With the proper knowledge, engineers can work alongside compressor manufacturers to ensure that an optimal air compression system is installed
There are various factors to consider when designing a compressed air system that help to improve the reliability and efficiency of compressors and ancillary equipment, reduce leakage and pressure drops, and minimize the compressor system’s lifecycle cost (Figure 1). This article provides guidance on several considerations that impact a compressed air system.
Compressor performance can vary based on ambient conditions. It is important to know the site elevation, ambient temperatures, relative humidity (RH) and airborne dust load prior to choosing a compressor system. Ambient air can also contain aggressive gases, such as hydrocarbons, hydrogen sulfide (H 2 S) or ammonia (NH 3), which require a suitable filtration system to protect compressed air equipment. Having this information on hand can help compressed air experts make a more informed decision when it comes to designing the best solution for a plant’s needs.
Centralized versus decentralized
Centralized and decentralized compressor systems each have their own advantages. A decentralized system is installed when compressed air is needed for applications where the compressor system must be located near the application, such as air blast for high-voltage electric breakers, pneumatic conveying of materials, pneumatic operation of forging tools and other applications that demand the air compressor be in close proximity.
In most other cases, a centralized system is preferred due to its added energy efficiency and decreased maintenance costs. A centralized system uses larger, but fewer, compressor units, as its air-intake filtration, ventilation requirements, cooling-water treatment, air cooling and drying are all located in the same area. Users can save time on labor and routine maintenance since centralized systems are well-suited for remote master control, load sharing and sequencing operations.
Sizing and selection
Selecting a correctly sized compressor requires a robust analysis of the following parameters:
- Flow demand
- The equipment manufacturers’ flow and pressure requirements
- Air consumption rates
- Utilization factor — the ratio of time that the equipment is in operation to the total working time
- Load factor — the ratio of actual flow to the full load flow during operation time
It is also necessary to check equipment specifications and evaluate whether or not the compressor has taken extra flow margins for leaks and pressure drops. In many cases, an additional margin on flow leads to various issues in the compressor’s operation and can reduce efficiency.
There are two basic principles of compression for air and other gases — positive displacement and dynamic compression.
In positive displacement compression, the air is drawn into one or more of the compression chambers, which are then closed from the inlet. Gradually, the volume of each chamber decreases and the air is compressed internally. When the pressure reaches the designed build in pressure ratio, a port or valve is opened and the air is discharged into the outlet system due to the continued reduction of the compression chamber’s volume. Positive displacement compressor types include piston, vane, scroll, liquid ring, rotary screw, tooth and blower. A typical compressor screw element is shown in Figure 2.
In dynamic compressors, such as radial and axial types, air is drawn between the blades on a rapidly rotating compression impeller that accelerates to a high velocity (Figure 3). The gas is then discharged through a diffuser, and the kinetic energy is transformed into static pressure. Most dynamic compressors are turbo compressors with an axial or radial flow pattern designed for larger-volume flowrates.
Selecting either of these technologies depends on the application. For instance, turbo technology is best suited for base load requirements, whereas positive displacement compressors are better for variable loads. For larger flow and variable demands, applications with a combination of both technologies work better for optimal utilization of compressed air and decreased energy consumption.
Flow and pressure units
An critically important first step is to decide which unit of measure will be used to indicate flowrate, based on the requirements of the specific process. There are several ways to measure the capacity of an air compressor, as follows:
- Inlet cubic feet per minute (ICFM), which is the inlet volume flowrate or intake volume as measured at the intake. For most processes, inlet flow is not usable flow; therefore, using the ICFM flow unit is not advised
- Cubic feet per minute (CFM), which describes the free air delivery as measured at the delivery point, downstream of the aftercooler
- Standard cubic feet per minute (SCFM) or normal cubic meter per hour (Nm3/h), which represent the standard or normal flow as measured at the delivery point and established by standard temperature and pressure (STP) or normal temperature and pressure (NTP) conditions. The reference condition will have a large impact on flow, which is why it is good practice to specify the reference condition you want to use when consulting with a compressor manufacturer. Typical reference conditions are given in Table 1
Required pressure can also be specified in psig, barg or kg/cm 2 g.
The primary function of an air receiver tank is to store compressed air, but it also serves as an additional condensate separator. Furthermore, the air receiver ensures a steady airflow and equalizes momentary pressure variations in the air piping network, which could cause frequent loading and unloading of the compressor. Normally, the air receiver comes with a safety valve, pressure gage, connection to fit the test-pressure gage, inspection cover and drain valve.
Air dryer and filtration selection
Atmospheric air contains water vapor that must be removed to a certain degree. Water concentration increases at higher temperatures and decreases at lower temperatures. Therefore, when the air is compressed, the water concentration increases.
There are different types of dryers available on the market, with the two main types being refrigerated dryers and desiccant dryers. Refrigerated dryers can reach a 37°F pressure dewpoint, whereas desiccant dryers are capable of handling negative dewpoints. Depending on what quality of air is needed for the application, either of these dryer types can be chosen.
When sizing your dryer, avoid selecting one straight from a manufacturer’s brochure. Inlet conditions can have a major impact on the dryer’s performance, so it is recommended to calculate sizing based on dryer inlet conditions.
With the heatless type of desiccant dryers, the dryers experience purge loss, which requires that designers oversize the compressors to meet that extra flow, resulting in higher energy consumption. There are also heat of compression dryers available with a zero-purge option to save on energy, as opposed to heatless dryers, since these dryers do not produce any air loss. Depending upon the application and level of energy efficiency, there are different options available to find the best solution possible.
For application or process filtration, the required filtration level and type of filters can be installed in the system piping (Figure 4). There are oil removal filters, dust filters and carbon filters available for use, depending on the quality of air required.
Compressor room ventilation
The total quantity of energy delivered to the compressor in the form of electricity is completely transformed into heat during compression. The majority of this heat is removed by a cooling medium — air or water. The remaining heat is not removed, and is referred to as the residual heat in compressed air.
In the case of water cooling, the heat is removed to outside the compressor room to the sump of the cooling tower. Conversely, with air-cooled compressors, all of the heat is dissipated into the compressor room if no dedicated ducting is provided.
Prior to deciding which type of compressor to install, make sure to procure installation proposals for both air-cooled and water-cooled options from the compressor manufacturer so that experts can help to properly size the required compressor room ventilation.
The cooling water quality must meet certain requirements as specified by the compressor manufacturer. It is recommended to have a water sample analyzed by a laboratory that can also advise on a suitable treatment solution if needed. The cooling water flow for compressors can be designed so that the total energy dissipated in the compressor can be evacuated with a reasonable amount of increase in water temperature.
Compressed air piping network
Compressors should be placed in a central location in close proximity to all relevant applications and processes in order to minimize the length of piping between compressors and points of use. The location must also take into consideration the quality of intake air, which should be cool, clean and dry. The compressor installation should be kept clear of steam, chemical vapors, engine exhaust and dust.
To reduce pressure drop, minimize the number of valves, bends, fittings and flow obstructions. Adequate space must be provided around the compressor for proper ventilation and for regular inspection and maintenance.
Compressed air pipes should be installed in such a way that they can be reached from all directions. Avoid pipe installations in subfloor trenches since they are difficult to maintain and repair. Also, these conditions make it difficult for adequate condensate drainage and air leak detection.
Horizontally installed pipes should slope 1 to 2% toward the air consumption point so the condensate is carried to predetermined locations where drains permit the condensate to be removed. While some may argue that properly installed and correctly sized dryers make sloping of compressed air pipes superfluous, the cost is minimal and sloping provides additional protection in the event that the dryer is out of service.
Pipe-to-tube bends should have generous radii to minimize turbulence. Bends are preferred to elbow couplings because they reduce turbulence and create less pressure drop.
To prevent condensate from entering the branch pipe, the latter should be branched on top of the subheader and condensate drain points should be provided at the lowest point of the ring network.
It is recommended to provide flanges so that sections of the air piping network can be isolated by inserting blind flanges. Maintenance work can then be carried out without completely halting production.
Adequate brackets, clamps or other supports will keep lines straight without sagging and prevent machinery vibrations that can lead to loosened pipe couplings.
Expansion joints should be used between the compressor and air network piping with proper support after the expansion joints, which ensure that the air net load is not transferred to the equipment.
Monitoring and control
Central controllers are an effective solution for improving the efficiency of a compressed air system, because they not only monitor the system’s operations, but can reduce lifecycle costs and help compressors to meet the requirements of environmental regulations. There are several advantages to central controllers for air compression systems, as described below:
- Potential energy savings (as high as 10%) on a typical compressed-air installation with mixed sizing and compressor technology
- Regulation of system pressure within a predefined and narrow pressure band to optimize energy efficiency
- Prioritized use of the most economic machines over older or less effective versions to reduce downtime
- Continuous use of variable speed drive (VSD) machines, which are the most energy-efficient machines for variable load
- Assurance that multiple VSD and/or turbo compressors are used in their most efficient performance zones when working together
- Shutdown scheduling to avoid costs during non-working hours
- Optimized pressure stability, which helps to reduce problems with air-operated equipment
- Workload equalization to avoid overloads on individual machines
- Reduced equipment maintenance costs
Comprehensive, flexible machine-sequence control ensures that installed machines are able to work in groups. The controller guarantees that the running hours of the system are equal across all machines in the same group. A central controller can also control other ancillary equipment, such as switching dryers, control valves and cooling water pumps. Users can add extra instrumentation, such as meters to monitor flow and vibration levels in order to prevent potential equipment failure. Safety parameters, temperatures and pressures can be monitored for all compressors and dryers that are connected to the controller. Furthermore, machines require fewer service visits and experience reduced costs, since they can all be serviced at the same time.
Designing a compressed air system that is right for a facility requires an understanding your applications, their demands and the different technology options available. After reviewing the information given in this article, engineers should be empowered to work knowledgeably with compressor manufacturers to design the best compression system for their application.
Edited by Mary Page Bailey
Deepak Vetal is a product marketing manager for oil-free screw and centrifugal compressors at Atlas Copco Compressors LLC (3042 Southcross Blvd., Suite 102, Rock Hill, SC 29730; Email: email@example.com). He has over 19 years of experience in sales, customer service, account management, product development and product marketing for oil-free screw and centrifugal compressors. He has handled national product marketing in key markets, including India and the U.S., and worked with many customers on their compressed-air systems in many industrial sectors, including automotive, metal, textile, pharmaceuticals, chemical, food–and-beverage, oil-and-gas, paper, power and electronics. He holds a B.Tech. degree in mechanical engineering from Doctor Babasaheb Ambedkar Technological University in Lonere, India.
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