Abstract
This Compressor Control Systems entry provides an outline of the strategies used to manage compressed air generation at a typical manufacturing facility and discusses the components of a compressed air system, typical control methodology, and energy saving strategies. Compressed air distribution, metering, and monitoring is also reviewed. This entry is meant to give the reader a basic understanding of compressed air generation and management fundamentals.
INTRODUCTION
Compressor systems are benefiting from increasingly advanced control technologies to realize improved efficiency, safety, and operational benefits. Compressor controls now use microprocessors, computer networking, sophisticated control algorithms, and Web-based monitoring to provide superior control capabilities and features.
Today’s control systems do more than just operate the air compressor. Using sophisticated technologies, advanced controls can significantly reduce energy expense and lower maintenance costs. Automation is another important feature, allowing much of the standard compressor system operation to be managed by software. By monitoring and controlling compressor auxiliary equipment, modern controls can help ensure that high-quality air at the lowest cost is reaching the end user. Zone management provides the ability to regulate air use in various plant departments. Remote and Web-based integration systems improve and streamline system control and management. Extensive data gathering and reporting provides real-time information to help drive business decisions, such as evaluating system expansion, identifying additional savings opportunities, and managing predictive maintenance.
Modern compressor control systems address the entire compressed air infrastructure. Today, a complete control approach can include the following facets of a compressor system: air production controls for air compressors and the motors that power them; air quality monitoring for dryers and other air conditioning equipment; distribution control in the form of zone management; and integration, management, and metering products to improve operations and decision making.
COMPRESSOR CONTROL SYSTEM COMPONENTS
Air Production Control
The supply side of a compressor system consists primarily of the compressor and the motor. Control systems have several important tasks: to ensure that the compressor system produces enough air to meet plant demand; to keep the compressor and motor running without costly shutdowns; to operate the system as efficiently as possible; and to prevent damage to the major pieces of equipment.
Older compressors use pneumatic and electro-mechanical control systems. Modern microprocessor-based controls improve performance by offering precise pressure regulation, networked capacity control, and additional features. For centrifugal compressors, advanced surge control can dramatically increase efficiency by reducing wasted air blowoff.
Electronic controls are available for all makes and models of compressors. To provide maximum efficiency and automation, a control system should electronically network the compressors, regardless of type or make. The primary goal of a control system is to ensure that the compressors produce enough air to meet plant demand.
Motor Control
Compressor systems begin with the prime mover, often an electric motor. A well-functioning motor is obviously crucial to the operation of a compressed air system.
Effective electric motor control prevents damage and helps alleviate common electric power concerns.
Primary Motor Control
Modern electric motor controls offer a number of features that improve motor reliability. Extensive motor protection is built into these products, preventing motor overload and burnout that can result from drawing too much power. User-friendly, man-machine interfaces, with Liquid Crystal Display (LCD) screens and micro keypads, are now common features. These allow for easy initial configuration and modification of the control’s operating parameters. Some motor controls are based on solid state components, which require less maintenance and replacement of costly electrical parts that otherwise wear out over time.
An important element of modern motor controls is the soft start function, wherein the motor is gradually brought up to its maximum power. Compared to a typical high torque start, a slow start greatly reduces mechanical and electrical system shock to the motor and attached compressor. This results in less wear and tear from starts and stops and reduces long-term maintenance of expensive motors. A soft start also reduces the initial electrical current inrush, placing less stress on the facility’s electrical system and alleviating related low power problems.
Ride Through Motor Control
An important new auxiliary motor control is the ride through controller. This device addresses the problem of momentary interruptions to the motor’s power supply, which, no matter how short, can often cause motor shutdown. Motor shutdown stops the supply of compressed air, halts the facility’s production, and results in lost productivity and increased costs. A ride through device has the ability to keep the motor operating during momentary power interruptions and voltage sags. Because these are lower cost auxiliary controllers, they can often quickly justify installation costs.
Compressor Control
The most fundamental part of a compressor control system is the controller on the individual compressor. These devices are designed to protect the compressor from damage, operate it as efficiently as possible, and automate recurring control actions. Modern compressor controls offer features far beyond those of their predecessors; powerful microprocessor-based controls are becoming increasingly popular as their significant advantages become more widely known.
Compressor Control Types
Pneumatic
Many existing air compressors utilize electro-pneumatic control systems. These systems use electric and mechanically activated devices such as pressure switches, solenoid valves, and metering pins. To address monitoring and compressor protection, electro-pneumatic systems typically feature a series of mechanical trip switches which shut down the compressor when pressures or temperatures reach critical levels. With their mechanically limited control logic, pneumatic controls offer only a basic set of operating functions—usually simple compressor modulation and monitoring. In addition, the response time of pneumatic controls is typically inferior to other types, reducing the effectiveness and efficiency of control actions.
Programmable Logic Controllers
Programmable logic controller (PLC) systems are in common use today, and they represent a significant improvement over pneumatic-based systems. Programmable logic controller control systems utilize digital and analog control and monitoring instruments. The responsiveness and accuracy of these devices enable greater compressor efficiency. Additionally, because PLCs are electronically based, they can offer more functions, such as sequencing and advanced control and monitoring. Fundamental speed and performance limitations of PLC hardware, however, still leave room for improvement.
Microprocessor
The market for microprocessor-based controls is growing rapidly due to their comprehensive features and superior performance. The control and monitoring accuracy of these systems is excellent, allowing for tight pressure regulation and advanced protection strategies. Because these controls are based on hardware similar to modern computers, they can offer additional features such as sophisticated networking, multiple compressor control, and extensive operational record keeping. Many of the control benefits and features listed in this document are best implemented by these types of control systems.
Protection and Safety
On the most basic level, compressor controls must keep the machine running safely and reliably, preventing both serious compressor failure and more common mechanical damage.
Plant Safety
Maintaining plant safety by preventing catastrophic compressor failure is one of the most fundamental responsibilities of a compressor control. One example of severe compressor failure is when incompressible water builds up in a reciprocating compressor’s compression chambers; if enough accumulates, the compression action can cause the equipment to physically fail, sometimes explosively. Other safety concerns include overheated and potentially combustible oil entering the air distribution system or violent vibration caused by a severe mechanical failure. Most modern control systems have the ability to shut down the compressor if a serious malfunction is detected.
Machine Protection
On a less extreme scale, compressor controls should guard the compressor from common wear and damage. Advanced control systems use analog and digital monitoring instruments to check the compressor for abnormal operating conditions. When the control system detects an unsafe measurement, it can either trigger a warning alarm or shut down the unit, depending on the nature of the situation.
Modern control systems examine every relevant operational value of a compressor and continuously check these values against standard ranges. Today’s systems can visually display a list of the current values and alarm parameters of every monitoring point, allowing an operator to know precisely what the compressor is doing at any moment. Another useful feature is a recorded history of past operation and alarm events. If a problem occurs, such a record can provide critical diagnostic information to help resolve the issue quickly.
Centrifugal Compressor Surge
On centrifugal compressors, a key control feature is the ability to reduce or eliminate surge. Surge is a phenomenon where compressed air rapidly oscillates backwards then forwards through the compressor; unabated, this can cause severe damage. From a maintenance and protection standpoint, modern controllers offer far superior surge prevention compared to older systems. Advanced systems, with mathematical models of the compressor’s surge line and fast control responses, work to keep the compressor out of surge. If surge does occur, these systems can detect the event and adapt the controller’s operation to avoid any recurrence.
ENERGY SAVINGS WITH MODERN CONTROLS
One of the primary benefits of an advanced compressor
controller is reduced energy expenses. Energy savings can
result from any or all of these factors:
• Precise pressure regulation reduces the average system pressure output.
• Networked capacity control coordinates production among multiple compressors for maximum efficiency.
• Advanced centrifugal control can reduce wasted air from blow off.
• Leak loss reduction is a byproduct of a lower average system pressure.
• Automated load scheduling can shut down or offload compressors when plant demand is lower.
• Proper intercooler control ensures better compressor efficiency.
Precise Pressure Regulation
Significant energy savings can be realized by lowering compressor discharge output pressure. Older systems are slow and inaccurate, resulting in large plant pressure swings. In order to keep pressure from swinging below the minimum, these older controls commonly maintain an average system pressure much higher than necessary. Modern controllers, with faster, more precise abilities, and sophisticated control strategies, can greatly reduce pressure swings. The smaller pressure range, typically within 2 psi of target pressure, allows a subsequent drop in average pressure setpoint. A 2-psi reduction in system pressure results in an approximately 1% drop in energy use; thus, the potential energy savings are substantial.
Networked Capacity Control
In plants with multiple compressors supplying a common system, uncoordinated compressor operations often offer opportunities for energy savings. Compressors that do not communicate with each other act independently, raising or lowering output as they detect changes in plant demand. This can often result in competing compressors; one compressor may be lowering its output while another is increasing its capacity. Unstable plant pressure levels are one result of this competition. Additionally, the compressors operate at inefficient part load capacity levels.
Sequencers, which were the first solution to this problem, assign compressors different fixed pressure levels at which point they come on or off line. Although this method does prevent competition between the individual compressors, it causes plant pressure to fluctuate along the range of assigned pressure levels (a four-compressor system with pressure intervals of 5 psi will have a 20 psi operating window) and tends to maintain an average system pressure higher than needed.
Advanced control systems with networked capacity control capabilities produce an efficient compressor system operation. Through network communication, the controls automatically operate as many individual compressors at their most efficient (full load) capacity levels as possible. Instead of multiple compressors operating at part load capacities, a single compressor is modulated to meet plant demand. The whole system is coordinated to maintain a single pressure setpoint, providing precise pressure regulation. Should demand fall, compressors are automatically shut down or unloaded, further saving energy. Rising demand will cause another compressor to come online, ensuring stable plant pressure. With such a system, multiple compressor installations can maintain plant pressure while operating individual compressors as efficiently as possible.
System Controllers
Built-in networked capacity control is limited to the more advanced control systems. When installation of these systems is not feasible, similar capabilities can be achieved with a system controller. In order to achieve the benefits of networked control with less capable, mismatched, or incompatible control devices, a system controller is sometimes used to coordinate the individual compressors. These master controllers offer many of the features of networked control, and can operate several compressors at a common plant pressure setpoint.
Surge Control and Blow off Reduction
For centrifugal compressors, blow off at minimum capacity is a significant energy waste. Modern controllers can reduce this waste using several methods. Advanced antisurge algorithms provide greater turndown, allowing a larger modulation range before blow off begins. Additionally, when the minimum capacity point has been reached, the controller can switch the compressor into an unloaded state, where it produces little air and thus blows off little air. In a networked capacity control system, other compressors can modulate output, allowing the centrifugal compressor to operate at higher and more efficient capacity levels. These combined features offer great energy savings potential.
Leak Loss Reduction
Reducing the average pressure setpoint also reduces the amount of air that escapes from existing leaks. Leakage easily can be the largest energy problem in a compressed air system, ranging from 2 to 50% of compressor system capacity. An average plant has a leak rate of about 20% of total air production.
Lower pressure air has less air mass in the same volume. Because the volume of leaking air remains constant at a given pressure, a lower pressure results in less air escaping from existing leaks. Precise pressure control is one way to lower average system pressure.
Load Scheduling
Load scheduling automatically matches the compressor’s output pressure with predetermined plant demand. During breaks and off-production periods, a significantly lower pressure often can be maintained in the plant. Any pressure reduction will save notable amounts of energy.
Compressor Intercooler Control
Effective intercooler control will help a compressor operate at top efficiency. Air that has not been cooled adequately by the intercooler will enter the next stage with a larger volume, reducing total compressor output. Alternately, air that is cooled too much can form liquid condensate, which can damage compressor components and increase maintenance costs. Modern control systems usually include intercooler control as a standard feature.
Automation
Automation is a key element of advanced control systems. Automated machine protection, data collection, and start/ stop, combined with capacity regulation and load scheduling capabilities, give modern control systems the ability to automate nearly every operation of a compressor system. Remote monitor and control capabilities further reduce dependence on at-the-controller compressor supervision.
Start/Stop
Modern control systems can automate a compressor’s start and stop procedures. With monitor and control connections to essential compressor subsystems (motor, lubricators, coolant, etc.), the controller can start and stop the complete compressor station while ensuring safe operation.
In a networked control system, automatic start capability can add additional system reliability. When one compressor is shut down because of a problem, the system can automatically compensate for reduced air supply by bringing additional compressor capacity online. Thus, plant pressure is maintained and production continues with limited interruption.
Scheduling
When start/stop capabilities are combined with a schedule, much of the day-to-day compressor operation can be automated with controls. Once an effective schedule is established, the system can essentially run on autopilot, with little need for immediate operator adjustments.
AIR QUALITY
An often overlooked element of compressor system controls is the equipment used to condition air, which primarily includes dryers, aftercoolers, and filters. These pieces of equipment have the essential task of ensuring that hot, wet air leaving the compressor is converted to high-quality, cool, dry air for use in the facility. The two largest air conditioning problems are insufficient drying and excessive differential pressure drops.
Wet air that enters the distribution system can eventually cause rust formation, leading to clogs and extra wear on end use equipment. If the problem becomes severe enough, it can require a shutdown of portions of the compressor system while corroded piping and failed components are replaced. These humidity problems can be caused by inadequate drying or poor aftercooling.
An improperly sized, poorly maintained, or outdated piece of equipment can cause an excessive pressure drop as air passes through it. Pressure drop across dryers alone can be greater than 6 psi; because a 2-psi pressure change roughly equates to 1% of energy used to compress the air, the opportunities for energy savings are substantial. Excessive pressure drop is a factor that can affect dryers, aftercoolers, and filters.
Monitoring Air Quality
Given that high quality air is so important, there are opportunities for mitigating these issues with monitoring techniques. By monitoring the pressure and quality of the air, both before and after conditioning has taken place, a facility can identify existing and potential problems. With continued monitoring, the results of equipment upgrades or maintenance actions can be verified for effectiveness. This approach is also able to flag when maintenance is necessary to upkeep the quality of air, reduce unnecessary pressure drops, and maintain system efficiency. Monitoring capabilities of this type are most often integrated into an overall compressor management system, where the metered values are displayed on a networked compressor management system workstation.
AIR DISTRIBUTION
Zone Management
An emerging control strategy for the distribution side of a compressed air system is known as zone management, which provides additional opportunities for energy savings and operational improvements.
Zone management technology gives the capability to monitor and control the demand side of a compressed air system. This strategy involves separating an air system into different distribution zones and regulating and metering the air supply to each one. The control of each zone can be scheduled for automatic operation, making complex zone management relatively easy. Zone management opens up new system operation options, such as running zones at different levels of pressure and shutting off zones when they are not in use.
With the ability to monitor the air consumption of each zone, users can gain much greater insight into the operational dynamics of a compressed air system. This advanced metering ability provides an accurate determination of the compressed air energy costs of different plant operations and can provide incentives and justifications for initiatives to reduce those costs.
On a design level, zone management involves logically separating the operations that use compressed air into different zones based upon factors such as concurrent air use, pressure setpoints, and air quality. The air supply to each zone is then individually controlled, allowing the air flow to each zone to be modulated according to the current use in that zone.
With a scheduling function, the system can be configured to automatically raise and lower pressures or to turn the air supply completely off to each individual zone.
Implementing zone management requires a combination of metering and control instruments for each zone, and a master control device to provide the monitoring, control, and scheduling functions. This master control functionality is often an add-on capability of a compressor management system workstation.
Benefits of Zone Management
A facility can realize one or more benefits from a zone management system: reduced compressed air energy consumption, zone cost regulation, increased data collection, air quality monitoring, and zone air leakage measurement.
Reduced Air Use
Zone management often results in a reduction in air demand due to its ability to lower the pressure or completely stop the flow of air to zones that are not in use. Less air is then used to maintain pressure in nonproduction areas and total demand is lowered. Less demand means less compressed air production, which directly reduces energy costs.
Regulate Air Costs
The ability to establish a separate cost center for each air use zone is another important benefit that comes from the capability of monitoring and metering the air use of each zone. With a comprehensive metering program, the facility has the ability to regulate the costs of compressed air for different segments of their production operations. Better information regarding air use and the costs of that use allows for better management decisions to be made.
End to End System Information
When zone management is paired with modern controls for other sections of the compressor system, a facility can have complete end to end system information and management. This system-wide approach enables better understanding of the operational dynamics of the entire compressed air system.
Monitor Air Quality
Air quality can be just as important as energy management. Correct pressure and humidity levels ensure the proper functioning of end use equipment and lower the lifelong maintenance costs of equipment based on wear and tear. Zone management allows constant monitoring of the pressure and dew point of the air entering each zone, ensuring that high-quality air reaches the end use point.
Measure Zone Leakage
Leakage is a frustrating aspect of compressed air systems and one that is difficult to measure. Zone management makes it easier to determine how much air is used and thus wasted by a particular zone when it is not in use. A majority of this wasted air comes from air leaks. By measuring the amount of air leaking from a zone, the facility can make effective decisions regarding leak control programs and the potential return from such efforts.
AIR MANAGEMENT, MONITORING, AND METERING
The final control layer for a compressed air system is an integration strategy that pulls all of the different control and monitoring features into one centralized location. This is usually accomplished through a compressor network, which is then routed to a human machine interface (HMI) workstation. This workstation can be a local monitor and control computer, or a Web-based management and analysis client server.
Facility Intranet Monitoring and Control
When connected to a network of individual compressor controllers, a remote computer station allows for monitor and control of an entire compressed air system from a single convenient location. With access to the entire range of monitored compressor data, the remote station can display the condition of the complete system at any given moment. A remote operator with security access to control capabilities can start, stop, and change the capacity of any compressor from the remote station.
Remote operation also allows an operator to pinpoint compressor problems as they occur. More immediate and precise information directs corrective actions, reducing compressor down time and associated costs from loss of adequate plant air supply.
A workstation connected to a network of compressor controls can also record and store data for the entire compressor system. This wealth of information is then available for analysis, which can quantify compressor system performance over time. Individual compressor problems can be identified and corrected before they become serious. This preventive maintenance reduces operating costs resulting from both inefficient operation and lost production due to compressed air equipment failure.
Below are some of the specific features of a typical HMI compressor workstation:
• A total system view, which displays the operating conditions of the complete compressed air system, including pressure, motor status, and alarm status.
• Complete remote control over every compressor on the network, including starting and stopping, capacity changes, and pressure setpoint changes.
• An individual compressor view, which displays the current readings of the major monitoring points, while graphically showing the user where the points are located on the unit.
• A complete list of all monitoring points for the individual compressor, with alarm and trip values and current status.
• Recorded history of all alarm and trip readings that occur.
• Recorded history of all operator events for the compressor.
• An ability to generate analytical performance reports based on the collected data. These reports can track compressor performance over time and spot trends before they become problems.
Web-Based Management Systems
A Web-based compressor management system offers many similar features to a local compressor network.
One of the central capabilities of a Web system, however, is the ability to provide access to multiple facilities from a standard internet connection.
Usually, a gateway device of some kind is used to connect the facility compressor network to a dedicated communication connection, such as a phone line or broadband connection. An off-site server then collects data from the system and a Web portal provides access to the information. From the site, a user can access real-time monitoring of plant air compressor systems, as well as the operating parameters of each individual compressor. For security purposes, a Web system often does not allow control actions; this prevents unauthorized tampering with the facility’s compressor operation. If a company operates multiple plants, the Web site can provide centralized monitoring and metering capabilities for the entire enterprise. The Web-based nature also means that a user can access compressor system information from anywhere there is an internet connection. Additionally, Web systems frequently incorporate built-in efficiency and operations analysis and reporting, giving management powerful tools to measure the performance of their air compressor systems.
Benefits from Whole System Integration
These centralized management, monitoring, and metering systems offer many benefits to the operation of a compressed air system. Some of the more common benefits are described in the following sections.
Internet-Based Access
A Web-based system is accessible from any standard internet connection. This provides operators with monitoring capabilities from office, home, or even Web-enabled phones or PDAs (personal digital assistants, such as Treo or Blackberry).
Cost Metering
An integrated management solution offers the ability to meter the costs of compressed air operations. Comprehensive whole system metering provides operators and management with improved understanding of the costs and operational dynamics of the compressed air system. Real-world data can then be used to identify opportunities and drive business and management decisions.
System Efficiency
The ability of a management system to accurately and consistently calculate the cost of compressed air for single or multiple facilities makes it easier to measure any changes in efficiency and compressed air production. As future investments are made in the compressed air system, a management and metering system can compare system efficiencies before and after improvements are made to validate savings. Using centralized monitoring and analysis capabilities, a management system can often help identify further efficiency gains from operational changes. For example, these systems can help find the most optimal, most efficient mix of compressors to use during different shifts at a facility. Once the optimal mix is found, automatic scheduling ensures that the most energy efficient solution is used.
Measure Air Leakage
Leakage is a notoriously hard quantity to measure in normal circumstances. A system management tool can easily measure the air flow rate of a facility during nonproduction hours; this flow will be a close approximation of a system’s air leakage.
Preventive Service Monitoring
The constant monitoring performed can be helpful in identifying and resolving problems with compressors before they become serious. Problems with sensors or other issues can be identified during periodic system reviews and resolved before the issue becomes serious.
Real-Time, Data Driven Troubleshooting
When a problem with a compressor does occur, the data recording and real-time monitoring capabilities of a whole system management solution can provide faster resolution to the problem. With a Web-based solution, experts from outside facility locations can view the current system status and help with troubleshooting.
Comprehensive Overview
Companies with multiple facilities find a Web system very useful in providing a centralized overview of all compressor networks. These tools provide an easy and accessible method to see the current status of every compressor on the system.
CONCLUSION
Modern compressor control systems now offer increased levels of sophistication and opportunities for control of the entire compressor system. A comprehensive, system-wide solution cannot only control and protect individual compressors, but it can also provide powerful new operational and management tools. This new control strategy provides powerful, efficient, and effective control of compressed air systems.
