Few perfect things are perfect in this world — though a baby’s smile, beachfront sunsets and a pitcher who retires 27 consecutive batters come to mind. Unfortunately, for those who operate industrial facilities, hot-water heating, process loops or closed-loop cooling-water recirculation systems are not among them.
Car radiators may be the closest thing to the perfect closed loop. Yet, even they can have challenges: glycol leaks, various metallurgies, air ingress from the expansion tank, temperature excursions (hot to cold, cold to hot) and debris in the system. All of these conditions can lead to severe issues in the system and require corrective actions. This is why car manufacturers recommend glycol changeout after 60,000 miles and every 30,000 miles thereafter.
Such a swap cannot be done in a facility’s closed loop, however. That is one reason why industrial closed-loop systems must be chemically treated to prevent scale, corrosion and biological growth. A component of effective treatment is the ability to add corrosion inhibitors to the system. Integral filtration also plays an essential role.
Focusing generally on closed-loop systems with capacities between 100 and 10,000 gallons, this article will identify traditional and modern challenges facing operators of industrial facilities. It also will describe how implementing filter feeders can improve system operation.
Closed loops are becoming a more important part of facility operations because they can enhance heat transfer and conserve water. Ensuring the efficient operation of a closed-loop liquid-recirculation system requires identifying and overcoming challenges, both old and new.
Some challenges are as old as the development of the closed-loop system itself:
- Ensure that the heat exchange properties of the system are maintained.
- Make certain that the fluid used to facilitate heat transfer reacts well with the system’s metallurgy to prevent corrosion.
- Ensure contaminants in the fluid do not precipitate, or lay out, on the heat transfer surfaces. Such a layer will reduce heat transfer.
- Ensure that particulates that can damage system internals, or foul or plug the equipment, are kept out of the system.
- Keep biological growth under control to prevent fouling and corrosion.
Enhanced tubes from a heat-pump system (left) and a cross-sectioned plate-and-frame heat exchanger (right) are two heat exchanger innovations that allow more rapid heat exchange. They also change the flow dynamics of the close-loop system.
Another challenge is today’s energy-efficient equipment such as plate-and-frame or other heat exchanger designs with small flow orifices or enhanced tubes. These design changes allow more rapid heat exchange, but they also change the flow dynamics of the close-loop system. In the past, mostly shell-and-tube heat exchangers were used. Using modern heat exchanger designs, closed-loop system designers engineers seek to exchange heat very rapidly, and the overall system footprint is smaller.
Shown here is an enhanced tube from an aluminum boiler system.
It does not take much to clog up these smaller flow orifices, however. The system’s main piping may not get clogged, but the heat exchanger surfaces can get blocked with dirt and debris. Any particles that collect on the heat exchanger also will attract other particles to the heat exchanger surfaces. This, in turn, kills efficiency and can increase corrosion due to underdeposit corrosion mechanisms. All of these mechanisms can increase in the amount of solids in the system.
At the same time, the metallurgies used for process equipment are changing. Aluminum, for instance, is a material used because it promotes rapid heat exchange. This also affects the overall system dynamics.
What Is a Closed Recirculating Loop and What Are its Issues?
The basic design of all closed-loop systems is shown in figure 1. Modern influences, however, mean these systems are getting more complicated.
A true closed loop is defined as one that loses less than 1 percent of the fluid that it holds in a month. (For example, with a 500-gal system, that is 5 gallons. For a 10,000-gal system, it is 100 gallons.) Anything else would be considered an open-recirculating loop.
FIGURE 1. The basic design of all closed-loop systems is shown.
Many systems in today’s facilities, however, can experience fluid loss of up to 5 percent per month — or greater. Thus, it is important to know how much water the system is making up per month. Having a water meter to measure the system’s water loss is important.
What is so important about water loss in a closed-loop system? New supply (makeup) water brings in dissolved minerals. It also can introduce oxygen and sometimes even suspended solids and bacteria. Even if the makeup water is municipal drinking water, it can contain up to 500 cfu/ml of heterotrophic bacteria. (It cannot contain any coliforms.) Suspended solids such as aluminum sulfate, aluminum silicate and iron oxides can be in the water as well. Suspended solids prevalent in drinking water systems can produce color and turbidity issues.
Where does the water loss come from? It can be a result of leaky pump seals; a drip leak somewhere in the plumbing or distribution system; a small leak at a fitting or valve; or even a dripping relief valve. Other potential sources are purposeful leaks or process blowdown, which is used to limit other issues in the system. In open-top or vented expansion tanks, this loss also can be due to evaporation.
All of these losses require replenishment of the lost liquid with new water or fluid. As noted, fresh makeup water brings with it new oxygen, which means increased system corrosion. If you have a closed-loop system, you may think that once you put water in it, there will be a little bit of corrosion, and then it will stop. But, it does not work that way. The corrosion process forms solids and deposits in the system. These deposits can increase corrosion through other mechanisms.
An expansion tank also can be a source of air or oxygen ingress. Most modern systems utilize bladder tanks that reduce the air ingress. At the same time, these tanks leak small amounts of air into the closed loop during the expansion and contraction process. (The effect is similar to how a tire loses air over time.) Air-blanket tanks and closed loops with sumps that are open to the atmosphere also are utilized. Thus, an expansion tank can be a source of extra air ingress, allowing air into the system and leading to corrosion.
Today, various metallurgies also are quite common. In the past, most systems just had copper, brass and iron. Modern systems can include iron, copper, brass, aluminum, stainless steel, nickel, galvanized steel and other metallurgies from plated parts or specialty coatings. Most of these new metallurgies are associated with changing heat transfer demands and are used in the name of energy efficiency. Engineers have been developing new energy-efficient heat exchangers using such materials for years. Unfortunately, design engineers who think “water is just water” do not understand the properties of water — and how minor changes can alter corrosion rates.
These changes in metallurgy mean different corrosion rates. They also may mean that different water/fluid conditions must be maintained to limit the corrosion on the specific metallurgies. Sometimes, these water- or fluid-condition limits are much restrictive for a specific metallurgy. Some may even conflict with the other metallurgies in the system. Thus, having a system move outside the limits for one metallurgy can easily lead to corrosion on another system metallurgy. For example, copper-piping corrosion can lead to severe corrosion on an aluminum heat exchanger.
Another issue with closed loops — and a normal part of the construction phase of any project — is precleaning the new system. Corrosion and fouling protection starts with the system precleaning. Dirt and debris, mill scale, pipe-cutting oils, pipe-flux agents and other contaminants are all introduced during system construction. Flash-rusting agents — utilized to protect the piping and systems after manufacture — also are part of the new system before water is ever introduced.
Likewise, corrosion can occur during hydrotesting, a process where the system is filled with water to verify that pipe connections and joints are watertight. When a leak is found, the system is drained and sometimes not immediately filled again with water. Thus, flash rusting or corrosion can occur during these drained periods. Not all specifications call for hydrotesting inhibitors or system precleaning before operation. Sometimes, because of time constraints, the mechanical contractor may leave this process until the end and just flush the system with water until it runs clear. Yet, as has been shown, leaving these materials in a system can be detrimental to the overall system reliability.
Filter feeders can be used to introduce solid or liquid chemicals into hot- or cold-water closed-loop recirculating systems while also filtering out contaminants that may foul the system. Depending on the needs of the application, they can be designed to filter particulate as small as 1 micron in size.
Mitigating Corrosion Risks
Taking precautions can help mitigate the issues that lead to system corrosion.
All new or refurbished closed loops need to be hydrotested first with a vapor-phase corrosion inhibitor, so the systems are protected during the draining and filling phase of the hydrotesting. The owner of the equipment or system and the engineers should verify that this step is included in the new construction specifications.
Hydrotesting is not performed just once — as one might normally expect. Hydrotesting can be performed several times, and the more times a system is drained and refilled, the more problems that can occur in the future. The effect of the fill/drain/refill process on the system is similar to constantly dipping a nail into a jar of water and then removing it. Corrosion will occur on the system metallurgy. If all of the water is not removed from the piping or system, corrosion can be even more severe within the high humidity environment of the enclosed piping.
New and refurbished systems also need to be precleaned properly. In all-new systems, precleaning can utilize normal surfactants and dispersants. In a refurbished system, however, you may need to include more aggressive cleaners to remove old deposits and foulants. This may be especially true if existing equipment is retrofit into the new system. In all parts of this process, filtration should be included. And of course, during the cleaning process, water testing needs to be done throughout the various steps: before cleaning, during cleaning, during flushing and after the post-passivation steps.
The testing should calculate many different factors, including water hardness, alkalinity, chlorides, silica, oils and grease, TOC, sulfates, pH, conductivity and iron, copper and aluminum levels. Most new energy-efficient equipment providers have limits on suspended solids and the size of the particles, so a particle-size distribution analysis should be performed on the final flush water or system water. The company performing the cleaning and testing should issue a report discussing the protocol utilized, the testing results and any oddities observed that may influence the system later. The report also should include any recommendations for future system operation. This report provides verification and certification that the specifications have been followed.
To protect the system during normal operation, corrosion inhibitors should be added. (Remember, corrosion can never be totally stopped, but effective inhibitors can slow its rate notably.) The corrosion inhibitors must meet the needs of each of the metallurgies involved in the system. Most water-treatment providers combine various corrosion inhibitors to protect various metallurgies in their formulations. Verify that the formulations do meet the requirements of your specific system as well as the various heat exchangers and metallurgies included within it. The water treater should be provided with all equipment submittals to verify that the treatment consists of the proper material for the system.
Most of the blended formulations also include polymeric dispersants to try to keep the suspended solids in suspension. Filtration can help to remove any solids left or formed in the system during operation. Traditional methods of removing contaminants include screen strainers and dirt/air separators. Such screens are quite large: approximately 60 mesh, which is equivalent to 250 microns. With some new energy-efficient equipment, filtration down to 5 microns or lower may be needed. One option is to utilize equipment that combines filtration to capture contaminants and particulates with the chemical-feeding capabilities to help protect against corrosion in the system.
It is important to specify filtration on closed-loop systems. If the system is dirty, you will decrease the efficiency of the heat exchanger equipment, which results in increased electricity and natural gas costs.
Many operators may incorporate separate cartridge filters and bypass feeders into the closed-loop recirculation system. Others combine the systems with a tie-in bag. With cartridge filters, the fluid goes through the system from the outside to the inside. Such a system effectively filters the system, but it cannot totally flush out the dirt and debris from the filter. Debris that drops on the outside of the filter will fall to the bottom of the feeder. Thus, when filtration is restarted, the materials move back onto the filter if it is installed properly. (If the filter is not installed correctly, the material goes back into the system.)
An alternative to these combined units is a filter feeder with a filter bag to trap any contaminants. The filter bag can be removed from the system and disposed of. For instance, a 5-micron bag filter can capture impurities that are smaller than the width of a human hair. Using a filter bag in a filter feeder helps ensure that all of the solid particles that have been captured are removed and not reintroduced to the closed-loop system.
In conclusion, while closed-loop recirculation systems will never be perfect, in an era of high energy-efficiency systems, their operation must come as close as possible to that nirvana of performance. High performance filter feeders have the capability to capture and control contaminants and inject the chemicals that help prevent corrosion. They can play a significant role in optimizing the performance of the closed-loop system.