What to consider when sizing your compressed air system
There are many factors to consider when sizing compressed air systems such as capacity and pressure, but also its role in the company short- and long-term.
- A review of overlooked factors when sizing a compressed air system including dryer purge rate, compressor control gap and more.
- Types of compressors – fixed and/or variable speed drive compressor – and their impact on efficiency in a compressed air system.
- Utilizing efficiency data sheets and other tools and formulas in right-sizing a compressed air system.
Compressed air insights
- Selecting the right air compressor is critical for efficiency and cost-effectiveness. The primary cost of ownership is often electrical power, accounting for up to 80% over ten years.
- Control gap, often overlooked, occurs when demand falls outside the capacity of compressors. Careful consideration of compressor types and capacities is necessary for stable plant pressure.
Selecting an air compressor for a facility presents many questions. The wrong size compressor can operate at an inefficient part-load condition or lack capacity to meet the peak demand in the plant, costing plenty in energy costs and down time.
When it comes sizing air compressors, there are often two parameters that come to mind: capacity and pressure. To be certain, those must be properly specified for the application to be a successful one.
Another consideration is the role of the compressor in the system. Will it be a base-loaded compressor that always runs at full capacity, or is it intended to be a trim compressor, which will run at varying loads?
Selecting the trim compressor entails a host of decisions, not only determining the appropriate full load capacity but also its turn down capacity (how far below full load it can operate) and how the turn down capacity compares to the capacity(ies) of the existing base load air compressor(s). When weighing these considerations, it is not just the turn down capacity, but also the efficiency of the machine at the part load(s) at which it will operate.
The compressor selection comes down to a simple question; what is the best business decision for the organization? In most cases, the best business decision comes down to the lowest cost of ownership with the highest degree of reliability. For industrial air compressors, the primary component of the total cost of ownership (TCO) is often the cost of electrical power.
Energy, efficiency and capacity standards
The U.S. Department of Energy Compressed Air Challenge estimates up to 80% of the ten-year total cost of ownership of an industrial air compressor is electrical power. Hence, the efficiency of the compressor, not only at full load, but at the partial load at which it may operate, become important points of comparison not only between competing manufacturers, but between competing capacity control technologies.
As it turns out, there is a lot more to sizing a compressor than actual cubic feet per minute (acfm) capacity and psig operating pressure. The good news is that there is a dependable and accessed source of information to help guide the decision-making process.
Manufacturers participating in the Compressed Air and Gas Institute (CAGI) performance validation program publish data sheets that provide information including package power (kW), capacity (acfm), specific power (kW/100 cfm) and isentropic efficiency at full load. For variable displacement and variable speed-controlled compressors, the package power, capacity and specific power information is published for part-load conditions.
The data sheets are available on most manufacturer’s websites or can be accessed through links found on the CAGI website. Users can navigate to the CAGI Air Compressor Performance Verification and Testing Results page and access the participant directory.
Consider the application illustrated in Figure 1. The peak demand for the facility is between 2,000 and 2,100 standard cubic feet per minute (scfm) and occurs 6.34% of the time. The compressed air system must be capable of meeting this demand. If this were an intermittent demand, the application of controlled storage would be appropriate. In this case, the demand is sustained, and the system must be capable of regularly generating the stated capacity.
The application engineer must also consider the other ranges of demand because the plant operates at other demand levels for much of the time. Specifically, the range of 1,400 to 1,500 scfm must be given priority as the plant operates in that range of demand about one-third of the time, or 56 hours per week.
In this example, oil flooded rotary screw air compressors will be considered, but the philosophy would be applicable to oil free rotary screw compressors as well. The common approach to sizing compressors of providing a baseload, trim-load and back-up compressor will be used. This arrangement can be desirable as it represents N+1 redundancy to minimize the risk of unplanned downtime, while providing the lowest cost of acquisition among alternatives.
Mind the control gap
A commonly overlooked aspect of sizing a combination of air compressors is control gap. Control gap occurs when the demand for compressed air falls into a range where neither the variable speed drive (VSD) nor the fixed speed compressor or compressors can effectively meet the demand. The result is often unstable plant pressure as the on-line compressor(s) seek to meet the demand which falls outside of the turn down range for the trim compressor.
In a two-compressor system, control gap can often be avoided by selecting compressors where the turn down range of the trim compressor is greater than the capacity of the fixed speed (baseload) compressor. The control gap is often associated with VSD controlled compressors but can be present with any capacity control technology.
For this application, a VSD trim compressor and a fixed speed base compressor will be selected to meet the demand profile. A back-up compressor will be selected to provide N+1 reliability.
The demand data provided in this example is shown in scfm, while the compressor capacity data is shown in acfm. The demand and capacity information must be expressed in consistent terms so the compressors can be properly sized. For this example, the demand data will be converted to acfm.
In many applications, the end user will stipulate the compressed air system must always provide a defined capacity at the specified pressure and air purity. Often, the capacity is expressed as scfm, and the design conditions (extreme summer and winter) are provided.
In the case of a rotary screw air compressor, the combined effects of altitude, temperature and humidity present in summer conditions result in the de-rating of the compressor capacity when the capacity is expressed as scfm instead of acfm. The relationship between acfm and scfm is provided below.
ps = standard pressure (14.5 psia for CAGI standard conditions)
pa = actual atmospheric pressure at location (corrected for altitude)
ppwv = partial pressure of water vapor at saturated condition, actual temperature
rh = relative humidity at site conditions
Ta = actual or design temperature
Ts = standard temperature
For this application, the specified site summer conditions are as follows:
1250 ft above sea level (14.1 psia atmospheric pressure)
90 °F (0.698 psia, not corrected for altitude)
60% relative humidity
Substituting values into the equation:
The CAGI data sheet shown in Figure 2 illustrates the full-load capacity of the compressor as 1,660 acfm @ 125 psig. Also note its turndown range is 1,225 acfm (1,660 acfm at full load – 435 acfm at minimum flow). A base load compressor complementing the trim compressor that provides a minimum capacity of 669 acfm is needed so the compressor meets the total demand required by the plant.
VSD and fixed speed
Another point of interest is the efficiency of the VSD controlled compressor (CAGI data sheet on the left). At the 1,500 to 1,600 scfm demand level (corrected to 1663 acfm for summer conditions) the VSD compressor is very near full load, representing an efficient operating point. Below this point, the VSD compressor will be on its control curve, where the specific power begins to increase (less efficient). However, the increase in package specific power is reasonable, such that at minimum flow, the specific power is 21.9 kW/100 cfm. What happens when the demand exceeds the capacity of the VSD controlled compressor? The fixed speed compressor comes on-line, and operates at full load, contributing its capacity to the plant demand. The VSD compressor output falls to 958 acfm, as it is reduced by an amount equal to the capacity of the fixed speed compressor (702 acfm). The reduced output of the VSD air compressor falls at a location on the control curve slightly more efficient than the full load specific power for the compressor.
In this scenario, the capacity of the back-up air compressor would need to be equivalent to the VSD controlled compressor, as it is the largest in the system.
With the technology available today in many of the on-board microprocessor controls, two VSD controlled compressors of equal capacity could be selected and operated in a load-sharing arrangement, where they modulate together to meet the varying demand. Fewer, smaller compressors often represent larger initial and maintenance costs, however.
Don’t forget the purge
It would appear the specifier has completed the analysis and is ready to place the purchase order for the compressors. Perhaps he or she is, or perhaps he or she has overlooked an important consideration: dryer purge loss.
If the plant is using regenerative compressed air dryers, the purge rate contributes to the total demand. It is also important to remember that most manufacturers state the average purge rate on their equipment specifications. The instantaneous purge rate can be higher. It is important to have a clear understanding of the dryer operation and its National Electrical Manufacturers Association (NEMA) purge cycle to accurately determine the demand associated with the dryers.
One last thought about sizing a compressor, or more appropriately, about selecting a compressor. An industrial air compressor, if properly maintained, should last ten years or more. Often, the compressor distributor will provide maintenance and service for the life of the compressor (and the associated equipment).
Considering the capabilities of the service provider is an important part of the selection process. It is so important the Compressed Air Challenge Best Practices for Compressed Air Systems includes an entire chapter on the topic, “Guidelines for Selecting a Compressed Air System Service Provider.” A trusted service team can ensure the perfectly sized compressed air system performs optimally.
In the end, many factors should be considered when sizing a compressed air system, not just capacity and pressure. Properly analyzing all variables – VSD, base load, control gap, dryer purge, and maintenance — will result in the most efficient and reliable system for an operation.