Steam Filtration for Process Cooling Applications
Stainless steel filter elements provide durable construction that avoids carbon shedding.
Steam is essential in the production of many consumables, including dairy products, processed foods, beverages, chemicals and pharmaceuticals. Similar to compressed air, steam often is considered a utility or energy source — generated at a central location and then distributed to various points of use throughout the facility.
While bacteria are not generally present in steam due to its high temperatures, filtration on steam lines is necessary to remove dirt, rust and scale that may leach into or originate in system piping. These contaminants are even more prevalent when steam is condensed, recirculated and reused in the process. Filtration helps ensure a consistently high quality of steam for both producers and consumers.
Differences in Steam Filter Elements
The composition of steam filters makes a difference in their performance and service life as well as in the purity of the final product downstream. Many applications put considerable stress on filter elements, and technologies must be considered that match those conditions.
The oldest and most common steam filters are activated carbon tubes. This filter media has been in use with variations since the 1950s; in fact, carbon filters are still referenced in some regulations dating back to that era. Generally, carbon filters can be effective in temperatures up to 500°F (260°C) and pressures to 400 psig. Carbon has its drawbacks, however, including potential media shedding during use. With a texture similar to sandpaper, some granulated carbon tubes can produce a visible residue upon handling (figure 1).
Alternatives to carbon tubes are available in the form of stainless steel steam filter elements. High grade stainless steel tolerates high pressure and is rated for use up to 700°F (371°C). These characteristics make stainless steel steam filters suitable — and even required — for certain applications.
FIGURE 1. Granulated carbon elements can sometimes shed particles during use.
Stainless steel filters are appropriate if your process has these specific goals or concerns.
- The process must meet culinary-grade steam standards.
- The process is regulated by the Pasteurized Milk Ordinance (PMO).
- The process has risks from water hammer.
Culinary-Grade Steam Requirements. There are two grades of steam: process steam and culinary-grade steam. In processes where steam is injected into food products or used to clean and sterilize food processing equipment, steam is required to be culinary grade.
3-A, the premier standards body for food processing, defines culinary grade as steam that is filtered to remove 95 percent of particulates 2 microns and larger. It must be “free of entrained contaminants, relatively free of water in liquid form and suitable for use in direct contact with food products or product contact surfaces.” This is why stainless steel is the required media under 3-A. Steel does not release fibers, resists the migration of filter media and is fabricated without binders, adhesive, additives or surface-acting agents that can leach into the process.
Typically, two sets of steam filters are recommended on a culinary-grade steam line:
- Prefiltration, to remove particulates 10 microns or larger.
- Point-of-use filtration, to remove 95 percent of particulates 2 microns and larger.
Stainless steel filters on the market differ widely in quality. Look for filters that provide retention efficiencies greater than 95 percent at 2 microns and meet food contact requirements in the federal code of regulations (CFR) Title 21.
Pasteurized Milk Ordinance Regulation. Dairy processors have another regulation to meet: the Pasteurized Milk Ordinance (PMO). Drafted in the 1950s, this regulation requires steam in dairy processes to be filtered. In fact, the pertinent section prescribes a specific brand of carbon steam filter but adds the qualifier “or equivalent.”
This latitude to use an equivalent filter solves a dilemma for processors worried about carbon shedding. One PMO-compliant alternative uses existing stainless steel technology but is sized to drop into most legacy carbon-tube housings installed in plants governed by the PMO (figure 2).
The stainless-steel alternative was developed following a water hammer event in a dairy production facility using carbon-tube elements. The carbon-tube elements were damage during the event, and carbon particles were found downstream in the process and finished products. The cleanup costs to reverse the contamination were considerable, and the processors urgently sought alternatives. It was this series of incidents that prompted the development of PMO-compliant alternative filters.
Because filter housings are connected to the larger steam system, the compatibility of these elements allows facilities to upgrade to steel elements without remodeling. This effectively reconciles the dairy regulation with more current industry processing standards such as 3-A. With stainless steel elements, production facilities can exceed 3-A guidelines for culinary steam while still meeting the filtration requirements specified by the PMO.
FIGURE 2. Identical in size to legacy carbon filters, stainless steel filters (right) can be dropped into the same housings.
Water Hammer Risks. Even if your process does not require culinary-grade steam, there is another reason to consider stainless steel construction: water hammer. This destructive phenomenon is a common problem in steam system operation. Water hammer occurs when a cooled steam system is reheated too quickly, rapidly propelling condensate through the piping. The rapid water movement shocks, or “hammers,” the entire system — and all valves, joints and filters along the way. Other causes of water hammer include sudden pressure or velocity changes.
Because water hammer can damage equipment, prevention and safety measures are essential. Carbon-tube elements are vulnerable to damage in water hammer conditions. Stainless steel filters are better able to resist cracking, chipping or shattering.
Water hammer is a common risk due to the volatile nature of steam and the size of many systems. At many facilities, consideration should be given to selecting filter elements constructed to withstand the shock and help protect system integrity.
Potential Benefit: Reducing Total Filtration Costs
A final reason to consider upgrading from carbon to stainless steel steam filters is economics — both in energy requirements and filter life.
The more restrictive an element is, the more energy it takes to push steam through the media. This degree of restriction is measured as pressure drop or differential pressure (DP). Higher filtration differential pressure drives higher energy costs.
For example, on a compressed air line, on just one filter designed for airflow of 1,000 cfm, every additional pound per square inch of pressure to overcome restrictions adds energy costs of more than $1,000 per year (based on an estimate of 8,000 hours per year at $0.10 per kilowatt hour). While steam lines vary, they also incur increased energy costs to overcome restrictive filter media.
It is easy to assume that all-metal filter media would be denser and more restrictive than carbon tubes. Filtration engineers have developed a pleated construction for fine layers of stainless steel, however. The result is an effective media that delivers a precise cut-off range. The pleated design expands the filtration surface area, which can greatly improve airflow. The alternative filters mentioned previously, for example, reduce differential pressure over comparable carbon-tube filters, based on laboratory testing. This helps support efficiency in energy costs (figure 3).
FIGURE 3. One design of stainless steel filters for steam production have been shown to reduce differential pressure in laboratory testing.
A larger filtration surface area also can reduce wear on the filter and lengthen filter life — another advantage of stainless steel. Unlike carbon filter elements, steel can be ultrasonically cleaned, which is highly effective at removing contaminate buildup. Ultrasonic cleaning is an abrasive process, but one stainless steel filter element design can be ultrasonically cleaned up to six times before replacement. In addition to savings in replacement filters, the benefits are reduced maintenance and downtime costs.
In conclusion, the design and maintenance of filtration systems is important. The robust nature of stainless steel filters make them suited for most high pressure, high temperature steam processes.
When purchasing any filter, review the materials and the data offered by filter manufacturers. Consider return on investment versus initial capital costs only. It can be helpful to obtain a comprehensive evaluation of your system by a qualified process filtration consultant. Work with suppliers who are knowledgeable of filter operation and maintenance needs from end-to-end and whose equipment is validated to meet objective standards. PC