. A material balance should be established for the plant and developed so air use for each department can be defined. Test the balance and update it. Providing incentives for conserving air can help drive down costs.
High header pressure
Every process in your plant that uses compressed air has a minimum pressure required to operate effectively. Except for the pressure drop in the delivery system, which should be minimized, extra pressure is a waste. Lower the pressure, save energy, right? Not when the perceived minimum pressure is high. The politically correct approach is to measure the equipment air flow and pressure required at the demand source. Once this is done, installing a receiver (a small vessel between the regulator and the process) or eliminating a restriction should help you find the real minimum.
What if a high pressure really is needed? You may want to supply the local high-pressure need with a secondary, smaller, “booster” rather than drive the whole system at a higher pressure. The filter press (Figure 3) required a 100-psig minimum while the rest of the chemical plant ran well on 90 psig.