Drying processes can consume from 5% to 25% of the total electricity consumed by various industrial enterprises that use dehumidification during production [
1,
2]. However, in the wood and textile industries, this value can reach 70% and 50%, respectively [
3]. Due to such high participation and the desire to obtain products or provide services based on ecological and economical technologies and processes, clients are looking for solutions that, after appropriate analysis, can reduce energy consumption to varying degrees and thus simultaneously minimise operating costs. There are several types of drying devices, but the most popular are those based on electric heaters and exploiting the thermodynamic processes of heat pumps. Dryers based on refrigeration systems have lower energy consumption than traditional solutions, which is why they are becoming an increasingly popular choice, especially in devices in which the heat demand is up to several kW. As noted by Lee and Kim [
4], the
SMER coefficient for devices with a refrigeration system is three times higher than that for heater dryers. However, in the report of the US Department of Energy [
5], it was determined that home clothes dryers with a heat pump are capable of using up to 50% less electricity but at a higher initial cost and longer drying time, which may also discourage potential buyers.
The drying process involves the removal of water from the material being dried. This is most often achieved by using hot and dry air. In the most commonly used drying devices, an electric heater is used to heat the external air, which in turn goes to the drying chamber, where water is evaporated from the material subjected to this process. The hot and humid air is then removed from the chamber, and another volume of the hot medium enters in its place. This system is called open-loop drying, which is characterised by low efficiency due to the loss of energy in the form of the waste heat contained in the hot air. The air circulation in a dehumidifier using a heat pump usually works as a closed system; the heated air on the condenser is directed to the drying chamber, and then it goes to the evaporator operating below the dew point temperature, where water vapour from the air condenses on its cold elements, the pipes and fins, intensifying the heat exchange process. The dehumidified air is directed back to the condenser, i.e., back to the starting point. Depending on the application, the heat pump can recover both latent and sensible heat from the working air, which has the potential for energy savings, especially for closed air circuits [
6]. In addition, the dried air is capable of absorbing more water in the next cycle, which translates into the legitimacy of striving to obtain a drying medium of the highest possible temperature and low relative humidity. Due to the potential energy savings and the ability to control the parameters of the air at the outlet, the application of this technology in various industries has been described by researchers: agriculture [
7,
8,
9,
10], medicine [
11,
12], the chemical industry [
2], the textile industry [
13,
14,
15], the wood industry [
16,
17], and the paper industry [
18]. Most scientists focus on researching innovation in the field of new drying methods and optimising the process and design to ensure lower energy consumption and improve overall efficiency [
19]. In his work, Dean [
20] built an analytical model simulating the dehumidification process in which he stated that the consumption of electricity was primarily influenced by the temperature and relative humidity of the air. Rezk and Forsberg in their work [
21] analysed the process of designing an internal duct tumble dryer based on CFD software. They obtained a reduced pressure drop of up to 23% and an improved uniformity at the outflow boundary. The benefits of this approach include a more time effective design process and knowledge sharing with the CAD-engineer. Another publication [
22] examined the influence of air temperature and flow velocity on the efficiency of the entire system. The validity of using an inverter compressor with variable rotational speed on the moisture extraction coefficient was also analysed [
23]. There are many publications dedicated to the design and optimisation of heat exchangers [
24,
25,
26,
27,
28,
29]; there is also a mathematical model for a complete set of exchangers (evaporator and condenser) [
30] in a finned air dryer. In a book, Zalewski and Niezgoda-Żelasko presented mathematical algorithms for calculating many types of single heat exchangers (i.e., finned heat exchangers) [
24]. In article [
25], the authors proposed an algorithm for the evaporation of melted water from a finned evaporator, which flowed into a container placed on the top of the compressor. In a publication [
26], Zalewski presented a method of conducting thermal calculations of the surface area of a finned heat exchanger with known computational efficiency used for a ground heat pump. In another article [
27], the same author presented a mathematical algorithm for thermal and hydraulic calculations that can be used for finned evaporators with a fan forcing the flow operating at different ambient temperatures. In the book [
28], Shah and Sekulić reviewed the methodology of designing and calculating heat exchangers, presenting the basis of calculations for the ε-NTU method. The same relationships and calculation examples are also presented in one of the chapters of the publications [
29].
As shown above, although the topic discussed in this article has been widely described by other authors, it has not been exhausted, especially in the context of optimising processes and construction. Due to the fact that heat pump-based dehumidifiers are an issue for the future, it is necessary to investigate the best possible efficiency of the refrigeration system as well as the optimal airflow in the zone where the dehumidified products are located. The influence of evaporation temperature on the efficiency of the process analysed in this work is an issue affecting the overall efficiency of the system. The connection of these two parameters would enable estimation of whether it is profitable to obtain lower evaporation temperatures that may result in a higher compressor pressure and increased electricity consumption in order to obtain a higher
SMER index, which can be calculated by Equation (1) as the ratio of the mass of evaporated water (
) to the electricity consumption of the dryer (
):
This article will enable the optimisation of the selection of refrigeration system components at the design stage, which will shorten the testing time of devices and speed up production. The analysis will be conducted for positive evaporation temperatures in order to ignore the effect of ice layer build-up on the evaporator elements.