This section provides general design guidelines for air cooled exchangers (also called air coolers). See API Standard 661 for air cooler nomenclature and illustrations and for mechanical design requirements. General Design Considerations Design Air Temperature Tube Side Design Viscosity Plugging Headers Layout Draft Fans Fins Winterization Estimating Rules of Thumb
600 Air Cooled Heat Exchangers Abstract This section provides general design guidelines for air cooled exchangers (also called air coolers) See API Standard 661 for air cooler nomenclature and illustrations and for mechanical design requirements Chevron Corporation Contents Page 610 General Design Considerations 600-2 611 Design Air Temperature 612 Tube Side Design 613 Viscosity Plugging 614 Headers 615 Layout 620 Draft 600-3 630 Fans 600-4 640 Fins 600-4 650 Winterization 600-4 660 Estimating Rules of Thumb 600-4 600-1 December 1989 600 Air Cooled Heat Exchangers Heat Exchanger and Cooling Tower Manual 610 General Design Considerations Air cooled exchangers are used to cool or condense low viscosity fluids They are usually designed by the manufacturer The HTRI ACE computer program may be used to rate air coolers and condensers, if air rate is known Fan design and rating is beyond the scope of the ACE program 611 Design Air Temperature Design air temperature is usually selected as the dry bulb temperature that is exceeded about 150 hours per year during the hottest consecutive months This corresponds to meeting design performance 98+% of the year, but not for a few hours a day during the hottest days Evaluated weather data can be obtained from the following sources: • “Evaluated Weather Data for Cooling Equipment Design,” by Fluor Products Company, 1958 • “Army, Navy and Air Force Manual Engineering Data,” by Departments of the Army, the Navy, and the Air Force, April 1, 1963 • “Evaluated Weather Data for Cooling Equipment Design, Addendum No 1, Winter and Summer,” by Fluor Products Company, 1964 Consult ETD Engineering Analysis Division if these or other weather data sources are not readily available Air cooling process streams to about 20°F over the design ambient temperature, or higher, is economic Water cooling or refrigeration is used if lower process temperatures are required 612 Tube Side Design Tube side design for air coolers and shell and tube exchangers involves the same principles Economic tube side velocities given in Section 220 and process side heat transfer coefficient given in Section 481 apply to air coolers Estimating rules of thumb specific to air coolers are given in Section 660 below 613 Viscosity Plugging Cooling viscous fluids is usually not practical in air coolers because of viscosity plugging The viscosity of viscous oil usually increases rapidly with decreasing temperature Air side maldistribution inevitably causes some tubes to run cooler than others This increases the viscosity in the cooler tube and therefore reduces flow through that tube, which causes further cooling and more flow reduction The result of this temperature-viscosity-flow effect is to virtually stop flow in some tubes and force flowing tubes into the turbulent flow regime, if the viscous oil pumps can develop the needed head Otherwise, flow essentially stops December 1989 600-2 Chevron Corporation Heat Exchanger and Cooling Tower Manual 600 Air Cooled Heat Exchangers Air coolers should not be used when the tube side outlet Reynolds number (defined in Appendix B) is less than 2000 Air recirculation described in Section 650 may be used to air cool fluids that become very viscous or freeze near ambient temperatures Tempered water cooling described in Section 330 is an alternative 614 Headers Most air coolers have header plugs opposite each tube on each end to facilitate inspection and cleaning Removable cover plates on each header is an alternative but is not economic and rarely used 615 Layout Air-cooled exchangers usually have four rows of staggered tubes Six rows are sometimes used for large process temperature range Aligned tube rows are never advantageous because of significant air side channeling and temperature maldistribution Air-cooled exchangers are often mounted over pipeways to minimize plant real estate Pipeway mounted units are usually 30 feet long Shorter bundles are common for grade mounted units More than one bundle (service) may be in the same bay with a common pair of fans, if overcooling some of the services is not a concern If overcooling is a potential problem, adjustable louvers for that bundle can be used, or the service can be built as a separate unit with dedicated fans 620 Draft Forced draft is required when the maximum outlet air temperature is higher than the manufacturer’s rated temperature of the fans or the auto-variable fan pitch control hubs Operation with the fans off and during steam out should be considered The air outlet temperature is approximately the same as the process inlet temperature when fans are off, and same as the steam temperature during steam out The plenum of induced draft units acts like a stack and produces a significant “fansoff” duty The “fans-off” performance of forced draft units is negligible Induced draft is sometimes specified for a column overhead condenser where temperatures permit Induced draft is normally considered if overcooling during rain storms is a problem or if design relief loads would be reduced by the superior fans off performance of induced draft units Rain affects the full bundle in forced draft units and only about half the bundle in induced draft units When the above considerations not govern draft, the manufacturer usually decides Induced draft provides better air flow distribution than forced draft; however, induced draft fans must move a greater volume of (heated) air than forced draft These effects are not large Chevron Corporation 600-3 December 1989 600 Air Cooled Heat Exchangers Heat Exchanger and Cooling Tower Manual 630 Fans There should be at least two fans per bundle with the fans covering at least 40 percent of the bundle Half of the fans are usually autovariable pitch fans During the summer, the manual fan is on and the autovariable fan adjusts air flow to control duty During the winter, the manual fan is usually shut off Auto-variable fan pitch control is usually justified by fan power savings rather than control needs, but serves both functions Fan blades are usually aluminum or plastic The best plastic fan blades are good to about 350°F Auto-variable fan pitch control hubs may have rubber seals that limit temperature to about 250°F 640 Fins Fins are almost always aluminum Three fin types are commonly used: footed, integral (extruded) and imbedded Maximum process inlet temperatures for these fin types are 250°F, 550°F, and 750°F, respectively API 661, Section 5.1, describes these fin types in more detail 650 Winterization Winterization is required for air coolers with fluids that may become immobile under expected winter conditions due to freezing, pour point problems or the viscosity plugging phenomenon described in Section 610 Winterization usually means building an enclosure around the air cooler with provisions to recirculate a portion of the outlet air back to the inlet A steam coil under the bundle is also normally provided to heat up the bundle if the fluid in the bundle ever becomes immobile Winterization of air coolers is discussed in detail in API Recommended Practice 632 (not included in this manual) 660 Estimating Rules of Thumb Air coolers usually have four layers of 1-inch O.D tubes with 10, 5/8-inch high fins per inch, arranged in a 2-1/2-inch pitch equilateral triangular layout The ratio of fin surface to bare outside surface (bos) is about 20:1; the required plot space is about 200 ft2 per 1000 ft2bos; the air rate is about 600 lb/hr/ft2bos; the installed fan power is about 20 hp per 1000 ft2bos; and the air side heat transfer coefficient is about 175 Btu/hr ⋅ °F ⋅ ft2⋅bos Note that the installed fan power (“pumping power”) is four times higher than that normally used for other types of heat exchangers This is because air cooler design is limited by warm weather operation Annual average fan power is usually about one fourth of the design value December 1989 600-4 Chevron Corporation ...600 Air Cooled Heat Exchangers Heat Exchanger and Cooling Tower Manual 610 General Design Considerations Air cooled exchangers are used to cool or condense... Corporation Heat Exchanger and Cooling Tower Manual 600 Air Cooled Heat Exchangers Air coolers should not be used when the tube side outlet Reynolds number (defined in Appendix B) is less than 2000 Air. .. forced draft These effects are not large Chevron Corporation 600-3 December 1989 600 Air Cooled Heat Exchangers Heat Exchanger and Cooling Tower Manual 630 Fans There should be at least two fans