**1. Introduction**

For more than 100 years ago, direct contact heat and mass exchangers (DCHME) have been widely used in various industries, including chemical process plant, food, and beverage industry, geothermal heat recovery, seawater desalination, waste heat recovery, energy storage systems, production of steam generation for the Rankine power cycle, air conditioning and refrigeration industries, and many so forth. DCHME is a device in which the two process streams are flowing and contacting each other to exchange heat and mass between these two streams,

which can be gas-solid, gas-liquid, liquid-liquid, liquid-solid, or solid-solid streams. The limitation of DCHME is the contamination of the streams depending on the degree of miscibility. Although it has a limitation, there are many advantages such as no corrode or foul or no degradation of heat transfer performance due to the lack of surfaces, a larger heat transfer surface area, much lower flow resistance compared with surface-type heat exchangers, and less capital and operational cost [1]. Thus, DCHMEs are widely used in air conditioning industry for cooling, heating and humidification, cooling and dehumidification, and cooling and humidification, such devices are swamp cooler or direct contact evaporative cooler, cooling tower, air washer spray chamber, cooling coil of air handling unit (AHU), direct expansion (DX) evaporator coil, indirect evaporative cooler, and M-cycle dew point evaporative cooler. Before developing the numerical model of these devices, the process of each device has required to analysis with the psychometric chart. There are total of eight basic air conditioning processes, which are plotted on a psychometric chart as shown in **Figure 1**. These processes are the air stream from the initial state O to the state (1) for sensible cooling, (2) for cooling and humidification, (3) for humidification only, (4) for heating and humidification, (5) for sensible heating, (6) for heating and dehumidification, (7) for dehumidification only, and (8) for cooling and dehumidification. In order to achieve the above eight processes, some processes need to transfer heat (sensible heat) only but some need to exchange heat and mass (sensible and latent heat) from the air stream. Process 1, cooling, and process 5, heating, are pure heat transfer processes only, which means removing heat from the air stream for cooling and adding heat to the air stream for heating without changing the moisture of the airflow. Process 3, humidification, and process 7, dehumidification, are pure mass transfer processes without variation in air temperature. The rest processes 2 (cooling and humidification), process 4 (heating and humidification), process 6 (heating and dehumidification), and process 8 (cooling and dehumidification) are both heat and mass (sensible and latent heat) transfer process that is adding or removing of heat and moisture to or from the air stream. For DCHME has a capability to transfer both heat and mass in the same process, almost all the processes except six can be accomplished by using DCHME in a single stage. The typical five DCHMEs mentioned above extensively used in the air conditioning industry are discussed with their basic air conditioning process.

(1) Swamp cooler or direct contact evaporative cooler, shown in **Figure 2(a)**, is a device in which the outside air flows through the medium of cooling pad or fill

**Figure 1.** *Basic eight air conditioning processes plotted on the psychometric chart.*

*Direct Contact Heat and Mass Exchanger for Heating, Cooling, Humidification… DOI: http://dx.doi.org/10.5772/intechopen.102353*

**Figure 2.** *Two fluids direct contact (TFDC) heat exchanger: (a) direct evaporative cooler/swamp cooler, (b) cooling tower, and (c) air washer chamber.*

that is wetted with water by dripping or spraying. The air becomes cool and humidified (process 2) after passing through the medium that results from the absorption of heat from the air by the evaporation of water droplets or water film of the cool pad. For it is a passive cooling device, it has a very efficient electrical energy consumption, and so that, it is a very efficient space cooling in the hot and dry region with the lowest initial cost and operation cost.

(2) Cooling tower, shown in **Figure 2(b)**, is not an air conditioner device, but it is used extensively as a heat sink or heat removal from the condenser of the refrigeration circuit in the air conditioning industry. Likewise, in a swamp cooler, the hot water, carrying the rejection heat from the condenser, can be cool down by evaporating its water droplet or water film from the fill. In the cooling tower, the hot water is cooling down as the air stream is heated up and humidified (process 2) by transferring heat and mass between these two fluids.

(3) Air washer spray chamber, shown in **Figure 2(c)**, is a typical DCHME, which is widely used as an air conditioner and works in synergy with the heating or cooling coil in the air handling unit (AHU). The air washer chamber has the capability to condition the inlet air into three desired outlet air state-points by controlling the spraying water temperature. Heating and humidification, like process 4, can be achieved by spraying hot water into the chamber if its spray temperature is higher than the inlet air dry-bulb temperature. Cooling and dehumidification, like process 8, can be achieved by spraying chilled water which temperature is lower than the dew point temperature of the inlet air of the chamber. The inlet air can also be cooled as per process 2 line without changing the moisture content of air by spraying the chilled water temperature, which is lower than the inlet air dry-bulb temperature but higher than its dew point temperature.

Different structures but similar to another type of DCHME are plate finned tube heat exchanger used as a direct expansion (DX) evaporator coil, cooling coil operating with chilled water, or non-volatile refrigerant, shown in **Figure 3(a)**. Indirect evaporative cooler, shown in **Figure 3(b)**, and M-cycle dew point evaporative cooler, shown in **Figure 3(c)**, is also another type of DCHME. These types of exchangers have three fluid streams in which two streams directly contact each other, and the last stream is separated by the copper tube wall or separator sheet, shown in **Figure 3**.

(4) Plate finned tube heat exchangers are the most common type of heat exchanger used extensively in the air conditioning industry using as a DX evaporator coil and cooling coil operating with chilled water or ethylene/propylene glycol. For this case, if the dew point temperature of inlet air is lower than the surface temperature of the exchanger, the air stream's water vapor starts turning into condensate as a form of droplets or film on its surface. Therefore, depending on the dew point and surface temperature, the exchanger, can be thoroughly wetted, fully dry, or partially wetted. The fully dry or dry part of a partially wetted exchanger cannot be presumed as a heat and mass exchanger but as a conventional heat exchanger that can be calculated by its traditional LMDT or NTU-effectiveness

#### **Figure 3.**

*Two direct contact with one non-contact fluids (TDCONF) heat and mass exchanger: (a) plate finned-tube heat exchanger operating with non-volatile or volatile working fluid, (b) indirect evaporative cooler, (c) M-cycle dew point evaporative cooler.*

*Direct Contact Heat and Mass Exchanger for Heating, Cooling, Humidification… DOI: http://dx.doi.org/10.5772/intechopen.102353*

method. However, the fully wetted or wet part of the partially wetted exchanger is a direct contact heat and mass exchanger because heat and mass are exchanged between the two fluids of the air stream and the film of water condensate [2], shown in **Figure 3(a)**. The process rendering in the wet region of this exchanger is cooling and dehumidification of the air stream as process 8.

(5) Indirect evaporative cooler and M-cycle dew point evaporative cooler, shown in **Figure 3(b)** and **(c)**, are a combination of a direct evaporative cooler and air to the air heat exchanger. These coolers are composed of multiple duo wet and dry channels separated by a thin film sheet. The working air flowing through the wet channel is directly in contact with a thin water film of wick material or water mist sprayed into the wet channel for the evaporation process. The required heat for the evaporation is transferred from the product air flowing in the dry channel through the separator sheet. Thus, the product air temperature is cooling down along process 1 without changing the moisture content. For the indirect evaporative cooler, the working air at the beginning of the wet channel will be cooled and humidified like process 2 due to evaporation, but later part of the wet channel, the product air is heated and humidified due to the heat that transfers from the product air, like process 4. Likewise, indirect evaporative cooler, M-cycle dew point evaporative cooler has a similar process, but there has a concise process 2 at the beginning of the wet channel because the working air is partially taken from the product air [3–5].

Based on the number of fluids flowing in the exchanger and their arrangement, DCHME for air conditioning can be categorized into two main groups, which are shown in **Figure 4**. In the first group, the two fluids streams, water, and air are flowing in parallel or counter flow direction and directly contacting each other to exchange heat and mass between these two fluids. This type of exchanger is noted as Type-1 two fluids direct contact (TFDC) heat and mass exchanger. Examples of these exchangers are: (1) air washer chamber, (2) cooling tower, and (3) swamp cooler or direct contact evaporative cooler, shown in **Figure 2**. A single discretized element of this type of TFDC exchanger is shown in **Figure 5**.

The second group of DCHME has three kinds of working fluids in which two fluids, water, and air, are directly contacting each other to exchange heat and mass between these two fluids. The third fluid is separated by a tube wall or a thin sheet

**Figure 4.**

*Categorization of direct contact heat and mass exchangers that widely used in air-conditioning industry.*

**Figure 5.**

*TFDC exchanger with nine discretized elements,* j *= 9 and* n *= 10, and water is spraying into a (a) parallel or (b) counter flow with air flow direction.*

layer to prevent mass transfer but only allow heat transfer. Examples of this type of heat and mass exchanger are shown in **Figure 3**, and it can be noted as Type-2, two direct contacts with one non-contact fluid (TDCONF) heat and mass exchanger. This TDCONF exchanger can also be divided into two groups based on the separator type. The first group, Type-2.1 has a separator with an extended surface. An example of an extended surface TDCONF exchanger is the wet region of the plate finned-tube cooling coil unit. Based on the nature of the working fluid, Type-2.1 can be sub-categorized into two groups. The first group, Type-2.1.1 of extended surface TDCONF exchanger, works with non-volatile refrigerants such as chilled water, ethylene/propylene glycol, etc., so that the working fluid temperature is changing continuously along with the coil rows depth. The second group, Type-2.1.2, works with volatile refrigerants such as R134a, R410, etc. Due to the evaporation in the working fluid flow, most of the exchanger has the same surface and fluid temperature profile. Although these two subgroups are the same exchanger, their mathematical models are different due to their different working fluid temperature profiles. Another group, Type-2.2 non-extended surface TDCONF exchangers, are using a plain sheet or tube as a separator, and examples of this type of exchangers are in the direct evaporative cooler and M-cycle dew point evaporative cooler. The mathematical model of this exchanger is the same with Type-2.1.1 of extended surface TDCONF exchanger except for the separator's area calculation.

Kays and London [6] introduced the definitions of effectiveness and NTU method to use in heat exchanger design in their 1955 publication. London et al. [7] used this method to fit the experimental data of the cooling tower, but this method is not generally consistent with all the other units [8]. The reason for the inconsistency is that the method was based only on the sensible heat transfer process, yet the mass exchanging process is excluded. Berman [9] introduced the log-mean enthalpy method (LMED) to reflect both the heat and mass transfer process of cooling tower design. Several studies used this method to analyze heat and mass exchangers, such as cooling towers and spray chambers [9–11]. LMED method can calculate the dependent parameters and effectiveness of the exchanger without using iteration process, but this method has some limitations to calculate all the parameters of the spatially distributed system. Thus, discretized volume with distributed lumped-parameters method is an alternative approach to develop the exchanger model with element nodes. This method is widely used to study the behavior of spatially distributed parameters of electrical systems [12], chemical reaction systems [13], heat and mass transfer [14, 15], microwave [16, 17], acoustics [18], and so many other systems. In this study, discretized volume with distributed lumped-parameters model was developed for both types of exchangers, Type-1 TFDC and Type-2 TDCONF, based on the principle of the graphical method mentioned in ASHRAE Fundamentals Handbook [19] and Systems and Equipment

*Direct Contact Heat and Mass Exchanger for Heating, Cooling, Humidification… DOI: http://dx.doi.org/10.5772/intechopen.102353*

Handbook [20]. The derivation of the model for Type-1 TFDC is explained in Section 3, and for Type-2 TDCONF exchanger is described in Section 4. The relation of convective heat transfer with mass transfer coefficients between air and water film, which are related to both models, is explained in Section 2.
