Compressors consume about 12% of total electricity use in refineries. The major energy users are compressors for furnace combustion air and gas streams in the refinery. Large compressors can be driven by electric motors, steam turbines, or gas turbines. A relatively small part of energy consumption of compressors in refineries, for instance, is used to generate compressed air. Compressed air is probably the most expensive form of energy available in an industrial plant because of its poor efficiency when it is used. Typically, efficiency from start to end-use is around 10% for compressed air systems. In addition, the annual energy cost required to operate compressed air systems is greater than their initial cost.
Compressors | Energy Conservation In Plants
Most commonly used compressors in industry are reciprocating compressor. For small flow rates they may be single stage or double stage for high pressure (above 8 Bar).
- Screw compressor for high flow rates
- Vane compressors
- Centrifugal compressor of higher flow rate
The compressor consists of the compressor driving “motor”, inlet filter, lubricating system, intercooler of multistage compressor and cooling system which may be air or water cooled.
A centrifugal compressor is essentially a constant pressure variable capacity machine.
An axial compressor is a constant capacity, variable pressure machine over a significant discharge pressure range.
Simply, the compressor work is affected by the pressure ratio, inlet pressure and inlet temperature, molecular weight and cooling efficiency of the compressor.
Axial compressors are more efficient than centrifugal compressor as much as 5% but have a much narrow range of operation.
All compressors have a minimum flow point called the surge limit, below which the operation of the machine is unstable. The surge limit is a function of the compressor type, design pressure ratio, gas properties, inlet temperature, blade angle and speed. Operation at or below the surge limit must be avoided. However, an anti-surge control loop is always there, sometimes a back up loop may be warranted, to protect this valuable process equipment.
Air Compressor Maintenance
Inadequate maintenance can lower compression efficiency, increase air leakage or pressure variability and lead to increased operating temperatures, poor moisture control and excessive contamination. Better maintenance will reduce these problems and save energy. Proper maintenance includes the following:
Blocked pipeline filters increase pressure drop. Keep the compressor and inter-cooling surfaces clean and foul-free by inspecting and periodically cleaning filters. Seek filters with just a 1 psi pressure drop. Payback for filter cleaning is usually under 2 years (Ingersoll-Rand, 2001). Fixing improperly operating filters will also prevent contaminants from entering into equipment and causing them to wear out prematurely. Generally, when pressure drop exceeds 2 to 3 psig replace the particulate and lubricant removal elements. Inspect all elements at least annually. Also, consider adding filters in parallel to decrease air velocity and, therefore, decrease pressure drop. A 2% reduction of annual energy consumption in compressed air systems is projected for more frequent filter changing. However, one must be careful when using coalescing filters; efficiency drops below 30% of design flow.
Poor motor cooling can increase motor temperature and winding resistance, shortening motor life, in addition to increasing energy consumption. Keep motors and compressors properly lubricated and cleaned. Compressor lubricant should be sampled and analyzed every 1000 hours and checked to make sure it is at the proper level. In addition to energy savings, this can help avoid corrosion and degradation of the system.
Inspect fans and water pumps for peak performance.
Inspect drain traps periodically to ensure they are not stuck in either the open or closed position and are clean. Some users leave automatic condensate traps partially open at all times to allow for constant draining. This practice wastes substantial amounts of energy and should never be undertaken. Instead, install simple pressure driven valves.
Malfunctioning traps should be cleaned and repaired instead of left open. Some automatic drains do not waste air, such as those that open when condensate is present. According to vendors, inspecting and maintaining drains typically has a payback of less than 2 years.
Maintain the coolers on the compressor to ensure that the dryer gets the lowest possible inlet temperature.
Check belts for wear and adjust them. A good rule of thumb is to adjust them every 400 hours of operation.
Check water-cooling systems for water quality (pH and total dissolved solids), flow and temperature. Clean and replace filters and heat exchangers per manufacturer’s specifications.
Specify regulators that close when failed.
Applications requiring compressed air should be checked for excessive pressure, duration or volume. They should be regulated, either by production line sectioning or by pressure regulators on the equipment itself. Equipment not required to operate at maximum system pressure should use a quality pressure regulator. Poor quality regulators tend to drift and lose more air. Otherwise, the unregulated equipment operates at maximum system pressure at all times and wastes the excess energy. System pressures operating too high also result in shorter equipment life and higher maintenance costs.
Proper monitoring (and maintenance) can save a lot of energy and money in compressed air systems. Proper monitoring includes the following:
- Pressure gauges on each receiver or main branch line and differential gauges across dryers, filters, etc.
- Temperature gauges across the compressor and its cooling system to detect fouling and blockages
- Flow meters to measure the quantity of air used
- Dew point temperature gauges to monitor the effectiveness of air dryers
- kWh meters and hours run meters on the compressor drive
- Compressed air distribution systems should be checked when equipment has been reconfigured to be sure no air is flowing to unused equipment or obsolete parts of the compressed air distribution system.
- Check for flow restrictions of any type in a system, such as an obstruction or roughness. These require higher operating pressures than are needed. Pressure rise resulting from resistance to flow increases the drive energy on the compressor by 1% of connected power for every 2 psi of differential. Highest pressure drops are usually found at the points of use, including undersized or leaking hoses, tubes, disconnects, filters, regulators, valves, nozzles and lubricators (demand side), as well as air/lubricant separators, after-coolers, moisture separators, dryers and filters.
- Leaks can be a significant source of wasted energy. A typical plant that has not been well maintained could have a leak rate between 20 to 50% of total compressed air production capacity. Leak repair and maintenance can sometimes reduce this number to less than 10%. Overall, a 20% reduction of annual energy consumption in compressed air systems is projected for fixing leaks.
- The magnitude of a leak varies with the size of the hole in the pipes or equipment. A compressor operating 2,500 hours per year at 6 bar (87 psi) with a leak diameter of ½ mm is estimated to lose 250 kWh/year; 1 mm to lose 1,100 kWh/year; 2 mm to lose 4,500 kWh/year; and 4 mm to lose 11,250 kWh/year.
- In addition to increased energy consumption, leaks can make pneumatic systems/equipment less efficient and adversely affect production, shorten the life of equipment, and lead to additional maintenance requirements and increased unscheduled downtime. Leaks cause an increase in compressor energy and maintenance costs. The most common areas for leaks are couplings, hoses, tubes, fittings, pressure regulators, open condensate traps and shut-off valves, pipe joints, disconnect, and thread sealants. Quick connect fittings always leak and should be avoided. A simple way to detect large leaks is to apply soapy water to suspect areas. The best way to detect leaks is to use an ultrasonic acoustic detector, which can recognize the high frequency hissing sounds associated with air leaks. After identification, leaks should be tracked, repaired, and verified. Leak detection and correction programs should be ongoing efforts.
Reducing the Inlet Air Temperature
- Reducing the inlet air temperature reduces energy used by the compressor. In many plants, it is possible to reduce inlet air temperature to the compressor by taking suction from outside the building. Importing fresh air has paybacks of up to 5 years, depending on the location of the compressor air inlet. As a rule of thumb, each 3°C will save 1% compressor energy use.
Maximize Allowable Pressure Dew Point at Air Intake
Choose the dryer that has the maximum allowable pressure dew point, and best efficiency. A rule of thumb is that desiccant dryers consume 7 to 14% of the total energy of the compressor, whereas refrigerated dryers consume 1 to 2% as much energy as the compressor. Consider using a dryer with a floating dew point. Note that where pneumatic lines are exposed to freezing conditions, refrigerated dryers are not an option.
Remembering that the total air requirement is the sum of the average air consumption for pneumatic equipment, not the maximum for each, the objective of any control strategy is to shut off unneeded compressors or delay bringing on additional compressors until needed. All compressors that are on should be running at full load, except for one, which should handle trim duty. Positioning of the control loop is also important; reducing and controlling the system pressure downstream of the primary receiver results in reduced energy consumption of up to 10% or more. Start/stop, load/unload, throttling, multi-step, variable speed, and network controls are options for compressor controls and described below.
Start/stop (on/off) is the simplest control available and can be applied to small reciprocating or rotary screw compressors. For start/stop controls, the motor driving the compressor is turned on or off in response to the discharge pressure of the machine. They are used for applications with very low duty cycles. Applications with frequent cycling will cause the motor to overheat. Typical payback for start/stop controls is 1 to 2 years.
Load/unload control, or constant speed control, allows the motor to run continuously but unloads the compressor when the discharge pressure is adequate. In most cases, unloaded rotary screw compressors still consume 15 to 35% of full-load power when fully unloaded, while delivering no useful work. Hence, load/unload controls may be inefficient and require ample receiver volume.
Modulating or throttling controls allows the output of a compressor to be varied to meet flow requirements by closing down the inlet valve and restricting inlet air to the compressor. Throttling controls are applied to centrifugal and rotary screw compressors. Changing the compressor control to a variable speed control has saved up to 8% per year. Multi-step or part-load controls can operate in two or more partially loaded conditions. Output pressures can be closely controlled without requiring the compressor to start/stop or load/unload.
Properly Sized Regulators
Regulators sometimes contribute to the biggest savings in compressed air systems. By properly sizing regulators, compressed air will be saved that is otherwise wasted as excess air. Also, it is advisable to specify pressure regulators that close when failing.
Sizing Pipe Diameter Correctly
Inadequate pipe sizing can cause pressure losses, increase leaks, and increase generating costs. Pipes must be sized correctly for optimal performance or resized to fit the current compressor system. Increasing pipe diameter typically reduces annual energy consumption by 3%.
Heat Recovery for Water Preheating
As much as 80 to 93% of the electrical energy used by an industrial air compressor is converted into heat. In many cases, a heat recovery unit can recover 50 to 90% of the available thermal energy for space heating, industrial process heating, water heating, makeup air heating, boiler makeup water preheating, industrial drying, industrial cleaning processes, heat pumps, laundries or preheating aspirated air for oil burners. Paybacks are typically less than one year. With large water-cooled compressors, recovery efficiencies of 50 to 60% are typical. Implementing this measure recovers up to 20% of the energy used in compressed air systems annually for space heating.
Adjustable Speed Drives (ASDs)
Implementing adjustable speed drives in rotary compressor systems has saved 15% of the annual compressed air energy consumption. The profitability of installing an ASD on a compressor depends strongly on the load variation of the particular compressor. When there are strong variations in load and/or ambient temperatures there will be large swings in compressor load and efficiency. In those cases, or where electricity prices are relatively high (> 4 cts/kWh) installing an ASD may result in attractive payback periods.
High Efficiency Motors
Installing high efficiency motors in compressor systems reduces annual energy consumption by 2%, and has a payback of less than 3 years. For compressor systems, the largest savings in motor performance are typically found in small machines operating less than 10 kW.