Like most of the world, we’re finding new ways to stay connected and keep everything moving forward in these challenging times. Fortunately, we are already well acquainted with online learning methods for our internal training and customer education. For a number of years now, webinars have been a cornerstone of our efforts to get the word out on best practices regarding compressed air, vacuum and blower systems.
Our next webinar is with Plant Services magazine and will expand on our recent blog posts on oversizing compressed air systems. Compressed air experts Neil Mehltretter and Werner Rauer will address why oversizing occurs, how to identify oversized systems, how they impact productivity and profitability, and strategies to reduce their effects on your compressed air system.
Webinar: Oversizing: A widespread compressed air problem that is costing you time and money Date: May 20, 2020 Time: 2:00 pm, EDT Hosted by: Plant Services Magazine Speakers: Neil Mehltretter, Engineering Manager, and Werner Rauer, Rotary Screw Compressors Product Manager
We are closely monitoring the COVID-19 situation and have implemented guidance from the CDC, as well as state and local agencies to minimize health risks to our employees, our customers and the general public. We have eliminated all non-essential travel, implemented stringent hygiene protocols, and now many of our team members are working from home.
At the same time we remain fully functional and ready to support the many essential operations that depend on compressors, blowers and related equipment. We have a healthy stock of equipment and parts, and maintain frequent contact with our supply chain partners. At this time there is no supply disruption for our products.
Our sales/engineering teams continue to consult on new systems and upgrades. Our national service network is available for maintenance and repair work. If you need help on site, let us know in advance of any special access constraints or personal protection requirements for visiting service personnel.
You’ve probably heard this from us before, but at Kaeser we believe that the more you know about operating air systems, the more you’ll get out of them. We are committed to offering you the most current information you need to operate and maintain your compressed air system. We’ve shared some advanced tips for optimizing your air system, but in today’s post we’re going back to the basics with a glossary of terms used frequently in our industry.
Air flow: Volume of free air in cfm
Air receiver tank: Tank used for compressed air storage.
Artificial demand: Additional air consumption caused by excessive system pressure.
Capacity: The amount of air flow delivered or required under some specific conditions. May be stated as acfm, scfm, or cfm FAD (free air delivered).
Cubic feet per minute (cfm): The most common measure of air flow/volume in the US.
Cubic feet per minute, free air (cfm FAD): cfm of air delivered to some specific point and converted back to ambient air (free air) conditions.
Actual cubic feet per minute (acfm): Flow rate of air measured at some reference point and based on actual conditions at that reference point.
Inlet cubic feet per minute (icfm): cfm flowing through the compressor inlet filter or inlet valve under rated conditions.
Standard cubic feet per minute (scfm): Flow of free air measured at a reference point and converted to a standard set of reference conditions (e.g., 14.5 psia, 68°F, and 0% relative humidity).
Demand: Flow of air under specific conditions required at a particular point.
Discharge pressure, rated: Air pressure produced at the compressor outlet.
Discharge pressure, required: Air pressure required from the compressor at the outlet.
Dual control: A type of individual compressor control in which the compressor runs at constant speed either fully loaded or fully unloaded (idling), and stops completely if it idles uninterrupted for a preset time to reduce energy consumption. Will automatically restart when line pressure drops below selected minimum pressure.
Duty cycle: Percentage of time a compressor unit operates during a specified period. Allowable duty cycle is the maximum recommended duty cycle for a compressor that does not compromise compressor performance or accelerate wear.
Dynamic control: Allows compressors to switch from “load” to “stop” at low motor temperatures, and to “idle” when the motor is hot. (The control may bypass “idle mode” if the motor temperature is low and more compressor starts are allowable.)
Flow meter: An instrument used to measure flow rate of a fluid or gas.
Load factor: The ratio of average compressor load to the maximum rated compressor load during a given period of time.
Modulation control: An individual compressor control system which will modulate the inlet air flow in response to variations in pressure near the discharge in order to maintain relatively stable system pressure. The compressor runs at a constant speed.
Pressure: Force per unit area.
Pounds per square inch (psi): Standard US metric for compressed air pressure.
Pounds per square inch absolute (psia): Absolute pressure above zero pressure.
Pounds per square inch gauge (psig): Pressure difference between absolute pressure (psia) and ambient pressure.
Pounds per square inch differential (psid): Pressure difference between two defined points in the system. May also refer to pressure drop between two points in a system.
Pressure dew point: Temperature at which water will begin to condense out of air at a given pressure. To ensure that no liquid water is present, the pressure dew point must be lower than the lowest temperature to which the compressor air will be exposed.
Pressure drop: Loss of pressure in a compressed air system due to friction or flow restriction.
Quadro control: An enhancement over Dual Control that includes an additional timer to fine-tune the “idle” period,while bypassing “idle mode” after periods of low air demand.
Vario control: Uses a “smart” timer to vary the idle time based on the frequency of motor starts — resulting in greater energy savings.
Variable speed drive/variable frequency drive: Air flow is controlled to maintain a specified discharge pressure by controlling electrical frequency (and therefore speed of the drive motor) in response to a pressure signal at the compressor discharge. This is the most advanced and energy-efficiency type of compressor control.
We’re picking up on the thread of our last post about over-sized compressed air systems, where we showed that the further away from full load a fixed speed compressor operates, the higher the energy cost is per cfm of compressed air. Energy may be the easiest cost per unit of air to recognize and measure, but it’s not the only component of cost, and it may not be the most significant cost in your operation.
Increased cycling associated with under-utilization has several negative effects on compressors, and we’ve found that for under-loaded systems, maintenance and repair costs increase as a portion of total operating cost. A review of service records showed that units with duty cycles had a significantly shorter mean time between failure (MTBF). Because compressors are usually serviced based on total run time rather than actual load time, a machine that idles a lot costs more in parts and labor per loaded (i.e. productive) hour. If you calculate the service costs based on cfm produced rather than hours of run time, you’ll find that PM and repair costs per cfm rises also.
Like highway miles vs city miles
Think of your car. Cost per mile for gas and maintenance goes down if most miles are highway miles. But highway miles are also gentler on your car (fewer starts and stops, etc.). City miles are notoriously inefficient with fuel, but they also accelerate wear on the motor, the brakes, steering and suspension.
Likewise, low-loaded compressors are more likely to show wear at an accelerated rate. Inlet valves, vent valves, and others, cycle many more times at low load. Motors starts are more frequent which can affect bearing and winding life. On direct drive units with polymer couplings, frequent cycling can reduce coupler life. Frequent starts and stops put more wear on thrust bearings in the airend.
Further, if the unit doesn’t run enough, it may not reach proper operating temperature, which results in moisture accumulation in the lubricant. This is a common cause of premature airend failures. Frequent changes in temperature can also cause metal fatigue on aluminum coolers. These conditions call for increased frequency of preventive maintenance and the likelihood of downtime for repairs.
Downtime and Scrap
Another downside to poorly sized systems is pressure fluctuation. Swings in pressure may result in defective products, and more sophisticated production machinery have sensors that will shut down the equipment if pressure is outside of design specifications. Depending on the cost of raw materials and value of finished product, the costs of downtime and scrap may far exceed the losses in energy efficiency and service costs.
Meeting the Challenge
If you are planning a compressed air system for a new plant or expansion, you may only be able to estimate your compressed air demands. So the smart money is spent splitting the estimated demand among multiple compressors and having good controls (and ample storage). Using variable output compressors as trim machines is part of a good strategy.
For existing systems, the first step is an accurate air system assessment to determine how well your system is sized and controlled. If your budget allows for replacing compressors, the ROI from lower energy consumption, lower service expenses and reduced downtime may justify replacing over-sized compressors and adding controls. In some cases, just adding one smaller machine can make the difference.
If your budget cannot accommodate new compressors, there are lower cost investments that can help mitigate over-sized compressors. Adding storage often reduces compressor cycling and can stabilize pressure. In some cases, flow controls may further improve the effect of storage. For systems with multiple compressors, adding a modern multi-unit controller will definitely help reduce starts/stops while stabilizing pressure and provide additional benefits such as remote monitoring and energy consumption information.
Downtime and scrap caused by pressure fluctuations, high service and repair costs, and high energy costs, are problems that many plants simply live with as expected costs of operating compressors. But they don’t have to be. The first step is an honest assessment of how well your compressed air system is working.
With over ten thousand air system audits under our belt, we’ve seen it all and learned a few things. One of the most common problems we see is that most systems have far more capacity than needed. On average, users operate at 44% of peak capacity. It’s so common, we’d say it is an epidemic, and even our own customers are not immune despite our efforts to inoculate with education.
How does this happen? In many cases, users select compressors based on what they already have, adjusted with some prognostication about whether they expect to grow, add or eliminate production lines, etc. Generally, very little measurement and analysis goes into it. Plant operators are usually comfortable up-sizing a compressor for the safety factor. They don’t want to hear complaints of equipment with low pressure alarms, nor do they want to re-revisit compressed air system design every few years as they grow. So they purchase as big as their budget allows at the outset. When involved, consulting engineers may add to the problem by making conservative assumptions that all pneumatic equipment will operate fully loaded, all the time. Then they take this bad estimate and add a safety factor. In nearly all cases, there’s fudge factor on top of fudge factor. All believe they are acting in the interest of reliability, without understanding the significant negative impact on energy consumption.
Compressed air efficiency is best measured in terms specific power, which is kW/100cfm, and the Compressed Air and Gas Institute (CAGI) has an excellent program that encourages compressor makers to publish the specific power for each compressor. This is a great point for comparing two compressors side by side, but it cannot be used to predict what the user’s actual system performance will be. As the car sellers say: “your mileage may vary.” So much depends on how the compressors are run. The CAGI datasheets for fixed speed machines assume 100% load, which rarely happens in practice. From our many system studies we know that systems are grossly over sized. Whether a single machine or multi-compressor system, under-utilized compressors do not operate at their datasheet spec.
Let’s look at some actual examples of over-sized systems and the costs that resulted.
The chart above shows how the performance of compressed air systems declines dramatically as demand decreases (shown for the most common types of screw compressors in the field). This is measured in specific power (kW/100cfm), which increases as compressors operate further away from their full output capacity. We’ve added data points showing where a few actual customers operate on this curve to show that this graph is actually showing ideal (e.g. laboratory) conditions. As you can see, some are off-the-charts inefficient, but achieving efficient operation is certainly possible.
This is a greenfield plant (i.e., new construction) where the company specified dual 125 hp compressors, (2) 230 cfm refrigerated dryers, 1000 gallons of storage, an air main charging valve, and a master system controller. They spent $1.10/1000 cubic feet! Their system could be replaced with a pair of 15 hp units.
The current facility operates with a 50 hp screw compressor, a 285 cfm refrigerated dryer, and a 400 gallon receiver tank. Typical operation showed the facility running ~11 hours a day Monday through Thursday, with no operation Friday through Sunday. According to the data on the screw compressor’s controller the average system pressure was approximately 115 psig. The peak demand measured was 65 cfm and the average flow was 22 cfm. This unit is over-sized for the current demand. The calculated system specific power was 65.54 kW/100 cfm. The company would be much better off with a pair of 10 hp compressors. They spend $1.09/1000 cubic feet for their air!
Retail equipment manufacturer
The facility currently operates with (3) water-cooled 200 hp compressors, (3)100 cfm refrigerated dryers, and 3,800 gallons of dry storage. The data was provided from the master system controller. This system is highly variably in demand (891 to 2417 cfm) but was designed with 3 units to supply this full range efficiently. While this is an outstanding example of a well-designed system, they could get even better specific performance if they drop their pressure below the average of 115 that they currently maintain. They spend $0.35/1000 cubic feet including their cost of cooling water.
The cost per unit of compressed air goes up as the % load goes down, which means that your yield on this costly input goes down as well. Don’t be yet another statistic with an over-sized and inefficient compressed air system. Educate yourself on the life cycle cost benefits of multiple smaller units that will provide low costs, high efficiency and reliability. Below are a couple of useful resources: