I don't know who will be interested with my topic. Any way I’ll try my best to squeeze out my time to write more.Today’s topic: Air-cooled Heat Exchanger DesignHighly recommended Technical Paper: “Effectively Design Air-cooled Heat Exchangers”, by R. Mukherjee, published on CHEMICAL ENGINEERING PROCESS / FEB 1997 Page 26 to 46. Abstract: This primer discusses the thermal design of ACHEs and the optimization of the thermal design, and offers guidance on selecting ACHEs for various applications. API 661—Petroleum, petrochemical and natural gas industries—Air –Cooled heat exchangers Applications:• • • • • • • • • • •Forced and induced draft air cooled heat exchangers Recirculation and shoe-box air cooled heat exchangers Hydrocarbon process and steam condensers Large engine radiators Turbine lube oil coolers Turbine intercoolers Natural gas and vapor coolers Combustion pre-heaters Flue gas re-heaters Lethal service Unique customizationsRecommend Vendor: Hudson Products Corporation GEA Rainey Corporation Jord International Korea Heat Exchanger Ind. Co., Ltd. FBM Hudson Italiana SpA Air Cooler Design Heat Transfer Basics Air cooled heat exchangers rely on thermodynamic properties of heat transfer. Specifically, heat transfer is energy released over time. Two standard formulas used to calculate heat transfer are as follows:• •Duty=Fluid Mass Flow * Cp * Delta T The overall heat-transfer coefficient, U, is determined as follows:1/U=1/airside heat transfer coefficient+1/tubeside heat transfer+tubeside fouling resistance + airside fouling resistance+ tube wall resistance•Duty=U * Area * LMTD o U is the inverse of sum resistance to heat transfer (defined as above) o Area is the cooler’s total finned heat transfer area o LMTD is the Log Mean Temperature Difference, or the driving force of heat transferGiven the above graph, recommending an absolute minimum of 10°C Delta T for most applications based on economies of scale. Of course smaller Delta T’s, such as 5°F, have been designed. Keep in mind as the ambient increases, the LMTD goes down reducing cooling ability. The optimum temperature is around 15°C-20°C more than the design ambient temperature. Flow Pattern & LMTD Effects There are three main types of flow patterns used in air cooled heat exchangers; countercurrent flow, co-current flow and cross current flow. Counter-Current Flow – By far the most common in the process industry, counter-current flow cools the hottest fluid with the warmest air, and the coldest fluid with the coldest air. In other words, the process fluid enters the heat exchanger and passes through the finned tubes at the top of the bundle. These top tubes are exposed to air warmed by the lower tube rows. As the process fluid cools and passes through the lower tube rows, the air temperature is lower as it has been exposed to less and less tube rows.Co-Current Flow – This flow pattern is typically used in processes with critical pour points as it provides the highest outlet process temperature control since it has the lowest efficiency. In this pattern the ambient air cools the hottest fluid, and the hottest air attempts to cool the coldest fluid. The shaded arrows to the right illustrate this flow pattern. Cross-Current Flow – Most common in the gas compression industry, the cross-current flow pattern exposes each pass of the process fluid to the same air stream. Therefore the pass plates inside the headers are vertical, rather than horizontal, to allow the fluid to pass perpendicular to the air stream. Minimizing Air Cooler Costs Through understanding the customer’s needs to size and design air cooled heat exchangers use commercially available software programs, typical HTRI, B-JAC etc. These programs, while not offering a thermal guarantee, can offer an advantage to customers when trying to compare air coolers from different manufacturers. Quick selection between multiple designs:• •• •• •Maximize tube length while maintaining >=40% fan coverage Design air cooler with a 1 to 3 ratio. For example, if your cooler is 30’ long it should typically be around 10’ wide. This helps reduce the header size, the most expensive portion of an air cooler, while still maintaining proper fan coverage. Minimize tube rows to increase heat transfer effectiveness of area, minimize header thickness. Typically between four to six tube rows . Try and maintain 1” tube diameters, depending on service. Even high viscosity services that appear to benefit from larger diameter tubes can typically be designed cheaper with more 1” diameter tubes. Use a counter-current flow where possible as it reduces surface and potentially minimize header plate thickness. Increase your allowable pressure drop. This allows more passes in the bundle reducing the cooler size.ComponentsAn air cooled heat exchanger consists of the following components:• • • • •One or more bundles of heat transfer surface. An air-moving device, such as a fan, blower, or stack. Unless it is natural draft, a driver and power transmission to mechanically rotate the fan or blower. A plenum between the bundle or bundles and the air-moving device. A support structure high enough to allow air to enter beneath the ACHE at a reasonable rate.• • • •Optional header and fan maintenance walkways with ladders to grade. Optional louvers for process outlet temperature control. Optional recirculation ducts and chambers for protection against freezing or solidification of high pour point fluids in cold weather. Optional variable pitch fan hub for temperature control and power savings.Typical components of an air-cooled heat exchanger Tube Bundle A tube bundle is an assembly of tubes, headers, side frames, and tube supports as shown in figure below. Usually the tube surface exposed to the passage of air has extended surface in the form of fins to compensate for the low heat transfer rate of air at atmospheric pressure and at a low enough velocity for reasonable fan power consumption.Typical construction of tube bundles with plug and cover plate headers The prime tube is usually round and of any metal suitable for the process, due consideration being given to corrosion, pressure, and temperature limitations. Fins are helical or plate type, and are usually of aluminum for reasons of good thermal conductivity and economy of fabrication. Steel fins are used for very high temperature applications. Fins are attached to the tubes in a number of ways:• ••An extrusion process in which the fins are extruded from the wall of an aluminum tube that is integrally bonded to the base tube for the full length. Helically wrapping a strip of aluminum to embed it in a pre-cut helical groove and then peening back the edges of the groove against the base of the fin to tightly secure it. Wrapping on an aluminum strip that is footed at the base as it is wrapped on the tube.Sometimes serrations are cut in the fins. This causes an interruption of the air boundary layer, which increases turbulence which in turn increases the airside heat transfer coefficient with a modest increase in the air-side pressure drop and the fan horsepower. The choice of fin types is critical. This choice is influenced by cost, operating temperatures, and the atmospheric conditions. Each type has different heat transfer and pressure drop characteristics. The extruded finned tube affords the best protection of the liner tube from atmospheric corrosion as well as consistent heat transfer from the initial installation and throughout the life of the cooler. This is the preferred tube for operating temperatures up to 600°F. The embedded fin also affords a continued predictable heat transfer and should be used for all coolers operating above 600°F and below 750°F. The wrap-on footed fin tube can be used below 250°F; however, the bond between the fin and the tube will loosen in time and the heat transfer is not predictable with certainty over the life of the cooler. It is advisable to derate the effectiveness of the wrap-on tube to allow for this probability. There are many configurations of finned tubes, but manufacturers find it economically practical to limit production to a few standard designs. Tubes are manufactured in lengths from 6 to 60 feet and in diameters ranging from 5/8 inch to 6 inches, the most common being I inch. Fins are commonly helical, 7 to 11 fins per inch, 5/16 to I inch high, and 0.010 to 0.035 inch thick. The ratio of extended to prime surface varies from 7:1 to 25:1. Bundles are rectangular and typically consist of 2 to 10 rows of finned tubes arranged on triangular pitch. Bundles may be stacked in depths of up to 30 rows to suit unusual services. The tube pitch is usually between 2 and 2.5 tube diameters. Net free area for air flow through bundles is about 50% of face area. Tubes are rolled or welded into the tube sheets of a pair of box headers. The box header consists of tube sheet, top, bottom, and end plates, and a cover plate that may be welded or bolted on. If the cover is welded on, holes must be drilled and threaded opposite each tube for maintenance of the tubes. A plug is screwed into each hole, and the cover is called the plug sheet. Bolted removable cover plates are used for improved access to headers in severe fouling services. Partitions are welded in the headers to establish the tube-side flow pattern, which generates suitable velocities in as near countercurrent flow as possible for maximum mean temperature difference. Partitions and stiffeners (partitions with flow openings) also act as structural stays. Horizontally split headers may be required to accommodate differential tube expansion in serviceshaving high fluid temperature differences per pass. The figure below illustrates common head types. Bundles are usually arranged horizontally with the air entering below and discharging vertically. Occasionally bundles are arranged vertically with the air passing across horizontally, such as in a natural draft tower where the bundles are arranged vertically at the periphery of the tower base. Bundles can also be arranged in an "A" or "V" configuration, the principal advantage of this being a saving of plot area. The disadvantages are higher horsepower requirements for a given capacity and decreased performance when winds on exposed sides inhibit air flow. Within practical limits, the longer the tubes and the greater the number of rows, the less the heat transfer surface costs per square foot. One or more bundles of the same or differing service may be combined in one unit (bay) with one set of fans. All bundles combined in a single unit will have the same air-side static pressure loss. Consequently, combined bundles having different numbers of rows must be designed for different face velocities. Typical Heat Transfer Coefficients for Air-Cooled Heat Exchangers Condensing service Amine reactivator Ammonia Refrigerant 12 Heavy naphtha Light gasoline Light hydrocarbons Light naphtha Reactor effluent Platformers, Hydroformers, Rexformers Steam (0 - 20 psig) Gas cooling service Air or flue gas @ 50 psig (DP = 1 psi) Air or flue gas @ 100 psig (DP = 2 psi) U 100 - 120 105 - 125 75 - 90 70 - 90 95 95 - 105 80 -100 80-100 135 - 20010 20Air or flue gas @ 100 psig (DP = 5 psi) Ammonia reactor stream Hydrocarbon gasses @ 15 - 50 psig (DP = 1 psi) Hydrocarbon gasses @ 50 - 250 psig (DP = 3 psi) Hydrocarbon gasses @ 250 - 1500 psig (DP = 5 psi) Liquid cooling service Engine jacket water Fuel oil Hydroformer and Platformer liquids Light gas oil Light hydrocarbons Light naphtha Process water Residuum Tar30 90 - 110 30 - 40 50 - 60 70 - 90130 - 155 20 - 30 85 70 - 90 90 -120 90 120 -145 10 - 20 5 - 10Coefficients are based on outside bare tube surface for 1-inch OD tubes with 10 plain extruded aluminum fins per inch, 5/8 inch high, 21.2:1 surface ratio.。