Improving Rutgers' Air Compressor System’s Efficiency and Performance David Kim [email protected]Jack Mamiye [email protected]Kyle Burns [email protected]Torie Rose [email protected]Joseph Zavodny [email protected]1. Abstract The purpose of this project was to detail and analyze Rutgers' compressed air system, ultimately finding ways for Rutgers to save money by using less energy. Our motivation for this task was to gain experience and a better understanding of the type of problems engineers face today. To do this, we first researched what factors affect a compressed air system’s performance and efficiency. With this knowledge, we set out to find problems with the compressed air system by visiting the Cogeneration (Cogen) Plant and several of the buildings it supplies compressed air to. We discovered a number of problems with the system. Due to the lack of time, however, we could not accomplish everything we set out to do. Therefore, we made recommendations on how the system could consume less energy and what the best possible solution would be for Rutgers to follow using what data we were able to gather. Our solution entails that Rutgers have the leaks in their system repaired. After this, we recommend the purchase of a newer, more efficient compressor to replace the current central compressor. Finally, we recommend that Rutgers cut off its longest distribution lines and that distant buildings establish their own compressed air systems. 2. Introduction Compressed air is commonly referred to as the fourth utility. A utility is something which is a basic necessity. For example, three common utilities are water, electricity, and natural gas. Utilities play a major role in the modern world – without them, today’s technologically advanced society could not function. Compressed air is simply air that is pressurized by an air compressor. It is known as a utility because it is a type of energy source which is a basic necessity in many industrial environments. It can be seen in over seventy percent of the world’s industrial facilities. Although compressed air systems are widespread, they are extremely inefficient. Compressed air systems typically consume more energy and cost more to operate than anything else in industrial environments.
Transcript
Improving Rutgers' Air Compressor System’s Efficiency and Performance
The purpose of this project was to detail and analyze Rutgers' compressed air system, ultimately finding ways for Rutgers to save money by using less energy. Our motivation for this task was to gain experience and a better understanding of the type of problems engineers face today. To do this, we first researched what factors affect a compressed air system’s performance and efficiency. With this knowledge, we set out to find problems with the compressed air system by visiting the Cogeneration (Cogen) Plant and several of the buildings it supplies compressed air to.
We discovered a number of problems with the system. Due to the lack of time, however, we could not accomplish everything we set out to do. Therefore, we made recommendations on how the system could consume less energy and what the best possible solution would be for Rutgers to follow using what data we were able to gather.
Our solution entails that Rutgers have the leaks in their system repaired. After this, we recommend the purchase of a newer, more efficient compressor to
replace the current central compressor. Finally, we recommend that Rutgers cut off its longest distribution lines and that distant buildings establish their own compressed air systems.
2. Introduction
Compressed air is commonly referred to as the fourth utility. A utility is something which is a basic necessity. For example, three common utilities are water, electricity, and natural gas. Utilities play a major role in the modern world – without them, today’s technologically advanced society could not function.
Compressed air is simply air that is pressurized by an air compressor. It is known as a utility because it is a type of energy source which is a basic necessity in many industrial environments. It can be seen in over seventy percent of the world’s industrial facilities.
Although compressed air systems are widespread, they are extremely inefficient. Compressed air systems typically consume more energy and cost more to operate than anything else in industrial environments.
Working with our mentors at the university’s Center for Advanced Energy Systems (CAES), we traveled to Rutgers’ Cogen Plant and several buildings on campus which use compressed air. During this time, we collected data that would help us to evaluate the system currently in place and to determine how much money might be saved by making various changes to the system.
3. Background
An air compressor is a machine driven by an electric motor that compresses air through a variety of different techniques and technologies. As a system, an air compressor has a variety of applications such as operating pneumatic tools and sandblasting, but constructing the system itself is complicated and must be engineered properly to match the demand and achieve optimal efficiency.
First, the system intakes air, usually through a filter, and sends it to a primary compressor to pressurize the air. Then this air is sent to an appropriately-sized storage tank. When compressed air is actually needed, the storage tank releases its air through either refrigerant or desiccant dryers to remove any of its moisture. These components, however, can be set up as a series according to the need and situation of the demand for compressed air.
The three most common types of compressors are the rotary screw, the centrifugal, and the reciprocating compressor: [1]
• Rotary screw compressors typically use two to three screws that mesh together to compress air
and force it out through a small opening.
• Centrifugal compressors often use several stages of rapidly spinning impellers to provide a continuous flow of air. This type, however, requires an inter-stage cooling at higher pressures in order to return it to its initial temperature before the next stage.
• Reciprocating compressors (the most common type, especially in smaller applications) use an electric motor to turn a crank shaft that operates a series of pistons and valves to intake, compress, and release air into a storage tank.
Air compressors, however, are very inefficient, usually having a determined energy efficiency of only ten to twenty percent. [6] The primary reason for this inefficiency in air compressor systems is directly losing heat to its surrounding environment due to the fact that the air is being pressurized and therefore increasing in temperature (according to the combined gas law) and because there is no effective nor practical way of containing this developing heat. Another inefficiency is that much heat energy is lost due to the friction in/against the motor and against the system's many moving parts.
Air compressors are also very inefficient due to many poor techniques used by numerous industrial facilities. One huge factor that many facilities ignore is the “quality” of the air intake. Essentially, the temperature, composition, and humidity of the intake air are three major components that affect the energy consumption and performance of air compressors. [8]
• The temperature of the intake air is important because it directly dictates what the density of the intake air will be. This can be explained by how the temperature of air particles is directly related to
their average kinetic energy. If the particles’ kinetic energies are high, they will travel “more actively” through space and therefore be less dense (less particles in any given space). Thus, the ideal temperature of air is cold because cold air (denser air) will require less energy to compress – or in other words, to make denser. Furthermore, the temperature of the intake air is directly proportional to the energy consumption of an air compressor – therefore, the amount of energy required to compress air decreases by how much cooler the intake air is.
• The composition of the intake air is important because it indicates what exactly is going into an air compressing system. Clean intake air is ideal because the air being compressed moves smoothly through the entire process without damaging the system. “Dirty” intake air, however, is detrimental to the system because its contaminants, such as dust particles, can accumulate in it over time and eventually cause premature wear and/or reduced storage capacity.
• The humidity of the intake air is important because it indicates how much moisture it has. Moisture, though, can also be detrimental to an air compressing system as it too can accumulate inside the system and rust its components, leading to premature wear or leaks and reducing the system’s storage capacity. Therefore, dry air is ideal simply because it is lacks moisture. [7]
While the compressor itself is inefficient, the system itself can detract from the overall efficiency of a compressed air system. For example, a lot of friction is created in long compressed air distribution lines and bends in the lines. Leaks in these lines grow as time passes and are the main cause for inefficiency and poor performance. Energy is also wasted when compressed air is used inappropriately. For example, one could inappropriately use compressed air to dry towels.
Furthermore, poorly-maintained systems can result in inevitable problems in the future. It is important that all the parts to an air compressor system is checked for leaks, premature wear of any parts, accumulation of contaminants in the air compressor, etc. to ensure that maximum efficiency and storage/operating capacity is reached. One problem past industries have faced is the accumulation of water in storage tanks that continue to grow and reduce storage capacity. Therefore, simple negligence to constantly check on the entire air compressor system can lead to wasting energy and money.
These all are the major problems facing air compressor systems. While compressed air can be a great resource for industry, the complicated nature of its production and distribution has led to studies concerning the possible improvements of an air compressor system to obtain higher efficiency and better performance.[5]
4. Experimental Design
On the Rutgers campus, buildings that receive compressed air from the campus’ Cogeneration Power Plant currently receive air at a lower
pressure than anticipated. The compressors are designed to produce 100 PSIG (pounds per square inch, the unit that compressed air is measured in gauged based on the atmospheric pressure). However, there have been significant PSI losses between the Cogen plant and the buildings it distributes compressed air to. These losses are caused by both leaks and the length of the lines. Our research aims to identify a solution for the pressure loss due to leaks, as losses due to friction are a fact of nature.[1]
In order to find ways for Rutgers to lower the cost of operating the compressed air system, we took multiple trips to Rutgers' Cogen Plant and the several of the buildings it supplies compressed air to. At the plant, we collected data pertaining to the compressed air system. We measured the capacities of the compressors and tanks. Then, we measured the pressure that the compressor was supposed to produce in contrast to how much it actually was producing. We also determined the operating hours as well as the load factor (the percentage of maximum capacity at which the compressor was operating) of the compressors. After collecting the serial numbers of the compressors, we called the manufacturers to get their efficiencies. The manufacturers were unable to provide us with efficiency data for some compressors, so we had to make educated guesses.
In the end, we used our limited knowledge to discover a solution for three simple scenarios that represent the main problems at the Cogen plant. It was necessary for us to simplify these scenarios.
4.1 Determining Cost of Leaks
First, we determined how much it costs to operate the current compressor system. This was done using the following equations:
N x HP x LF x UF x H EFFwhere,
N = number of compressorsHP = horse power of compressors,
converted to kilowatts at a rate of 0.746 kW per HP
LF = load factor, ratio of actual load to maximum loadUF = utilization factor, maximum
demand of system over its capacityH = operating hours of compressorsEFF = efficiency of compressor motor
Next, we determined the cost of the consumed energy using the equation
EC = EU x ER
where,
EC = energy costEU = energy usageER = rate of efficiency (kW/hr)
We simulated Rutgers' current system using compressed air from one centralized compressor, outputting air at 100 PSIG. For the sake of simplicity, we only include three of the buildings on campus. The system pressure at each building is less than 100PSIG as a result of leaks and the length of lines. System pressure varies from building to building because leaks are not evenly distributed throughout the system, and because some buildings are further from the Cogen Plant than others.
Figure 1: This diagram shows different pressure loss situations for three different buildings each being supplied compressed air at 100 psig.
4.2 Centralized Versus Decentralized
The second scenario was motivated by our findings at the separate buildings. We visited Smithers Hall, the Library of Science and Medicine (LSM), and
University of Medicine and Dentistry of New Jersey (UMDNJ). All of these buildings we visited had their own compressors. Some buildings used these compressors as a back up to the centralized Cogen system, while others used them as primary compressors, with the Cogen line as a back up. The buildings’ decentralized compressors brought up a good point. The centralized system at the Cogen plant may not actually be necessary.
Figure 2: This diagram shows a centralized system with one compressor operating at 200 horse power supplying 100 HP and 50 HP worth of compressed air to three different buildings.
We decided to determine whether it was more efficient to keep the centralized
system that was in place, having the one compressor operating at 200 HP at the plant supply compressed air to all of the buildings, or to cut off the power plant all together and put a compressed air system that met each individual demand in each building as shown in Figure 3.
Figure 3: This diagram shows each building with its own compressor that operates at the exact horsepower to produce the amount of compressed air that it requires.
To determine which scenario would be more efficient we called manufacturing companies to find out the motor efficiencies of the centralized 200 HP compressor versus the individual 100 HP and 50 HP compressors.
5. Results In order for us to be able to
analyze Rutgers’ compressor air system, it was necessary to gather data and learn about the current system in place. We began our data collection at the Cogen, which is the center of Rutgers' compressed air system. Our first task was to find all compressor-related equipment and to sketch the layout of the system. Had the main compressor
been functioning, we would have installed data loggers to monitor the system. Although it was not possible for us to do this, Rutgers' CAES provided us with their log data. Additionally, we acquired a list of complaints with the current system and came across some key problems. Finally, we discussed and analyzed our data, proposing a number of scenarios for the compressed air system.
5.1 COGEN PlantRutgers has five air compressors
in the Cogen Plant. The main compressor is a rotary screw compressor made by Atlas-Copco. There is also a reciprocating compressor made by Ingersoll-Rand, which was the main compressor until Rutgers purchased the Atlas. This Ingersoll-Rand now functions as a backup compressor. Rutgers also owns three small reciprocating compressors made by Quincy. These are used when starting up the Cogen Plant. There are two
storage tanks, one for the Atlas and one for the Ingersoll-Rand. Rutgers owns two air dryers, one of which is a mechanical refrigerant dryer; the other is a desiccant dryer. Once air is compressed, it enters one of two storage tanks. Air then flows through the
mechanical dryer, then through the desiccant dryer. After leaving the desiccant dryer, compressed air leaves the Cogen Plant. Here is a rough diagram of the compressed air equipment at Rutgers’ Cogen Plant
Control AirControl Air
Atlas Copco Air Compressor
Storage Tank
Storage Tank
Quincy Compressors
Quincy Compressors
Ingersall -Rand
Compressor
Ingersall -Rand
Compressor
Storage Tank
Storage Tank
Mechanical Dryer
Mechanical Dryer
Schematic of Cogen Plant’s Air Compressor System
5.2 Data LoggingCompressor data was measured
and logged using a type of data logger known as a “HOBO”. System pressure and in one case electrical current draw by the compressor was logged. This
data was provided by the members of CAES, who had recently recorded the data before we joined the project. We graphed the data so that it would be easier to analyze. Notice in the first three graphs that pressure tends to vary through the night.
5.3 Problems and Complaints at Rutgers
In order to analyze and improve the efficiency of Rutgers’ compressed air system, it was vital to talk to managers and ask questions. However, this turned out to be a major obstacle along the course of our project; most people were unable to answer our questions. Here are some of the few pieces of information we learned:
The Atlas compressor fails often, especially in summer
The Cogen Plant is having trouble meeting pressure requirements
LSM has trouble with low pressure caused by leaks in the system
Waksman, the farthest building from the Cogen Plant, has the most complaints They are having trouble with
water in the lines Rutgers is willing and looking to buy
a new compressor It is possible to move the intake and
exhaust lines from the compressors at the COGEN Plant
Some of the major questions which could not be answered: How often the compressor
equipment is serviced When the system was last checked
for leaks
5.4 Scenario Data
First we needed to calculate how much it costs Rutgers to run their compressed air system. Since the cost of electricity is based on the amount of energy consumed ($0.10 per kilowatt-hour), we must calculate the energy consumed by a compressor. As work (energy) is equal to the product of power and time, we first need to find power. We can find the power output of a single compressor by first converting horsepower to watts (using a conversion factor), then by multiplying the result by the load factor of the compressor. For our 250HP Atlas compressor, this gives:
P = 250HP x 0.746 kilowatts/HP x 80% = 150 kilowatts
To calculate the time a compressor is running, we multiply the number of hours in a year by the utilization factor, or the percentage of time the compressor is running.
T = 8766 hours per year x 91% = 8000 hours/year
Multiplying these numbers gives the following for work output:
W = 150 kilowatts x 8000 hours/year = 1,200,000 kilowatt-hours
Finally, as motors used to drive compressors are not 100% efficient, they draw more power than the work they do. To find power consumed by the motor (and thus the compressor), we divide energy output by the efficiency of the motor. Since our Atlas is 10 years old, we estimate the motor efficiency to be 85%.
EU = 1,200,000 kilowatt-hours / 85% = 1,412,000 kilowatt-hours
At the current cost of electricity, $0.10 per kilowatt-hour, it costs Rutgers approximately $141,200 a year to run their Atlas compressor.
A large portion of the energy consumed by the air compressor is being lost due to leaks in the system. The amount of money Rutgers is losing is directly related to the amount of energy lost due to leaks. All scenarios and data from this point on will assume that all leaks have been fixed. For the sake of example, we will estimate that Rutgers is losing roughly 20% of their energy as a result of leaks. If there are fewer leaks than we believe, the cost savings will be less. However, it is likely that the leak situation is worse than we have estimated and that cost savings will be greater. If Rutgers fixed the leaks and continued to use their Atlas compressor, the load factor of the compressor would decrease and the compressor would consume 20% less power resulting in a 20% cost savings. By doing nothing other than fixing leaks, this would result in over $28,000 saved per year.
However, the current Atlas compressor uses an older, more inefficient motor than found in modern compressors. Rutgers is already looking to buy a new compressor as the Atlas is unreliable, so we have decided to compare the cost of operating the Atlas to the costs of running newer, more efficient compressors.For scenario 2A, we have replaced the Atlas compressor with a 200HP compressor. With the leaks fixed, a lower HP compressor can be used (resulting in a higher load factor). Also,
the replacement compressor will use a newer, more efficient motor rated for 95% efficiency. Previously we calculated that the Atlas is currently putting out 150 kilowatts. With leaks fixed, the power requirement of the compressed air system should drop 20% to 120 kilowatts. Multiplying this by the operating hours and dividing by motor efficiency gives the energy usage of the compressor motor:
EU = 120 kilowatts x 8000 hours/year / 95% = 1,010,000 Kilowatt-hours / year
Giving,1,010,00 kwh x $0.10 / kwh = $101,000
This is an additional savings of $15,000 for a total savings of $40,000 per year.
For scenario 2B, we look at the option of decentralizing the compressed air system by placing a number of smaller compressors across campus. To simplify this type of scenario, we look at a system with one medium and two
small compressors. In this system, we use a 100HP compressor with an efficiency of 93% and a 50HP compressor with an efficiency of 91.5%.
Cost of operating 100 HP compressor:
60 kilowatts x 8000 hours/year / 93% x $0.10 per kilowatt-hour = $51,600
And for the two 50HP compressors:
60 kilowatts x 8000 hours/year / 91.5% x $0.10 per kilowatt-hour = $52,500
Combined cost= $104,100
However, it should be noted that we have not taken the efficiencies of the actual compressors into account. Larger compressors are usually more efficient than smaller ones; it is possible that a new 200HP motor may be as much as 10% more efficient than a new 50HP compressor. For this reason, we believe that we have underestimated the cost for scenario 2B.
6. Discussion/ConclusionsThe compressed air system at
Rutgers has much room for improvement. First, it is necessary for Rutgers to have the majority of leaks repaired as soon as possible. Leaks are responsible for the majority of wasted energy; fixing them will result in the greatest cost savings.
We propose that all the buildings which are far from the Cogen plant be cut off from the centralized compressed air system, while all the nearby buildings remain connected to the current centralized air system. This is so that the majority of buildings on campus can benefit from a more efficient, centralized compressor. It is a good idea to disconnected distant buildings because the longer lines are the second largest waste of energy with leaks being the first.
We have concluded that the best course of action is to eliminate the long compressed air lines to UMDNJ and LSM in favor of a decentralized system, providing both facilities with their own in-house compressors. The buildings closer to the Cogen plant, that are experiencing better compressed air service, will keep the centralized supply because our calculations for the centralized system have proved more cost effective. The central compressor would then be replaced in favor of a smaller, more efficient compressor that would do the same job of the current Atlas compressor, assuming most leaks had been repaired. A new central compressor should be purchased because more efficient motor will save additional
$15,000 a year, and because the Atlas is not working properly.
Finally, Rutgers can achieve additional savings by moving air intakes. According to our temperature measurements, the Cogen Plant’s air temperature is roughly 90 degrees Fahrenheit. The air temperature outside is much less, especially during the winter season. The potential for energy savings, just by moving the intake outside the plant, is tremendous. For example, in the winter the air compressor system may take in air that is 40 degrees Fahrenheit instead of 90 degrees Fahrenheit. The air compressor would consume 9.1% less energy during the winter than it does currently in the summer.
90 Fahrenheit (F) = 32.2 Celsius (C)40 F = 4.4 C
32.3 C = 305.3 Kelvin (K)4.4 C = 277.4 K
277.4 K / 305.3 K = 90.9%9.1% difference
7. Related Work
This project is centered around determining the most efficient model of a compressed air system, especially focusing on the most cost-effective. While this is a common goal in industry where compressed air is involved, one such case at a bottling plant bears a close resemblance to our issue at the Rutgers Cogen facility. The Southeastern Container Company, a blown plastic bottle production facility, was looking to
drastically increase the efficiency of its compressor system and subsequently cut operating costs. Enlisting the help of NCSU’s Industrial Extension Service, they were able to manipulate the system and make several improvements that ended up saving the company over $180,000 a year in operating costs in regard only to the operation of the compressor system. [5] Through NCSU’s collaborative study with Southeastern Container Company, they were able to determine that major leaks and a lack of efficient moisture removal from the compressed air led to poor operations and issues later on down the bottling line. The addition of a dryer and a comprehensive compressed air leak study and repair helped the bottling plant to recover costs. Also, distributing the load of the two compressors onto a third, new compressor helped to actually lower costs to save the company a substantial amount of money, the project only producing a buy-back of less than half a year. [5]
As was our goal at the onset of the project, we were looking to determine the causes of malfunction and a major cause of inefficiency within the Rutgers compressed air system, as well as determine which system configuration is superior. Our project differed from the bottle production plant in that we were focused on air distributed to multiple buildings while the production plant contained exposed lines providing easy and viable fixes. While the systems differ, recommendations that we hoped to make are similar in that their effect is aimed at reducing over-expenditure energy-wise in the plant and hopefully, significantly reducing costs.
8. Future Work
We would have liked to determine the exact length of compressed air line at which delivering the compressed air from a centralized compressor becomes more inefficient than a localized compressor. From this we could determine the proper compressed air distribution scheme for the Busch campus as well as UMDNJ. We also would have liked to acquire a line diagram for the campus as well as determine the exact amount of leakage and frictional losses that occur within the line. Also, we would have liked to shut off the entire system to estimate the severity of the leaks present. However, because Rutgers is a research university, compressed air is required by many research facilities at which such work has been ongoing for decades. Therefore, such a disruption in service would ruin decades’ worth of research. Since shutting down the entire system is not possible, we would have liked to shut off at least one of the buildings on the Busch campus to compressed air to obtain an estimate as to the leakage inside the building which is shown by loss in pressure.
We also would have liked to determine the cost savings of drying air before the compressed air tank by placing an air dryer before the compressed air tank as well as how this configuration would maximize the use of the air tank. If the factor is considerable, then dryers should be placed before the tank whenever possible. However, some compressed air applications require completely dry air. In this case, the dryer must be placed before the tank so that the stored air is dry and ready to be used in the case of a spike in load. We would have liked to determine the amount of moisture that builds up inside the tank in each case of dryer placement and how
often drainage of the water would be required.
We also would have liked to experiment with how the use of a variable frequency drive (VFD), which modifies the frequency of the alternating current power source, would optimize efficiency and save money while operating a partially loaded compressor. A VFD increases efficiency of the compressor by lowering the motor RPM as load decreases, causing the motor to use less energy.
9. Conclusion
Our group’s task was simply analyzing Rutgers’ air compressor system and making recommendations on what the university could do to save money. To do this, we set out to collect and analyze data from Rutgers’ air compressor system and several of the buildings it supplied compressed air to. Despite the obstacles we faced, we used what data we could acquire to compare the approximate costs for different systems Rutgers could implement. After figuring out the pros and cons of each one, we decided that using a combination of a centralized air
compressor and a number of individual (decentralized) air compressors would be Rutgers’ best solution to its current system.
In retrospect, we discovered that an appropriate analysis of any topic (in our case, an air compressor system) required the consideration of all possible factors that affected it. Our research may have pointed out the obvious improvements that could be made with Rutgers’ air compressor system, yet the solution we suggested for Rutgers was only developed by considering real-life problems that theoretical information could not cover.
Despite our findings and recommendations, however, we feel as if we could have improved our performance if we had more time and information available to us. If we were to visit this problem again, perhaps we could investigate the intensity of leaks in the air compressor system at Rutgers or develop a much-needed plan to eliminate the need for long delivery lines. One thing for certain is that this project opened our eyes to the magnitude of society’s many problems. Yet we are engineers – it’s our job to go out and address them.
10. Acknowledgments
Thank you to all who made this research possible:
The Governor's School of Engineering and Technology (GSET):Dean Brown, Blase Ur, The Governor’s School Board of OverseersCaleb WilliamsWho coordinated the effort for this research project.
The Center for Advanced Energy Systems (CAES) Project Advisors:-Mike Cesarano-Joshua Kace-Malik Kahn-Don Kasten-Dr. Michael Muller, Jill MesonasFor supplying us with all of the required knowledge, resources, and site-visits to pursue our research.
Sponsors:Rutgers University, Rutgers University Center for Advanced Energy Systems
Citations
1. “Compressed Air Terminology.” Air Technology. 13 July 2008. http://www.airtechnology.co.uk/compressedairterms.html.
2. “Air Compressor Uses.” Air Compressor Equipment Company. 13 July 2008. http://www.acec-ark.com/Techinfo/Uses/uses.html.
