Page 155 - Low Temperature Energy Systems with Applications of Renewable Energy

Heat pumps in the drying industry 145

Classification of heat
pump dryers

Mode of techn. Number of Product Number of Auxiliary heat Operation of
process of drying drying stages temperature HP stages supply a heat pump
dryer

Below Above
Single- Multi- freezing Interrupted Cyclic
stage stage freezing
point point
Continuous
Serial Continuous Single-stage Multi-stage
dryers dryers HP dryers dryers

by heat Others:
by convection
conductivity -radio-frequency,
-microwave,
-infrared
Fig. 4.10 Classification of heat pump dryers.

Fig. 4.11 SMER versus inlet temperature of the drying agent.

configurations, depending on the relative location of air and energy flows. An estimate
of the efficiency of heat pump dryers was carried out for some arrangements in
Ref. [24].
Two-stage heat pump systems are used in drying technology to achieve higher en-
ergy efficiency and improved temperature and humidity control of the drying agent
(Fig. 4.13).
Figure 4.13 gives an example of a 2-stage HPD in a parallel configuration, where
the evaporator of the first stage operates at a higher pressure relative to the evaporator
of the second stage [25]. In terms of energy efficiency, a 2-stage heat pump can regen-
erate more latent heat of humid air, due to a higher COP. Comparative data of the COP