It’s Not Just About Energy: The Sins of Over-sizing, Continued

By: Kaeser Compressors, Inc.

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. 

Read our white paper titled “Using Master Controls to Improve the Performance and Efficiency of Industrial Air Compressors” to learn more about how to minimize equipment run times, maintain stable air pressures, and deliver rapid payback in operational and energy savings.

Over-sized and Under-utilized: An Epidemic

By Matt McCorkle

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.

Shoemaker

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.

Cabinet manufacturer

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:

Or, request a free system walk-through!

Our Top 5 Compressed Air Blog Entries for 2019

By: Kaeser Compressors, Inc.

If you are looking for some quick tips to improve your compressed air system, consider starting with our most read blog posts from 2019.

#5 This is Why You Don’t Use PVC: Using PVC in a compressed air system poses significant safety risks. This post covers what you need to know if you are considering using it.

#4 Applying Motor Temperature Ratings: A perennial favorite, this blog post offers useful information to help you apply motor temperatures ratings. Motor temperature ratings are given by the type of insulation used on the wire as well as the utilization rate. These two parameters determine the expected lifetime of the motor windings.

#3 Some Like It Hot…Your Compressor Room Doesn’t: If you are having problems with compressor room overheating, read this post for tips on better temperature regulation.

#2 Choosing Between an Air-cooled or Water-cooled Compressor: This post outlines four questions to answer when deciding between an air-cooled and water-cooled compressor.

#1 The Art of Dryer Sizing: This post has been at the top of the list since it was published in 2015 and is the most viewed post again in 2019. Read this post to understand how temperature and pressure impact water content and to learn how to make sure dryers are properly sized.

Bonus:

Our most popular post published in 2019 was this summer’s How to Keep that Trusty Recip Going! If you have a reciprocating compressor because it’s a good fit for your shop, here are some tips to avoid some common issues as well as some maintenance tips to keep your recip unit going.

Do you have a topic you’d like us to cover in 2020? Let us know in the comments.

Compressor Purchasing Criteria for Energy Efficiency

By: Michael Camber

During the purchasing decision process, it is common for prospects to compare compressors with some sort of utility criteria. In other words, how much air will they get for their money. Below, we address some common approaches we encounter:  

Compressor cost per horsepower 

This is a quick comparison that can be done using basic product literature, but it is a very superficial metric for comparing compressors. Since the requirements of air tools and equipment are not rated in compressor horsepower, and since the flows among compressors of the same nominal hp can vary by 20% or more, this doesn’t tell you how much air (cfm) you are getting or whether a compressor will meet your air demands (assuming you know them). Our experience with hundreds of thousands of systems has shown that without knowing your actual system needs, you are more likely to oversize your system, which leads to higher power and maintenance costs and reduced longevity (see our blog post on oversizing).    

Compressor cost per cfm

This can also be done with literature and is a step forward for basic comparison, and if you know your actual flow demands it will help avoid sizing mistakes. Like the first method, its shortfall is that it only considers initial cost. It does not reflect energy efficiency, so it is not a predictor of the largest component of compressed air life cycle costs:  electricity usage.

Compressor cost and specific power (kW/100 cfm)

Specific power is the true measure of a compressor’s efficiency, so combining this with unit cost is a better indicator of compressor value. Keep in mind, however, that specific power is based on a fixed set of conditions and assumes the compressor is running at maximum capacity, which they rarely do.  Nonetheless, when choosing machines it is very useful to compare the specific power (AKA “specific performance) of the compressors. Most major manufacturers provide this information in CAGI data sheets on their websites or by request (see our blog post on how to read them).  

System specific power

Because most compressors run partly loaded for a variety of reasons (demand fluctuation, oversizing, changes in production), the best metric for energy efficiency (and therefore compressor selection) is system specific power. This metric reflects the ability of the total system to maintain efficiency throughout the full range of production demand and is a far better metric for operational efficiency. This is not easy to assess for new plants (unless there is a similar sister plant in operation), but it is easily done for upgrades on existing systems with tools like ADA/KESS that data log  parameters including compressor run time, system pressure, power consumption and flow, and then select the best mix of machines to meet the need. We strongly recommend assessing system performance anytime you are adding or replacing compressors — even if you plan to simply replace a compressor with another of same size. This is an ideal time to baseline the system and identify inefficiencies in pressure drop, storage, sizing, and controls.

Because compressed air demand changes as plants increase or reduce production levels or upgrade pneumatic equipment, it can be a challenge to maintain optimal system performance. The best approach in multi-compressor systems is a combination of proper sizing of compressors and the use of adaptive smart controls. These learn system dynamics and switch compressors on/off in the most efficient manner while maintaining desired system pressure, balancing load hours and minimizing idle time.