Experimental Investigation of the Effect of Using Porous Internal Sub-cooler on the Performance of Refrigeration System: R422A Case Study

Using porous materials in the flow passages of heat transfer fluids is one of the innovative techniques that can be used to enhance the heat transfer performance of thermal systems by improving the heat transfer rates and the heat transfer coefficients. Many of the heat transfer researchers in literature focused on different methods to improve the refrigeration capacities and coefficient of performance of refrigeration systems. Klein et al. [1] studied, theoretically, the effect of using liquid suction heat exchanger on the performance of refrigeration system. Gaffar et al. [2]. Examined, experimentally COP enhancement of domestic refrigerator by sub-cooling and super-heating using shell and tube type heat exchanger. Gustavo et al. [3]. Conducted experimental and numerical investigations on the effect of condenser sub-cooling on the performance of vapor compression systems. Bukola Bolaji et al. [4] performed a comparative analysis of the performance of hydrocarbon refrigerants with R22 in a sub-cooling heat exchanger refrigeration system. Joaquín Navarro et al. [5] conducted an experimental evaluation of internal heat exchanger influence on a vapor compression plant energy efficiency working with R22, R134a and R407C. Bolaji et al. [6] tested the influence of sub-cooling on the energy performance of two eco-friendly R22 alternative refrigerants. Prayudi et al. [7] investigated the effect of degree of sub-cooling on the performance of refrigeration with CFC, HFC and hydrocarbons refrigerant. Muhammad et al. [8] studied the effect of liquid suction heat exchanger sub-cooler on performance of a freezer using R404a as working fluid. Cecep et al. [9] studied the performance improvement using sub-cooling on freezer with R22 and R290 as refrigerants for various ambient temperatures. Prayudi et al. [10], performed heat transfer analysis on the effect of sub-cooling performance of vapor compression refrigeration system with cooling load variation. Boda Hadya [11] analyzed a vapor compression refrigeration system with sub- cooling and super-heating with three different refrigerants for air-conditioning applications. Tarawneh [12] conducted an experimental comparative study on the combined effect of using sub-cooling regenerator and porous evaporator on the performance of a refrigeration system using R422A and R22 as refrigerants. Sun et al. [13] studied the sub-cooled flow boiling with sintered porous coatings in small Channel. They conducted the experiments at atmospheric pressure for fully-deionized water as working fluid. Tarawneh [14] conducted an experimental performance study on the evaporation and pressure prop of low temperature refrigerant blends in porous media. Tarawneh concluded from this experimental performance study of the heat transfer behavior of different refrigerant blends that the refrigerant R422A has a heat transfer performance better than that of the refrigerant R404A in all of the experimental tests. Tarawneh et al. [15] investigated the effect of super-heating of different low-temperature refrigerant blends on the performance of a refrigeration system using porous evaporators. They concluded that, the coefficient of performance as well as the refrigeration capacity of the analysed refrigeration system, showed a reasonable increase when using porous evaporators. Cabelloa et al. [16] studied the energy influence of the use of internal heat exchanger with R-22 drop-in and long-term substitutes in refrigeration plants. Selbas et al. [17] conducted a thermo-economic optimization of sub-cooled, Tarawneh [18] derived a generalized correlation for the condensation and evaporation heat transfer coefficients of different refrigerants in porous horizontal tubes. Up to the on-hand references and up to the author knowledge, no one in literature studied the effect of using porous internal sub-coolers on the performance of a refrigeration system. This research comes to fill this gap in literature. In this experimental investigation a porous internal sub-cooler is used as a heat exchanger inserted between the condenser and the expansion valve in a vapor compression refrigeration system. The heat is exchanged between the worm liquid refrigerant coming from the condenser and the cold gaseous refrigerant coming from the evaporator, accordingly the liquid refrigerant is further sub-cooled before entering the expansion valve and the cold gas is super-heated before entering the compressor. This action will lead to actual increase in the refrigeration effect of the evaporator and save the compressor from probable moisture that can come from the evaporator. Using porous inserts in the low and high-pressure sides of the internal sub-cooler will increase the effectiveness of this heat exchanger, eventually, refrigeration capacity, coefficient of performance will increase and compressor power consumption per ton of refrigeration will decrease. Refrigerant R422A as one of proposed alternative low temperature refrigerant is considered as case study during this experimental work.

Different performance parameters were considered during this experimental study. Refrigeration capacity, coefficient of performance, relative capacity index, effectiveness of the internal sub-cooler, power consumption per ton of refrigeration and pressure drop, were calculated for different sub-cooling temperatures, different porosities of sub-cooler and constant condensing and evaporating temperatures. Using the P-h diagram in Figure 3, the refrigeration capacity can be calculated according to Eq. (1):

$\dot{Q}_{r e f}=\dot{m}_{R}\left(h_\acute{19}-h_\acute{10}\right)$        (1)

where, $\dot{m}_{R}$ is mass flow rate of R422A in (kg/s) and it can be calculated according to Eq. (2);

$\dot{m}_{R}=\dot{V} / v$                (2)

$\dot{V}$ and v are volumetric flow rate in (m³/s) and specific volume of the refrigerant R422A in (m³/kg), respectively. $h_\acute{19}$ and $h_\acute{10}$ are the refrigerant enthalpies at states $\acute{19}$ and $\acute{10}$, respectively as shown in Figure 3. The coefficient of performance of the cycle can be found using Eq. (3):

$C O P=\frac{\dot{Q}_{r e f}}{\dot{W}_{c o m p}}$    (3)

where, $\dot{W}_{c o m p}$ is the compressor power in (kW), which can be calculated according to Eq. (4):

$\dot{W}_{c o m p}=\dot{m}_{R}\left(h_\acute{1}-h_{\acute{9}}\right)$   (4)

h1' is the refrigerant enthalpy at the inlet of the compressor.

The effect of using internal sub-cooler on the cycle refrigeration capacity can detected by calculating the relative capacity index (RCI) according to Eq. (5) [1]:

$C O P=\frac{\dot{Q}_{r e f}-\dot{Q}_{r e f n s c}}{\dot{Q}_{r e f n s c}} \times 100 \%$            (5)

$\dot{Q}_{r e f}$ is the refrigeration capacity in (Kw) with an internal sub-cooler.

$\dot{Q}_{r e f n s c}$ is the refrigeration capacity of the refrigeration system in (Kw) without, using an internal sub-cooler but, using the same condensing and evaporating temperatures. Referring to Figure 2, the effectiveness of the internal sub-cooler (EISC) can be calculated according to Eq. (6):

$E I S C=\frac{T_\acute{19}-T_{19}}{T_{20}-T_{19}} \times 100 \%$               (6)

where, T19' is the vapor temperature leaving, the internal sub-cooler, T19 is the vapor temperature entering the internal sub-cooler andT20 is the liquid temperature entering, the internal sub-cooler.

The compressor power consumption per ton of refrigeration PCPTR in (kW/ ton of refrigeration) can be calculated according to Eq. (7):

$P C P T R=\frac{3 .5 \dot{W}_{c o m p}}{\dot{Q}_{r e f}}$         (7)

The pressure-drop (ΔP) in (kPa) in the internal sub-cooler is detected as in Eq. (8):

$\Delta P=P_{19}-P_\acute{19}$          (8)

where, P19and P19' are the pressures in (kPa) at the inlet and exit of the internal sub-cooler, respectively. The degree of sub-cooling (ΔT(sub)) in ℃ can be found as in Eq. (9):

$\Delta T(s u b))=T_{20}-T_\acute{20}$     (9)

T20 and T20' are temperatures of liquid at the inlet and exit of the internal sub-cooler, respectively.

It can be concluded from this experimental work, that using porous internal sub-cooler is one of the innovative techniques that can enhance the performance of refrigeration systems working on low temperature refrigerant blends. The refrigeration capacity of R422A as well as the COP and RCI were improved when using porous internal sub-cooler. It was noticed that, the coefficient of performance (COP)of the refrigeration system can be increased by decreasing the porosity of the internal sub-cooler. Low power consumption per ton of refrigeration (PCTR) was recorded at low porosities of the internal sub-cooler. Low power consumption was recorded at high sub-cooler effectiveness. As degree of sub-cooling increases, the sub-cooler effectiveness increases with an average percentage increase of about 80%. It can be concluded also, that the installation of internal sub-cooler prevents the presence of vapor in the line entering the expansion valve and prevents also the presence of liquid refrigerants in the lines entering the compressor.  High values of pressure drop in the porous sub-cooler were compensated by the valuable increase in COP and valuable decrease in power consumption. It can be concluded from this experimental research that, the power consumption per ton of refrigeration (PCPTR) can be decreased by increasing the degree of sub-cooling and increasing the porous internal sub-cooler effectiveness.

This experimental work is supported technically by Engineering Geniuses Company (www.egeniuses.net)-Jordan/Amman.