Analysis of energy consumption during convective drying of fruits and berries

The work is dedicated to the calculation of energy costs for the realization of the process of convective drying of fruits and berries in a suspended layer. The energy consumption for the fan drive for organizing the air flow, providing the phenomenon of fluidization of fruits and berries, as well as the costs for supplying heat to the dehydration object have been calculated. The energy consumption was determined for various options of energy supply: using a heat pump and due to the operation of thermoelectric heaters (TEH). It is found that the largest proportion of the energy consumption for air circulation organization. It has been established that from the energy point of view, of all the investigated freons, the refrigerant R410 is the most efficient, the total energy consumption for dehydration of 1 kg of irgi berries with it is 7102 kJ, for honeysuckle 9765 kJ / kg, for lingonberry 7989 kJ / kg. Comparative analysis revealed that the use of a heat pump installation of convective drying fruits and berries in the fluidized bed reduces the power consumption by an average of 13% in comparison with drying by using heaters to heat the coolant.


Introduction
This work is devoted to the analysis of energy costs when using a heat pump in the processes of convective drying of fruits and berries. [1] The use of energy-efficient plants and technologies for processing food raw materials allows to reduce this cost item, and hence the cost of the product, which is an important factor for increasing the competitiveness of manufacturing enterprises.
The use of low-temperature technologies in drying processes makes it possible to one way or another to reduce energy consumption and shorten the dehydration time. Quite a lot of domestic works are devoted to the comparative analysis of energy costs when using certain methods of dehydration. [2,3] Most of the works show that modern innovative drying equipment can achieve high results in terms of energy savings through the use of heat pumps and refrigeration units. This work is devoted to the analysis of energy costs when using a heat pump in the processes of convective drying of fruits and berries. Some background information on the technological regimes adopted on the basis of studies conducted earlier. [4,5,6]

Materials and methods
To calculate the energy consumption for the operation of a heat pump during convective drying of fruits and berries in a suspended layer, first of all, it is necessary to determine the thermal load on the condenser of the refrigerating machine.
Let's consider the calculation using the example of irgi. The duration of convective drying of irgi at 50 ° C is 8 hours. According to the results of the studies presented in Chapter 5, during this time it is possible to remove 70% of the mass of raw materials. We define the amount of moisture removed in the average for 1 hour where m is the initial mass of the product, kg; τ -drying time, hour.
However, in addition to the heat removed from the berry, it is also necessary to take into account heat losses to the environment and heat inflows to the evaporator. Based on the calculated data, we will accept the above heat losses qp = 450 kJ / h. These heat losses were calculated taking into account the loading of the working chamber of 10 kg. With respect to 1 kg of product, specific heat losses will be respectively 45 kJ / h Then the total load on the heat pump condenser is determined by the sum of the heat loads: Taking the final temperature difference in the evaporator Δtи=tи2-t1=1-4=-3° С, we calculate the evaporation temperature: At a given evaporation temperature, the air leaving the evaporator will have a temperature of 12 ° C and a relative humidity of about 10%, which ensures the most effective drying process, therefore, using a lower evaporator temperature is impractical, which is confirmed by the data from the graph in Fig.   Fig. 2. Dependences of the relative humidity of the air leaving the evaporator on the boiling point of the refrigerant.
Based on the selected convective drying modes, the condensation temperature should be 50 ° C. We accept the temperature difference in the condenser 5°С.
Using the T-S diagram for refrigerant R134A, determine the cycle set points (Table. 1). We accept the internal adiabatic efficiency of the compressor η1i=0,8. Determine the inner workings of the compressor: The specific heat load of the evaporator will be calculated as follows: Next, we determine the specific thermal load of the capacitor.
Find the specific heat load of the pre-capacitor.
‫ݍ‬ = ݈ + ‫ݍ‬ = ‫ݍ‬ + ‫ݍ‬ pc = 175 kJ/kg We calculate the mass flow rate of the refrigerant. where v1 is the specific volume of the working substance sucked in by the compressor; Ocompressor delivery ratio,, O=f(phg/pbc). Here phg -pressure to which the process of compression of the working substance in the compressor is carried out; pbc -pressure of the working substance entering directly into the working cavity of the compressor. We determine the power spent on overcoming friction and driving auxiliary devices using the empirical formula: where pif = (40y90)•10 3 Pa friction pressure. Next, we find the indicated compressor power: ,଼ = 39,26 kJ (13) where Kicompressor indicator efficiency.
Then the capacity of the chiller Ne (kJ), which must be provided for organizing the convective drying process is determined as follows. ܰ = ܰ + ܰ = 39,26 + 7,83 = 47,09 kJ (14) In addition to the above power, it is also necessary to take into account the energy consumption for organizing air movement..
Determine the volume of air required to dry 1 kg of product using the following formula: ܸ = 3600 ⋅ ߱ ⋅ ‫ܨ‬ ⋅ ߬ = 3600 ⋅ 4 ⋅ 0,176 ⋅ 8 = 20275m 3 (15) where ω -air speed, m / s (we accept 4 m / s [4]); Fс -air duct cross-sectional area, m 2 ; τdrying time, hour. Determine the Archimedes number by the formula:  where Gp -product weight (accepted Gp= 1kg); Fр -area occupied by the product on the grid, м 2 . Next, we find the aerodynamic resistance of the supporting grid with cells of size 3u3mm and free cross-section for air passage E=0,308.
where Kb -Fan efficiency, for centrifugal fans the efficiency is approximately Kb=0,76.

Results and Discussion
Thus, from the energy point of view, of all the investigated freons, the most effective refrigerant is R410, the total energy consumption for dehydration of 1 kg of irrigated berries is 7102 kJ. For the rest of the studied berries, the average energy consumption is: -for honeysuckle -9765 kJ/kg; -for lingonberry -7989 kJ/kg; -for black currant -17755 kJ/kg; -for sea buckthorn -12428 kJ/kg. It should be noted that the largest share is made up of energy costs for organizing air circulation, therefore, the above energy costs will differ significantly due to different drying times.
To compare the efficiency of using a heat pump instead of heating elements, the energy consumption for convective drying was also calculated in the case of using heating elements for heating air. The power required to organize air heating due to heating elements was determined by the following: where Q -the required amount of heat to organize the drying process, kJ/h; η -Efficiency of heating elements; kп -coefficient taking into account heat loss to the environment.

Conclusion
Total costs consisted of costs for heating and air heating elements on air circulation organization costs.
In fig. 3 shows graphs comparing energy consumption for convective drying of berries using heating elements and using a heat pump. Comparative analysis shows that the use of a heat pump in a unit for convective drying of fruits and berries in a suspended layer can reduce energy consumption by an average of 13% compared to drying when using heating elements for heating the coolant. Thus, from the point of view of energy costs, the efficiency of using a heat pump for organizing the process of drying fruits and berries has been proven.