Recuperative Multi-Tube Cooler
Cooling of high-temperature bulk materials (up to 1,000°C) of all kinds in an indirect air cooled rotary tube. Using the dust-free 200 - 300°C cooling air can save up to 15% primary energy in firing systems.
Function
The Recuperative Multi-Tube Cooler is an indirect air-cooled rotary tubular cooler for solids (see diagram, items 1, 2 and 3). The contract between the solid (i.e. the product to be cooled) and the cooling air (diagram, items 4, 5 and 6) is via the metal tube walls of the apparatus. Thus, there is no direct material contact - the heat is exchanged indirectly via thermal conduction. Product and cooling air pass through the cooler in counterflow. The product flow is distributed to several tubes (diagram, item 7) and the cooling air flows around these tubes in a meandering movement. The tube filling level is 5 - 15% (referred to the tubes' empty volume) - this provides for an optimum relation between product flow characteristics and heat exchange. The product passes through the apparatus as a result of the rotary movement and slight inclination of the drum. A fan draws ambient air into the apparatus (diagram, item 4) to serve as cooling air. The air flow velocity is so high that an optimum is obtained between achieved cooling capacity and required fan capacity.
Design features
The cooler basically consists of a drum jacket that rotates around its longitudinal axis and is supported by two tyres (diagram, item 8) and four tread rollers and driven via one girth gear (diagram, item 9) and one pinion. The drum is closed off at one end by a product feed housing and by a discharge housing at the opposite end (diagram, items 10 and 11). Special seals arranged between these stationary housings and the rotating drum jacket provide for a suitable air seal. The product tubes (diagram, item 7) through which the product to be cooled (diagram, item 2) passes are located inside the jacket in a revolver-type (or planetary) arrangement. A series of specially designed baffle plates causes the cooling air (diagram, item 5) to pass around the product tubes inside the jacket in a meandering movement.
Advantages
The indirect heat transfer between product and cooling air precludes any dust emission into the air. The heated cooling air is thus suitable for further energy saving utilisation without any problems. Even pieces of equipment sensitive to dust (such as burner nozzles, heat exchangers etc.) can easily handle this air. The cooling air leaving the cooler contains the heat removed from the product. Plant concepts utilising this energy can save a corresponding amount of primary energy (fuel, electricity) - in plants supplied so far this amounts to up to 15%. Depending on the individual case, the exhaust temperature is usually at a relatively high and efficiently usable level of 150 - 300°C.
Typical Applications
From an economic point of view all those applications are of interest in which a solid is treated or produced in a hot-gas fired oven or kiln (such as a rotary kiln) at high temperatures. In these cases heating the oven/kiln usually requires a high amount of primary energy; on the other hand the finished product normally needs to be cooled down for handling and further processing. The primary energy consumption can be reduced if the exhaust from a Recuperative Multi-Tube Cooler is used as pre-heated supply or combustion air (recuperate use or recuperation). Such solutions are often selected in combination with direct-fired rotary kilns used for titanium dioxide production and also for other inorganic chemical products that are discharged from the kiln at temperatures of 800 - 950°C.
Limits of Application
For design and process engineering reasons, a Recuperative Multi-Tube Cooler can be built for product inlet temperatures of 400 to 1,000°C, requested product outlet temperatures of 80 to 150°C and heat quantities of 150 to 5,000 kW to be removed (these are only approximate figures). The solid must be suitable for being conveyed by means of the drum rotation, i.e. it must not agglutinate or form cakings. Coarser product lumps, however, are usually no problem as long as the fine-grain portion predominates. Recuperative Multi-Tube Coolers supplied within the performance range mentioned above have drum diameters of 1.0 to 4.0 m and drum lengths of 10 to 40 m.
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