Page 168 - Low Temperature Energy Systems with Applications of Renewable Energy

158 Low-Temperature Energy Systems with Applications of Renewable Energy

Fig. 4.24 Operational efficiency of a heat pump and traditional dryer as a function of the outlet

relative humidity of a drying agent, 4 mix :1 e t mix ¼ 65 S;2 e t mix ¼ 55 S;3 e t mix ¼ 45 S.

presented in Figs. 4.24 and 4.25 [46]. The graphs pertain to the theoretical convective
low-temperature process of drying wheat grain in the first drying period.
As can be seen from Fig. 4.24, an increase in the regulated relative humidity of the
drying agent leads to a significant increase in the operational efficiency of the recircu-
lating heat pump dryer plant. In the range of variation 4 mix ¼ 30e70%, h dp increases
by 1.5e5 times for a HPD compared to a traditional dryer plant.
It should be noted that at 4 mix < w30%, the h dp of the HPD does not differ much
from that of the traditional system. That is due to the low moisture content in the
exhaust dryer agent which is a source of low-potential heat for the heat pump dryer
plant.
The efficiency of the HPD increases somewhat with a decrease in the dryer agent
temperature (Fig. 4.25); this comes about because of the shrinking of the temperature
limits of the cycle and, consequently, an increase in the COP. Therefore, it is in the
low-temperature processes of grain drying that the use of the HPD can be recommen-
ded [47]. In general, the low-temperature drying process using heat-pumps as the main
source of heat can include auxiliary heat sources for use during start-up to warm up

Fig. 4.25 Operational efficiency of a heat pump and traditional recirculating dryers as a function
of the temperature of a drying agent t mix at 4 mix ¼ 40%: 1 e traditional dryer; 2 e real cycle
heat pump; 3 ideal Carnot heat pump.