Selecting Air Treatment Systems
As we have mentioned, compressed air can be treated from dirty, oily, wet state to dry, clean, oil-free air that is many times cleaner than the ambient air we breathe. However, with increased air quality there is a corresponding increased cost. This cost comes in many forms:
- Increased capital equipment cost
- Increased maintenance cost
- Increased energy cost
- Dryer energy consumption (in electric or compressed air)
- Pressure Drop (requiring compressors to use more power to overcome losses)
1. Determine Required Air Quality
Though an obvious step, this can be more difficult than it sounds. For many years, production equipment manufacturers have specified air quality in ambiguous terms such as “clean, dry air”, or through ignorance have specified requirements far in excess of the actual requirement. When quantifying air quality, it is also important to recognize both the most stringent requirements and the overall average requirements.
As a reference to help facility engineers and OEM’s specify compressed air quality for solid particulates, humidity, and oil, the International Organization for Standardization (ISO) has developed ISO 8573.1:2010.
The air quality is expressed as X.Y.Z, where X is the particulate class, Y is the humidity (moisture) class, and Z is the oil class, as outlined in the table below.
If no specification for installed equipment is available, some guidelines are as follows:
Typical Manufacturing and Woodworking in a Heated Facility using air tools, cylinders, blow-offs and CNC machines typically requires elimination of liquid water and reduced particulate and oil content (ISO 8573.1:2010 class 2.4.2). This can be met using a refrigerated air dryer and general purpose coalescing filter.
Manufacturing in Cold Environments and Unheated Spaces will require lower moisture dewpoints to prevent moisture and line freezing (ISO 8573.1:2010 class 2.2.2). This can be met by a desiccant dryer with coalescing prefilter and particulate afterfilter, or by a membrane dryer with coalescing prefilter if the demand is low.
Laser Cutting of Ferrous Metals typically requires low moisture and oil content (ISO 8573.1:2010 class 1.2.1). This requires a desiccant or membrane dryer with particulate filter, extra fine coalescing filter and vapor filter.
Laser Cutting of Stainless Steel at certain thicknesses may require the use of Nitrogen or another shield gas, rather than compressed air. This can be generated from compressed air through the use of a Membrane Nitrogen Generator or Pressure Swing Adsorption (PSA) Nitrogen Generator.
Instrument Air as specified by the Instrument Society of America (ISA) standard 7.0.0.01-1996 requires pressure dewpoint to be no greater than 18°F below the lowest temperature to which any part of the instrument air system will be exposed to during any season of the year, in any case not to exceed 35°F.
Particulate level is limited to 3 Microns, and oil content is not to exceed 1 part per million by weight. This puts instrument air at ISO 8573.1:2010 class 2.3.3 in most instances, requiring a desiccant dryer with coalescing prefilter and particulate afterfilter, or a membrane dryer with coalescing prefilter.
In fully heated facilities, this requirement can very nearly be met by ISO 8573.1:2010 class 2.4.3, requiring only a refrigerated air dryer and coalescing filter.
2. Split Overall and Point of Use Quality Requirements
It is rarely cost effective to supply an entire facility with the highest air quality required by the most stringent end use or process. Often the best solution is to treat all air to the “lowest common denominator” or at least the average required quality, then use point of use air treatment for the more sensitive processes.
For example, consider a woodworking shop in a cold climate that is heated but has dust collectors that require compressed air located outside the building. The vast majority of the shop requires only a refrigerated dryer and coalescing filter, but the dust collector will require freeze protection that only a desiccant dryer can provide. It is far less expensive to purchase and operate a system with a central refrigerated dryer feeding the plant, and small desiccant dryer feeding only the dust collectors than to install and operate a central desiccant dryer.
Another example is a plant that requires only general shop air, but some outdoor instrument quality air as well. If the instrument air requirement is widely dispersed around the facility, two separate sets of compressed air lines may feed the plant, with the “plant air” treated only by a refrigerated dryer and oil removal filter, and the “instrument air” treated with a desiccant dryer and appropriate filtration.
3. Sizing Components
When sizing compressed air treatment, many factors come into play, such as compressed air flow (CFM) capacity, operating pressure, ambient temperature, and inlet temperature. It is often assumed that a 250 CFM compressor requires a 250 CFM dryer, which may not necessarily be the case.
Virtually all compressed air treatment in the United States is rated in SCFM (Cubic Feet per Minute, measured at Standard Temperature, Pressure, and Relative Humidity Conditions) at 100 PSIG inlet pressure, 100°F Inlet Temperature, and 100°F Ambient Temperature. Variations to these factors affect the sizing of the filtration in different ways:
Changes in inlet pressure change the velocity of the air through the component for any given flow rate. For example, 50 SCFM at 80 PSIG is not as compressed as 50 SCFM at 100 PSIG, and flows through the treatment over 20% faster than it would at the treatment’s 100 PSIG rated pressure. This requires sizing components larger to accommodate this. Conversely, higher operating pressure would act oppositely, allowing a smaller component to meet the requirement.
Changes in inlet temperature affect the amount of moisture in the air and the ability of oil aerosols to coalesce. As a rule of thumb, a 20°F increase in temperature doubles the moisture capacity of compressed air, greatly reducing the effectiveness of dryers and filtration. Cooler inlet temperature causes reduced load on dryers and filters, allowing them to be scaled down.
Ambient temperature primarily affects the condensing temperature of a refrigerated dryer, with higher ambient temperature reducing the dryer’s capacity. More importantly, when air-cooled compressors are used, the ambient temperature directly affects the compressor discharge temperature. For example, a rotary screw compressor with an aftercooler will have a discharge air temperature of about 15°F above ambient temperature. If ambient temperature is 100°F, then compressor discharge temperature (and air treatment inlet temperature) is 115°F, thus de-rating the air treatment.
Correction factors for all of these conditions are typically printed on air treatment literature for aid in sizing.
Central air treatment equipment can often be reliably sized using the compressor capacity and correction factors. One potential complication is when there is a large volume of air storage upstream of the air treatment. In this case, an excessive short duration demand may allow high flow rates of air from the storage tank to exceed the treatment capacity. If air demand is expected to occur in large bursts of usage, treatment must be oversized or (preferably), a larger “dry” receiver tank should be installed downstream of the air treatment.
Point of use treatment sizing can be problematic, but may be done through one of these methods:
- Published air usage data for the tool or process being supplied
- Estimated air usage, calculated by a knowledgeable engineer
- On site flow testing using a flowmeter
It is NOT reliable to size air treatment based upon line size. For example, installing a ½” filter on a ½” line may or may not yield the desired results.
4. Size for Energy Efficiency
While air quality is a critical concern to most facilities, energy efficiency is highly important as well. Virtually all styles of air dryers offer some method of matching energy usage to compressed air demand. If a dryer is frequently operated at part load (as most are), then energy savings can easily outweigh the added cost.
Additionally, most components can be oversized to reduce pressure drop with little or no effect on air quality. Sizing a treatment component 50% larger than required, for example, will cut pressure drop approximately in half. Since each 2 PSI of pressure drop that the compressor overcomes requires 1% more power, pressure drop can be a significant cost. Additionally, it may limit the ability to control multiple compressors in an acceptable range.
5. Single or Multiple Air Treatment Trains
Traditionally, groups of multiple compressors would feed a common receiver tank, then feed a single compressed air treatment system with a bypass to enable service of the air treatment while the remainder of the system is in operation.
In recent years, it has become increasingly common for each installed compressor to have its own air treatment train installed. In fact, many rotary screw compressors are now available with refrigerated air dryers built into the compressor package for reduced footprint and increased modularity of the system. This has certain advantages and disadvantages.
On the positive side:
- Separate trains provide redundancy for the air treatment. Operation of the system without treatment is reduced when each air supply train can be shut down in the event of a compressor or treatment malfunction.
- Expansion of the system is made easier, as well. If an existing system has reached the limits of its air treatment system, the addition of a compressor necessitates complete treatment replacement, unless the new compressor is installed with a dedicated air treatment system.
- Floor space is conserved when dryers are packaged into the compressor, reducing the overall size of the compressed air station.
However, dedicated treatment trains have some disadvantages as well:
- Dedicated treatment trains tend to maximize the pressure drop losses of a system. If multiple compressors are installed, the intent is generally for only one compressor to operate part loaded. Any other operating compressors should be operating at full load for best energy efficiency. If a 500 CFM compressor is operating at full load through a 500 CFM rated dryer, it will see the full rated pressure loss of the treatment. In systems with multiple compressors feeding a single dryer, pressure drop is substantially less when the system is operating at less than full dryer capacity.
- When dryers are packaged into compressors, there is little flexibility in sizing for adverse conditions. High temperature and low pressure applications will often provide reduced performance.
- Increased cost. A system of three 500 CFM dryers will cost more to purchase and operate than a typical 1500 CFM dryer.
6. Parallel Treatment Trains
As previously mentioned, the traditional system would consist of an air treatment train with a service bypass, which allows contaminated air to go around the treatment when it is malfunctioning or being serviced. As quality requirements become more and more stringent, it may be advantageous to install two complete treatment trains, in parallel. This allows redundancy in case of malfunction without the need to feed the air system with contaminated air. Additionally, when both trains are operating correctly, they may be operated in parallel, reducing the flow through each to ½ of the designed capacity and reducing pressure drop through the treatment to ¼of the pressure drop of a single train.