Compressed Air System

Calculating compressed air efficiency

CAGI sheets can help to make a comparative decision on compressed air systems.

By Ron Marshall, Compressed Air Challenge

In brief:

  • The data sheets made available by the Compressed Air and Gas Institute (CAGI), in addition to resources from Compressed Air Challenge, can make a real difference in helping you choose a compressor.
  • There is now a wealth of information about compressor and air dryer energy consumption available within the CAGI Compressor Data Sheets published by participating manufacturers.

It’s probably been years since you’ve even thought about buying a new air compressor, but now your old unit is broken, the boss is howling, and you need to make a decision. But there are so many choices. How do you decide the best one? The data sheets made available by the Compressed Air and Gas Institute (CAGI), in addition to resources from Compressed Air Challenge, can make a real difference in helping you choose.

Too often, the purchasing decision comes down to price. The difference between various compressor brands and models may be only a few hundred dollars, making the cheapest machine the obvious choice to the purchasing department. But what about operating costs?

At $0.10/kWh with a five-day, two-shift operation of about 4,200 hrs/year, a typical 100 hp compressor would require about $37,000 in annual electrical costs. Over a 10-year period, these costs can add up to about 76% of the total lifecycle cost of the air compressor with the purchase price representing only 12% (Figure 1). Even a 5% change in operating efficiency can equal an $18,500 difference over 10 years.

Figure 1. Over 10 years, the operating costs of an air compressor is mostly energy.
Figure 1. Over 10 years, the operating costs of an air compressor is mostly energy.

Data sheets can help

Figure 2. Typical CAGI Data sheet for a fixed speed, lubricated screw compressor.
Figure 2. Typical CAGI Data sheet for a fixed speed, lubricated screw compressor.

When making decisions about energy efficiency, we need some accurate information to plug into power cost formulas. In past years, there has been a jumble of information based on a variety of test conditions. Fortunately, there is now a wealth of information about compressor and air dryer energy consumption available within the CAGI Compressor Data Sheets published by participating manufacturers.

Figure 2 shows an example data sheet for a selected 100-hp, 125-psi, air-cooled, fixed-speed, lubricated rotary screw air compressor. Using it and others you can make and compare cost projections on the electrical needs of fully loaded operation between different makes and models of compressors Table 1 shows a summary of comparisons of other compressors, including more efficient compressors from the same companies (shown as Opt 2).You can see the numbers vary, but the common ground is the specific power number. Specific power is like a gas mileage rating for compressors and shows the ratio of the total package power input kW of a complete compressor package, including cooling fans, pumps, and other electrical loads for every 100 cfm of output, not just the break horsepower on the screw inlet shaft.

Compare carefully

It should be noted that this table is not an apples-to-apples comparison. Two of the manufacturers have chosen to test their compressors at 115 psi, and this lower pressure results in lower specific power numbers by about 5%. To compare fairly, the numbers would have to be adjusted for a common pressure by about 1% for every 2 psi increase in pressure. The manufacturer should be consulted to provide assistance finding out the actual package kW consumption at the pressure you choose to run your system. Besides lower kW consumption for fixed-speed compressors, this lower pressure allows most VSD-controlled machines to produce more cfm output through an automatically programmed increase in maximum speed.

Table 1. Specific Power can range for different choices (various selected CAGI members).
Table 1. Specific Power can range for different choices (various selected CAGI members).

An additional item has been added for interest in Table 1 as Item E, a 100-psi-rated compressor. This shows the potential savings in specific power gained if you operated your air compressor at 100 psi, rather than 125 psi. The resulting 9.5% reduction would reduce the lifecycle power costs significantly. Unregulated compressed air loads reduce flow at these lower pressures resulting in even more savings.

Part loads

If the compressor in question is operating at part load in load/unload trim duty, a different kind of calculation is required that takes the effective storage receiver capacity of the system into account. Figure 3 shows the relative power consumed by a lubricated screw compressor at various part loads and system storage sizes. These are the characteristic of lubricated screw compressors with a 10-psi wide pressure band and 40-sec blow down time and can vary with different conditions.

Figure 3. Part Load Performance of typical lubricated screw compressors, load/unload control.
Figure 3. Part Load Performance of typical lubricated screw compressors, load/unload control.

It can be seen, for example, that at 3 gal/cfm capacity the part-loaded air compressor might consume 75% of its full load power while delivering 50% of its full load flow. This would mean the specific power at the 50% point would be roughly 50% higher than at full load (75/50 = 1.5 times full load sp). At a lower load of 25% of full load, the power consumption would be about 55% of full load, resulting in a specific power that is more than double the full load number (55/25=2.2 times full load sp)

Evaluation of VSD power consumption

Figure 4. Typical VSD curve from 125 psi CAGI data sheet (CAGI)
Figure 4. Typical VSD curve from 125 psi CAGI data sheet (CAGI)

The availability of a new style of CAGI data sheet for VSD compressors is a fairly recently developed resource. Figure 4 shows a table and curve from a sample data sheet for a 100-hp, 125-psi-rated, VSD-style screw compressor.

It can be seen from the typical VSD curve provided that this machine would be in the midrange of the specific power numbers shown in Table 1 if it operated at 100% load, even beating some fixed speed units. However, at 50% load it would have a very attractive number in the 20 kW/100 cfm range, compared, for example, to about 31 kW/100 cfm for a partly loaded load/unload screw compressor following the 3 gal/cfm line in Figure 3. At lower-percent loading, the difference in efficiency would be even more pronounced.

The shape of the various VSD curves can vary between compressors, with some units having very flat curves and smaller turndown ranges, while others may have wider but steeper curves yielding higher specific power numbers at lower flows. When comparing the energy consumption between various units, the curve shape can make a significant difference depending on the characteristics of the compressed air demand so they shouldn’t be ignored. The new CAGI sheets for VSD compressors are valuable resources to assist potential compressor purchasers select the best option.

Comparison example

Two fixed speed: Consider the following example calculation using a simplified load profile where two different options with different compressor combinations are compared. The load profile has been determined to be 600 cfm for 3,000 hrs and to be 200 cfm for 2,500 hrs. The system has about 3 gal/cfm storage (1,200 gal). This profile might be the result of a compressed air audit of a system. This profile would require two running compressors, one fully loaded and one part loaded for the 600 cfm flow and would only need a single compressor part loaded at the lower 200 cfm flow. It is assumed the compressor control scheme allows the second unit to turn off when not required.

The energy required at the various loads is calculated using the rated specific power numbers for each compressor, if fully loaded, or adjusted specific power, according to the CAC Performance chart (Figure 3) if part loaded. Consider Option 1, using two compressors with the characteristics of Item A in Table 1, each capable of producing 414 cfm at 21.0 kW/100 cfm. At the 600 cfm flow level one base compressor would be fully loaded with the second trim unit covering the remaining flow of 186 cfm (600 – 414 = 186 cfm). This trim compressor would be 45% loaded (186/414= 45%).

At the 600 cfm level the kWh for the fully loaded base compressor is calculated as follows:

For the trim:

Specific power at 45% load would be 33.6 kW/100 cfm (from Figure 3 the unit consumes 72% of full load power at 45% which is 1.6 times the full load sp).

At the 200 cfm load (only one compressor running):

This trim unit would be at 48% load (200/414). Specific power from Figure 3 = 1.56 x 21 full load rated specific power = 32.8 kW/100 cfm

Total for all hours = 612,308 kWh

Purchasing a new compressed air system is typically a once-in-a-career endeavor. A well-organized approach will be met with the needed information and support. Read this article on your compressed air legacy to help you make the right decisions.

Option 2 using more efficient Item A - opt 2 as base load and the VSD from Figure 4 as trim.

The fixed speed base compressor would be fully loaded at its rated flow with the VSD trim unit covering the remaining flow of 148 cfm (600 – 452 = 148 cfm). This trim compressor would be 30% loaded. Energy consumption would be calculated as follows for the 600 cfm level.

For the base:

For the trim:

Specific power at 30% load would be 21.8 kW/100 cfm from Figure 4.

For the 200 cfm load:

Specific power of the VSD at 41% load (200/488) from Figure 4 = 20.2 kW/100 cfm

Total for all hours = 454,076 kWh

Savings Option 2 vs. Option 1 = 612,308 – 454,076 = 158,232 kWh

Note further adjustment to kWh readings may be required if the system is operated at a lower discharge pressure than the rated 125 psi. Ignoring any pressure reduction, at $0.10/ kWh, choosing Option 2 over Option 1 would represent a 26% savings in electrical costs worth $15,823 /yr.. Further savings could be gained by addressing other items such air dryers, filters, piping pressure drop, leaks, and inappropriate end uses.

Ron Marshall is a member of the Project Development Committee at the Compressed Air Challenge. Contact him at rcmarshall@hydro.mb.ca and (204) 360-3658.

If Option 2 cost $20,000 more per year to implement, the return on investment would be slightly more than 15 months.

The resources available from the CAGI data sheets and Compressed Air Challenge materials can assist compressed air users in evaluating their options when replacing compressors. There are more CAGI resources available to help you do similar calculations for air dryers, too.