By Roger Desrosiers
Evaporative condensers reject heat from refrigeration and air conditioning systems while using minimum quantities of energy and water. As shown in Figure 5A, water is pumped from the basin section and is distributed over the exterior of the condensing coil by a series of distribution troughs or spray nozzles.
The flow rate of water need only be enough to thoroughly wet the condensing coil to provide uniform water distribution and prevent accumulation of scale. Therefore, minimum pumping horsepower is required.
A fan system forces air through the falling water and over the coil surface. A small portion of the water is evaporated, removing heat from the refrigerant, and condensing it inside the coil. Therefore, like the cooling tower, all of the heat rejection is by evaporation, thus saving about 95% of the water normally required by a "once-through" system.
The evaporative condenser essentially combines a cooling tower and a refrigerant condenser in one piece of equipment. It eliminates the sensible heat transfer step of the condenser water which is required in the cooling tower/refrigerant condenser system. This permits a condensing temperature substantially closer to design wet-bulb temperature, and consequently, minimum compressor energy input.
The temperatures and water flow rate shown in Figure 5B are typical of an evaporative condenser applied to a refrigeration or air conditioning system at the designated design wet-bulb temperature with either ammonia or a halocarbon refrigerant. These conditions result in an economical evaporative condenser selection. However, a lower condensing temperature and lower compressor energy input could be obtained with a larger condenser at this same wet-bulb temperature. The evaporative condenser offers a number of important advantages over other condensing systems:
Low system operating costs
Condensing temperatures within 15°F of design wet-bulb are practical and economical, resulting in compressor horsepower savings of 10% or more over cooling tower/condenser systems and more than 30% over air-cooled systems.
Fan horsepower is comparable to cooling tower/condenser systems, and is about one-third that of an equivalent air-cooled unit. Because of the low pumping head and reduced water flow, water pumping horsepower is approximately 25% of that required for the normal cooling tower/condenser installation.
Initial cost savings
The evaporative condenser combines the cooling tower, condenser surface,
water circulating pump and water piping in one assembled piece of equipment. This reduces the cost of handling and installing separate components of the cooling tower/condenser system. Since the evaporative condenser utilizes the efficiency of evaporative cooling, less heat transfer surface, fewer fans, and fewer fan motors are required resulting in an initial material cost savings of 30 to 50% over a comparable air-cooled condenser.
The evaporative condenser saves valuable space by combining the condensing coil and cooling tower into one piece of equipment, therefore eliminating the need for large water pumps and piping associated with the cooling tower/condenser system. Evaporative condensers require only about 50% of the plan area of a comparable sized air cooked condenser.
Most refrigeration and air conditioning systems are subject to wide load variations and substantial changes in ambient temperature conditions. Where refrigerant control requires a reasonably constant condensing pressure, some form of capacity control is required.
Fan cycling is the simplest method of capacity control on evaporative condensers. However, this method can result in relatively large fluctuations in condensing pressures. On ammonia systems, most evaporators are fed by high pressure or low pressure float valves, or float switches which are less sensitive to variations in head pressure. On this type of system, fan cycling of the evaporative condenser will usually provide satisfactory capacity control on the high side of the system.
This is particularly true on larger ammonia systems, where the evaporative condenser may have several fan motors which can be cycled in steps. Halocarbon systems generally utilize evaporators controlled by thermal expansion valves. A reasonably constant pressure differential across the thermal expansion valve is required for its proper operation. Therefore, this type of system requires a closer degree of evaporative condenser capacity control than can be obtained with fan cycling.
Variable Frequency Drives
Precise capacity control and energy savings are achieved with the variable frequency drive (VFD) option. VFDs offer a more efficient and durable way to reduce fan speed compared to fan cycling, fan discharge dampers or mechanical speed changers. The inherent ability for VFDs to provide soft starts, stops and smooth accelerations prolongs the mechanical system life (fans, motors, belts, bearings, etc.). Sound levels are also reduced at lower fan speeds, and start-up noise is eliminated with the soft start feature. NOTE: An inverter duty motor is required for all models operating with a variable frequency drive.
Two-Speed Fan Motors
The number of steps of capacity control can be doubled by using two-speed fan motors in conjunction with fan cycling. This is particularly useful on single fan motor units which normally have only one step of capacity control using simple fan cycling.
Normally the two-speed fan motor will be selected so that the low speed is half of the full speed, such as 1800/900 rpm. An evaporative condenser will deliver approximately 58% of its rated capacity at half speed.
An additional benefit of two-speed fan motors is reduced fan horsepower at low speed. Brake horsepower varies as the cube of the fan speed, so the unit will use only about one eighth of the full load brake horsepower when operating at low speed. Maximum load and maximum wet-bulb temperature occur infrequently, so the unit will be operating at half speed and hence sharply reduced brake horsepower much of the time.
Another benefit of two speed motors is that when an evaporative condenser is operating at low speed, it will have substantially lower operating sound levels. The sound pressure levels of both centrifugal and propeller fan evaporative condensers will be reduced by four to ten decibels, depending on the sound frequency.
Modulating Fan Discharge Dampers
Modulating fan dampers, located in the fan discharge of centrifugal fan units, provide an infinite number of capacity control steps. Modulating dampers also affect a reduction in fan motor horsepower which is approximately proportional to the reduction in CFM as the dampers move toward the closed position.
Due to evaporative condensers 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.
By 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 corrosives. 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. 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.
Roger J. Desrosiers
About the Author: Roger is a contributing faculty member of HVACReducation.net. He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified. More About Roger