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Process air heat exchangers

  1. Feb 14, 2012 #1

    I am working on a small project where I have to look into/verify the selection of a replacemant heat exchanger for a process loop.

    Just about all of the information I have been able to locate is about getting rid of heat from the liquid going through the heat exchanger with the air blown through it being essentially incidental.

    The particular application is essentially a nitrogen enriched air (NEA) grinding circulation loop where the material is processed by an air classifier mill (ACM). The ground material is drawn off by the air classifier and passed through a bag house type dust collector with the filtered NEA has to be cooled and returned to the ACM.

    We have a water chiller that delivers chilled water at around 4 C, and the manufacturer rates the cooling capacity of the chiller at around 100 kW with a 30 kW input.

    The NEA exiting the dust collector comes through a 300 mm dia duct at a rate of around 236 litres/second and then enters the tube/fin heat exchanger chamber to pass through the exchanger core that is 1.37 m x 1.38 m and 440 mm thick. I have worked out nominal air speeds at the entry into the chamber at around 3.32 m/s and at the first face of the core to be around 0.125 m/s assuming that the NEA is distributed evenly across the core. This gives a contact time through the core of around 3.5 seconds.

    What I am trying to find is something that can point me to whether this contact time is enough to get the desired temperature drop. All of the information I can get talks about the rate of heat gain or loss through a surface. What I need is a coefficient that can give me the rate at which air/nitrogen/NEA can gain or lose heat. We have temperature indicators at the inlet and outlet of the chilled water through the core, but we don't have temperature indicators as such on the NEA loop other than an overtemperature indicator.

    It comes down to what contact time we need to have through the heat exchanger core to get the temperature of the NEA down to what is desirable. The suppliers guard the selection methodology only the existing unit has not functioned as claimed and so we are a touch cautious.

    I have looked into other types of heat exchanger, however the plant and has to be thoroughly dismantled and cleaned down inbetween each change of product. As such, completely enclosed exchangers that cannot be taken apart cannot be used.

    Anyhow, any pointers would be appreciated.

  2. jcsd
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