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Business: Industrial


Compressed Air

How Much Will I Save?

You can lower energy consumption and save money in many ways. These examples describe what you can do, telling you how much you can save by improving a typical compressed air system.

Compressor Operating Mode
You can control air compressors in a number of ways depending on the load profile. Choosing the appropriate operating mode for your application can produce significant savings.

Compressed Air Energy Efficiency Reference Guide

A 3/8 inch leak will cost you $41,000 annually in wasted energy (for a system delivering 100 cubic feet per minute (cfm) at 125 psi over a year.)

Decreased Operating Pressure
Every two psi decrease in compressor operating pressure reduces energy consumption by approximately one percent.

Heat Recovery
Only 20 percent of the energy entering the compressor compresses the air. Eighty percent converts into heat. Recovering some of this heat and using it for other processes in your plant can save much energy.

Air Dryers
Air from compressors is normally 100 percent saturated with water vapour. An air dryer removes this moisture that would otherwise condense as the air cools in the distribution system. Drying can use from one to twenty percent of the system's energy, depending on the dryer.

Condensate Drains
Air under high pressure at room temperature cannot hold as much moisture as can the atmospheric air ingested by the compressor. Condensate drains lie at common collection points in the system, removing the excess moisture that condenses out. Improving the condensate drains can save you money.

Compressor Operating Modes

You can control the efficiency of your system by choosing the best operating mode. When compressors run at peak efficiency in the right mode, they can also directly affect the success of other energy management measures such as load and leak reduction.

Most compressed air systems do not operate at full load all the time. Production cycles vary with the time of day and the day of the week. Compressed air demands usually follow a seven-day cycle, with highest demand at mid-day during the week, and lowest demand in the evenings and on weekends.

Such varying loads usually need one compressor in a system running at part load some of the time. Often, in poorly controlled systems, all compressors run at part load. The best tactic for reducing energy consumption is to have all base duty compressors fully loaded during peak hours and turned off during low demands periods while one trim duty compressor runs at part load.

Base compressors should be chosen for highest full-load efficiency, and trim compressors for highest part-load efficiency. Mode of operation greatly affects efficiency, especially if the trim compressor must run many hours at lower than 50 percent of its full load capacity.

Although full-load efficiency is important when comparing makes and models of compressors, it should not be the only deciding factor for the trim compressor. Often, less than one percent difference in efficiency makes one compressor a better choice than another one. For more information, read Purchasing Tips.

There are five common operating modes for screw compressors:

  1. Modulation (with or without blowdown) (if it can be with or without an option, then the word “option” is redundant)
    Modulation consists of throttling the input valve, allowing the compressor output to vary according to demand. You can usually throttle down to 40 percent capacity. Below 40 percent, you can use blowdown, by opening a blowdown valve on the compressor sump and venting the air to control the output pressure.

  2. Variable Capacity (with or without blowdown)
    Output is varied in discrete steps by opening ports inside the compressor to allow intake air to bypass the compression cycle.

  3. Load/Unload
    The compressor runs either at full-load or at no load, while the motor speed remains constant. Here, energy savings depend on storage receiver size.

  4. Variable Speed
    A Variable Speed Drive (VSD) controls the frequency of the voltage input to the compressor motor, thus controlling compressor speed to match the demand.

  5. Start/Stop (suitable only for small compressors with very large storage)
    The compressor motor is either on or off.

For a description of operating modes, see the section on Compressed Air System Controls in the CAC Sourcebook; for a description of typical power vs. capacity curves for operating modes, see the Compressed Air Storage section. Note that the capacity curves assume that compressor controls are set up according to the manufacturer's specifications. For example, if you poorly adjust the unloading controls on a compressor, you can increase its unloaded kilowatt consumption to 50 to 60 percent rather than the usual 25 percent. If you poorly adjust compressor pressure switches and modulating controls, you can prevent unloading, and greatly increase the power consumption.

How Much Can I Save? – Compressor Operating Modes

Table 1 lays out estimates of the annual operating cost of a typical 100 hp, 125 psi air-cooled lubricated screw air compressor using 20 kW per 100 (cfm) output at various part loads.

Table 1
Annual Energy Cost and Dollars Saved for Various Operating Modes

  % Load Modula-
ting with Blow
Capacity with Blow
Load Unload – Small Reser-
Load Unload – Large Reser-
Variable Speed Drive On/Off
Cost 100% $ 73,584 $ 73,584 $ 73,584 $ 73,584 $ 73,584 $ 73,584 $ 75,056 $ 73,584
75% $ 68,065 $ 68,065 $ 59,603 $ 59,603 $ 69,905 $ 59,787 $ 55,188 $ 55,188
50% $ 62,546 $ 62,546 $ 46,358 $ 46,358 $ 63,282 $ 45,990 $ 37,528 $ 36,792
25% $ 57,028 $ 44,150 $ 45,622 $ 35,320 $ 49,301 $ 32,193 $ 21,339 $ 18,396
15% $ 54,820 $ 34,584 $ 44,886 $ 28,698 $ 40,471 $ 26,674 $ 11,773 $ 11,038
0% $ 51,509 $ 18,396 $ 44,150 $ 18,396 $ 18,396 $ 18,396    
Saved 100%             ($ 1,472)  
75%     $ 8,462 $ 8,462 ($ 1,840) $ 8,278 $ 12,877 $ 12,877
50%     $ 16,188 $ 16,188 ($ 736) $ 16,556 $ 25,019 $ 25,574
25%   $ 12,877 $ 11,406 $ 21,707 $ 7,726 $ 24,835 $ 35,688 $ 38,635
15%   $ 20,236 $ 9,934 $ 26,122 $ 14,349 $ 28,146 $ 43,047 $ 43,782
0%   $ 33,113 $ 7,358 $ 33,113 $ 33,113 $ 33,113 $ 51,509 $ 51,509

Based on 10¢ per kWh at 8760 hours a year. For other rates, multiply the cost/savings times the actual cost per kW divided by 10. For other operating hours, multiply the cost times actual hours/8760. For other horsepower, multiply cost times the actual nameplate hp/100.

Important: some systems run with multiple modulating compressors, all running at part load. Consider a system with one 100 hp modulating compressor running at 50% load and another at 25%. This system would cost more than twice as much to operate as a similar sized system using a single, well-controlled VSD compressor running at 75%, with the second compressor turned off by automatic control.

Compressed Air System Leaks and End Uses

Reducing leaks and eliminating poor end uses will lower the demand. Properly controlled compressors can greatly reduce operating costs. Table 2 shows the estimated cost of supplying several loads with a well-controlled compressed air system running at 125 psi with a compressor rated at 20 kW/100 cfm, 8760 hours per year at 10¢ per kW.

Table 2
Annual Cost of Leaks of Various Sizes

Diameter cfm cf/day Cost/year
1/64 inch 0.45 576 $ 87.84
1/32 inch 1.6 2,304 $ 280.32
3/64 inch 3.66 5,270 $ 641.23
1/16 inch 6.45 9,288 $ 1,130.04
3/32 inch 14.05 20,880 $ 2,540.40
1/8 inch 25.8 37,152 $ 4,520.16
3/16 inch 58.3 83,952 $ 10,214.16
1/4 inch 103 148,320 $ 18,045.60
5/16 inch 162 233,280 $ 28,382.40
3/8 inch 234 336,960 $ 40,996.80

If the compressors are poorly controlled, the savings will be minimal. Table 3 illustrates savings gained by load reductions for three different compressor operating modes. These figures are based on a typical 100 hp, 125 psi air-cooled lubricated screw air compressor consuming 20 kW per 100 cfm output.

Table 3
Annual Savings for Various Control Strategies

  $ saved by control stragegy
CFM Modulating Load/Unload – Large Reservoir Variable Speed Drive
50 $ 2,628 $ 6,570 $ 8,784
100 $ 5,256 $ 13,140 $ 17,520
150 $ 7,884 $ 19,710 $ 26,280
200 $ 10,512 $ 26,280 $ 35,040
300 $ 15,768 $ 39,420 $ 52,560

This shows clearly that leak or end use reduction, combined VSD compressor control, would produce significantly more savings than a modulating compressor system.

Decrease Operating Pressure

Every two psi reduction in operating pressure lowers the compressor power consumption by approximately one percent. Reduced operating pressure also lowers the flow to unregulated demands by a further 0.6 to 1.0 percent.

Table 4 shows how you can save much by reducing operating pressure. For example, dropping the pressure from 125 psi to 110 psi will save $20,639 per year, (with a VSD).

Table 4
Annual Savings by Lowering Pressure from 125 psi

Modulating Load/Unload – Large Reservoir Variable Speed Drive
120 $ 2,334 $ 9,618 $ 13,666
115 $ 4,667 $ 12,693 $ 17,152
110 $ 7,001 $ 15,768 $ 20,639
105 $ 9,335 $ 18,843 $ 24,125
100 $ 11,668 $ 21,917 $ 27,611

Based on a 100 hp compressor originally running at 125 psi at 80 percent average load for 8,760 hours a year. Power costs are assumed to be 10¢ per kW.

As with other measures, the mode of compressor operation affects the savings potential.

Heat Recovery

Often the heat generated by air compressors can be used for other purposes. It can be used to supplement or displace other process- or building-related heat sources for energy cost savings.

Most air compressor vendors have heat recovery options for new or existing compressors.

For more information, see the section Heat Recovery and Compressed Air Systems in the CAC Sourcebook.

How much can I save? – Heat Recovery

Energy Savings Calculations (from the CAC Sourcebook)

Energy savings
 =  0.80 bhp compressor  x  2,545 
 x  hours of operation

Example: A 100-hp compressor running two shifts for five days a week

Energy savings = (0.80) x (100 bhp) x (2,545 Btu/bhp-hour) x
(4,160 hours a year)

  = 846,976,000 Btu per year

Where:   0.80 is the recoverable heat as a percentage
of the unit's output
2,545 is a conversion factor.

Cost savings
= (Energy savings in Btu/year)/(Btu/unit of fuel) x ($/unit fuel)
Primary heater efficiency

Example: Waste heat will replace heat produced by a natural-gas, forced-air system having an efficiency of 85 percent

Cost savings = (846,976,000 Btu per year)/(100,000 Btu/thermal) x ($0.60/thermal)

  = $5,979 per year

Note: The cost of operating an additional fan for duct loading has not been included.

Air Dryers

Air drying is an important part of a compressed air system. The air produced by an air compressor is typically 100 percent saturated with water vapour. The air dryer removes this moisture, which would normally condense in the cooler environment of the distribution system. Without an air dryer, free water will likely show up in the piping. This water would mix with compressor lubricant and pipe scale, producing a sludge that would contaminate downstream equipment and processes. Often, excessive drainage downstream removes the liquid. This drainage, however, wastes energy.

Depending on the dryer, the cost of drying compressed air can range from one percent to 20 percent of the compressed air system's energy consumption. Higher percentages are common in lightly-loaded systems with oversized air dryers. In general, the drier that the air must be, the higher the drying cost.

You can find a brief description of available air dryer types in the Proven Opportunities at the Component Level section of the CAC Sourcebook. It is important to know that for dryers with no energy management controls, the energy costs per 100 cfm listed in the Sourcebook are for the dryer nameplate ratings, not the actual flow through the dryer. A 1,000 cfm rated, fixed-cycle desiccant dryer, for example, will consume a constant 20 to 30 kW of compressed air system energy even though it may only be drying at 25 percent of its capacity.

To save energy with air-drying systems, dry the air only as much necessary, and control the dryer energy consumption according to the moisture loading in the inlet air. Unless there are special process requirements, or the compressed air distribution pipes are exposed to temperatures near freezing or below, most industrial plants only require compressed air dryness levels in the range a refrigerated air dryer can produce. If you choose this type of air dryer, you can save energy by installing a refrigerated dryer operating in a cycling on/off mode, using a thermal mass to maintain a more constant dewpoint output.

When drier air is required and a desiccant air dryer is chosen, you can make significant savings by choosing dewpoint-sensing controls. These limit costly dryer regeneration to only that needed to maintain the rated dewpoint output through all compressed air system flows and moisture levels.

How Much Will I Save? – Air Dryers

Table 5 shows the approximate annual operating costs for various types of compressed air dryers based on a 100 hp, 125 psi air compressor feeding a 400 cfm air dryer, 8760 hours per year at 10¢ per kWh. Also shown are savings gained by better dryer control.

Table 5
Annual Operating Costs for Various Types of Air Dryers

  Regrigerated Desicant Membrane
Loaded % Standard Cycling Standard Controlled Standard Controlled
100 % $ 2,190 $ 2,190 $ 10,512 $ 10,512 $ 14,016 $ 14,016
75 % $ 2,190 $ 1,643 $ 10,512 $ 7,884 $ 14,016 $ 10,512
50 % $ 2,190 $ 1,095 $ 10,512 $ 5,256 $ 14,016 $ 7,008
25 % $ 2,190 $ 548 $ 10,512 $ 2,628 $ 14,016 $ 3,504
10 % $ 2,190 $ 219 $ 10,512 $ 1,051 $ 14,016 $ 1,402

Loaded % Refrigerated Desiccant Membrane
75% $ 548 $ 2,628 $ 3,504
50% $ 1,095 $ 5,256 $ 7,008
25% $ 1,643 $ 7,884 $ 10,512
10% $ 1,971 $ 9,461 $ 12,614

When buying air dryers, aside from efficiency, consider pressure differential. Air dryers with lower pressure differentials reduce the needed compressor operating pressure and save energy. For more information on savings, see Decrease Operating Pressure.

Condensate Drains

Condensate drains can waste a great deal of energy. Condensate forms in a compressed air system because air at high pressure and at room temperature cannot hold as much moisture as the atmospheric air ingested by the air compressor. One or more condensate drains are necessary to remove this moisture at various common collection points in the system. A system with a properly-sized air dryer should have no condensate forming downstream from the cleanup equipment.

The most common condensate drains are manual drains, float-type drains, timer drains, and no air-loss drains. Manual drains that are cracked slightly open and draining constantly are the most costly to operate. Float-type drains or no air-loss drains are the least costly to operate. Regardless of type, it is important that there be a way of testing and inspecting the drains to ensure that they are working. This is the failing of some internal float-style drains that are installed inside filter assemblies and separators. The drains cannot be inspected easily. A single failed drain can disrupt the proper operation of even the best designed filter and dryer systems.

Table 6 shows the operating cost and savings for three types of condensate drains based on a compressor operating at 20 kW/100 cfm running 8760 hours at 10¢/kWh.

Table 6
Annual Cost and Savings for Various Types of Condensate Drains

Drain Comparison cfm used Annual Cost Saved
Manual 10 $ 1,752  
Timer 5 $ 876 $ 876
Timer 5   $ 1,752

Next: Purchasing Tips