Maintaining Evaporative Condensers
Refrigeration systems are used around the world for all kinds of applications. From cooling or storing produce to keeping a building cool on a hot summer day, refrigeration systems remove heat from people, places and things. Every refrigeration system has a condenser, and the condenser is a critical system component (figure 1). It fills the vital role of rejecting heat from a refrigerated space to the atmosphere by condensing the refrigerant. Several different condenser types exist, each with its own features and benefits.
Air-Cooled Condensers. Most often found where water is not available, in an air-cooled condenser, a fan blows air over a coil containing refrigerant. Air movement over the coil cools the refrigerant and allows condensation to occur. Air temperature determines the refrigerant's temperature. Energy requirements tend to limit this type of condenser to smaller systems.
Evaporative Condensers. Evaporative condensers employ evaporative cooling to condense refrigerant and reject heat to the atmosphere. Evaporative cooling is an age-old process where water cools as it is evaporated. The effect of evaporative cooling is easily explained through a simple example: Wet one of your fingers, then blow on it. The cold you feel is due to evaporative cooling. Heat from your finger evaporates a small amount of the water on your finger. Blowing on your finger improves the evaporation rate.
In an evaporative condenser, refrigerant is condensed inside a tubular heat exchanger coil. The coil is housed in a structure where water is sprayed over the outside surface of the tubes. A portion of the water is evaporated through contact with the tubes and outside air passing through the structure. Evaporation of the spray water removes heat from the refrigerant and rejects it to the atmosphere. The temperature at which the refrigerant is condensed is actually below the temperature of the outside air. Through the use of evaporation, evaporative condensers use wet bulb temperature to drive the process of condensation.
Due to their ability to provide lower condensing temperatures while maintaining low electrical costs, evaporative condensers are effective and a popular means of condensing refrigerant in industrial applications. However, evaporative condensers move a large amount of air and water: approximately 3 gal/min of water is evaporated for every 100 tons of refrigeration. As a result, the condenser's efficiency is tied directly to the effectiveness of evaporation.
Evaporation can be hindered by a number of factors, including condenser water quality, condition of condenser coils and condition of the mechanical equipment. Therefore, it becomes important to system operation that the condenser be in good working order. In fact, maintenance is the single most important factor affecting the life of an evaporative condenser.
Recirculated Water SystemBy definition, evaporative condensers evaporate water. But, this is not a completely clean process. Just as when water is boiled on a stove, impurities in the water are left behind in the condenser when it evaporates. If left uncontrolled, minerals will build up in the system until they precipitate out, leaving a layer of scale on the coil and sheet metal. To control the buildup of impurities, manufacturers supply condensers with bleed lines to remove a portion of water from the system. Throwing away a small quantity of water (usually equal to the evaporation rate) may seem wasteful, but it is more important to keep scale from building on the coil.
A mere 0.03" of scale will reduce the capacity of an evaporative condenser roughly 30%. Therefore, controlling the mineral level in the water is important. The scale potential for water can be measured in terms of calcium hardness. Calcium hardness is measured in parts per million (ppm), where there are so many calcium carbonate molecules for every million water molecules.
In addition to controlling scale, water chemistry must be maintained to avoid creating a corrosive environment for the evaporative condenser. The vast majority of condensers are built from galvanized steel. While zinc provides good corrosion resistance for its cost, it also is a reactive metal that must be protected.
Monitoring the pH of recirculated water is perhaps the most important aspect of the water chemistry. The pH measures the water's acidity or alkalinity and is a quick indication of corrosivity. An extremely low or high pH indicates that the water chemistry is corrosive to galvanized steel. A high pH also may indicate white rust. White rust is the formation of white, porous deposits that are fluffy or waxy. Products of zinc corrosion, white rust forms mostly in new condensers operating at pH's of 8.3 or greater. Preventing white rust is best accomplished through passivation of the condenser during the first weeks of operation. Maintaining a near neutral pH and moderate levels of hardness and alkalinity will allow a protective barrier of zinc carbonate to form. This protective layer inhibits zinc reaction.
Occasionally, water chemistry cannot be controlled through employing the bleed line on the condenser. In these cases, a reputable water treatment specialist familiar with local conditions should be consulted to control the water's hardness and pH.
Above all, processors should avoid soft water systems. Water always seeks equilibrium with its surroundings. Soft water has been stripped of its mineral content, and when in contact with galvanized steel, it strips zinc from the base metal in an effort to reach equilibrium. Certainly, there are occasions where water must be softened. However, the 50-ppm minimum hardness level must be taken into account. Also, batch feeding of chemicals, especially acid, should be avoided as wild fluctuations in pH typically result in corrosion.
Biological contamination of a condenser can have a dramatic effect on performance. Biological fouling can have the same insulating effect as scale on the condenser's heat rejection capability. Certain strains of algae also can present health risks to employees, so it is important to keep the condenser clean and free from the dirt and debris that act as breeding grounds for bacteria. Biocides often are routine parts of a treatment program and usually are implemented on the system's initial startup.
Fan-and-Drive SystemsThe fan-and-drive system comprises the moving parts of the condenser. The fan and pump motor, fan shaft bearings and belts that drive the fans require routine maintenance. Without these components operating at peak efficiency, the condenser cannot perform as needed.
Regularly scheduled maintenance is the best way to ensure long product life for an evaporative condenser. Manufacturers of evaporative condensers typically publish operating instructions or checklists to assist owners with maintenance requirements.
How Condenser Type Affects MaintenanceThere are several types of evaporative condensers, and each type has its own features. These features can enhance the condenser's ability to be maintained. Whether it is a forced-draft centrifugal fan condenser, forced-draft axial fan con- denser or induced-draft axial fan condenser, the condenser's configuration contributes to its own maintenance.
Forced-draft centrifugal fan condensers often are used on smaller systems and for indoor installations. In a forced-draft design, the mechanical equipment is located at the base of the condenser in the dry, entering airstream, making the fan motor, bearings and belts easily accessible. Owners gain access to the basin area through openings in the condenser walls. This design lets little light into the condenser, inhibiting bacteriological growth.
Due to its use of squirrel cage fans, the forced-draft centrifugal fan design is quiet. However, the quietness, along with the ability for indoor installations, requires higher fan motor horsepower.
Similarly, forced-draft axial fan condensers have mechanical equipment located near the condenser's base. However, the axial fan design allows for lower fan motor horsepower than centrifugal fan models. Originally developed in the 1960s, the forced-draft axial fan condenser quickly took over the market from centrifugal designs due to its energy-conscious design.
In the induced-draft design, mechanical equipment is located in the leaving airstream. Air is drawn through the condenser instead of being forced through as in the other designs. The basin area is open for visual inspection even during operation. Induced draft condensers also utilize axial fans for low motor horsepower and are quiet due to fan location.
The induced draft design is relatively new to the industrial refrigeration market. Available in small tonnages in the 1970s and early 1980s, it has gained significant market share in the last 20 years because of its maintenance-oriented design.
Accessories That Benefit MaintenanceEquipment furnished with an evaporative condenser can improve its operation and life span. Accessories such as ladders and service platforms ease owners' access to the water distribution system and mechanical equipment, thereby improving the condenser's operation. One accessory that provides a good means of condenser fan motor maintenance is a motor davit or jib boom. This device allows the fan motor to be removed and reinstalled with ease.
A well-conceived equipment design - in combination with a sound preventive maintenance program and diligent monitoring of a condenser's water chemistry - will provide long equipment life and trouble-free operation.