Plant engineers know the importance of compressed air in the industrial environment. Because it's as vital as electricity, gas and water to plant operation, compressed air is often regarded as a "fourth utility." But unlike the other three, which are purchased from outside sources, a plant's compressed air supply is generated on-site, using its own equipment. That's why every industrial plant, whatever its size, needs a dependable, efficient plant air system. Selecting the right one requires a careful, informed approach, guided by the principles of common sense engineering and design simplicity.
While the primary responsibility for specifying a new or expanded compressed air system usually rests with the plant engineer, others may be involved in the decision-making process. A specifier has two options: a plant-wide centralized compressed air system or a distributed system using multiple localized compressors (see Figure 1).
Figure 1. Heat from a compressor can be recovered and used for space heating, thus reducing utility costs.
Reliability and efficiency
To determine the most suitable option, evaluate two factors to optimize plant operations. First, determine specific demands and work toward achieving an air system with long-term dependable performance. The second factor is system efficiency, analyzed from two perspectives: the generation and distribution of compressed air, and the processes or machines that require that air.
As you begin the process of choosing between a centralized and a multi-compressor system, answer the following questions:
- Is this a new plant or an expansion of an existing installation?
- What is the system's overall size and scope?
- What are the critical air needs of selected users?
- What are the overall system demands?
New system versus expansion
Although designing an air supply for a new facility is a large task, it's often easier than expanding an existing one. New installations offer almost complete control over design variables. The system can be optimized without making concessions for the existing air supply and its downstream components, such as piping, filters, dryers and lubricators.
In an expansion, new users are added to a system designed for other purposes, and accommodating new operations may compromise the original system. However, you can either expand the existing plant system or install a larger dedicated compressor.
Expanding an existing multi-compressor system requires balancing costs and the efficiency and cost of a central system. In some cases, the efficiency and reliability of a dedicated compressor may be the best choice.
Keep the initial cost of centralized and multiple systems in perspective. Annual operating costs for either option probably will exceed the first cost by a factor of three to four. For this reason, system efficiency should receive higher priority than initial cost.
System size and scope
Start with the basics, such as deciding whether the system is to serve one building or multiple buildings within a complex. Many single-building applications may be larger than a multi-building application in terms of piping length, system demands and general air requirements.
The multi-building system offers unique challenges, such as distribution and distance. Distance may make a centralized system impractical. On the other hand, overall air demand, including pressure and volume, may require a centralized approach.
Distribution and distances between compressors and individual users dictate piping size and downstream air quality. Balance installation, maintenance and operating costs with reliability and efficiency requirements. If they are not properly sized and insulated, external pipe runs to remote buildings will result in pressure loss and condensation.
Examine the availability and cost of electricity, water and sewers. Check load factors and rate schedules. For example, a central compressor may require a costly electrical service upgrade, possibly a new substation. Smaller, multiple compressors may be able to operate from the existing electrical service, but will consume more electricity.
Compressor heat recovery (see Figure 2) is a common technique for increasing overall efficiency. In air-cooled systems, the low-grade heat that's recovered can be used for space heating, thereby reducing purchased utility costs. However, consider the installation and operating costs associated with heat recovery equipment.
Figure 2. Piping layouts are similar for multiple and centralized systems. Connecting valves allow multiple systems to back up each other.
Heat recovery from water-cooled compressors has become increasingly popular. A closed-loop cooling system reduces utility costs because it slashes water and sewer use. Consider these utilities carefully, because availability may be limited in areas unprepared for industrial-size loads. In many industrial areas, sewer charges far exceed water charges. In some areas, they are triple the cost of water.
Location and access
Easy access to equipment for maintenance and replacement is a requisite for both central and multi-compressor systems. When compressors are located in remote outdoor enclosures, physical access is often a challenge. If a compressor is walled off, provide adequate ventilation to cool the unit and to meet air intake needs.