*2.3. M-Cycle Evaporative Cooling (MEC)*

Maisotsenko-Cycle (M-Cycle) evaporative cooling technique is an advanced method for achieving the dew point temperature as compared to the other two traditional systems where cooling limit touches the wet bulb temperature [44,48]. In this system, the cooling effect is produced by evaporative cooling and heat transfer where dew-point temperature is achieved instead of wet-bulb temperature [49,50]. Figure 6 illustrates the working principle of MEC system. This system comprises of three channels in which wet channel is sandwiched in between the two dry channels. The ambient air cools down due to the convective heat transfer between the dry and wet channels when it passes through the dry channel [51]. In this system, the humidity ratio of the inlet and outlet air remains same while the enthalpy decreases [45,52]. The studied experimental systems have been developed at lab-scale and analyzed for poultry air-conditioning. However, when developed at large scale, factors like pressure drop, fan power, availability of fresh water for evaporative coolers, availability of the evaporative media, direction of the system installation in the poultry shed, and the energy consumption should be taken into consideration.

**Figure 6.** Illustration of evaporative cooling phenomenon. (**a**) Typical evaporative cooling phenomenon using evaporative pads. (**b**–**d**) DEC, IEC, and MEC systems phenomenon along with air transformations on psychrometric chart.

### **3. Materials and Methods**

*Mathematical Models for Poultry Heat Generation*

Heat production in poultry birds changes with their body weight and muscle growth due to the consumption of more energy produced from the feed intake [21,22]. Heat production in poultry birds based on live weight is governed by Equation(4) given in literature [15].

$$\mathbf{Q} = \mathbf{60.65} + \mathbf{0.04W} \tag{1}$$

where, Q is the heat production (W/kg) and W is the live weight (kg). Sensible heat production (SHP), and latent heat production (LHP) based on broiler age at various temperatures were calculated using gates model Equations (5)–(12) given in literature [53]. The set of Equations (5)–(12) were replicated from [53] using the self-defined chicken age, and specific temperature ranges (presented below) to investigate the performance of the developed EC systems.

All brooding temperature relations encompass the broiler chickens of age below 15 days. Whereas the rest of relations (i.e., for brooding temperature 15.6 ◦C, 21.1 ◦C, and 26.7 ◦C) are concerned with heat and moisture production of broilers above 15 days and below 48 days.

For all brooding temperatures:

$$\text{SHP} = \text{Kexp}\left(-6.5194 + 2.9186\mathbf{x} - 0.24162\mathbf{x}^2\right) \tag{2}$$

*SE* = 0.284 K; 3 ≤ x ≤ 5

$$\text{LHP} = \text{K}\left(-42.961 + 27.415\text{x} - 2.84344\text{x}^2\right) \tag{3}$$

*SE* = 0.296 K; 2 ≤ x ≤ 5 For temperature (t = 15.6 ◦C):

$$\text{SHP} = \text{K}\left(38.612 - 2.6224\text{x} - 0.072047\text{x}^2 - 0.00066\text{x}^3\right) \tag{4}$$

*SE* = 0.045; 20 ≤ x ≤ 41 LHP = K 22.285 <sup>−</sup> 0.78279x <sup>+</sup> 0.011503x<sup>2</sup> <sup>−</sup> 0.000038x<sup>3</sup> (5)

*SE* = 0.192 K; 20 ≤ x ≤ 43 For temperature (t = 21.1 ◦C):

$$\text{SHP} = \text{K}\left(36,070 - 2.1307\text{x} - 0.058862\text{x}^2 - 0.00051\text{x}^3\right) \tag{6}$$

*SE* = 0.110 K; 20 ≤ x ≤ 39

$$\text{LHP} = \text{K}\left(11.221 + 0.40495\text{x} - 0.02727\text{x}^2 - 0.000353\text{x}^3\right) \tag{7}$$

*SE* = 0.069 K; 20 ≤ x ≤ 43 For temperature (t = 26.7 ◦C):

$$\text{SHP} = \text{Kexp}(5.3611 - 0.1617 \text{\AA}) \tag{8}$$

*SE* = 0.052 20 ≤ x ≤ 23

$$\text{LHP} = \text{K}\left(20.094 - 0.70318\mathbf{x} + 0.015182\mathbf{x}^2 - 0.000108\mathbf{x}^3\right) \tag{9}$$

*SE* = 0.022 K; 20 ≤ x ≤ 42

Where SHP, LHP represent the specific sensible, and latent heat production (W/kg), x represents bird age (days) and SE denotes the standard error of regression.

Temperature-humidity index (THI) is a direct combination of DBT and WBT. The further insights of THI for broilers and layers is given below in Equations (10) and (11) given in literature [54,55].

$$\text{THI}\_{\text{broilers}} = 0.85 \text{t}\_{\text{db}} + 0.15 \text{t}\_{\text{wb}} \tag{10}$$

$$\text{THI}\_{\text{layers}} = 0.60 \text{t}\_{\text{db}} + 0.40 \text{t}\_{\text{wb}} \tag{11}$$

where, tdb, twb represent dry-bulb and wet-bulb temperatures (◦C) respectively and THI represents temperature-humidity index.

Temperature-humidity-velocity index (THVI) is used to analyze the ability to maintain an internal condition constant by including velocity as one of the factors. The further insights of THVI and ET can be seen in Equations (12)–(15) with normal, alert, danger, and emergency regions of homeostasis for the broilers given in the literature [56].

$$\text{THVI} = \text{THI} \times \text{V}^{-0.058} \quad (0.2 \le \text{V} \le 1.2) \tag{12}$$

For 1 ◦C temperature rise

$$\text{ET} = \left(2 \times 10^{29}\right) \times \text{THVI}^{-17.68} \tag{13}$$

For 2.5 ◦C temperature rise

$$\text{ET} = \left(4 \times 10^{13}\right) \times \text{THVI}^{-7.38} \tag{14}$$

For 4 ◦C temperature rise

$$\text{ET} = \left(3 \times 10^{11}\right) \times \text{THVI}^{-5.91} \tag{15}$$

where, THVI represents temperature-humidity-velocity index, and ET stands for exposure time in minutes. Wet-bulb temperature is calculated by Equation (16), as given in literature [57,58].

$$\begin{aligned} \mathbf{T\_{wb}} &= \mathbf{T\_{db}} \tan^{-1} \begin{bmatrix} 0.151977 + (\mathbf{RH} + 8.313659)\_2^1 \\ -\tan^{-1}(\mathbf{RH} - 1.676331) \\ +0.00391838 \,\mathbf{RH}\_2^3 \tan^{-1}(0.023101 \mathbf{RH}) - 4.686035 \end{bmatrix} \end{aligned} \tag{16}$$

### **4. Results and Discussion**

Climate control strategy starts with the estimation of ambient weather details for a region. Such kind of analysis makes it visible that what kind of changes occur in the ratio of DBT and RH in a day. Evaporative cooling systems (DEC, IEC, and MEC) were evaluated in the laboratory under summer conditions in Multan. These systems appreciably reduced the ambient temperature and increased relative humidity to meet the threshold THI limit for poultry birds. Figures 7 and 8 show the experimental analysis of DEC, IEC, and MEC systems for the climatic conditions of Multan. Table 1 shows the summary of performance profile of the experimental DEC, IEC, and MEC under the climatic conditions of Multan (Pakistan) for poultry air-conditioning.

**Table 1.** Summary of performance profile of the experimental DEC, IEC, and MEC under the climatic conditions of Multan (Pakistan) for poultry air-conditioning.


∆ denotes the gradient/difference in ambient the supply air-conditions.

Broiler heat production increases as its weight increases. It is directly proportional to the physiological growth of birds being grown up under healthy conditions. Broilers need optimal environmental conditions to thrive in tropical regions. In these dry and humid regions, broilers need evaporative cooling effect in hot conditions to maintain their body heat and moisture loss. These birds are sensitive to the slight change in temperature and humidity values and become accustomed to high mortality rates. In Figure 9, it is mentioned through graphical representation that with the increase in broiler age, their body weight keeps on increasing. The way this body weight increases, the need for encountering heat and moisture production arises.

For this purpose, an optimal air-conditioning and ventilation technique needs to be devised. The heat from the broiler started increasing from 60 till 155 W/m<sup>2</sup> with the corresponding increase in the weight up to 2700 g. Sensible and latent heat production of broilers is studied with respect to its weight under different temperatures according to Gates model. A set of regression equations were developed to study the heat production patterns in broilers. This situation is graphically presented in Figures 10–13, to understand the effects of high temperature and difference in heat production on the overall welfare of poultry birds. These models made it clear that broilers with growing age attain physiological maturity and meanwhile, their sensible and latent heat production is dependent on ambient temperature. These graphs also illustrate that total heat production with varying

temperatures increase initially and comes to rest. On the other hand, sensible and latent heat production makes narrower gap at lower temperatures while this gap gets wider at higher temperatures.

**Figure 7.** Experimental analysis of DEC, IEC, and MEC systems in the months of April, May, and June for ambient conditions of Multan, Pakistan.

**Figure 8.** Experimental analysis of DEC, IEC, and MEC systems in the months of July, August, and September for ambient conditions of Multan, Pakistan.

**Figure 9.** Relationship of broiler weight and heat production of chickens and broilers.

**Figure 10.** Representation of Broiler age [days] as the function of sensible, latent, and total heat production at all brooding temperatures.

**Figure 11.** Representation of Broiler age [days] as the function of sensible, latent, and total heat production at 15.6 ◦C temperature.

**Figure 12.** Representation of Broiler age [days] as the function of sensible, latent, and total heat production at 21.1 ◦C temperature.

**Figure 13.** Representation of Broiler age [days] as the function of sensible, latent, and total heat production at 26.7 ◦C temperature.

This can be controlled with the help of management guide designed for broilers where THI values can resolve this deficiency. The difference between sensible and latent heat production is seriously important to get a know how about the assimilative capacity inside the poultry environment. The wider the gap is, poorer is the resilience of the environment surrounding the poultry birds.

The sustenance capacity of poultry birds to survive heat stress increases with the increasing velocities as depicted in different layouts of THVI with thermal exposure time (ET) in Figure 14. Figure 14 was reproduced from published literature using the tabular data in a research conducted by Tao et al. [56]. These figures categorize exposure time of broiler chickens with THVI and state that it is the air movement that tells the story other way round if not checked properly.

In these figures, it is shown that increasing velocity to some extent makes it easier to achieve thermal comfort zone for poultry birds. THI is the summation of different percentage compositions of dry- and wet-bulb temperatures. Dry-bulb temperature is measured by simple thermometer while wet-bulb temperature is measured with soaked wet cloth wrapped over the measuring segment of the thermometer. In Figure 15a,b, it is stated that there is correlation between Tab and THI to describe the increasing trend from

the daily data of weather for broilers from daily weather conditions to calculate THI which is a function of Tdb and RH.

**Figure 14.** Thermal comfort zone for poultry birds with respect to THVI [◦C] and ET [min] at (**a**) 0.2 m/s, (**b**) 0.4 m/s, (**c**) 0.6 m/s, (**d**) 0.8 m/s, (**e**) 1.0 m/s, (**f**) 1.2 m/s velocities, respectively, reproduced from tabular data published by [56].

From Figure 15, an empirical equation is obtained with appreciable R<sup>2</sup> value. Figure 15 concludes that the range of THI variation is less in broiler chickens (Figure 15a) as compared to relatively higher range of THI variation in egg-laying chickens (Figure 15b). This also explains the assimilative capacity of natural environment to resist heat stress to some extent in both cases of broilers and layers, respectively. Figure 16a,b proposes a THI pyramid (i.e., amalgamation of THI) based on daily wet-bulb temperature range. In Figure 16, the green, blue, and red color lines overlaid on top of the regression lines represent boundaries of

different zones based on allowed THI of both broilers and egg-laying chickens. In Figure 16, chart area covered underneath the green line represents the threshold zone, chart area covered between green and blue line represents the alert zone, area covered between blue and red line represents the danger zone, and area covered above the red line shows the emergency zone based on the allowed/comfortable THI for both cases. The resilience of layers chicken is more in pyramid (i.e., amalgamation of THI as shown in Figure 16) as compared to broiler chicken with enlarged elliptical trend (in Figure 16). Figure 17a,b explains dry bulb representation with the RH and THI relationship for broilers and egglaying chickens and states that this trend for broiler chickens was found to be more tolerant to the thermal stress (i.e., heat stress due to ambient air conditions) effectively as compared to layer chickens, which justifies the published literature against the thermal stresses of broilers and layers as shown in Equations (13)–(14). According to Figure 17a, the broiler chicken has higher range of temperature-humidity-index at a specified relative humidity indicating relatively more thermal resilience as compared to layer chicken (Figure 17b) at same relative humidity conditions. Feasibility calendar of EC (DEC, IEC, and MEC) systems for the ambient conditions of Multan (Pakistan) is presented in Figure 18. The ambient conditions of the study area were recorded for a year using standard temperature sensor and were later analyzed for poultry air-conditioning.

**Figure 15.** Representation of dry-bulb temperature and THI as a correlating factor for (**a**) broiler chickens, and (**b**) egglaying chickens.

**Figure 16.** Representation of wet-bulb temperature and THI as correlating factor for (**a**) broiler chickens, and (**b**) egglaying chickens.

tan, Pakistan.

**Figure 17.** Representation of dry-bulb temperature as a function of relative humidity and THI for (**a**) broiler chickens, and (**b**) egg-laying chickens. *Sustainability* **2021**, *13*, x FOR PEER REVIEW 20 of 23

**Figure 18.** Feasible calendar showing the applicability of evaporative cooling systems for the ambient conditions of Mul-**Figure 18.** Feasible calendar showing the applicability of evaporative cooling systems for the ambient conditions of Multan, Pakistan.

economic loss. Heat stress causes severe impacts on poultry health such as mortality rate and body weight increases. Economically, these birds are the cheapest source of proteins in South Asia. Furthermore, poultry farming is catching its momentum with millions of capital investment by many in a quest to gain much more in a short span of time. In view of presented work, the air-conditioning process for broiler chickens carries significant importance. The major issue revolves around is the optimal control of temperature and humidity such as THI. Any minor fluctuations of index wreak havoc for the investor and birds ultimately. For all this there was an open window for the current research to oversee an energy-efficient evaporative cooling system that could condition the air without raising humidity levels. In this regard, the EC systems (DEC, IEC, and MEC) were studied under the weather conditions of Multan in line with the regression equations from Gates model. In fact, the empirical relations of sensible and latent heat production minimized the need to erect a whole new setup to raise poultry birds for studying the heat and moisture production. Moreover, the experimental results of the studied EC systems conclude that the MEC system could be considered as a viable alternate option as compared to the traditional DEC systems used for poultry air-conditioning due to the psychrometric and climatic (i.e., monsoon season) limitations of the DEC system. However, all the studied standalone evaporative cooling systems are still limited by the ambient air conditions.

**5. Conclusions** 

### **5. Conclusions**

Poultry industry is affected by (sensible/latent) heat stresses and results in substantial economic loss. Heat stress causes severe impacts on poultry health such as mortality rate and body weight increases. Economically, these birds are the cheapest source of proteins in South Asia. Furthermore, poultry farming is catching its momentum with millions of capital investment by many in a quest to gain much more in a short span of time. In view of presented work, the air-conditioning process for broiler chickens carries significant importance. The major issue revolves around is the optimal control of temperature and humidity such as THI. Any minor fluctuations of index wreak havoc for the investor and birds ultimately. For all this there was an open window for the current research to oversee an energy-efficient evaporative cooling system that could condition the air without raising humidity levels. In this regard, the EC systems (DEC, IEC, and MEC) were studied under the weather conditions of Multan in line with the regression equations from Gates model. In fact, the empirical relations of sensible and latent heat production minimized the need to erect a whole new setup to raise poultry birds for studying the heat and moisture production. Moreover, the experimental results of the studied EC systems conclude that the MEC system could be considered as a viable alternate option as compared to the traditional DEC systems used for poultry air-conditioning due to the psychrometric and climatic (i.e., monsoon season) limitations of the DEC system. However, all the studied standalone evaporative cooling systems are still limited by the ambient air conditions. This problem can be resolved by further research on experimental desiccant dehumidification-based evaporative cooling systems for poultry air-conditioning. Therefore, the present study concludes the MEC as the best alternate option to the traditional DEC system used for poultry air-conditioning in Multan, Pakistan.

**Author Contributions:** Conceptualization, K.S., M.B., and M.S.; data curation, K.S. and M.B.; formal analysis, K.S. and M.B.; funding acquisition, M.S.; investigation, K.S., M.B., H.A., M.F., T.M., U.S., I.A., and M.I.H.; methodology, K.S, M.S., M.B., M.F., and T.M.; project administration, M.S.; resources, M.S. and T.M.; supervision, M.S. and T.M.; validation, M.S. and T.M.; visualization, M.B., H.A., U.S., I.A., and M.I.H.; writing—original draft, K.S.; writing—review and editing, M.S., M.B., H.A., M.F., T.M., U.S., I.A., and M.I.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** The Bahauddin Zakariya University, Multan-Pakistan funded this research under the Director Research/ORIC grant titled "Development and performance evaluation of prototypes of direct and indirect evaporative cooling-based air-conditioning systems.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data is contained within the article.

**Acknowledgments:** All this work is part of the Ph.D. research of Khawar Shahzad (1st Author). This research work has been carried out in the Department of Agricultural Engineering, Bahauddin Zakariya University, Multan-Pakistan. The Bahauddin Zakariya University, Multan-Pakistan funded this research under the Director Research/ORIC grant titled "Development and performance evaluation of prototypes of direct and indirect evaporative cooling-based air-conditioning systems," awarded to Principal Investigator Muhammad Sultan.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **Abbreviations**


### **References**


*Article*
