**5. Heat exchanger: selection and sizing**

As it has been described previously, a conventional absorption heat pump includes, at least, four heat exchangers (generator, condenser, evaporator, and absorber), and the coefficient of performance (COP) is strongly affected by the heat and mass transfer efficiency of these components. The fact that the heat exchangers are widely used in many industrial applications allows that the new developments on heat transfer subject promote new designs on absorption heat pumps. Annex 33 of the International Energy Agency (IEA) Heat Pump Program, which was aimed at promoting use of compact heat exchangers in heat pump systems, included three main goals [26]:

**1.** Identify compact heat exchangers, either existing or under development, that may be applied in heat pumping equipment. This has the aims of decreasing the working fluid inventory, minimizing the environmental impact of system manufacture and disposal, and/or increasing the system performance during the equipment life, thereby reducing the possible direct and indirect effects of the systems on the global and local environments.


common working fluids [12]. However, both systems have their limitations, which make nec-

According to research, the 2,497,819 and 4,151,721 U.S. patents proposed an arrangement for absorption of refrigeration systems based on aqueous solutions of hydroxides in order to present alternative mixtures with the aim to avoid the problems presented by the conventional working fluids. The patents propose soluble basic hydroxides such as sodium and potassium [15–17], and cesium hydroxide was considered later [15]. Subsequently, an aqueous mixture of ternary hydroxide was developed [18], and its performance was theoretically compared with the solution of lithium bromide showing promising results [19]. To calculate and optimize absorption processes, accurate data about the different properties of the mixtures are required. Unfortunately, the available information is scarce, and in most cases, it does not cover the full range of concentration and operating temperatures. However, **Table 3** presents different properties for sodium hydroxide and potassium

Based on this lack, we can conclude that there is an area of knowledge yet to be explored.

As it has been described previously, a conventional absorption heat pump includes, at least, four heat exchangers (generator, condenser, evaporator, and absorber), and the coefficient of performance (COP) is strongly affected by the heat and mass transfer efficiency of these components. The fact that the heat exchangers are widely used in many industrial applications allows that the new developments on heat transfer subject promote new designs on absorption heat pumps. Annex 33 of the International Energy Agency (IEA) Heat Pump Program, which was aimed at promoting use of compact heat exchangers in heat pump sys-

**1.** Identify compact heat exchangers, either existing or under development, that may be applied in heat pumping equipment. This has the aims of decreasing the working fluid inventory, minimizing the environmental impact of system manufacture and disposal, and/or

essary to research different working pairs [14].

**Table 3.** Properties for sodium hydroxide and potassium hydroxide.

**Property Sodium Potassium** Vapor pressure Olsson et al. [20], Balej [21] Balej [21]

Viscosity Olsson et al. [20] Kelly et al. [25]

Density Olsson et al. [20] Akfcrlof et al. [22], Kelly et al. [25] Enthalpy Olsson et al. [20] Biermann [23], Ginzburg et al. [24]

hydroxide according to the analysis of authors.

**5. Heat exchanger: selection and sizing**

tems, included three main goals [26]:

**4.2. Hydroxides**

50 Sustainable Air Conditioning Systems

Plate heat exchanger (PHE) is a compact heat exchanger and has been used for absorption system applications [27–30]. Design, sizing, and selection of a PHE for absorption systems are restricted by the thermodynamic properties of the working mixture because they limit the heat transfer rate; consequently, the heat transfer area is in function of this. Other parameter to consider in heat exchanger selection is the operating conditions (temperature and pressure). Aqueous solutions such as LiBr, NaOH, CaCl, LiCl operate at vacuum pressure conditions (from 0.8 to 7 kPa) [31–35], but there are configurations that include a high-pressure generator (from 150 to 300 kPa) [30, 36, 37].

To the performance, a thermal or heat transfer analysis to heat exchanger is suitable to apply some of the methods such as LMTD, ε-Ntu, and P-Ntu. The methodologies have subtle variations but in essential are the same. P-Ntu method is often used for the calculation of the correlation factor F for the first method. LMDT and ε-Ntu methods have been widely applied in industrial practice [38]. A calculation procedure for plate heat exchanger and useful charts was developed as functions of the number of transfer units (Ntu) and the heat capacity ratio (R) for different heat exchanger configurations. Number of channels, number of passes of each fluids, and flow arrangement were the terms to classify the heat exchangers [39]. The ε-Ntu method avoids a rather cumbersome iteration through logarithmic terms, necessary in the LMTD method, and provides a very elegant method using dimensionless parameters that can be applied in easy way to new design and performance rating problems of heat exchangers.

The effectiveness (ε) is the ratio of heat transfer rate (Q̇ ), to the maximum heat transfer potential rate (Q̇ max),when the heat exchanger area is infinite:

$$
\varepsilon = \frac{\dot{Q}}{\dot{Q}\_{mn}} \tag{6}
$$

Heat capacity rates are obtained by multiplying the specific heat and mass flow rate of the fluid. The fluid with the higher heat capacity is designated **Cmax** and the lower one **Cmin**. If the cold fluid has the minimum heat capacity, then the effectiveness is defined as:

$$\varepsilon = \frac{\mathbf{C}\_{\text{max}}(T\_{b,i} - T\_{b,o})}{\mathbf{C}\_{\text{min}}(T\_{b,i} - T\_{c,i})} = \frac{\mathbf{C}\_{\text{min}}(T\_{c,o} - T\_{c,i})}{\mathbf{C}\_{\text{min}}(T\_{b,i} - T\_{c,i})} \tag{7}$$

where *Th*,*<sup>i</sup>* and *Th*,*<sup>o</sup>* are the inlet and outlet temperatures of the hot fluid and *Tc*,*<sup>i</sup>* and *Tc*,*<sup>o</sup>* are the inlet and outlet temperatures of the cold fluid. The ratio of capacities is defined by:

$$R = \frac{C\_{\min}}{C\_{\min}} \tag{8}$$

**5.1. Condenser and evaporator**

where *m*̇ is the mass flow rate, T<sup>i</sup>

heat transfer area, and As

respectively.

the project.

**6.1. Solar radiation**

**6.2. Solar thermal technologies**

the heat transfer rate is calculated according to [43]:

, To

and Ts

*<sup>Q</sup>*̇ <sup>=</sup> *<sup>m</sup>*̇*Cp*(*To* <sup>−</sup> *Ti*

**6. Solar thermal technologies selection**

These devices for air-conditioning applications are designed considering the coil flooded with two-phase refrigerant and also a wall temperature equal to the refrigerant in general [42]. The outer side heat transfer coefficient and the physical properties are assumed constant. Thereby,

*<sup>Q</sup>*̇ <sup>=</sup> *<sup>ϕ</sup> As*(*Ts* <sup>−</sup> *Tm*) (12)

The equation number 12 is the heat transfer rate, Tm is the mean flow temperature over the

and ε are the heat exchanger effectiveness.

Around the world are different types of sources of renewable energy. However, solar thermal energy is the most abundant, and the interest in their development has increased in recent years [44, 45]. This subsection describes the technologies used with solar thermal energy, advantages and disadvantages, and a guide to choose one of them depending on

The interest in solar energy has grown since the environmental problems caused by burning fossil fuels have become severe. One of the most important parameters for the use of solar energy is the estimate of solar radiation [46]. Solar radiation is compound of three elements: *direct radiation*, which is received direct from the sun without diffusion by the atmosphere and is used by the solar energy technologies; *diffuse radiation*, formed with sunlight diffused by atmosphere when in the sky, air molecules, dust and cloud interfere the natural path of the rays, "fragmenting" the sunlight; and *albedo*, which is the radiation reflected by the floor [43]. In order to reduce the greenhouse gases pollution, there exist technologies that transform the

Solar energy technologies are special kind of heat exchangers that transform solar radiation energy to thermal energy storing it in a fluid. The most important component of any solar technology is the solar collector, which is a device that absorbs the incoming solar radiation, converts it into heat, and transfers that heat to a fluid flowing through the collector [48].

solar energy in thermal energy, which is called "solar thermal technologies".

) = *ε m*̇*Cp*(*Ts* − *Ti*

Design and Construction for Hydroxides Based Air Conditioning System with Solar Collectors…

) (11)

http://dx.doi.org/10.5772/intechopen.72188

53

, are the inlet, outlet, and surface temperatures,

The dimensionless parameter Ntu (number of transfer units) expresses the size of the heat exchanger and is commonly used in heat exchanger analysis and is expressed as [40]:

$$Ntu = \frac{\text{LIA}}{\text{C}\_{\text{min}}} \tag{9}$$

Effectiveness correlations for different types of heat exchangers are summarized in **Table 4**.

If there is a phase change in a heat exchanger, the heat capacity of the fluid-changing phase becomes infinite, and Cr is zero, then effectiveness correlations reduce to:

$$
\varepsilon = 1 - \exp(-Ntu) \tag{10}
$$

Regardless of the type of heat exchanger.


**Table 4.** Effectiveness relations for different types of heat exchangers [41].
