ISHS Acta Horticulturae 148: III International Symposium on Energy in Protected Cultivation
EVALUATION OF POLYTUBE HEAT EXCHANGERS FOR GREENHOUSE VENTILATION |
| Authors: | E. Brundrett, D.A. Lawson, T.J. Jewett, R. Quist |
| Abstract:
Polytube heat exchangers appear to provide a low cost method of extracting heat from greenhouse exhaust air. The recovered heat can be used to warm fresh air, thereby reducing ventilation heat demands. Simple two tube counter-flow heat exchangers were examined, both analytically and experimentally, and were found to require long lengths to provide adequate heat exchange. A more compact seven tube-in-shell design is described. The unit provides a thermal effectiveness of up to 70% for lengths of 30 meters, based upon a model of the heat transfer process. The model has been constructed to take into account condensation, in addition to conventional heat transfer properties of the air streams. The model provides an iterative solution of the heat exchanger, yielding thermal effectiveness, inlet and outlet air temperatures, location of condensation, and heat transfer rate. Two heat exchangers have been constructed and tested. The experimental results are in very good agreement with their model values, with the model values being slightly conservative. Condensation that occurs within the heat exchangers can be directed to collection points, via condensate lines, since it occurs on the shell or exhaust air side. The passage sizes are large enough that frost formation at the cold end can be ignored, thereby saving the cost and energy required for defrost equipment. The heat exchangers were mounted on end walls of the greenhouses, but could be mounted under growing benches or other appropriate locations. | |
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