In plants that use chillers to cool machine components and keep production lines and process machinery running smoothly, these pieces of equipment often are the single largest consumers of energy. As energy prices increase, chillers therefore can be identified as a potential source of significant energy savings. Preventive maintenance procedures such as keeping a daily log and performing weekly, monthly and annual operational reviews can help identify ways to improve chiller efficiency. For water-cooled tubular chillers that extract heat, a preventive maintenance program should include regular tube cleaning.
Chiller efficiency is affected more by the cleanliness of the heat transfer surfaces than any other factor. In a water-cooled tubular chiller, the heat transfer surfaces are the tubes. As contaminants accumulate on the tube surfaces, efficiency declines rapidly. Contaminants can range from mud, sand and algae to hard mineral scale, all of which increase thermal resistance and cause the chiller to work harder to meet demand, thereby consuming more energy.
Given that a water-cooled tubular chiller can have several miles of tubing comprising the condenser and evaporator, it makes sense to keep tubes as clean as possible by using a combination of water treatment and planned outages for tube cleaning and other major maintenance tasks.
Tube Cleaning MethodsMany methods have been used to clean chiller tubes, some with greater success than others. The following is a discussion of the various methods and their advantages and disadvantages.
Rod-and-Brush Method. Perhaps the oldest and least pleasant method for chiller tube cleaning is the rod-and-brush method. This technique involves using a length of rod, usually metal of some sort, with a nylon or wire brush larger than the tube’s inside diameter attached to one end. Typically, the tubes are flushed with water from a hose, then the brush is pushed manually through the tubes. The tubes then are flushed again with water.
The advantages of this method are its low cost and simple operation. Little training or expertise is required to carry out this cleaning method properly. However, this method is time- and labor-intensive. Additionally, because the brush bristles are folded over, they tend to swab the tube more than brush it. Tubes cleaned at the beginning of the job tend to dry with debris still in them, making the final water flush ineffective. This method also requires enough space to use a rod slightly longer than the tubes to be cleaned; otherwise, rod sections must be coupled together, further increasing the time needed to complete the job. The rod-and-brush method is not a pleasant task, so often it either does not get done or it is accomplished poorly.
Chemical Cleaning. Less prevalent now than in years past, this method uses acid solutions that are circulated through the tube bundles to break down or soften scale deposits in the tubes. Under the right conditions, tubes can be cleaned to bare metal using this method. In other cases, mineral scale can be softened enough to allow brush cleaning as a secondary operation.
However, the cleaning chemicals can be costly and are an expense that is incurred every time the tubes need cleaning. Additionally, the cleaning process also is time consuming, and the chillers must be offline for at least 24 hours while the cleaning is under way. The chemicals present employee health and safety risks as well as risk employer liability. Significant training and expertise are required to perform this method safely. And, with all the local, state and federal restrictions on hazardous waste disposal, getting rid of used chemicals may be difficult, costly and carry some legal implications.
However, this method has limited capabilities in terms of the types of deposits it can remove. Most projectiles are suitable only for light deposits such as mud and algae. Before cleaning, both end-bells must be removed from the chiller to allow the projectiles to be recovered, which increases labor requirements. Additionally, high-water-pressure units are costly and require considerable maintenance.
Safety also is a consideration: Some guns are not vented to eliminate the possibility of “blowback,” which occurs when one attempts to shoot a projectile into a plugged tube. The tube becomes pressurized with air; when the gun is removed from the tube, the projectile is launched back at the operator. (This condition only occurs with air/water guns that do not incorporate a pressure-relief feature.)
Online Systems. Two basic online tube-cleaning systems are in use today. One uses plastic brushes that are “trapped” in each tube with plastic baskets that have been attached to each end of the tube using an epoxy. Periodically, the direction of the water flow through the tube bundle is reversed, causing the brush to travel to the other end of the tube. This back and forth brushing action is designed to keep the tubes clean. With the other type of system, a quantity of foam balls is circulated continuously through the system. In theory, a foam ball will travel through every tube in the bundle frequently enough to keep the tubes clean.
Provided the water treatment is adequate, these systems do work, and the maintenance personnel need not even think about tube cleaning. However, these systems require a high initial outlay of capital to purchase and install. They also are not suitable where scaling commonly occurs, such as in areas where hard water is present or where water treatment is poor.
The initial outlay for this method is minimal, and the cost of consumables is reasonable. It also is simple to use; little training or expertise is required, and only one operator is needed to perform the cleaning. Only one end of the chiller needs to be opened for cleaning, and the tubes are left thoroughly cleaned for better heat transfer.
Drawbacks of this method are that it is slower than tube cleaning guns, and untrained operators might tend to break flexible shafts.
Internally Enhanced TubesThe internally enhanced tube is a relatively new feature in the evolution of chillers. These tubes are manufactured with “rifling,” or spiral grooves inside the tubes (figure 1). They often have external enhancements as well. The idea behind the internally enhanced tube is to provide more surface area for heat transfer. The internal enhancement also creates more turbulence in the water passing through the tube, allowing it to absorb more heat. The high points of the rifling are called the lands, and the low points are the grooves.
Using internally enhanced tubes, chiller manufacturers are able to get more cooling out of smaller chillers, thereby reducing costs and enhancing efficiency. However, if internally enhanced tubes become fouled, they are difficult to clean properly. With the rod-and-brush method, the grooves would remain filled with deposits. Acid cleaning will work, but the drawbacks of working with chemicals are substantial. Projectiles from tube-cleaning guns move through the tubes too quickly to remove deposits effectively. Online systems also are not that effective at keeping the grooves clean. Rotary tube cleaners can effectively clean internally enhanced tubes; however, to optimize their effectiveness, the design of these cleaners has undergone several modifications.
The original machines were single-speed, single-direction units that were well suited to the smooth-bore tubes used in chillers (figure 2). However, when internally enhanced tubes were introduced, chiller manufacturers asserted that the best way to clean enhanced tubes effectively was to use a rotary tube cleaner from both ends of the tubes. This suggestion was not well received as it effectively doubled the labor cost of a tube cleaning.
Research by equipment manufacturers proved that several features could improve cleaning performance for internally enhanced tubes. For example, it was learned that different chiller manufacturers used different enhancement patterns in their tubes. This fact is important because it was also learned that keeping the bristles in the grooves depended largely on how fast the brush rotates in the tube. This understanding led to rotary-tube cleaning machines that incorporated variable speed as well as bidirectional rotation.
Lastly, researchers learned that deposits could be flushed from the tubes more effectively using a higher water flow and pressure than simply using line pressure. Machines with augmentations such as pressure-boosting pumps are more effective at cleaning internally enhanced tubes.
Chiller maintenance is important for optimizing efficiency and extending equipment life. Training and knowledge are required to perform chiller maintenance properly. The single biggest determinant to chiller efficiency is the cleanliness of heat transfer surfaces - primarily the tubes. By understanding the different cleaning options, maintenance personnel can ensure that chiller tubes are cleaned quickly and with minimal effort, thereby saving the plant money on both maintenance and energy.
Sidebar:While regular tube cleaning is crucial to optimize the efficiency of water-cooled tubular chillers, issues such as water treatment, refrigerant charge, the condition of the motor and electrical components, and lubrication also can affect the chiller’s performance.
Beyond Tubes: Other Common Chiller Problem Areas
Water Treatment. When chillers use an open cooling source such as atmospheric cooling towers, some sort of water treatment is needed to control scale, biological growth and corrosion, all of which can lead to fouling of the chiller tubes. The more the chiller becomes fouled, the more energy the chiller consumes to satisfy the cooling demand.
Refrigerant. It is important to maintain the proper refrigerant charge to minimize the load on the compressor and enhance efficiency. Refrigerant leaks, along with the introduction of air and moisture into the system, decrease system efficiency.
Motors and Other Electrical Components. The chiller motor might be one of the largest consumers of electricity in the building, so proper maintenance is important. Make sure that cooling air vents are kept clean on the motor and that shaft seals are checked periodically. Electrical connections, wiring and insulation must be maintained properly for peak performance.
Lubrication. Compressor lubricant should be tested in a laboratory annually for moisture content, acids and other contaminants that affect performance.