512 Fermentation and Biochemical Engineering Handbook cooling water temperatures vary with the season of the year. At some locations a 30C difference between summer and winter water temperatures is experienced. At such locations a tempered water system may be used in order to reduce both pumping costs and maintenance costs. tempered water system requires a pump to recycle part of the heated cooling water in order to maintain a constant inlet water temperature Evaporative-cooled condensers in many applications give greater heat transfer than air-cooled or water-cooled condensers. the evaporative equipment can do this by offering a lower temperature sink. Evaporati cooled condensers are frequently called wet-surface air-coolers. Perhaps the best description for this type of equipment is a combination shell-and-tube exchanger and cooling tower built into a single package The tube surfaces are cooled by evaporation of water into air cially attractive at locations where water is scarce or expensive to treat. Even when water is plentiful, air coolers are frequently the more economical alternative. Elimination of the problems associated with the water side of water-cooled equipment, such as fouling, stress-corrosion cracking, and water leaks into the proce advantage of air-cooled equipment. In many cases, carbon steel tubes can be employed in air-cooled condensers when more expensive alloy tubes would otherwise have been necessary. The use of air-cooled heat exchangers may eliminate the need for additional investment in plant cooling water facilities Maintenance costs for air-cooled equipment are about 25% of the maintenance costs for water-cooled equipment. Power requirements for air- coolers can vary throughout the day and the year if the amount of air pumped is controlled. Water rates can be varied to a lesser degree because daily water temperatures are more constant and because water velocities must be kept high to reduce maintenance. The initial investment for an air-cooled condenser is generally higher than that for a water-cooled unit. However, operating costs and maintenance costs are usually considerably less. These factors must be considered when selecting water or air as the cooling medium Air-cooled condensers employ axial-flow fans to force or induce a fl of ambient air across a bank of externally finned tubes. Finned tubes are used because air is a poor heat transfer fluid. The extended surface enables air to be used economically. Several types of finned-tube construction are avail- able. The most common types are extruded bimetallic finned tubes and fluted tension-wound finned tubes The most common fin material is aluminum Air-cooled heat exchangers generally require more space than other types. However, they can be located in areas that otherwise would not be used(e. g, on top of pipe racks). A forced draft unit has a fan below the tube512 Fermentation and Biochemical Engineering Handbook cooling water temperatures vary with the season of the year. At some locations a 3OoC difference between summer and winter water temperatures is experienced. At such locations a tempered water system may be used in order to reduce both pumping costs and maintenance costs. A tempered water system requires a pump to recycle part of the heated cooling water in order to maintain a constant inlet water temperature. Evaporative-cooled condensers in many applications give greater heat transfer than air-cooled or water-cooled condensers. The evaporative equipment can do this by offering a lower temperature sink. Evaporative￾cooled condensers are frequently called wet-surface air-coolers. Perhaps the best description for this type of equipment is a combination shell-and-tube exchanger and cooling tower built into a single package. The tube surfaces are cooled by evaporation of water into air. Air-cooled condensers are especially attractive at locations where water is scarce or expensive to treat. Even when water is plentifbl, air coolers are frequently the more economical alternative. Elimination of the problems associated with the water side of water-cooled equipment, such as fouling, stress-corrosion cracking, and water leaks into the process, is an important advantage of air-cooled equipment. In many cases, carbon steel tubes can be employed in aircooled condensers when more expensive alloy tubes would otherwise have been necessary. The use of air-cooled heat exchangers may eliminate the need for additional investment in plant cooling water facilities. Maintenance costs for air-cooled equipment are about 25% of the maintenance costs for water-cooled equipment. Power requirements for air￾coolers can vary throughout the day and the year if the amount of air pumped is controlled. Water rates can be varied to a lesser degree because daily water temperatures are more constant and because water velocities must be kept high to reduce maintenance. The initial investment for an air-cooled condenser is generally higher than that for a water-cooled unit. However, operating costs and maintenance costs are usually considerably less. These factors must be considered when selecting water or air as the cooling medium. Air-cooled condensers employ axial-flow fans to force or induce a flow of ambient air across a bank of externally finned tubes. Finned tubes are used because air is a poor heat transfer fluid, The extended surface enables air to be used economically. Several types of finned-tube construction are avail￾able. The most common types are extruded bimetallic finnedtubes and fluted tension-wound finned tubes. The most common fin material is aluminum. Air-cooled heat exchangers generally require more space than other types. However, they can be located in areas that otherwise would not be used (e.g., on top of pipe racks). A forced draft unit has a fan below the tube