Converting Air to Dollars

Achieving efficiencies in compressed air systems can deliver significant savings to sawmills.

At a time when achieving every possible operating efficiency is a primary concern for sawmill managers, they should be aware that the air around them in the mill may not be "free" at all and could in fact be costing the operation upwards of $75,000 a year.

Most mills utilize a variety of energy sources—electric, steam, hydraulic and compressed air. But it is probably safe to say that compressed air is the most expensive to generate and the easiest to waste because it’s not considered an obvious prime culprit when it comes to wasting energy. Other energy sources usually get higher consideration when it comes to energy efficiency programs, but the energy savings with compressed air can be substantial.

A quick look at a basic power source such as an electric motor driving an air compressor illustrates this point. All the energy from the driving motor is converted to heat in the process of compression and approximately 75 per cent of this heat is expelled from the oil/aftercoolers. A further eight per cent is lost through radiation and cooling fans, leaving only approximately 15 to 17 per cent retained in the compressed air to actually perform useful work. Applying this to a 100-hp compressor, the mill ends up with only 17 hp doing the work.

This is a key reason why compressed air can be very expensive, under-lining the need for its proper production and maintenance. But this kind of inefficiency is not readily visible— the mill manager doesn’t "see" it, nor does the maintenance crew. Air has no color or taste and is non-toxic. This costly area usually ends up as a low-priority item from a maintenance perspective—this is clear when evidence shows that most sawmills have wasteful air leak levels averaging +/- 500 cubic feet per minute (cfm).

One hp of electric energy consumed by most compressors should deliver about 4 cfm FAD at 100 pounds per square inch (PSI) pressure. (FAD is Free Air Delivery at a nominal 100 psi measured after the aftercooler). If a mill has 500 cfm of air leaks, this translates into a direct energy loss of about 125 hp or approximately $40,000/year based on the sys-tem being pressurized on a 24- hour basis. That figure may appear high, but the wasted air was probably first put through a desiccant air dryer and operating a desiccant air dryer is not a cheap prospect.

There have been several initiatives in British Columbia to reduce these costs, however. In 1990, BC Hydro embarked on a vigorous energy management program, "Power Smart", and compressed air ranked very high on their agenda. A Vancouver company, Airflow Energy Ltd., was brought in under contract to BC Hydro to study and carry out performance testing at some 300 plants. Approximately 100 of these plants were sawmills and other wood processing facilities. Another BC utility company, West Kootenay Power, has initiated a similar program which is still ongoing.

During the BC Hydro program, over 200 multi-orifice test flow meters were installed, including one in just about every sawmill in BC. These meters allow sawmill maintenance crews to perform their own system troubleshooting.

Looking back on these progressive initiatives, it could be said the programs were a bit ahead of their time simply because before the increased focus on efficiency, the system benchmarks for maintenance and engineering personnel were set by equipment sup-pliers and efficiency wasn’t a major criteria.

However, these initiatives are now recognized as among the best compressed air energy efficiency programs in North America.

Today, the terms "energy and efficiency" are readily linked together. The US is just now—a decade after the start of the Canadian programs—embarking on a massive program called "Compressed Air Challenge", which shows the US Department of Energy recognizes the huge potential for energy savings in air systems, and all US industries are being asked to get involved.

While the BC Hydro and West Kootenay Power programs were carried out in BC, they identified production areas where energy/ efficiency can often be improved with-in sawmills regardless of their location with minimal costs.

It’s not unreasonable for the average sawmill to realize an annual savings of between $50,000 to $75,000 by reviewing some of the following operating procedures related to compressed air systems:

An electrician would not troubleshoot an electrical problem without a volt and amp meter. Mechanical maintenance personnel should carry out periodic testing using meters to troubleshoot compressor behavioral problems and to quantify air leak waste. If a mill operates multiple compressors, and the work profile is different between each operating shift, it’s almost certain a considerable amount of money can be saved with a multi-compressor sequencer.

When a system is properly managed, air leaks should not exceed 15 per cent of net production. For example, if a mill requires a nominal 2,000 cfm for production, the air leaks should not exceed 300 cfm. This is not difficult to achieve with a well-structured preventive maintenance program.

Systems should not be operated at higher-than-required pressures. If a system’s air pressure is above 100 psi in the compressor room, it’s probably safe to say the pressure has been "jacked up" to correct some system piping problem or an equipment problem. Generally, the higher the operating pressure, the higher the air consumption. Pressure of 100 psi in the compressor room, and 85 to 90 psi in the plant should be sufficient in 90 per cent of all cases.

Some mechanical superintendents may disagree, but there’s little justification for operating dual tower desiccant dryers through the summer months for sawmills which do not have pneumatic instrumentation. A properly sized refrigerating dryer can perform the moisture removal duties for six months of the year in most facilities. Enclosed plants, such as ply-wood and particleboard mills, could operate year-round with a refrigerating dryer. A refrigerating dryer will operate at a fraction of the cost of a desiccant unit.

If compressed air is used for cleaning off conveyor belts, optical scan equipment, or cleaning dust off the product as is the case in plywood manufacturing, a considerable savings can be realized by using blowers for these applications.

It definitely pays to do a periodic review of mill production air needs and applications. There can be numerous applications in wood processing plants where hydraulics already exist and some air end-use applications might better be served by hydraulic energy. Energy reductions of 50 per cent are attainable in some applications.

Companies may be reluctant to spend money on upgrades, particularly during tough market times. But it begs the question why money is readily available to pay the utility for the excessive power the mill might be consuming with sys-tem inefficiencies, but not available for quick-payback improvements.

A recent system air study performed at a large BC interior sawmill confirms the savings that can be achieved.

A test meter was installed and the following inefficiencies were uncovered within minutes: An air compressor with 300-hp 1,200-cfm capacity was only capable of delivering 800 cfm due to a misplaced inlet throttle valve on its throttle shaft; wasteful plant air leaks amounted to 1,000 cfm; some old, oversized steam cylinders were being operated with air; four large compressors (with a total of 1,000 hp) were manually controlled.

Action was subsequently taken to make corrections based on the study recommendations. The 300-hp delivery shortfall was repaired and the unit is now delivering close to its rated 1,200 cfm. Air leaks have been reduced from 1,000 cfm down to 450 cfm, with ongoing efforts to reduce it further. A multi-compressor PLC sequencer was installed to allow match-up of compressor capacity to plant demand. The sequencer has been programmed to manage air dryer purge to air consumption, rather than to a factory setting of purge based on full dryer capacity of 3,000 cfm. Several steam cylinders have been converted from air to hydraulic power. Additional work is underway at the operation to address an inefficient air system in the planer mill.

The end result: a reduction in energy of 1,500,000 KWh, representing a savings to the mill of about $75,000 a year. An additional savings of 500,000 KWh, representing a further $25,000, is possible once the planer mill changes are completed.

All this was achieved because the mill manager, along with the mechanical maintenance and electrical group, took a genuine interest in energy efficiency and made things happen. The group realized that air, compressed air anyways, is not "free".

Lanny Pasternak is a principal of Airflow Energy Ltd. and spent more than 25 years with the international firm of Atlas Copco in positions of design, marketing and management.

Airflow Energy performs air system studies and performance testing of air systems and is actively engaged in performing system study/audits/testing and technical training seminars. It also produces test meters for air compression systems.