Processed Air Ensures Food Quality

The accepted practice for supplying air under pressure for contact with milk, milk products, and product contact surfaces involves the use of a coalescing filter.1 The practice requires that the final filter efficiency, as measured by the dioctyl phthalate fog method, be >99%; when commercially sterile air is required, the final filter efficiency should be >99.999%.

In addition to the accepted practices, a number of other standards have been developed regarding the level of allowable contaminants in compressed air, including DIN ISO 8573-1, which defines various classes for solid matter, water content, and oil content.

Using a Coalescing Filter

A coalescing compressed air filter provides a simple, powerful, and reliable method of removing moisture, particulate matter, and oils from compressed air with a minimum of maintenance. The filter consists of a matrix of borosilicate glass hollow fibers in a fluorocarbon resin binder and is resistant to water, hydrocarbon lubricants, and synthetic lubricants. The filter(s) are housed in an assembly in which the liquids are continuously trapped and are directed to a drain via a passive design so that the filter can continue to remove liquids for an unlimited time without loss of efficiency or flow capacity. The filter system is easy to use on a continuous basis, requires only an annual filter element change, and is extremely reliable because it contains no moving parts. A coalescing air filter is typically employed as part of a compressor installation .

A broad range of coalescing filters is available (Parker Hannifin Corporation; Haverhill, Mass.). The filters are designed to withstand a maximum pressure of 250 psi and cover the flow range from one standard cubic foot per minute (SCFM) to 65,000 SCFM with an initial pressure drop of <2 psi. These filters can be used over a temperature range of -150ºF to 300ºF (-100ºC to 149ºC).

Several grades of coalescing filters are available, beginning with a single stage DX filter, which is able to remove >93% of 0.01 µm particles and droplets and is commonly used for general purpose applications such as purifying plant compressed air. For instrument grade air, two-stage filtration is used (a DX filter is followed by a BX filter, which can remove 99.99 % of the 0.01 µm particles and droplets); a three-stage filter system (a DX filter followed by a BX filter and a CI filter) is normally used to remove trace compressor oil vapor. The filter(s) are self-gasketing, which negates the need for end caps and means element replacement is less expensive. The filter system requires minimal maintenance and is extremely reliable and quiet in operation.

Coalescing filters are often used in conjunction with other devices, like chillers, to reduce the workload of the filter. In many facilities, for example, a DX filter is placed upstream before a refrigerated dryer, desiccant dryer, or membrane dryer to prevent most of the oil and water from entering the dryer.

Benefits of Coalescing Filter

A coalescing compressed air filter purification system provides a number of significant benefits:

• A coalescing compressed air filter does not require any external power. Many techniques used to dry compressed air require considerable power, increasing the cost of the process. When a chiller or refrigerated dryer is employed, for example, the compressed air is cooled to condense the water vapor and then brought back to the desired temperature. A refrigerated air chiller can be quite large, requires routine maintenance on a periodic basis, and uses valuable floor space. In addition, because a fan is used to cool the refrigerant, refrigerated air chillers may be quite noisy. Refrigerator-based dryers cannot achieve dew points below freezing; the optimum level that can be attained is approximately 36ºF (2ºC). It should be noted that the dew point may be difficult to control without sophisticated automatic control systems.
• The use of the coalescing compressed air filter is a continuous process and requires a minimum amount of maintenance. Once a coalescing compressed air filter is installed with an automatic trap valve, the operator won’t need to perform any maintenance activities—other than an annual change of the filter element—or constantly monitor the performance. When a desiccant dryer is used, however, timed valves are needed to switch from one desiccant vessel to another, and heat is required to drive the adsorbed water during the recycling process. If heat is used to regenerate the desiccant, a considerable amount of energy is expended.
• A coalescing compressed air filter system is small and does not require any floor space in the facility. A variety of filter housings are available, and they can be line mounted as part of the overall piping system, saving considerable space in comparison to a refrigerator or chiller.
• A coalescing compressed air filter eliminates the need for the use of refrigerants. Many refrigerants are environmental hazards and are not compatible with food processing facilities. In addition, the fluid level of the refrigerant must be checked from time to time.
• A coalescing compressed air filter operates silently. Refrigerated dryers have been used at point-of-use applications; however, their physical size, noise level, and electrical requirements often contribute to equipment operator complaints.

Sterile Air Filters

Sterile air (SA) filters, used for direct contact with food and food surfaces, are designed to remove all viable organisms from the air at 99.9999+% efficiency for 0.01 µm particles. These filters are at least 30 times better than the accepted standard for SA filters. A recent study by D.A. Evans, PhD, at the University of Massachusetts, indicated that a Balston SA filter produced commercially sterile air, and, to the limits of detection, no viable colonies of microorganisms were found.