Baselining, the term currently in vogue for this process, either estimates or measures the base data. Measuring is by far the most accurate and reveals operational characteristics that may lead to inexpensive changes that can reduce energy savings greatly. Knowing what, when and where to measure are the keys to establishing a good baseline.
Of course, measuring everything always establishes the best baseline. In a new facility, it's quite easy and cost-effective to include the monitoring equipment in the plant design. In an existing facility, a few simple measurements, a calculator, a pad of paper and a telephone are the tools one needs to start reducing energy costs. The primary measurements are power and pressure. Flow measurements add to understanding the dynamics of the system, but are not an absolute requirement for baselining energy consumption.
Don't confuse this simplified baseline with a compressed air system audit. A complete audit performed by a qualified compressed air specialist gathers much more data concerning system dynamics and user characteristics. The information gathered in the simplified baseline is important to the specialist and may reduce audit costs by reducing the time spent collecting and analyzing data.
What to measure
The first priority is determining the plant-specific factors that might affect power consumption. Do different shifts or different days have different production levels? If production is stable, data collected at any time is representative of the overall power used. If production varies by shift or day, you'll need to collect data at each production level.
Do production levels show seasonality? It's a characteristic that can be difficult to model. Compressed air systems are nonlinear. A 50% reduction in production will probably not reduce system power by 50%. If it's the busy season, look for a shift or day that runs at production levels similar to the slow season. For example, a Saturday during the busy season might be similar to a weekday during the slow season. Data collected on Saturday could serve as a proxy for slow season power consumption.
In addition to variations in power consumption, power cost may vary by time of day or season. It may be possible to average, or blend, power costs to simplify calculations. However, if you are subject to a substantial demand charge imposed at a certain time of day or season, using a blended power cost is inappropriate. Small changes to operation during peak times may yield short paybacks that can't be correlated to a blended power cost. Moving an operation or process from one shift to another also may dramatically change power costs without a corresponding physical modification to the compressed air system.
Develop a plan
Believe the adage that states, "If you can't draw it, you don't understand it" because it certainly holds true for a compressed air system. Draw a simple block diagram of the compressed air system. It doesn't need to be P&ID-quality because its purpose is to identify locations where pressure readings are to be taken. Don't rely on OEM pressure gauges supplied with the compressors. Most have an accuracy that may result in as much as a five-psig difference in readings between compressors.
Pressure readings are taken at the compressor discharge, before and after filters, before and after dryers, at the main air distribution pipe and at the most critical user. If possible, take readings in several locations throughout the distribution system. When taking pressure readings at the critical end user, read both the pressure in the distribution piping and the pressure at the point of use. Many times, the largest pressure drop occurs between the distribution pipe and the point of use. Simultaneous pressure readings help identify potential problem areas.
If the system has multiple compressors, verify the pressure range for each controller. This may require compressors to be isolated from the system and run through their entire control pressure range. If each compressor has dedicated dryers and filters, isolate the entire supply subsystem and record pressures at various points. Use a single, calibrated pressure gauge to measure each compressor.
From these readings, develop a pressure profile showing each compressor's control pressure range, pressure drop across treatment equipment, pressure drop through the distribution system and final pressure at the critical point of use.
Finding the tools
Most manufacturing operations lack an extensive collection of recording power meters, pressure transducers and data loggers. The local utility may help with recording compressor power consumption. Other sources for assistance are state energy offices and local universities.
Recording power meters also can be rented. If the plant air pressure is stable, a meter that records the power level every 10 seconds and averages the data once a minute probably will work fine. On the other hand, sampling every second may be required if there are abrupt swings in system pressure. Keep in mind this is merely a simple baseline and not a complete system audit. Collecting too much data only makes analyzing and understanding it that much more difficult.
Proper data analysis requires knowing how much power the system is supposed to use. Call the compressor vendor and ask for the unit's full-load power consumption. Just because a compressor has a 200-hp motor doesn't mean it should be running at the full-load current shown on the motor nameplate. Changes in pressure settings affect power requirements. The unit may be designed to run slightly into the service factor or significantly below. Given the pressure settings, a qualified compressor distributor or manufacturer can provide a reasonable estimate of full-load power for the particular model of compressor.
Collecting pressure data can help identify problem areas in the facility. Pressure data often can identify the operation or process that determines supply-side pressure requirements. It's not uncommon to find a pressure drop of 25 psig or more between the distribution pipe and the point of use. If that pressure drop forces the entire system to operate 25 psig higher than necessary, the power required may be 10%to 12% higher than needed, depending on how the compressor controls are set.
System pressure behaves dynamically
Using a calibrated gauge to take static pressure readings provides a snapshot of the system. This may be useful in identifying problem areas, such as plugged filters or restricted supply lines, that can be corrected inexpensively and easily. A snapshot of the system tells you little about where it's going, how fast it's moving, whether it's full and, most importantly, whether it's in control. Getting more than a snapshot requires equipment that shows how the system responds to events and suggests ways to improve the response.
Data loggers and pressure transducers can be rented. Recording and interpreting the information is a little more complex than reading and recording the power. Before renting this equipment, contact local compressor suppliers. They may be able to help with rental, installation and data interpretation. State energy offices and local power companies also may know where to find instruments to monitor system pressure.
After establishing the baseline, start making system improvements. Begin by implementing the energy efficiency measures a qualified air system supplier or system specialist suggests.
In a typical system, a good baseline and minimal effort can result in 15% to 25% energy savings.
Wayne Perry is a technical director at Kaeser Compressors. He can be reached at Wayne.Perry@Kaeser.com or 540-898-5500.