**2. Proposed system**

#### **2.1 Design concept**

The current system consists of the following major components: a heat exchanger (HE), a mixing chamber (MX), fans, a water pump, and nozzles as shown in **Figure 2**. The heat exchanger consists of three identical channels designated for supply air (1 middle channel) and working air (2 side channels). The channels are constructed using a conductive impermeable wall. Spacers are used to maintain the required channel gap to avoid sheet bulging at high air flow rates. The mixing chamber is constructed using acrylic sheets. The chamber is equipped with water spray systems (nozzles, pump, and supplementary water float inlet), and working air manifolds. The sump of the humidifier contains water that is used in a recirculation manner for cooling the working air. As the water is consumed during evaporation, the supplementary water is fed to the system from the reservoir through a float-controlled valve. Besides, the system is also equipped with two axial fans for product air and working air supply. The working cycle of the system is presented below.

In the proposed system the hot outer air undergoes sensible cooling (1 to 2) in the middle (dry) channel of the heat exchanger. The outdoor air flows from the bottom of the channel to the top. An axial fan is employed to maintain the required flow rate and overcome the channel frictional pressure drop. This air stream rejects the heat to the working air in the two side channels flowing in the counter-current direction (from top to bottom). While the working air stream first enters the mixing chamber (as hot outer air) where it is mixed with fine water mist generated using atomizing nozzles. This air-water meshing causes the hot air to release its heat through evaporation and gain high humidity (100%) (1 to 3). This cold humid working air carrying fine water particles enters the heat exchanger and extracts heat from the dry channels. The humidity drops and the temperature rises as the working air moves along the heat exchanger because of latent and sensible heat transfer respectively (3 to 4). The product air (dry and cold) is supplied to the conditioned space and the working air (hot and humid) is discarded to the outer environment.

#### **2.2 Experimental test rig**

Based on the above-presented design concept, an experimental test rig was developed as shown in **Figure 3**. Different components of the system are labeled as, A: heat exchanger, B: mixing chamber, C: outer air inlet to heat exchanger, D: product air outlet, E: air inlet to MX, F: working air inlet to HE, G: working air outlet, and H: atomizing nozzles. The heat exchanger was constructed using 0.025 mm thick high thermal conductivity (235 W/m K) Aluminum sheets. The channel gap was maintained at 5 mm using closed foam spaces. The acrylic sheets were used to support the heat exchanger structure and act as adiabatic walls for working air channels. The

*Advancements in Indirect Evaporative Cooling Systems through Novel Operational… DOI: http://dx.doi.org/10.5772/intechopen.107305*

**Figure 2.** *Schematic diagram of the proposed system.*

#### **Figure 3.** *Experimental test rig front and side view.*


#### **Table 1.**

*Geometric characteristics of the system.*

inlet and outlet manifolds of the heat exchanger were developed using an in-house 3D printing facility. The design characteristics of the system are summarized in **Table 1**.

A real-time data acquisition system is used to monitor and record temperature data during experimentation. For this purpose, dry and wet bulb temperature sensors are installed at different locations in the system. General purpose thermistor probes are used for temperature measurements. For wet bulb temperature, the measuring station is developed using high capillary action felt material with a continuous water supply. Meanwhile, the flow rates in the system are measured using a hot wire anemometer (Testo 405i) at product air and working air outlets.

*Advancements in Indirect Evaporative Cooling Systems through Novel Operational… DOI: http://dx.doi.org/10.5772/intechopen.107305*


#### **Table 2.**

*Process parameters.*


**Table 3.** *Instrumentation details.*

Detailed experimentation of the developed system was conducted to capture the performance picture of the system. For this purpose, the system is operated at varying outdoor air temperature conditions. The process parameters considered in the study are presented in **Table 2**. The instrumentation details are summarized in **Table 3**.

#### **2.3 Performance assessment**

The performance of the cooler is measured in terms of heat extraction (cooling) capacity from the product air stream as it moves along the heat exchanger from 1 to 2. It is calculated in terms of airflow rate, specific heat, and temperature drop [40, 41].

$$
\dot{Q}\_{1-2} = \dot{m}\_{PA} c\_p \left( T\_1 - T\_2 \right) \tag{1}
$$

The heat rejected by the dry channel air stream during processes 1–2 is taken by the working air stream in the wet channel (3–4) as a combined latent and sensible heat. This heat transfer process is given as:

$$
\dot{Q}\_{1-2} = \dot{Q}\_{3-4} = \dot{m}\_{WA} c\_p \left( T\_4 - T\_3 \right) + \dot{m}\_{vapor} \lambda\_{\text{fg}} \tag{2}
$$

The cooler performance is also measured in terms of the cooler performance indicator (CPI) which is the ratio of cooling produced to the energy consumed [42].

$$CPI = \frac{\dot{Q}\_{1-2}}{\dot{E}\_{input}} \tag{3}$$

The energy consumption is calculated in terms of fan energy and pumps energy used to maintain the required air and water flow rates, respectively. It can be used in terms of fluid flow rate, pressure differential, and component efficiency [43].

$$
\dot{E}\_{in} = \dot{E}\_{blower} + \dot{E}\_{pump} = \frac{\dot{V}\_{air} \Delta P\_{HE}}{\eta\_{blower}} + \frac{\dot{V}\_{water} \Delta P\_{water}}{\eta\_{pump}} \tag{4}
$$

The pressure drop in the heat exchanger channels is calculated in terms of friction factor, channel length, flow velocity, and equivalent diameter [40, 44].

$$
\Delta P\_{ch} = f \frac{L}{D\_h} \frac{\rho \, V\_{air}^2}{2} \tag{5}
$$

The fan and pump power are calculated using pressure drops of air and water with corresponding flow rates. The maximum power input calculated is 4 W which is constant for all operating conditions.

The above equations are valid under the following standard assumptions.


#### **3. Results and discussion**

The performance of the proposed indirect evaporative cooler was investigated in terms of temperature drop, cooling capacity, and cooling performance index. The most important parameter in indirect evaporative coolers is the supply air temperature. This is because all other performance indices are governed by the supply air temperature. For this purpose, the cooler was tested under different outdoor air conditions to record the supply air temperature trend for 4-to-5-hour continuous operation. **Figure 4** shows the typical product air temperature trends for an outdoor air temperature of 40 � 0.5°C. It shows that the product air was obtained at a uniform temperature of 25 � 0.5°C. It implied the steady cooler performance during the whole operational time producing the uniform cool product air. Therefore, the cooler is suitable for continuous longer operations without the development of any longitudinal heat conduction effect or heat storage in heat exchanger walls. The visual inspection during experimentation shows the mist evaporation on the walls which extracts heat from the wall thus keeping it at the same temperature. Meanwhile, it is also important to emphasize that the product air was supplied at constant absolute humidity of 10 g/kg because of sensible cooling.

The temperature trends for the working air stream are shown in **Figure 5**. It is observed that the hot outer air entered at 40 � 0.5°C in the mixing chamber. It was cooled to the wet bulb temperature of 23 � 0.5°C by mixing with water mist. During *Advancements in Indirect Evaporative Cooling Systems through Novel Operational… DOI: http://dx.doi.org/10.5772/intechopen.107305*

**Figure 4.** *Product air temperature trend.*

**Figure 5.** *Working air temperature trend.*

mixing the working air achieved 100% relative humidity (ω = 18 g/kg). This cold and humid air then enters the heat exchanger and extracts heat from the product air stream. **Figure 6** shows the temperature trends for working air at the heat exchanger inlet and the product air at the heat exchanger outlet. It showed that the cooler performed close to wet bulb temperature with a maximum temperature differential across the air streams of 2–3°C thus giving the highest cooling performance.

**Figure 6.** *Product air and working air temperature trend.*

The performance of the cooler in terms of product air temperature at different outer air conditions is presented in **Figure 7**. It shows that the product air temperature increased as the outer air inlet temperature increased. A temperature rise of around 7° C (from 22.6 to 29.3°C) was observed for the outer temperature rising from 29 to 43° C. It suggested that the cooler can be used for any other higher or lower outer air temperature conditions with slight variation in the product air temperature. However, it is also important to emphasize that the variation in product air temperature is less

**Figure 7.** *Product air temperature trends at different outer air temperatures.*

#### *Advancements in Indirect Evaporative Cooling Systems through Novel Operational… DOI: http://dx.doi.org/10.5772/intechopen.107305*

compared to the outer air conditions. It showed the performance of the cooler close to the wet bulb temperatures for all the operational conditions.

The effect of outer air temperature on the cooling capacity is presented in **Figure 7**. Correlated to the temperature drop, the cooling capacity of the cooler was observed to be increasing with the increasing outer air. For instance, the cooling capacity varied from 50 W to 110 W as the outer air temperature increased from 29 to 43°C. This increase in cooling capacity was achieved due to increasing temperature drop at higher outer temperature conditions. Similarly, the cooling performance index at different outer air temperature conditions is presented in **Figure 8**. It showed that the cooling performance index increased at higher outer air temperature conditions. The increase in CPI was due to an increase in the cooling capacity. For instance, the CPI increased from 13 to 28 as the outer air temperature increased from 29 to 43°C. Meanwhile, it is also worth mentioning that the power input was considered constant for all the cases because of the fixed pump and fan installation. However, less power is required (theoretically) at lower outer air conditions because of low water requirement and less working air velocity. Therefore, intelligent system control regulating the air and water supply commensurate to the outer air temperatures can offer the same higher CPI at lower outer air temperature conditions (**Figure 9**).

Besides promising energy performance, the proposed system also resolves the issue of high humidity in conventional water-based cooling systems. The outlet of the system for all operating conditions remains within the comfortable zones recommended by ASHRAE (Winter: RH = 75%, T = 21°C, Summer: RH = 53%, T = 27°C) and ISO (Winter: RH = 30–70%, T = 23°C, Summer: RH = 30–70%, T = 26°C). So, the issues associated with high humidity are eliminated while using the proposed system. One of the issues in this regard is Legionnaires which commonly occur due to airborne water droplets. It is important to emphasize that the supply air in the proposed system does not interact

**Figure 8.** *Cooling capacity at different outer air temperatures.*

**Figure 9.** *Cooling performance index at different outer air temperatures.*

with water during any operational stage. Therefore, there is no chance of mixing water droplets with the supply air stream and associated problems. Moreover, the working air can be used for beneficial use like hydroponics or water harvesting to further mitigate the potential issues with wet air delivery to the ambient.

The system is also convenient to integrate with the building particularly using the existing ducting network. This is because the system provides cool dry air at the required temperature as a single outlet. The outlet of the system can be connected to the ducting network in the buildings to supply air at different points. The duct fans can be used to facilitate air delivery at the required points by overcoming the pressure drop in ducts. It is also important to mention that in industrial-scale systems with high-velocity air delivery, the velocity of air is managed in the distribution duct to achieve the required comfortable range of supply air velocity as per ASHRAE standard 55.
