**1. Introduction**

Pig breeding is a highly productive branch of animal husbandry due to fast-growing pig breeds. Thanks to its nutritional value, availability and extraordinary cooking characteristics, pork meat plays a significant role in the human diet. In Russia, pork meat accounts for 25.6% of the meat resources consumed. The number of pigs in the Russian Federation amounted to 25.9 million by the end of 2021, 90.2% of which were industrially grown [1]. Such enterprises use flow technologies, high animal concentration, and intensive use of breeding stock to achieve maximum productivity. All these factors require a high level of production at all stages. Indeed, any deviation from the optimum production mode inevitably leads to losses.

The indoor climate is determined by a combination of temperature, relative humidity, chemicals, air composition, contaminants, micro-organisms, light, etc. Each of these indicators has a significant impact on animal productivity and must be maintained within strict limits based on the physiological needs of the animals. The most important indicators are temperature and relative humidity [2]. It is reasonable to use these indicators as regulators for the heating and ventilation system.

The body of a pig is covered with very sparse wool. It does not actually protect against external temperature influences. A stable body temperature is maintained by the thermoregulation system, in which the body uses energy to maintain a constant temperature. This energy consumption rate is minimal at an optimum temperature (Figure 1) [3].

Currently, genetics companies have significantly lowered the fat content of pork meat by reducing the thickness of the subcutaneous tissue that acts as natural thermal insulation in pigs. As a result, breeds developed with modern genetic techniques are more sensitive to temperature drops.

Figure 2 shows that a high relative humidity (ϕ > 75%) results in a decrease in pig weight gain (by 20%) and an increase in feed consumption (by 40%) [3].

**Citation:** Ignatkin, I.; Kazantsev, S.; Shevkun, N.; Skorokhodov, D.; Serov, N.; Alipichev, A.; Panchenko, V. Developing and Testing the Air Cooling System of a Combined Climate Control Unit Used in Pig Farming. *Agriculture* **2023**, *13*, 334. https://doi.org/10.3390/ agriculture13020334

Academic Editor: Massimo Cecchini

Received: 22 November 2022 Revised: 11 January 2023 Accepted: 27 January 2023 Published: 30 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Figure 1.** Influence of ambient temperature on the performance of fattened pigs.

**Figure 2.** Effect of relative humidity on Large White pig productivity.

Dry air (relative humidity under 50%) also negatively affects the animal's body, causing irritation of the mucous membranes of the eyes and the respiratory tract, decreased local immunity, increased thirst, and consequently reduced appetite and nutrient absorption.

The above materials show the considerable influence of indoor climate on the productivity of animals.

Indoor climate indicators and animal excretion rates are taken by planners as input data from their manuals. It should be noted that the updated norms currently operating in Russia—"RD-APK 1.10.02.04-12 methodological guidelines on engineering design of pig farms and facilities"—are based on the All-Union Science and Technical Regulations 2–96. The requirements have not changed much as to the emission of heat and harmful gases by animals and indoor climate parameters.

Russian pig breeders have achieved tremendous results in ensuring early maturity and increasing prolificacy [4]. The animal's body has become longer. The ratio of the surface area of a pig to its volume has changed; the thickness of the rump has decreased, which inevitably influences the parameters of its heat exchange with the environment. By comparing domestic and foreign excretion rates and indoor climate requirements, we can see a number of differences [5].

The entry "Nursing sows with piglets" in RD-APK provides data for animals weighing up to 200 kg, although the weight of a typical sow is 200–300 kg. These data are also available in the DIN 18910 standard operating in Germany (Berechnungs- und Planungsgrundlagen fuer das klima in geschlossenen ställen—norms for calculating parameters and indoor climate planning in closed stables (barns)).

The standard indoor climate parameters are also different, as shown in Table 1.

**Table 1.** Standard indoor climate parameters (data taken from RD APK and DIN 18910).


The data presented in Table 1 show that the requirements are basically the same, but DIN 18910 is stricter with regard to the requirements for relative humidity and temperature. However, the CO2 concentration requirements are stricter in the RD-APK and amount to 0.2%, compared with 0.3% in DIN 18910. In our opinion, this is due to the fact that in the Soviet Union central heating systems were used everywhere to provide the required concentration of carbon dioxide. However, decentralized heating systems operating on natural gas are more common now due to their lower cost, as described in the thesis written by D. Tikhomirov [6].

Open combustion heat generators direct the flue gases indoors. The main component of natural gas is methane. The products of its combustion include carbon dioxide and water. With these systems, a carbon dioxide concentration of 0.2% is theoretically unachievable in pig houses during the coldest period, but 0.3% is still achievable.

Climate control systems used in crop production [7] and livestock farming aim to achieve normative values for temperature, relative humidity, and pollutant concentration [8]. Temperature is an important indicator of the indoor climate in livestock buildings [9].

In winter, a good supply of heat is required for heating the air supply [10]. Therefore, exhaust air heat recovery systems offer an effective way to reduce heating energy consumption [11,12]. Supply and exhaust air units with heat recovery exchangers utilize the heat in the exhaust air without mixing exhaust and supply air [13], thus supplying

clean heated air to the production facilities. There are also ventilation systems with spiral recuperators [14,15] for heat recovery in winter and air cooling in summer.

Regenerative heat exchangers are widely used in the industry, e.g., in transcritical CO2 heat pumps [16] and in CO2 heat pump systems with compression/ejection for simultaneous cooling and heating [17].

In decentralized pit-type systems it is important to ensure reliable separation of supply and extract air [18].

However, in summer the challenge is no less important for decreasing the indoor temperature [19]. This is often achieved with individual cooling systems [20].

The importance of energy-saving systems using heat recovery can hardly be overestimated; they can be found in both mobile and stationary equipment [21]. In our opinion, it is reasonable to consider retrofitting supply and exhaust heat recovery units with cooling systems [22]. Such a solution would increase the intensity of equipment use, reducing the payback period.

However, in some cases the use of modular coolers is feasible [23].

This work aims to develop and test an air cooling system in a combined climate control unit used in pig farming.

To achieve this goal, it is necessary to solve the following tasks:


Air temperature can be decreased to the required values in different ways.

For pig farms, air cooling methods can be divided into two types: water-evaporative cooling systems and vapor-compression refrigerating units (Figure 3).

**Figure 3.** Classification of cooling systems.

The vapor-compression refrigeration unit is a closed hermetic system consisting of an evaporator, a compressor, a heat exchanger condenser, a filter-dryer and a throttle connected by pipelines. The unit is filled with a refrigerant with a boiling point lower than that of the cooled medium. The cooled air comes into contact with the surface of the evaporator and transfers its heat to the refrigerant, turning it into vapor. The compressor forces the refrigerant vapor into the condenser, where the latter is turned into its liquid state. Vaporcompression refrigeration systems are used extensively in industry and civil engineering, but they are rarely used in animal husbandry. However, where strict temperature limits are

set, regardless of weather conditions, vapor-compression refrigeration units are the only feasible solution.

With water evaporation cooling, the air is cooled by being blown through a spray chamber or a cartridge package soaked in water and subjected to consecutive heat exchanges. Another option is spraying water into the cooled air stream via nozzles. The evaporated moisture absorbs the heat of vapor formation and the air is cooled.

Water-evaporative cooling is fundamentally simpler and less demanding in terms of maintenance and operating conditions, but it has a number of limitations and its efficiency depends significantly on the temperature and relative humidity of the outdoor air.

In spite of their fundamental similarity, water-evaporative cooling systems are available in a wide range of designs. The most noteworthy are water spray systems and systems with sprayed surfaces. Table 2 presents the results of their comparative assessment.


**Table 2.** Analysis of cooling system efficiency.

Water is evaporated by absorbing the heat of vapor formation. The energy is thus spent to provide the required air exchange and water supply to the evaporation zone [24]. Ejection and nozzle systems do not directly consume energy for the cooling process. However, their operation requires water supplied at a given flow rate and pressure, which results in electricity costs at the water plant [25]. These costs have been taken into account in estimating the costs of electric power for the production of 1 kW of cold.

The following conditions and assumptions are taken into account when comparing cooling systems:


Among the water-evaporative cooling systems, systems with sprayed surfaces are the most effective in terms of reducing the air supply temperature. They are also the most energy efficient.

Based on the above and considering the fact that the climate control unit has a large heat exchange surface area, it is advisable to spray water on it to use the effect of waterevaporative cooling of the supply air.

Water-evaporative cooling is widely used in the climate control systems of various production facilities due to its combination of low cost and high efficiency [26]. However, the system's operation is highly dependent on outdoor air parameters [24]. When designing climate control systems, it is important to analyze the main process indicators [27] and to predict the efficiency of water-evaporative cooling in different climate zones. In this connection, it is advisable to design a computational model of the combined unit [28] to link geometric parameters of the cooling unit based on the recuperative heat exchanger, its performance, total aerodynamic resistance of the system and air parameters at the inlet and outlet of the refrigerating unit.

#### **2. Materials and Methods**

#### *2.1. Description of the System*

The heat-exchanging surfaces are to be cleaned periodically in the process of operation. Consequently, the spraying system performs cleaning and disinfecting functions in addition to cooling [29]. This requires the use of detergent and/or disinfectant solutions. Disinfection can affect both the working surfaces of the unit and the ventilation air, making it possible to disinfect the supply and exhaust air of the serviced area.

The spraying system consists of a pipeline with nozzles, a time switch and a solenoid valve, and is connected to a water supply system for periodic water spraying in the exhaust and supply ducts above the heat exchanger. The sprayed liquid can be used in a flowthrough or a cyclic mode. In the latter case, it requires a storage tank, which can take the form of a sump unit [30].

The offered solution ensures a more intensive use of the equipment, helps save on the number of cooling units and provides for the implementation of previously unavailable functions that are extremely important in terms of longevity, operational efficiency and biological safety—thus increasing the economic efficiency of the system as a whole.

The block diagram is shown in Figure 4.

**Figure 4.** Schematic diagram of the combined climate control unit.

During the warm season, the ventilation system is used to remove excess heat, and the ventilation rate is significantly higher than in winter. In particular, in winter the specific air consumption per 1 kg of live weight of fattened piglets is 0.17 to 0.21 m3/kg/h, and in summertime this value increases to 1.1 to 2.5 m3/kg/h, that is, 5.2 to 14.7 times higher.

In warm periods, air cooling occurs as follows. The indoor air is extracted by separate window or roof fans. The exhaust fan of the unit is reversed, and both channels provide the air inflow. A process diagram of the prototype of the combined climate control unit equipped with the water-evaporative cooling system is shown in Figure 5. The surface of the heat exchanger is sprayed with water via nozzles. On contacting the heat exchanger surface, the water wets it in a film or droplet pattern, creating a large cooling area. The air supply passes through the heat exchanger, comes into contact with the heat exchanger surface, is cooled there and then discharged into the room by the fans.

**Figure 5.** The cooling mode of the combined climate control system: 1—inlet window; 2—inlet duct; 3—heat exchanger; 4—exhaust fan; 5—sump; 6—discharge pipe; 7—supply fan; 8—recirculation opening; 9—recirculation damper; 10—pipeline with nozzles; 11—exhaust duct; 12—pipeline with nozzles.
