This section describes time-proven measures to improve the energy efficiency of compressed air systems, including:
- Identifying and repairing air leaks
- Minimizing pressure drops
- Minimizing end use of compressed air
- Examining compressor heat recovery
- Optimization of air production equipment
a. Compressed Air System Leaks
Air leaks can be a significant contributor of wasted energy in a compressed air system, and in some instances lead to productivity losses. It is not unusual to encounter 20 to 30 percent of a compressor's output in the form of air leaks at typical industrial facilities. Proactive leak management programs (detection and repair) can reduce leaks to less than 10 percent of a plant's compressed air production.
Experience has shown, time after time, that fixing air leaks is most often the top priority for any compressed air system optimization. Typically you will find that your efforts will have a simple payback of less than 6 months.
In addition to being a source of wasted energy, leaks can also contribute to other operating losses. There is strong cause and effect relationship between the number and magnitude of air leaks with the overall compressed air system pressure. For example, lower air pressure can affect air tools and equipment by reducing the mechanical output and decreasing the resulting productivity of the process.
Indifference to air leak management can lead to purchasing unnecessary air compressor capacity, thereby increasing capital expenses.
Figure 26 shows the approximate annual cost for electricity ($0.10/kWh) for different size leaks based on one, two and three shift operation.
The type of compressor control can have a large effect on the results of any leak reduction effort. For example, a leak reduction effort that reduces the air consumption by 10%, in a system with a single modulating compressor, would only achieve about 3% in energy savings because of the limited turndown capability of the modulating compressor control. This same reduction when applied to a system with a VSD compressor would result in energy savings of about 10%.
|Leak size||1 Shift
Estimating Total Air Leaks
A good first step in addressing air leakage in a plant is to do a low load test during a non-production time. This might be fairly easy if there is an existing accurate flowmeter already installed in the system or if the air compressors have capacity gauges. If not, a special test can be performed using one or more plant air compressors.
If the plant compressors already operate in load/unload mode (a compressor service provider can assist in determining this) a leak estimate can be made by measuring the loaded and unloaded times while the compressor is feeding the leaks. For example if a 100 HP compressor rated at 400 cfm is loaded for 2 minutes and unloaded for 3 minutes, the leak load can be estimated by taking the loaded time and dividing the total loaded plus unloaded time, or for this example 2/5 = 0.4. This indicates the compressor is loaded 40% of the time. The leak load would then be 40% of 400 cfm or 160 cfm. If another compressor was loaded during this time its capacity would be added to this calculated value. Generally the output capacity of any compressor operating around 100 psi would be about 4 times the compressor nameplate horsepower rating.
This test can also be done with modulating compressors using an accurate pressure gauge and a stopwatch. This test causes wide pressure fluctuations so it is important to determine if critical equipment will be affected.
If the plant can be run on one compressor, test the leak load by turning the compressor off and measuring the time it takes for the pressure to drop from a point 10 psi lower than the normal system pressure to a point 30 psi lower (20 psi drop). The test is done at this lower point to prevent compressor modulation during the test.
For the second part of the test, turn on the compressor and measure the time it takes for the pressure to rise though the same two pressure points. Repeat the test a number of times, being careful not to exceed the 4 motor starts per hour. The compressor loaded ratio is determined by taking the rise time and dividing by the total time (rise plus fall). As in the previous example, the leak load is estimated by multiplying this ratio by the compressor cfm output. If a second compressor was required to get to the required pressure its capacity would be added to the total.
The approximate cost to feed these leaks at 100 psi can be determined as follows:
0.2 x leak cfm x hours per year x cost per kWh
A 100 cfm leak rate would cost about 0.2 x 100 x 4,250 x $0.10 = $8,500 per year to maintain at 10 cents per kWh blended rate.
How to Track Down Air Leaks
Air leaks are very difficult to see or hear in environments with high background noise (e.g., fans and machinery).
When the plant is shut down, you can often hear the air leaks. If background noise is present, you will probably need to use an ultrasonic leak detector. Ultrasonic acoustic detectors are handheld devices that recognize the presence of air leaks by their ultrasonic sound patterns. Once a general location of an air leak is determined, soapy water may be applied to suspected areas. The soapy water method is very reliable, however it is time consuming to undertake.
The best time to find air leaks is when the plant is not operating, usually at night or on weekends. Walk the length or perimeter of the compressed air distribution system. Stop every so often and listen for air leaks. Look for damaged fittings or cracked hoses. Write down and sketch the location of the air leaks. Use tags to mark the location of air leaks for repairs. Repeat the process periodically as part of your maintenance routine.
Caution: Always use appropriate vision and hearing protective equipment, and follow proper safety procedures when detecting air leaks or when working at elevated heights.
Experience has shown that air leaks occur most often at joints and connections. Fixing leaks can be as simple as tightening a connection or replacing root cause faulty equipment including:
- Pipe sections
- Drain Traps
- Valve stems
Preventing Air Leaks
Here are some tips to help prevent leaks from happening in the first place:
- Install fittings properly with appropriate sealants where applicable.
- Isolate non-operating equipment with a valve in the distribution system.
- Lower the air pressure of the system where possible. A lower pressure differential across an air leak reduces the rate of flow by a small amount. This however is not a cure for fixing air leaks.
- Select high quality fittings from reputable suppliers including air hoses, tubing, disconnects.
Remember that once leaks have been repaired, the compressor control system often needs to be adjusted so as to achieve the true energy savings potential.
b. Lower Compressor Discharge Pressure by Minimizing Pressure Drops
Compressor discharge pressure affects the efficiency of an air compressor. In rare cases the compressor discharge pressure will have been inadvertently set too high for no valid reason. In these cases energy can be saved by simply readjusting the compressor control setpoints to a lower level. This should be done carefully and in small steps so as not to affect sensitive plant equipment.
Most often, however, compressor discharge pressure is set artificially high to overcome various system pressure drops between the compressor and critical end uses. Pressure drop is caused by restriction to flow that is internal to system pipe work and components. Too much pressure drop can result in poor system performance and excessive compressor energy consumption.
The higher the discharge pressure from the air compressor, the more money and electricity it costs to produce the compressed air. Often times, the air compressor's discharge pressure is set at higher pressures than what would be normally required. A rule of thumb for systems in the 100 psig range is: for every 2 psi increase in discharge pressure, energy consumption will increase by approximately 1 percent at full output flow. This chapter deals with the subject of pressure drops and what you can do to reduce and minimize them.
Problematic areas for pressure drop include aftercoolers, filters, water separators, dryers, pipes and check valves. Flow restrictions of any type in a system require higher operating pressures, resulting in higher energy consumption.
In general, a properly designed compressed air system should have a pressure loss (or drop) of much less than 10 percent of the compressor's discharge pressure, measured from the point of discharge to the point of end use.
Ways to Minimize Pressure Drops
The following list discusses some common ways to minimize pressure drops in a compressed air system:
- Equipment components for air treatment including aftercoolers, moisture separators, dryers, and filters should be selected with the lowest practical pressure drop at specified maximum operating conditions. Once installed, the manufacturer's recommended maintenance procedures should be followed and documented.
- Maintain air filtering and drying equipment to manage the impact of moisture, e.g. pipe corrosion.
- Properly design the distribution system with appropriate pipe diameter size and looped system configurations where possible.
- Reduce the distance the air travels through the distribution system.
In cases where the air compressor's discharge pressure can be reduced, energy savings can be realized. Before lowering compressor discharge pressure, it is important to check with end use equipment specifications to determine the minimum pressure required by air tools and equipment for proper operations.
- Assess the pressure level requirements of the end-use applications. Minimize the compressed air system pressure to match the end use requirements.
- Check the air pressure at the inlet to the air tools to determine if sufficient pressure is being supplied. It is not uncommon to measure 30 to 40 psi pressure drop between the distribution header drop and the end use. This pressure drop is commonly caused by undersized air lines, quick couplers, filters, regulators and lubricators. Significant pressure loss is also common in end use hoses. Often, there will be a long coiled hose, or a series of hoses, supplying the end use. The resulting high pressure differential will negatively affect the power available to do useful work and often forces main system pressures to rise.
- Examine each end use point and determine the one having the highest pressure requirements. Reduce this pressure to the amount to maintain functionality, and then lower the overall system pressure.
- Specify pressure regulators, lubricators, hoses, and connections with the lowest pressure differential and the best performance characteristics. Size components for the actual flow rates, and not the average flow rates.
End use tool pressure differential can be easily diagnosed by simply making up a test gauge setup that is inserted using quick couplers in the air feed near the end use. Comparing the air pressure with or without the end use consuming air will show the pressure differential.
- Use larger size couplings for reduced pressure differential. For example, at the same flow, a 3/8 inch quick coupler has one-sixth the pressure differential of a 1/4 inch connector.
c. Minimizing Compressed Air End Use Energy Requirements
Here are some tips to help you to minimize the total energy requirements of your compressed air system.
- Replace inappropriate end use applications (such as open blowing) with efficient models (vortex nozzles, atomizers).
- Install a flow controller to lower plant pressure and reduce artificial demand caused by higher than required pressures.
- Turn off air consuming equipment, using electric solenoids or manual shutoff valves.
- Avoid operation of air tools without a load, as this consumes more air than a tool under load.
- Replace worn tools, as they often require higher pressure and consume excess compressed air than tools in good shape.
- Lubricate air tools as recommended by the manufacturer. Keep air used by all end uses free of condensate in order to maximize tool life and effectiveness.
- Where possible and practical, group end use air equipment that has similar air requirements of pressure and air quality.
Air Compressor Heat Recovery
Approximately 80 percent of the electrical energy used by an industrial air compressor is converted into heat. For many facilities, a properly designed heat recovery unit can recover 50-90 percent of this available thermal energy to offset space and water heating. For new or expanded compressed air systems, the heat recovery potential should influence the final location of the air compressor within the facility.
Packaged rotary screw compressors are ideal candidates for heat recovery for space heating. Generally, ambient air is heated by passing it across the compressor's aftercooler and lubricant cooler. As packaged compressors are enclosed in cabinets, and generally come equipped with heat exchangers and fans, only ducting and HVAC fans need to be installed to extract heat. The ducting can include a vent that is controlled by a thermostat. The vent could direct heated air to the outside during warmer parts of the year. As an energy efficiency measure, approximately 50,000 BTU per hour of heat can be extracted for each 100 scfm air for a compressor operating at full load.
It is not uncommon to be able to heat air to 15 to 25°C above the cooling air inlet temperature with 80-90 percent heat recovery efficiency. It is important to realize in using this heat that any heat recovery ventilation duct must not restrict the compressor cooling air flow. Booster fans are usually required if extensive ductwork is installed.
With an appropriate heat exchanger, waste heat can be extracted from the lubricant coolers in packaged water cooled, reciprocating or rotary screw compressors. Some manufacturers offer this as optional equipment. This can be used to produce hot water for use in central heating or boiler systems, industrial cleaning processes, plating operations, heat pumps, laundries, or any other application where hot water is required. Heat exchangers also offer an opportunity to produce both hot air and hot water, and allow the operator some ability to vary the hot air/hot water ratio. As many water cooled compressors are large (>100 HP), heat recovery for space heating can be an attractive opportunity.
Typically, 50-60 percent of compressor heat can be practically recovered for water heating applications. It is important to realize in using this heat that any heat recovery strategy must not restrict the compressor cooling water flow or overheating could occur.
d. Implement More Efficient Compressor Control
The discussion in a previous section titled "Compressor Controls and System Performance" showed that significant power can be saved by running compressors in more efficient operation modes. This should be considered for systems that have existing compressors that are capable of such operation. Local compressor service providers can assist in upgrading or modifying controls for more efficient service.
With regard to compressor control the following additional points should be considered:
- For optimal energy and operational performance, systems with multiple compressors require more advanced controls or control strategies (cascaded pressure bands, network or system master controls) to coordinate compressor operation and air delivery to the system.
- Remember to consider the element of time when designing or tuning a compressor control system. Compressors require time to start up and be brought up to speed. This may require extra storage receiver capacity.
- Cascaded pressure bands single pressure bands need to be adjusted from time to time.
- The "trim compressor" should be the one most capable of running efficiently at partial loads.
Install Storage Capacity
Receivers can help compressed air systems operate more efficiently and can help stabilize system pressures as discussed in "Receivers and Air Storage".
The following points should also be considered:
- Where practical, locate the receivers as close to the air compressors as possible.
- For most facilities with load/unload rotary screw compressors, install air receiver capacity of 10 US gallons per cfm of compressor capacity.
- When receivers are exposed to subfreezing temperatures, precautions need to be taken to prevent freezing in the condensate drains. In some cases receivers rated for lower temperatures are required.
- Select a slightly larger receiver than what may be currently required. This will generally result in improvements to stabilizing system pressure and also respond to intermittent demands.
- In cases where the air needs to be dried, it is sometimes beneficial to install two receivers - one before and one after the dryer.
e. Optimize Air Dryers
Air dryers can consume significant compressed air or electrical power and often have limited turndown capabilities as discussed in "Air Dryers" on page 43. It is possible the existing air dryer could be upgraded or replaced with good savings results. Consider the following points with regard to dryers:
- For new purchases of refrigerated air dryers always consider the energy savings cycling style.
- Avoid drying the air to a dew point level that is lower than what is needed for a specific application.
- Use energy saving dew point controllers for all types of regenerative desiccant dryers.
f. Reduce System Drainage
Condensate drains are a common point of compressed air loss. Consider airless drains as replacements for timer drains or manual drains that are partially cracked open. The following points should be considered:
- Where possible, procure condensate drains having a gauge glass. This will provide a visual indicator if the trap malfunctions.
- Regularly test automatic drain traps for proper operation.
- Piping should be sloped slightly downwards and away from the compressors.
- Locate drains at the bottom of main headers in order to allow condensate to collect and flow by gravity.
- Avoid using open manual drain valves.
Purchase a More Efficient Compressor
A good energy management strategy may be to purchase a new more efficient compressor as a replacement for an older existing unit. Often the existing unit can be retired to standby duty, providing backup capacity for increased system reliability. Consider the following when purchasing a compressor:
- Purchase the most energy efficient compressors, including ones equipped with premium efficiency motors.
- In situations involving multiple compressors, operate the base load units at maximum capacity rather than partially loaded.
- Consider purchasing and operating at least one Variable Speed Drive compressor to supply variations in flow above the base load.
- Purchase of a two stage compressor might provide better system efficiency if used as a base compressor
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