How Membranes Can Help Dry Compressed Air

How Membranes Can Help Dry Compressed Air

Non-electric technology for drying compressed air in electrical cabinets eliminates sparking risk in hazardous environments.

By David J. Connaughton, Product Manager, Parker Hannifin Corp., Filtration and Separation Division

The air in hazardous industrial environments poses two challenges. First, it needs to be instrument-grade quality, meaning it must be clean and dry to ensure effective operation of the facility's pneumatically operated safety equipment. Without clean, dry air, quality-control and process-control systems might not function properly, resulting in costly downtime.

Second, the means for achieving this clean, dry air must be without any risk of sparking, because such locations may have high concentrations of flammable gases, vapors, combustible dusts, or ignitable fibers and flyings. Refineries, chemical processing plants, mines and grain mills are examples of locations with the potential for hazardous atmospheres. Just a small spark can lead to a horrific explosion.

While industrial plants typically have centralized instrument-grade air lines that deliver air to the instruments, the air often is contaminated with water and will benefit from a point-of-use drying system that guarantees clean, dry air. Traditional drying systems using pressure swing adsorption (PSA) or refrigerants require often-expensive modifications to operate within hazardous areas.

Alternatives to such systems are air dryers made from hollow fiber membranes. This technology can dry compressed air without using electricity, making it safe for hazardous environments.

Types of Hazardous Areas

According to the National Electric Code (NEC®), three types of hazardous areas exist.

Class I areas have flammable gases or vapors at concentrations considered potentially explosive or ignitable. Examples of Class I locations include:

• Refineries.

• Gas storage and dispensing areas.

• Dry cleaning plants.

• Spray finishing areas.

• Aircraft hangars and fuel servicing areas.

• Utility gas plants.

• Sites that store or handle LPG or natural gas.

Class II areas have combustible dusts. When suspended in air, these fine dusts, because of their large surface area, can cause explosions as strong as those that might occur at refineries. Examples of Class II locations are:

• Flour mills.

• Feed mills.

• Grain elevators.

• Plastic manufacturers.

• Starch and candy producers.

• Fireworks factories.

• Spice, sugar and cocoa factories.

• Coal and other carbon handling sites.

Class III areas have combustible fibers or flyings. These fibers collect around equipment or light fixtures where they can be exposed to heat, hot metal or a spark, which may cause a fire. However, they aren't likely to be explosive. Examples of Class III sites include:

• Textile and cotton mills.

• Wood processing facilities.

• Processing sites that generate wood or combustible fibers.

• Coal mines.

Hazardous Air Conditions and Groups

The NEC also specifies the type of condition under which hazards are present. Those conditions are Normal, or Division 1, and Abnormal, or Division 2.

Normal hazards include the flammable gases, vapors, combustible dusts, or ignitable fibers and flyings commonly found in plants. Abnormal conditions refer to failures such as leaks or activation of emergency pressure relief valves.

Further classification addresses the nature of the hazardous substances. These “Groups” are a function of the ignition temperature and explosive pressure.

• Group A contains acetylene, which has extremely high explosive pressure, and is the only material in this group.

• Group B contains hydrogen and a few other materials.

• Group C contains ether.

• Group D contains most hydrocarbons, fuels and solvents.

• Group E contains metal dusts.

• Group F contains carbon, coal and other dusts.

• Group G contain flours, starches, grains and other explosive dusts.

Equipment Considerations

Equipment located in hazardous areas must be designed specifically to prevent ignition and explosion. Electrical enclosures found on the three types of traditional compressed air drying equipment (see below) must be strong enough to contain an explosion within the cabinet. Therefore, the walls must be thick and heavy. Sizes range from 3 in. x 3 in. x 3 in. to 24 in. to 36 in. x 10 in., according to the National Electrical Manufacturers Association (NEMA).

The internals in the cabinet must operate at temperatures below the ignition temperature of the hazardous material. The cabinet also must be designed so that any ignition inside wouldn't immediately exit the cabinet; in other words, the ignited gases must be quenched so that the escaping gases don't cause an explosion outside the cabinet.

The added weight and heavy-duty designs of such equipment increases the cost and space requirements required for the three types of traditional compressed air drying equipment, which are:

1. Refrigerated dryers work by cooling the air to low temperatures and condensing much of the water vapor. It's not possible to achieve dew points below freezing with a refrigerated dryer. Optimally designed refrigerated dryers can produce air with dew points to about 36°F (2°C). Because some water vapor is left in the air, these dryers shouldn't be used in water-sensitive applications.

2. Chemical dryers use a process of passing the compressed air over beds of chemicals, typically calcium chloride and lithium chloride, which attract the water vapor. The chemicals become saturated with water vapor and are discarded. The lowest dew point achievable with -this type of dryer is 27°F (15°C). Installation of a high efficiency, coalescing filter upstream from the chemical dryer is essential, because the life of the chemicals is significantly reduced if liquid water enters the dryer. A particle removal filter is needed downstream to prevent carryover of the chemical particles.

3. Desiccant dryers pass the compressed air over a bed of desiccant material, which adsorbs water vapor molecules. When the bed capacity is nearly saturated, the airflow is switched to a second bed of desiccant material. The first bed is then regenerated. Timers or dew point monitoring equipment can be used to control the regeneration phase. Desiccant dryers can deliver air at consistently low dew points, typically -40°F/°C or less. This technology is a good choice when the compressed air is subject to freezing conditions.

The two types of desiccant dryers include heated and heatless. Heated desiccant dryers use heat to remove water vapor from the desiccant material not in use at that point in the cycle. These dryers need large amounts of steam or electricity to operate. Heatless desiccant dryers use the dry air generated by the dryer to remove water vapor from the desiccant material. The advantage of this technology is the reduced dependence on excessive outside services for heat, such as steam, electricity or gas. A regenerative desiccant dryer can be located near the point-of-use to deliver dry compressed air at dew points to -100°F (-73°C). Heatless desiccant dryers are ideal for delivering instrument quality air for critical applications.

Membrane Technology

Membrane air dryers are space-saving, cost-effective and safe alternatives to traditional compressed air-drying equipment. They're designed for instrument-quality air, air exposed to freezing temperatures, and water-sensitive applications that require flow rates ranging up to 100 scfm (170 Nm3/hr). Typically, compressed air with a dew point of -40°F (-40°C) is reasonable for water vapor-sensitive applications.

Prior to entering the membrane-drying module, compressed air passes through a high-efficiency coalescing filter to remove oil and water droplets and particulate contamination. No oxygen, nitrogen or any other component of air permeates the membrane. [Click to Enlarge]

To free up floor space, the membrane air dryers can be wall-mounted. They don't have any refrigerants or freons, thus making them environmentally safe. Also, because they don't use electricity, they don't pose any sparking threats. Finally, the membrane air-drying technology is safe for Class I, Class II or Class III hazardous locations.

Membrane Air-Drying Process

The air-drying operation employed by membrane air dryers is straightforward. Prior to entering the membrane-drying module, compressed air passes through a high-efficiency coalescing filter to remove oil and water droplets and particulate contamination with an efficiency of 99.99%. The liquids are removed by the filter cartridge and continuously drain from the filter cartridge to the bottom of the housing, where they're automatically emptied by the auto-drain assembly (see Figure 1 and Figure 2).

Therefore, the air leaving the pre-filter is laden only with water vapor, which is then removed by the membrane module. This module contains bundles of hollow fiber membranes that permeate water vapor only through the wall of the membrane. No oxygen, nitrogen or any other component of air permeates the membrane. Water on the outside surface of the membrane is evaporated in a vapor state by a sweep of low-pressure dry air (see Figure 3, Figure 4 and Figure 5).

The driving force that pushes the water vapor through the wall of the membrane is the difference in partial pressure of water inside the hollow fiber (high) to the partial pressure on the outside of the fiber wall (low). This is why drying of the compressed air is accomplished without using electricity.

Membrane air dryers are also exempt from ATEX certification because they don't have enough energy to cause an ignition and the air movement is unlikely to produce static electric charges.

Ongoing Challenge

Drying compressed air at point-of-use is a continuous challenge. For industries with hazardous areas, selecting the safest, smallest and most cost-effective dryer is critical.

Parker Hannifin Corporation Filtration and Separation Division, based in Haverhill, Mass, is a participating EncompassTM Product Partner in the Rockwell Automation PartnerNetwork™. The company supplies filtration, separation and gas generation products, including cabinet dryers, nitrogen generators and air filters.

The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.

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