Page 169 - Low Temperature Energy Systems with Applications of Renewable Energy

Heat pumps in the drying industry 159

Fig. 4.26 HPD with an auxiliary heater. C hp , heat pump condenser; E vhp , heat pump evaporator;
H, auxiliary heater.

empty hoppers or at times of heavy load on the compressor of the heat pump. One of
the options for implementing such a technology is presented in Fig. 4.26.
Figure 4.27 presents a schematic diagram with recirculation of the exhaust drying
agent into a mixing chamber after the primary air flow has passed through the auxiliary
heating device, H (option (A) in the figure). This allows for adjusting the humidity and
cooling the heated drying agent to the required parameters. Auxiliary heat source
dryers can also be made with recirculation before the auxiliary heating device (option
(B) in the figure).
Using auxiliary heat sources in drying units in addition to heat pumps, using the
heat of an exhaust drying agent, and the latent heat of condensation of evaporated
moisture, all lead to stabilization of the dryer operation, improvement in controlling
dryer operation, equalization of load on the heat pump, and, consequently, to reducing
external energy for moisture removal from the grain.
Figure 4.28 illustrates a grain HPD with “warm” and “cold” drying chambers, and a
regenerative heat exchanger installed between the HP condenser and the electric heater
[67]. According to the authors, this system significantly reduces energy costs by using
the heat of the exhaust drying agent. Exhaust air is directed to a regenerative heat
exchanger, where its temperature decreases, thereby reducing the thermal load on
the cooler-evaporator in a regeneration cycle and consequently decreasing the energy
consumption. Using the exhaust air heat provides an increase in the temperature of
fresh air entering the electric heater, while at the same time reducing the energy costs

Fig. 4.27 Heat pump drying plant with an auxiliary heater, H, and two different modes of
recirculation, options (A) and (B).