Many plants link together a number of compressors into a single compressed air system. In such a system, all compressors but one should be operated at full capacity and optimum efficiency. The one so-called trim compressor is capacity controlled to satisfy system demand at a relatively constant pressure. This also minimizes the amount of storage capacity required. Sophisticated sequencing control panels can allow a change of trim compressor to even out run time and changes where demand varies from shift to shift. The limited capacity control range of many centrifugal compressors restricts their capability as trim compressors. Constant-speed oil-free rotary compressors have similar limitations but those with VSD are capable of a wide range of capacity reduction. Oil-injected VSD rotary compressors also are available. VSD compressors can maintain a supply pressure within ±1 psi. (Centrifugal compressors don’t come with VSD because the pressure-head-making capability varies as the square of the rotating speed and the required discharge pressure can’t be maintained at the reduced rotating speeds.)
Pressure/flow controllers also can be used to provide a relatively constant supply pressure (±1 psi) while demand varies but may require an increased compressor discharge pressure. Where used, they should be located downstream of the primary air dryers(s) and primary air receiver. The controller should be guaranteed by the supplier for operation over the entire anticipated range of rates of flow.
The same considerations apply to multiple compressors in different systems. The advantage of separated systems is that each can have its compressor(s) and compressed air treatment equipment tailored to the specific required conditions of operation without being influenced by the requirements of another system. One potential disadvantage is the need for adequate standby compressors and treatment equipment in each system. When systems are segregated to allow operation at different pressures, it’s possible to arrange for supply from a higher pressure system to a lower pressure system in times of emergency — provided the higher pressure system is adequately sized.
Whichever type of unit provides compressed air, it’s crucial to ensure the quality of air suffices for its intended use. Process equipment manufacturers often note air quality requirements. Where this isn’t given, ask the vendor to specify the necessary level, preferably with reference to a published standard. ISO 8573-1 is the International Standard for Air Quality Classes. It defines allowable levels of solid particles, moisture and liquid condensate, and lubricants. In some environments hydrocarbon gases can be ingested at the compressor inlet, so that the air delivered may not be truly oil-free and may require appropriate filtration.
A major consideration in many chemical plants is the quality of the product(s) being produced and the compressed air that comes in contact with it. As a general rule, air shouldn’t be dried more than required for the given application. This avoids unnecessary operating and maintenance costs. However, the limitations of compressed air drying equipment should be understood.
Refrigerant-type dryers cool the air to allow condensate to be drained off but at 32°F freezing will occur, so drying is limited to a pressure dew point of 35°F to 38°F. While this is adequate for many industrial applications, it generally doesn’t suffice for many chemical processes.
Standard regenerative-desiccant-type dryers use a material that adsorbs moisture and provide a pressure dew point of -40°F. Others are available down to -100°F. This type of dryer generally has twin towers, allowing one tower to be drying the compressed air going to the system while accumulated moisture is being removed from the desiccant in the other. Some dryers use heated purge air; a cooling option may be needed where processes are temperature sensitive. Specify a dew-point-sensing controller. A standby dryer allows for maintenance without shutting down the process. In segregated systems, drying can be tailored to meet the specific needs of the system.
As with drying, filter only to the extent needed for the particular process. Use ISO 8573-1 to determine the appropriate classes. There are three basic types of filtration: particulate, coalescing and adsorption. Specific applications may require combinations of types.
One common problem is that filters may be sized to match the size of the piping in which they are to be installed. The anticipated system flow rate should match the recommended flow rate of the filter. Determine piping size not by the size of the filter connections but by the anticipated maximum system flow rate, with a pipe velocity not to exceed 30 ft/sec.
Locate filters where they can be readily serviced; arrange standby filters, with bypass valves, to allow continuous operation during maintenance.
Install a good differential pressure gauge across filters and take regular readings to establish trends for scheduled maintenance.
David M. McCulloch is principal of Mac Consulting Services, Pittsboro, N.C. E-mail him at email@example.com.