4.1.1. Cooling Capacity, Compressor Work and COP at Various Compressor Speeds

The effect of compressor speed on cooling capacity, compressor work and coefficient of performance (COP) of the heat pump system with a high-pressure side chiller during the cooling mode is shown in Figure 2a. The variation of cooling capacity, compressor work and coefficient of performance with compressor speed were experimentally recorded at HVAC inlet air conditions of temperature of 25 ◦C, relative humidity of 60% and flow rate of 450 m<sup>3</sup> /h, air velocity of 3 m/s and coolant inlet conditions of 35 ◦C and volume flow rate of 10 L/min. As presented in Figure 2a, the cooling capacity increased from 3.8 kW to 4.7 kW with the rise in the compressor speed from 4000 rpm to 6000 rpm. The cooling capacity increased with compressor speed due to the increase in refrigerant flow rate [31]. The cooling capacity increased from 3.8 kW to 4.4 kW with an enhancement of 15.8% for the increase in the compressor speed from 4000 rpm to 5000 rpm. However, the cooling capacity increased from 4.4 kW to 4.7 kW with an enhancement of 6.82% for the increase in the compressor speed from 5000 rpm to 6000 rpm. Beyond the compressor speed of 5000 rpm, the increase in the cooling capacity reduced due to the rapid rise in the pressure ratio [31]. The rise in the pressure ratio affected the cycle capacity by increasing the degree of superheat associated with vapor refrigerant discharges from compressor and reducing the degree of subcooling of liquid refrigerant discharges from the condenser. In addition, the pressure ratio affected the compressor work. As presented in Figure 2a, the compressor work also increased with the increase in compressor speed. With the increase in the compressor speed from 4000 rpm to 5000 rpm, the compressor work increased from 1.07 kW to 1.37 kW with an enhancement of 28.0%. The compressor work increased from 1.37 kW to 1.71 kW with an enhancement of 24.8% for the increase in the compressor speed from 5000 rpm to 6000 rpm. Unlike cooling capacity, the percentage enhancement in compressor work was not reduced drastically with the rise in compressor speed. The variations of cooling capacity and compressor work with various compressor speeds affected the coefficient of performance. The coefficient of performance is the ratio of cooling capacity to compressor work. With the increase in the compressor speed, the percentage enhancement in the cooling capacity decreased drastically but the percentage enhancement in compressor work was not

decreased drastically. Therefore, the coefficient of performance decreased with the increase in the compressor speed as shown in Figure 2a. With the increase in the compressor speed from 4000 rpm to 5000 rpm, the coefficient of performance decreased from 3.54 to 3.21 with a reduction of 9.32%, whereas, the coefficient of performance decreased from 3.21 to 2.75 with a reduction of 14.3% for the increase in the compressor speed from 5000 rpm to 6000 rpm. The percentage reduction in the coefficient of performance increased beyond a compressor speed of 5000 rpm due to characteristic variations of cooling capacity and compressor work with compressor speed, as presented in Figure 2a. The working of the heat pump system with a high-pressure side chiller for various compressor speeds is shown on a P-h diagram in Figure 2b. With respect to the P-h diagram for the refrigeration cycle, as compressor speed increased, low-side pressure decreased to under 2 bar. However, high-side pressure was stable due to similar coolant conditions to be supplied. The portion of heat transfer rate between a refrigerant and a coolant at the developed chiller showed about one-third of the condensing heat capacity along with compressor speed. 2a. With the increase in the compressor speed from 4000 rpm to 5000 rpm, the coefficient of performance decreased from 3.54 to 3.21 with a reduction of 9.32%, whereas, the coefficient of performance decreased from 3.21 to 2.75 with a reduction of 14.3% for the increase in the compressor speed from 5000 rpm to 6000 rpm. The percentage reduction in the coefficient of performance increased beyond a compressor speed of 5000 rpm due to characteristic variations of cooling capacity and compressor work with compressor speed, as presented in Figure 2a. The working of the heat pump system with a high‐pressure side chiller for various compressor speeds is shown on a P‐h diagram in Figure 2b. With respect to the P‐h diagram for the refrigeration cycle, as compressor speed increased, low‐side pressure decreased to under 2 bar. However, high‐side pressure was stable due to similar coolant conditions to be supplied. The portion of heat transfer rate between a refrigerant and a coolant at the developed chiller showed about one‐third of the condensing heat capacity along with compressor speed.

*Symmetry* **2020**, *12*, 1237 8 of 22

from 1.37 kW to 1.71 kW with an enhancement of 24.8% for the increase in the compressor speed from 5000 rpm to 6000 rpm. Unlike cooling capacity, the percentage enhancement in compressor work was not reduced drastically with the rise in compressor speed. The variations of cooling capacity and compressor work with various compressor speeds affected the coefficient of performance. The coefficient of performance is the ratio of cooling capacity to compressor work. With the increase in the compressor speed, the percentage enhancement in the cooling capacity decreased drastically but

coefficient of performance decreased with the increase in the compressor speed as shown in Figure

**Figure 2.** *Cont*.

*Symmetry* **2020**, *12*, 1237 9 of 22

**Figure 2.** Effect of compressor speed on (**a**) cooling capacity, compressor work and coefficient of performance (COP) and (**b**) P‐h diagram of heat pump system with chiller during the cooling mode. **Figure 2.** Effect of compressor speed on (**a**) cooling capacity, compressor work and coefficient of performance (COP) and (**b**) P-h diagram of heat pump system with chiller during the cooling mode.

#### 4.1.2. Cooling Capacity, Compressor Work and COP at Various Coolant Inlet Temperatures 4.1.2. Cooling Capacity, Compressor Work and COP at Various Coolant Inlet Temperatures

The variation of cooling capacity, compressor work and coefficient of performance of heat pump system with a high‐pressure side chiller for various coolant inlet temperatures during the cooling mode is presented in Figure 3a. The behaviors of cooling capacity, compressor work and coefficient of performance for various coolant inlet temperatures were experimentally recorded at the HVAC inlet air conditions of temperature of 25 °C, relative humidity of 60% and flow rate of 450 m3/h, air velocity of 3 m/s, coolant volume flow rate of 10 L/min and compressor speed of 5000 rpm. The coolant inlet temperature had very little effect on the cooling capacity. As shown in Figure 3a, with the increase in the coolant inlet temperature from 35 °C to 45 °C, the cooling capacity remained constant at 4.15 kW, whereas, with an increase in the coolant inlet temperature from 45 °C to 55 °C, the cooling capacity decreased from 4.15 kW to 4.06 kW, a reduction of 2.2%. With the increase in the coolant inlet temperature, the compressor work increased because of the increase in the high‐pressure side of the compressor. The compressor work increased from 1.36 kW to 1.58 kW with an enhancement of 16.2% for the increase in the coolant inlet temperature from 35 °C to 45 °C. However, with the increase in the coolant inlet temperature from 45 °C to 55 °C, the compressor work increased from 1.58 kW to 1.67 kW, an enhancement of 5.70%. With an increase in the coolant inlet temperature, the percentage enhancement in the compressor decreased as shown in Figure 3a. The coefficient of performance varied with coolant inlet temperature due to variations in cooling capacity and compressor work with coolant inlet temperature. As shown in Figure 3a, the coefficient of performance decreased with the increase in the coolant inlet temperature because the cooling capacity was not affected much, and compressor work increased with coolant inlet temperature. Because of the characteristic variation of cooling capacity and compressor work presented in Figure 3a, the coefficient of performance decreased from 3.06 to 2.64 with a reduction of 13.7% for the increase in the coolant inlet temperature from 35 °C to 45 °C, while it decreased from 2.64 to 2.43, a reduction of 7.95%, for the increase in the coolant inlet temperature from 45 °C to 55 °C. The refrigeration cycle of the heat pump system with a high‐pressure side chiller for various coolant inlet The variation of cooling capacity, compressor work and coefficient of performance of heat pump system with a high-pressure side chiller for various coolant inlet temperatures during the cooling mode is presented in Figure 3a. The behaviors of cooling capacity, compressor work and coefficient of performance for various coolant inlet temperatures were experimentally recorded at the HVAC inlet air conditions of temperature of 25 ◦C, relative humidity of 60% and flow rate of 450 m<sup>3</sup> /h, air velocity of 3 m/s, coolant volume flow rate of 10 L/min and compressor speed of 5000 rpm. The coolant inlet temperature had very little effect on the cooling capacity. As shown in Figure 3a, with the increase in the coolant inlet temperature from 35 ◦C to 45 ◦C, the cooling capacity remained constant at 4.15 kW, whereas, with an increase in the coolant inlet temperature from 45 ◦C to 55 ◦C, the cooling capacity decreased from 4.15 kW to 4.06 kW, a reduction of 2.2%. With the increase in the coolant inlet temperature, the compressor work increased because of the increase in the high-pressure side of the compressor. The compressor work increased from 1.36 kW to 1.58 kW with an enhancement of 16.2% for the increase in the coolant inlet temperature from 35 ◦C to 45 ◦C. However, with the increase in the coolant inlet temperature from 45 ◦C to 55 ◦C, the compressor work increased from 1.58 kW to 1.67 kW, an enhancement of 5.70%. With an increase in the coolant inlet temperature, the percentage enhancement in the compressor decreased as shown in Figure 3a. The coefficient of performance varied with coolant inlet temperature due to variations in cooling capacity and compressor work with coolant inlet temperature. As shown in Figure 3a, the coefficient of performance decreased with the increase in the coolant inlet temperature because the cooling capacity was not affected much, and compressor work increased with coolant inlet temperature. Because of the characteristic variation of cooling capacity and compressor work presented in Figure 3a, the coefficient of performance decreased from 3.06 to 2.64 with a reduction of 13.7% for the increase in the coolant inlet temperature from 35 ◦C to 45 ◦C, while it decreased from 2.64 to 2.43, a reduction of 7.95%, for the increase in the coolant inlet temperature from 45 ◦C to 55 ◦C. The refrigeration cycle of the heat pump system with a high-pressure side chiller for various coolant inlet temperatures on the P-h diagram is shown in Figure 3b. High-side

temperatures on the P‐h diagram is shown in Figure 3b. High‐side pressure increased with coolant

pressure increased with coolant inlet temperature in order to have certain temperature difference. The proportion of the heat transfer rate in the developed chiller with the increase in coolant temperature varied from one quarter to three-quarters of the total condensing heat capacity due to heat transfer potential, such as temperature difference among fluids. *Symmetry* **2020**, *12*, x FOR PEER REVIEW 11 of 25

(**b**)

**Figure 3.** Effect of coolant inlet temperature on (**a**) cooling capacity, compressor work and coefficient of performance (COP) and (**b**) P‐h diagram of heat pump system with chiller during the cooling mode. **Figure 3.** Effect of coolant inlet temperature on (**a**) cooling capacity, compressor work and coefficient of performance (COP) and (**b**) P-h diagram of heat pump system with chiller during the cooling mode.
