Selecting Compressed Air Dryers for Dust Collectors

By William J. Ulrich

Dust collectors, or baghouses, play a vital role in reducing emissions and waste while ensuring compliance with environmental air quality regulations. Clean and dry compressed air plays a key role in the cost-effective and reliable operation of pulse-jet type baghouses.

For environmental and safety compliance, baghouses remove suspended dust from ambient air to increase indoor air quality and reduce outdoor emissions. When used for product recovery, raw materials can be collected and separated for reuse in manufacturing processes. Product recovery can substantially reduce waste and disposal costs. For example, a cement producer may recover and reuse cement dust, or a chemical producer may recycle certain bulk materials.

Compressed Air for Bag Cleaning

Dusty air flows into a dust collector and passes through a series of felt or woven bags which capture the dust. Periodic bursts of compressed air clean the bags at regular intervals by dislodging the particles, which fall to a collection chamber for disposal or recovery.

Moisture in the compressed air lines can have disastrous consequences for baghouse operation. All air contains moisture that becomes concentrated during compression. The moisture then condenses as it cools in plant air lines. Any condensation present in the compressed air system during pulse cleaning contaminates the bags with moisture. Wet bags not only resist air flow, but are prone to tears from the excessive weight. In cold areas, this moisture turns to ice, which blocks air flow and promotes bag rupturing. In addition, water contamination corrodes or freezes solenoid valves. Blocked orifices cause incomplete pulse cleaning and can even shut down the entire baghouse, resulting in costly downtime.

Properly treated compressed air prevents such occurrences. Clean and dry compressed air offers related benefits for baghouse operation, including increased bag life, reduced bag replacement costs, reduced maintenance, and increased product recovery.

Compressed Air Dryer Selection

When choosing a compressed air dryer for protecting a dust collector, determine whether the dryer will serve the baghouse alone or the plant's entire air system. Ease of installation may be a consideration. A point-of-use dryer may be the simplest answer in existing plants, while a newly constructed facility or one undergoing a major air system overhaul may benefit from a large central air drying system.

The next step is selecting from among the three main compressed air dyer types. A single tower deliquescent dryer operates with a low pressure drop, can be installed indoors or outdoors, and requires no electricity for operation, which makes it ideal for hazardous environments.

With no moving parts, it is also the simplest of dryer types. Air flows into the lower area and upward through a bed of deliquescent desiccant. The desiccant absorbs moisture vapor and slowly dissolves. Dry air exits through the top and condensate falls to the claim area inside the dryer vessel to be drained.

Single tower dryers provide a 20°F dew point reduction, achieving a dew point that is 20°F lower than the inlet temperature to the dryer. Desiccant is periodically added and condensate can be drained automatically with a pneumatic or electric drain valve.

Regenerative desiccant dryers use an adsorbent desiccant, such as activated alumina, to remove moisture from the compressed air system. These systems consist of dual towers. One tower dries the compressed air while the other tower regenerates the desiccant in preparation for the next cycle when the process shifts.

Three types of regenerative desiccant dryers are distinguished by the method of regeneration. The heatless type uses a dry purge air stream of approximately 15% of operating capacity to strip desiccant of accumulated moisture. Due to purge air costs, heatless dryers are usually specified for systems less than 2,000 scfm. Heated purge type dryers use about 8% of operating capacity for desiccant regeneration, but require additional electricity for heaters, which aid moisture removal during regeneration. Typical flow ranges for heated purge dryers are 500 to 5,000 scfm. Blower purge dryers use an external heated blower for desiccant regeneration and consume no compressed air. Blower purge dryers are typically specified for high capacity air systems up to 20,000 scfm.

All regenerative desiccant dryers provide a standard -40°F dew point for superior moisture protection and freeze-up prevention. Models are also available to provide a -100°F dew point for critical applications. Most commonly installed indoors, regenerative desiccant dryers, particularly the heatless type, can be installed outdoors if surrounded by a protective enclosure. Because regenerative dryers require electricity, caution must be used when considering one for installation in a hazardous area.

Refrigerated dryers are not typically used for dust collector compressed air lines that are subject to ambient temperatures below 32°F. Refrigerated dryers cannot protect against freeze-ups, as minimum dew points are approximately 35°F, and they must be installed indoors. Also, dusty conditions reduce the efficiency of heat exchangers.

Sizing Dryers for Dust Collectors

Air requirements for dust collectors are usually rated in scfm, which can be misleading. Before sizing the dryer, this rating must be converted to an instantaneous flow, which accurately represents the greater air use during pulse cleaning when the solenoid valves open for approximately 1/10 second or less. Without sizing the dryer to allow for these short, high volume air bursts, the dryer can be easily overloaded.

Instantaneous flow = rated dust collector flow/pulse period in seconds x 60 seconds per minute.

Example: 100 scf used in 10 seconds

100scf / 10 seconds x 60 seconds / minute = 600 scf / minute = 600 scfm

A dryer rated for 600 scfm (100 scfm) should be specified.

In general, a single tower or heatless regenerative dryer may be used to treat compressed air for the a dust collector. For protection of the entire system, a heated regenerative dryer can be sized large enough to protect the entire facility, including the dust collector and all other air-operated equipment and processes.

Additional equipment to be considered for proper air system operation include filters for lubricated compressor systems. Filters extend the life of desiccant beds and increase dryer efficiency. A surge tank can be placed downstream of the dryer and before the dust collector. This prevents possible overloading of the dryer when the dust collector is in operation by providing back-up air capacity.

A qualified compressed air treatment distributor or manufacturer can assist in selecting a dryer for dust collectors based on the plant layout and conditions. Once in place, air drying equipment will protect the baghouse and prolong its reliable and cost-effective operation.