Keeping process cooling fluids clean is not as easy as it might seem at first glance. An optimum solution must take into account a number of factors, including:
- The fluid source.
- The source, volume and nature of contaminants present in the fluids.
- The heat exchanger materials and technology being used.
- Pressures and temperatures within the process environment.
- The amount, if any, of acceptable downtime for filter maintenance.
- The initial capital and long-term operating cost of the filtration solution.
Even the most cursory examination of that list should make it clear that no prepackaged, one-size-fits-all solution is likely to be successful. In reality, selecting a filtration strategy for process cooling fluids can be every bit as challenging as developing a filtration solution for the process products themselves.
A successful solution is likely to include complementary technologies selected based on specific process conditions. For example, a system using raw river or ocean water as a source might well include:
- Intake bars to keep out large floating debris.
- A screen with 0.375" openings.
- A strainer to trap anything larger than 0.03125" in diameter.
- A filter to remove any particles larger than 25 microns.
The water source also will influence the type of media used in the strainer or filter. Sand, for example, does not change size or shape, so it is trapped easily in a porous filter medium. By contrast, pond scum presents an entirely different challenge that is best handled with mechanically cleanable media. Other organic contaminants, which typically are present in quantity in surface water after rainstorms, present their own challenges that have to be quantified and accommodated based upon specific local conditions.
Once in the process system, the water may be further treated to inhibit corrosion in carbon-steel piping and heat exchangers. Or, it may be filtered again to remove even smaller particulate as necessary. At that point, the process cooling water represents a significant investment, and it will likely be recycled through a secondary heat exchanger and recirculated. That, of course, means it also must be re-filtered to remove any scale or corrosion particles picked up from pipes and heat exchangers.
Strainers and Filters for Industrial Cooling Fluids
Strainers and filters perform essentially the same function, but filters can remove particles of much smaller size. The general rule of thumb is: “If you can see it, you can strain it.” In practical terms, this means that particles down to range of 0.003 to 0.004" (0.07 to 0.1 mm) — or in other terms, 75 to 100 microns, or 200 to 150 mesh — can be removed effectively with a simple strainer. For anything smaller, a filter, which can remove particles as small as a few microns, will be necessary.
Regardless of whether a strainer or a filter is in place, the buildup of particles trapped in the media must be removed periodically. For a basic basket-type strainer, this can be as simple as shutting off the flow, manually removing the basket and dumping the trapped debris. Simple filters also may use a filter bag constructed of woven polyester material that is maintained in essentially the same way.
Obviously, stopping the flow and manually cleaning the filter element means that the process is either shut down during the maintenance period or it is unprotected while the filter or strainer is bypassed. Neither situation is optimal. The solution is a self-cleaning filter, which usually is available in two basic designs.
An automatic self-cleaning filter uses a rotating hollow internal arm to collect debris deposited on the filter media. As particles are trapped on the media and build up, the pressure drop through the filter increases until it reaches a predetermined value. At that point, a valve is opened, allowing fluid to exit the filter through the rotating arm. It carries the accumulated debris with it.
This type of filter typically removes particles larger than 50 microns and can handle debris loads up to about 200 ppm. Automatic filters are best used in high-volume situations where fluid losses up to 5 percent of total flow during cleaning are acceptable.
A mechanical self-cleaning filter uses a mechanical disc to scrape accumulated debris off the filter media. Again, as trapped debris increases the pressure drop across the filter, the scraper is actuated at a predetermined value. The debris is deposited at the bottom of the filter housing, where it can be removed without interrupting the flow through the filter. The cleaning action of the mechanical filter makes it suitable for removing particles less than 10 microns in diameter. These filters are able to handle higher debris loads and more frequent purge cycles than automatic filters. They also consume a much smaller volume of fluid during cleaning than an automatic filter.
Note that either a strainer or a filter creates a pressure drop and also a flow restriction. Both factors must be accounted for when designing the system. Adding either as an afterthought may require up-sizing pumps to maintain adequate flow volume and pressure.
It should be clear by now that choosing a process cooling filtration solution is a complex process. Perhaps an example of a successful application will help demonstrate why the effort involved is well worthwhile.
Case in Point: A Pharmaceutical Producer
Process chilling is a critical element in the production of biological buffers, which are a key element used by chemical companies in the production of common household drugs such as aspirin as well as many prescription medications. One of the country’s largest producers of biological buffers determined it was time to replace the old, inefficient chillers the company had been using for decades. The chillers were becoming too difficult to repair and susceptible to frequent breakdowns.
To improve efficiency and mitigate potential maintenance costs, the company installed process-critical chillers with plate-and-frame heat exchangers. The new chillers were more efficient, but their design led to contamination and clogging in hard-to-clean areas of the plates. This clogging caused the equipment to shut down and required weekly cleaning by an outside contractor — an inefficient and messy process. The entire system was shut down during each cleaning, resulting in four to five hours of unplanned downtime each week. During high production cycles, additional contaminants were introduced into the process cooling water, resulting in emergency calls for additional unscheduled cleanings.
To address these concerns, plant officials sought a self-cleaning filtration solution. Process goals included effectively filtering contaminants and preventing the chillers from clogging and interrupting the process.
The customer’s engineer explained that they had expected the switch to the new chillers to generate some contaminants but not at the level they were experiencing. Moreover, the clogging was occurring in areas that were difficult to clean, and the unexpected downtime required for manual cleaning was costly. What they wanted was a solution that continuously cleaned itself so they did not have to “babysit” the operation.
After reviewing the customer’s operation, needs and goals, the filtration system manufacturer recommended a filter that offered a higher level of filtration (30 micron) than the previous solution (60 micron). Its design and small footprint permitted installation in a tight space on the second floor of the production facility.
Designed to handle fresh water from treated sources, the filtration system’s magnetically coupled actuation eliminated the need for dynamic seals. The technology eased access for maintenance, reduced potential leaks and required few moving parts while providing good service life. The filters were installed upstream of the new chillers to filter 100 percent of the condenser water supplied to the process chillers. In effect, the contaminants were filtered out of the water source before it reached the process.
The filtration system was installed in late August 2015, and the pharmaceutical company reports excellent results so far. The system has eliminated both the need for regular weekly cleaning and the associated cost and downtime. In addition, the unscheduled downtime for emergency cleaning that had adversely impacted productivity has been eliminated. The heat exchanger stays clean and runs efficiently without regular attention.
With the filtration system in place, the biological buffer production process is flowing without interruption. That has eliminated headaches for plant operators. Investing the time and effort to choose an appropriate technology solution pays big dividends in the long run. PC