Abstract
Compressed air is a valuable resource for manufacturers, allowing the use of pneumatic-driven hand tools, which can be an ergonomic boon to employees. This resource comes with a price, however, in the form of higher energy costs. This article describes the use of compressed air and the creation and delivery of compressed air from both a supply side and demand side approach. A major focus of this article is on the costs associated with the generation of compressed air and ways to reduce the waste of this resource.
INTRODUCTION
The first section of this entry focuses on the use of compressed air and how it is generated. The section on generation is then separated into a discussion of the supply side and demand side components of a compressed air system. Finally, the costs associated with compressed air, as well as sources of further information, are found at the end of the entry.
OVERVIEW
Compressed air systems could be considered a unique source of energy despite the fact that they are actually powered by electricity.This similarity stems from the fact that compressed air lines can be designed to allow modular tools to plug into the air lines, just like electrical devices can be powered by tapping into electrical outlets.
By far, the most common use of compressed air is to drive pneumatic tools, ranging from nail guns to jackhammers to large drill presses. Pneumatic tools are favored over electric motor driven models because:
• They’re smaller, lighter, and more maneuverable.
• They deliver smooth power and are not damaged by overloading.
• They have the ability for infinite, variable speed and torque control, and can reach these very quickly.
• They can be safer because they do not pose the potential hazards of electrical devices, particularly where water and gases are present.
Additional uses for compressed air in the manufacturing sector may include: filtration or control systems, driving conveyors, dehydration, aeration, or refrigeration.
As these latter applications do not have the need for portability, and can be performed more cheaply without the additional process step of compressing air, their use is fairly limited. The economics of compressed air will be discussed later in this article.
COMPRESSED AIR SYSTEMS
Although a relatively simple-looking, self-contained air compressor can be purchased at a hardware store, they are limited in size and these small units (battery-powered, gas-powered, or plug-in models) are typically only to be used to fill tires or inflate rafts. Our discussion from this point onward will focus on larger, commercial compressed air systems.
The typical compressed air system is composed of:
• One or more in-series compressors
• An air dryer and air filters
• A receiving tank (for storage)
• Piping
• End uses
Compressed air systems should be perceived as possessing both a supply side and a demand side. Fig. 1 shows a typical block diagram of a industrial compressed air system, with both the supply and demand side noted. These block diagrams are a very helpful first step in understanding how to better manage compressed air systems, as recommended by Ref. 1.
• Improving and maintaining peak compressed air system performance requires addressing both the supply and demand sides, as well as how the two interact in order to have dependable, clean, dry, stable air delivered at the proper pressure. A well-planned balanced system will yield the cheapest and most energy efficient results.
Fig. 1 Schematic of a compressed air system.
COMPONENTS OF A COMPRESSED AIR SYSTEM
Supply Side
A thorough understanding of the end-use compressed air needs, from both a volume and usage profile perspective, is necessary in order to select the appropriate number and size of air compressors. It is rare to find a manufacturing plant that has a constant, uniform use of compressed air throughout the day. Most manufacturing plants have cyclical flow and volume demands due to production schedules, and also desire back-up supply, so engineers typically plan for more than one air compressor to meet a facility’s needs. A good strategy is to size a compressor for a base load, and have one or more compressors staged to come online to meet additional compressed air demand. In designing a compressed air system, altitude, inlet air temperature, and relative humidity should be considered, as they impact compressor capacity. More information on how to calculate the influence of these design considerations can be found in Ref. 2, pp. 9-10. It may also be helpful to have different size compressors, so that they can be tailored to fit the operating conditions. Additionally, a small compressor or separate booster may be appropriate for off-shift operations or a special high pressure, periodic application.
The vast majority of industrial compressors are of the rotary screw variety, but double-acting reciprocating or centrifugal compressors are also available for specific applications. Rotary screw compressors come in two configurations: lubricant-injected or lubricant-free. Both have various pros and cons associated with their use. Lubricant-free rotary screw compressors require higher electrical demand, but assure no lubricant carryover. This may be crucial when ultra-clean air is required. On the other hand, lubricant-injected rotary screw compressors have the ability to trim to partial loads to meet usage needs, which can further save on their already lower power costs.
Another issue that can greatly impact the energy efficiency of air compressors is their control strategy. Start/stop, load/unload, and modulating (or throttling) control strategies can be used, depending on the facility’s compressed air usage profile.
In order to deliver clean compressed air, filters are installed downstream from the air compressors. The filters remove particulates, some condensate, and lubricant. Regular replacement of filters is necessary to prevent pressure drop, which results in a throttling effect. To illustrate the filter’s importance, see the following example:
Example (Replacement of a Compressed Air Filter Element)
Assume a 100 hp compressor that operates continuously with an energy cost of seven cents/kWh, resulting in an annual energy cost of $55,328. As the filter becomes clogged, assume the pressure drop increases to six psi across the filter (as compared to a two psi pressure drop for a new filter). Consider that this four psi increase can cost two percent of the annual required energy, or $1100, as compared to $375 for a new filter element.
Another component of a compressed air system is the dryer(s). The compressing of air will condense out the moisture from the natural water vapor found in atmospheric air. This liquid water can cause rust problems in the lines or, should compressed air supply lines connect between buildings, freeze in the winter. Compressed air should be dried to a dew point at least 18°F below the lowest ambient temperature of the demand side.
The various types of dryers are:
• Refrigerated: This is the most common type, with both low initial and operating costs. It can be subject to freezing if operating at low capacities.
• Regenerative desiccant: Typically operated in tandem between two twin dryers, with one operating and the other regenerating. The required volume of purge air needed to regenerate can increase the load or even cause an idle compressor to be started. Heaters can be used in place of purge air, but present their own energy penalty.
• Heat of compression: Similar to the regenerative desiccant dryer, this type of dryer is available for lubricant-free rotary screw compressors and utilizes the hot discharge compressed air to regenerate the desiccant. Their efficiency is affected by changing air temperatures and additional heat may be required for low load situations.
• Deliquescent desiccant: A dissolvable desiccant is used. Regular replacement of this resource is necessary, requiring labor and material costs.
• Membrane-type: A porous membrane separates water vapor from the air and suppresses the dew point. Although there is a low initial cost, these dryers are appropriate only for low-volume applications.
Air receivers can be found on either the supply side (immediately after the compressor or the dryer) or on the demand side, close to the application end use. Air receivers store compressed air and help cover peak events of short duration. If sized properly, they can greatly reduce the frequent loading and unloading of the compressor, saving both energy and maintenance costs. They also stabilize system pressure, which improves performance of the end use.
Other components associated with the supply side may include after coolers or intercoolers (for lubricant-free systems), moisture separators, and condensate drains. Depending on the manufacturer, these latter items may be packaged in a single housing with the compressor itself.
Demand Side
Besides a downstream air receiver, the demand side consists of the distribution system or piping, and the end-use applications. Correct sizing of the distribution piping is a critical feature in compressed air system design in order to minimize energy costs.
The piping typically consists of rigid metal or plastic piping from the air compressor room to the general area of the end-use equipment. From this point, flexible rubber or plastic tubing is used, which may be plumbed directly to the end use, or have a shut-off valve with quick-connect attachment points. This flexible tubing may be subject to being run over by foot or equipment traffic and can wear out over time. As a result, air leaks can grow to epidemic proportions, and greatly increase the demand on the compressor. In fact, it isn’t unusual to find a poorly maintained system running a compressor that is only feeding leaks. Some facilities will bury large portions of their distribution piping, which make finding and repairing leaks an expensive proposition. A 3/16″ in. hole in a system operating at 100 psig can cost over $5000 a year.
Another operating consideration associated with the demand side is the cost of “normal production.” Decisions to add additional applications should undergo a realistic cost evaluation. Consider the following example of an end-use application:
Example (Addition of an End-Use Application)
A quarter inch orifice required to operate a pneumatic hand tools at a recommended pressure of 100 psig was found to have a flow rate of 63.3 scfm (standard cubic feet per minute). After a year of constant use, this equates to 33.3 MMcf (million cubic feet) of compressed air. If compressed air generation costs $300/MMcf, then the power cost for this application will be approximately $10,000/ year. If we add additional operating costs of $170/MMcf to account for the operator maintaining the compressed air equipment and the maintenance, lubricant, and repair costs for the system, we find that the cost of this new application use is over $15,000/year. Compare this with less than $2000/year to operate a comparable electrical tool.
High costs can also be incurred through the artificial demand associated with setting the compressor pressure level higher than needed. According to Ref. 3, p. 56, supplying 20% extra psig will force the system to consume 20% more air flow, resulting in 20% waste. Poor applications, such as stuck condensate drains, personnel use of compressed air for cooling or drying, or sparging (aerating of liquids), also use up precious compressed air.
ESTIMATING NECESSARY PRESSURE SET POINT
The determination of the pressure set point for the air compressors needs to be equated. Because of natural pressure drops associated with the components of a compressed air system, as well as unrepaired air leaks, the final point is more difficult to find than just dialing in the pressure recommended by the end-use equipment manufacturer. In fact, it is not unusual for plant personnel to reach the desired pressure by trial and error, increasing the set point until equipment operators stop complaining about low pressure. When possible, pressure measurements should be made after each component of the compressed air system to monitor system performance. Flow or electrical readings can also provide useful performance data. More information on how to calculate optimum compressed air system settings can be found in Ref. 2, p. 205. Fig. 2 provides an example of the pressure drops that can occur along the line.
COSTS OF COMPRESSED AIR
To operate a one hp air motor, seven to eight hp of electrical energy are required. This large energy penalty, along with the common employee perception that compressed air is essentially a free resource, makes it a challenge to control the costs of compressed air. Inadequate compressor control schemes can cause multiple compressors to run at partial loads, rather than turning them off. Problems with poor maintenance can increase consumption or cause pressure variability. In fact, it isn’t unusual to find that compressed air can be the largest end user of electricity.
ARTICLES OF FURTHER INTEREST
The U.S. Department of Energy’s Industrial Technologies Program sponsors compressed air training and Air-Master + tools through their Best Practices programs.
Estimating Pressure Drop
Measurements can be taken at various points in a compressed air system to monitor the associated pressure drop from each component. The pressure profile shows the lowest pressure seen by the end-uses.
| Compressor operating range: | 115-105 psig | Air/Lubricant Separator | 5 psic |
| FRL (Fliter, regulator, lubricator) | 7 psid | Hose and Disconnects | 4 psic |
| Aftercooler | 3 psid | Dryer | 4 psic |
| Filter | 3 psid | Distribution System | 3 psic |
Fig. 2 Estimating compressed air system pressure drop.
