One of the most effective ways to save energy when it comes to compressed air is to reduce system flow. This requires dealing with the end uses and abuses of compressed air in an effort to eliminate or reduce the flow of compressed air that results from wasteful practices.
One often-cited opportunity for improvement is leak reduction – a valuable endeavor, but more can be done. Additional significant savings can be achieved by finding and fixing “inappropriate uses,” which we can define as some sort of process that could be more cost-effectively supplied by another energy source. This is not a simple process: There are hundreds if not thousands of individual uses of compressed air in most plants. With that in mind, though, let’s explore some of the more common potentially inappropriate uses of compressed air and look at examples of how plants have addressed these energy-wasters.
If enough investigation is done, you will become skilled at finding ways to amend end uses to reduce compressed air costs. The following case studies are examples of actual projects done over the past few years; they illustrate what can be done with some careful thought and action to reduce inappropriate compressed air use.
Conversion of compressed air agitation
A compressed air audit was performed at a parts machining plant that made transfer case parts for large farm machinery. Once the parts are machined, they’re painted, but to ensure the paint adheres properly to the metal, the part must be carefully cleaned and treated. The part is dipped into a bath to remove any contamination. To ensure proper mixing of the chemicals in the bath, the plant chose to agitate the liquid with 10 cfm of 100 psi compressed air as shown in Figure 1. The energy consumption of this flow of compressed air was 2.8 kW, equivalent to about $2,450 in annual energy consumption.
It was discovered that the actual pressure requirement at the submerged nozzles was about 2 psi; the flow of compressed air was being throttled off with a ball valve. Plant personnel investigated installing a regenerative-style blower to use in place of the compressed air agitation as per Figure 2. Trial with a test apparatus found that the submerged piping actually produced more agitation air than was previously provided, causing a better mixing action. The blower’s required power was measured at 0.42 kW – about 15% of the equivalent compressed air power consumption.
This particular plant had about four other containers of chemicals with agitation in place. Conversion of the compressed-air-powered agitation with blower style achieve a simple payback on the project of slightly more than two years, which was reduced to one year by a utility incentive.
More-efficient cabinet cooling
A furniture-making facility had a number of complex CNC machines that were controlled by PLC. The PLCs were installed within sealed metal enclosures near the machine in an often-warm environment, with the enclosure’s internal temperature reaching high levels thanks to a power transformer installed within the same cabinet. The high temperature often caused thermal failure of the control, and plant personnel recognized that they needed some sort of cooling to prevent this problem. Compressed-air-powered coolers were chosen because of their simplified design and ease of installation. To save costs, these coolers had no temperature controls installed, and therefore they ran 24/7, even when the plant was down during evenings and weekends.
A compressed air audit found that this style of cooling was quite expensive. Each cooler consumed a continuous 20 cfm of compressed air while producing 1,500 btu/hr of cooling – equivalent to about 4 kW of air compressor power. There were about 12 similar coolers, operating in the plant, contributing about $42,000 per year in operating costs.
The plant sourced some refrigerated panel coolers to use in place of the compressed-air-powered units. The units were installed with temperature controllers to ensure that the cooling circuits turned off when their use wasn’t required. Each of these coolers produced 3,000 btu per hour of cooling, with the average power consumption measured at about 4% of that of the compressed-air-powered coolers. Estimated payback for this conversion was 1.3 years based on $40,000 per year savings.
Through the years a number of similar cooling applications have come up. It is very common to see cabinets with a small cooling fan installed but no secondary ventilation hole to allow the air to circulate. In some of these cases simply providing better fan-powered ventilation with no refrigeration cooling provides the necessary solution.
It should be noted that site personnel installed thermostatic controls on the compressed-air-powered coolers and realized a savings of 25%. This measure could prove a valuable solution in situations where compressed-air-powered coolers must be used.
Reverse pulse baghouse improvements
A protein processing facility was using a number of reverse pulse baghouses to filter and capture processed protein out of the air stream. The facility had been having trouble with the filtering process; the filter elements would become clogged, choking off the air flow and plugging the filter. Plant personnel tried everything but finally settled on increasing the pulse duration from a blast of 100 milliseconds to about 350 milliseconds and increasing the pulse frequency to one pulse every six seconds. The filters worked satisfactorily but not optimally. A total of 100 blast valves operated at random cleaning filters during peak plant production.
This change had a significant impact on the total flow of compressed air in the plant. Whereas previously the plant was able to run on one air compressor, now a second compressor was required. This additional required capacity significantly increased compressed air operating costs and reduced the facility’s air quality because the air dryer was sized for only one compressor. The poor air quality allowed moisture to pass downstream and caused the filters to work poorly during some periods of hot weather. Air pressure also fell significantly because of high velocity in the undersized dryer.
An auditor immediately recognized that the blast valves were operating inefficiently, just by their sound. Properly operating valves should have a short and forceful pulse to shake the bags clean. These units had a less-forceful and long whoosh of air – less effective in freeing the captured dust. Investigation revealed that some piping issues were causing low pressure at the blast valves and that the feed lines to each manifold on the blast valve assembly were restricted. The manifolds on the blast valves were sized too small to provide the air volume needed for a blast that would fully clean the filter elements.
The auditor suggested installing secondary storage receivers of about 30 gallons as close as possible to each baghouse manifold. This move significantly increased the potential energy available to each blast valve and noticeably changed the cleaning force. For example, a 6-foot section of 3-inch pipe used as a manifold holds only about 2 gallons of compressed air. A 30-gallon receiver increases the available volume by 16 times.
Piping was improved and receivers were installed. The pulse duration and frequency have fallen enough that the plant now runs on only one compressor. The compressed air flow has decreased by 26%, but because a compressor has turned off, the actual total reduction in compressor power is 33%. The payback for this project is well under a year, with a savings of $32,000 annually in energy costs.
Lowering humidifying costs
A printer of lottery tickets is in a location that gets very cold in winter. Because of the volatile nature of the inks used in printing, a large flow of ventilation air is needed to ensure the environment is not contaminated with hazardous vapors. This ventilation draws in very cool and dry outdoor air, making the environment much too dry for proper operation of the printing presses.
The paper used to produce the tickets expands and contracts with changes in the ambient humidity level. In extreme cases, as in winter, the paper contracts so much the product becomes out of specification, causing quality issues. For this reason, the plant must humidify the ambient air to keep the relative humidity above a certain minimum level. When first placed in production, the plant used compressed-air-powered atomizers, with each nozzle using about 2 cfm of compressed air. There were about 150 nozzles in use at the site consuming an average of 60 kW of compressed air power, worth about $31,400 in electrical costs.
The plant sourced a high-pressure water humidification to use as a retrofit instead (Figure 3). This system uses 2,000 psi water pressure supplied by electric pumps to produce atomized air without the need for compressed air. This new system provides the same humidity level but at the much lower cost of only $8,500 per year – a reduction of 73 percent. At the same time, the plant’s compressed-air system needed to be replaced. The reduction of the compressed air flow saved the plant on purchase costs because the new compressor and air dryer could be sized smaller.
Lower blowing costs
A cabinet-making plant used a special machine that drilled holes in the side panels for wood dowels. Some of these holes would plug up with sawdust and prevent the dowels from being inserted properly. To correct this, a blowing device – basically a pipe with holes drilled in it – was connected to the compressed air system to provide a forceful blow. This plant operated on a two-shift, 4,000-hours-per-year schedule, and the drilling machine sent product through with a duty cycle of about 10%, yet the blowing was installed to blow continuously at 20 cfm, equivalent to 3.6 kW. Annual costs were $2,200.
This blower was retrofitted to include a proximity detector that turned off the blowing if a panel was not present. This reduced the operating cost by 96% to about $100 per year, thanks to the low duty cycle.
Optimized breathing air control
A bus manufacturer used breathing air purifiers in its paint booths to prevent carbon monoxide from affecting workers who were using compressed air in painting hoods. Because the facility manufactured highway transport buses, the possibility that one or more of the air compressors could inadvertently suck in exhaust from a motor was real. The company had five breathing air purifiers that consisted of a fixed-cycle desiccant dryer on the intake side to remove moisture and a catalyst element to remove any carbon monoxide. Each desiccant dryer consumed about 15% of its rated flow on a 24/7 basis even though the breathing air only needed to be available for a daytime eight-hour shift for five days a week. Total purifier purge load was estimated at 200 cfm of constant flow, costing $25,000 per year in electricity charges.
Flow meters were installed on the purifiers, and it was determined that breathing air is being used only about 15% of the time, and during operation the flow was about 20% of the purifier’s rating. Plant personnel sourced a retrofit for the purifier dryers (Figure 4). New dew-point controls were installed that turned off the dryer purge when the dewpoint was adequate for the operation of the onboard catalyst. Resulting purge flow decreased by 90%, saving about $22,900 in operating costs per year.
Although time-consuming, searching your industrial plant for potential inappropriate uses is often is rewarding in terms of reducing operating costs and increasing profits. Lowering compressed air flow will usually result in a significant reduction in power if the compressors are operating in a well-controlled system. This is a key point: Proper control of the plant’s air compressors is an important requirement. If not properly done, most of your efforts will be wasted.
Often hiring a reputable compressed air auditor to do an end-use study can be a good first step in addressing your end uses. Make sure your auditor is prepared to poke his or her nose out of the compressor room and into the workings of the plant.