THE DEVELOPMENT OF A LOW-TEMPERATURE HEAT PUMP GRAIN DRYER
Abstract
Heated-air, forced convection drying is an energy-intensive process in the production of grain in this country. Liquefied petroleum gas in high-temperature dryers is the predominant method in use today. Escalating costs, unreliable supplies, and the current emphasis on energy conservation have provided the impetus for examining alternative drying methods. One potential alternative is low-temperature heat pump grain drying. The objective of this work is to synthesize a prototypical heat pump dryer which is both energy-efficient and cost-effective. In view of the fact that heat pumps have never been designed specifically for grain drying, this work is based on the premise that a viable system must be designed on the basis of a fundamental understanding of heat pump operation. Consequently, two analytical tools--a heat pump design and simulation model--are developed. Both are based on the fundamental heat transfer and fluid dynamic processes present in the major components of a heat pump system. These mathematical models are presented in depth. A rationale for determining the best heat pump system in conjunction with the design model is developed. Moreover, the simulation model is used to explore the conditions necessary to control evaporator frost formation via recirculation of the drying airstream. Pertinent details of the heat pump configuration, fabrication, and instrumentation are presented. Validation of the analytical methods is achieved from analysis of test results acquired during a full-term, corn drying field demonstration of the developed system. As a standard for comparison, the results from a concurrent test of an electric resistance dryer are presented. Each major heat pump component mathematical model is individually compared to the experimental results. Moreover, the system of component models as embodied in the simulation model is evaluated by comparing predicted results to experiment. The results of this effort indicate that a low-temperature heat pump grain dryer is a viable alternative drying system. Evaporator frost control is effectively achieved by recirculation of the drying airstream if executed according to the control method deduced from simulations of the heat pump drying system. Generally, the accuracy of the component and simulation models is quite good.
Degree
Ph.D.
Subject Area
Mechanical engineering
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