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.

Drop and give me 20!

By: Michael Camber

During a recent set up of a new controller installed to manage three compressors (two 40 hp and one 75 hp), our field rep mistakenly set the system pressure 20 psi lower than planned. A week or so later, during a system check, the technician discovered the error. Meanwhile, the plant equipment ran fine. Nobody in the plant noticed any production issues. So in addition to a 13% power reduction from better compressor management, the customer got another 10% power benefit by running at lower pressure.

We certainly don’t recommend this approach to finding your proper system pressure, but this incident highlights a very common mistake in compressed air systems: many compressed air systems are running at higher pressures than needed. A rule of thumb for typical plant air systems is that every 2 psig increase in pressure requires 1% more power. So turning up the compressors from 100 to 110 psig increases power consumption about 5%. This practice does not increase productivity. It just uses more energy— and often causes premature wear in pneumatic equipment.

If you have any doubts at all (or even if you don’t), we advise turning down the pressure to see if it affects production, but with a conservative approach. Try 1 psi per week until someone in production complains. This is a no-cost solution that immediately saves money. And the bigger the system and the higher your utility rates, the more you save. The added bonus is reducing the volume lost through leaks, and this also reduces flow demand and compressor run time.

If you are trying to overcome pressure drop between the compressor and points of use, the ideal solution is to minimize the source (s) of the pressure drop (e.g. replace clogged filters, make sure ball valves are fully open, replace undersized piping and fittings). And if it does become necessary to set pressure higher, do it incrementally. People tend to bump up the pressure 5 or 10 psi at a time without trying to adjust it back down.

This is a tip you can take to the bank.

For additional tips visit our website!