Page 153 - Low Temperature Energy Systems with Applications of Renewable Energy

Heat pumps in the drying industry 143

In general, drying systems that incorporate a heat pump are less electricity intensive
than systems with simple resistance heaters because HP systems use 2e3 times less
electricity to run the compressor than is consumed in the electric heaters.

4.2 Experience with heat pumps for various drying
applications

For many decades, heat pumps have been known as an effective method for increasing
the potential of low-temperature and low-potential heat sources, which contributed to
the growth of their widespread use for the purpose of supplying heat to public facil-
ities. Moreover, their ability to capture the latent heat of vapor condensation of a
low-potential working fluid (by means of its cooling and drying) and transfer that en-
ergy to an air flow at a high temperature level made them suitable for use in heat en-
gineering processes of drying. Following the general trend of improving product
quality, controlling the drying process and reducing energy consumption, many re-
searchers recognized the specific features of the application of heat pumps in drying
technology. This led to a rapid growth of both theoretical and applied studies of
heat pump drying, as can be seen in Table 4.1 [19].
In addition to reducing the energy consumption of a drying plant, one of the main
advantages of using heat pump technology includes creating well-controlled drying
conditions of temperature and humidity of the drying agent. Many researchers have
demonstrated the ability to create precise drying conditions for drying a wide range
of products and improving their quality. A variant of the necessary optimization of
components and the system design for increasing an energy efficient heat pump dryer
is given in Ref. [20]. Any convection dryer may be equipped with a heat pump of the
appropriate design. Heat pumps are most often used in tunnel-type chamber dryers, but
it is also possible to use them in dryers of other types, e.g., rotary kilns. However, in-
stallations requiring high temperatures of a drying agent (above 80 C), such as spray-

ing facilities, are not suitable for the application of heat pump technology.
Figure 4.10 shows the general classification of heat pump dryers (HPDs) by mode
of operation, the number of stages of drying, the number of stages of the heat pump,
and types of auxiliary heat sources. Many installations from the following classifica-
tion have been proposed over the last two decades.
As mentioned above, the effectiveness of the HPD under specified conditions is
determined by the efficiency of the heat pump operation, which is measured by the co-
efficient of performance, COP. In relation to drying processes, researchers from some
countries characterize the efficiency of the drying plant by the specific moisture evap-
oration coefficient, which is commonly known as SMER, defined as follows:
SMER ¼ Amount of moisture evaporated; kg;

per the amount of energy consumed; kWh.

SMER depends on the maximum air temperature in the dryer, relative humidity of a
drying agent, evaporation and condensation temperatures of the refrigerant and the