By: Neil Mehltretter
One of the most popular methods to identify ways to lower compressed air system energy consumption is to complete a comprehensive compressed air assessment, or “air demand analysis.” If you’re bringing in an outside party to perform the service, make sure your vendor is not cutting corners and that you understand:
- Your goals from an air demand analysis (address low pressure points in the plant, air quality issues, et.al)
- The necessary steps for a vendor to provide an accurate system “snapshot”
- The industry standard power consumption goal that will contribute to the vendor’s system recommendations
- The importance of controllers in most optimization plans
- Systems can always be improved
Conducting a Site Survey
It is important to list and understand all the equipment in a compressed air system before installing any measurement devices so that 1) the devices are properly placed, and 2) system dynamics are properly understood. The person responsible for collecting information should note: environmental conditions, physical layout, details on all air system components including clean air treatment, piping, storage and controls. Processes at the facility should also be documented. Many auditors will have a questionnaire which they will complete to compile the details into one document.
Measuring and Quantifying kW / 100 cfm
For many systems — power, flow, and pressure should be measured for a period of approximately 10-days to obtain an accurate system snapshot (other data points such as pressure dew point [to determine air quality], vacuum, and temperature should be measured as needed based on system requirements). The measurement period should include nights, weekends or other downtime to identify non-productive demands. True power used by the air compressors is measured using kilowatt meters which would monitor amperage, voltage, and power factor. Data loggers should record data points on each air compressor every 0.5 seconds, and average the data over a preset recording interval such as 20-second resolution.
A suitable Key Performance Indicator (KPI) for all compressed air systems is specific power consumed (kW) per 100 cubic feet per minute (cfm) of compressed air used in the plant. This provides an idea of how efficient a compressed air system is – regardless of varying plant output levels. Recommended systems have a KPI below 21 kW / 100 cfm.
Understanding the System Dynamics
The system information collected should be thoroughly analyzed and areas of improvement should be identified. Multiple scenarios should be considered to lower the compressed air system’s energy consumption, including an analysis of the major compressed air users within the facility to determine whether compressed air is the most efficient option for each application.
Implementing a Recommendation to Improve kW/100 cfm
Many system assessments focus on reducing compressed air consumption by fixing compressed air leaks or through the elimination of “inappropriate uses” of compressed air. Using engineered air nozzles, for example, to replace perforated pipe for blow-off applications will reduce compressed air demand. Energy savings, however, will ONLY be realized if the controls on the air compressors can capitalize on these gains and when appropriate this includes proper use of a variable frequency drive (VFD). The costs involved with adjusting compressor controls are almost never included in the ROI calculations. Many modulating air compressors will continue to consume the same amount of electricity (kW) within broad ranges of compressed air flows (cfm). Users cannot assume a linear relationship between compressed air use and power consumption.
Verifying Performance of kW/100 cfm
Utility rebate programs sometimes require energy savings be verified by an additional compressed air audit performed after the air system is optimized. In cases where utility rebates do not require such a degree of verification, facilities should still consider implementing some form of a post-optimization assessment. The common mantra, “If you can’t measure it, you can’t manage it” applies. So while this step is actually optional for most systems, it should be contemplated.
All systems can be enhanced – even systems designed using air audit data and are newly installed. The additional assessment should verify the compressed air audit findings, as well as assist in identifying additional areas of improvement. In many cases, it is recommended to monitor a compressed air station over time and make improvements in a step-by-step fashion, then at each step measure the change in system efficiency. Some system controllers actually track and store ongoing energy consumption data, so for many, a review with a master system controller provides enough information for internal evaluation and can be used to verify improvement metrics.
To give an example of information that can be determined in a follow-up compressed air audit, see the chart below using data from an actual customer. The verification results showed that although the overall air demand in the factory had gone up, the VFD unit’s efficiency was well within the target kW /100 cfm. The annual energy savings were verified at $19,165 per year – a whopping 42% reduction in energy cost and slightly higher than the original conservative estimate.
|Annual flow (cf/year)||128,010,406||130,600,422|
|Existing Energy Consumption||685,792 kWh/yr||699,668 kWh/yr|
|Proposed Energy Consumption||395,942 kWh/yr||404,822 kWh/yr|
|Annual Energy Savings||289,850 kWh/yr||294,846 kWh/yr|
|Energy Cost||0.065 $/kWh||0.065 $/kWh|
|Existing Energy Cost||$44,577 / yr||$45,478 / yr|
|Proposed Energy Cost||$25,736 / yr||$26, 313 / yr|
|Annual Energy Savings||$18,840 / yr||$19,165 /yr|
|Original System Specific Power KPI||32.14 kW/100 cfm||32.14 kW/100 cfm|
|Specific Power KPI||18.56 kW/100 cfm||18.60 kW/100 cfm|
It is critical to focus on improving the specific KPI of kW/100 cfm. Compressed air users can ensure return on investments (based upon energy savings) on air system audits by working with firms who measure at least power, flow, and pressure over a period long enough to obtain an accurate system snapshot, often 10 days. Further, the data obtained from the audit must be thoroughly analyzed and recommendations should be made based on conservative estimates of power reduction.
When implementing an optimization plan, it is important to remember that without proper system control, the system’s overall energy consumption may not be significantly lowered. In the case described in the chart above, adding a VFD unit allowed the system to generate compressed air at the lowest acceptable pressure, thereby reducing energy consumption and cost. In many cases, a master system controller is the best solution as master controllers use compressors at their most efficient design point or turn them off.
Finally, users that complete a compressed air audit and optimize their systems will surely reduce energy consumption and see lower operating costs. However, even the best, newly optimized systems can always be further improved. Facilities should be advised that additional savings can be determined by completing a leak detection audit in conjunction with a full evaluation of the demand-side of the facility for appropriate use of compressed air. Facilities should determine which services are best suited for them (leak detection, supply-side compressed air audit, and/or demand-side compressed air audit), and develop a plan for how often those services should be performed.