Drying is one of the most energy intensive unit operations. In many applications the drying temperatures required are low enough to make the inclusion of a heat pump in the system worthy of consideration. Five drying/dehumidification systems, including three with heat pumps, have been compared theoretically on the basis of specific power consumption (SPC), (i.e. the energy supplied per unit of moisture condensed) and primary energy consumption (PEC) which is (SPC) divided by the efficiency of primary energy conversion. The efficiency of each system is improved as the relative humidity of the air leaving the dryer is increased. The optimum is, however, very flat and a heat pump should be advantageous when a minimum relative humidity of 30 per cent is acceptable within the drying chamber. A closed cycle dryer is shown to be the most advantageous but requires careful matching.

Experimental data from the heat pump dehumidifier described in Part II (Tai et al, 1982) are presented in terms of the specific power consumption (SPC). Plots are presented of (SPC) against superheat at the exit of the evaporator for various air bypass ratios and (SPC) against approach velocity of air to the evaporator for various dry bulb temperatures. These all show a minimum value of (SPC). A plot of (SPC) and (COP)A against the superheat at the exit of the evaporator shows that the minimum value of (SPC) coincides with the maximum value of (COP)A.

An experimental heat pump dehumidifier is described. Actual coefficients of performance (COP)A are plotted against the gross temperature lift (TCO ‐ TEV) for various bypass ratios and air velocities. Interpolated values of (COP)A for a specified temperature lift were obtained by fitting each set for various dry bulb temperatures of air leaving the humidifier using a linear equation. These values of (COP)A are plotted against the linear velocity of the air stream approaching the evaporator at different dry bulb temperatures. The curves show a maximum of (COP)A at approach velocities in the region of 1·6 ms−1.

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