Collected data were processed in a spread sheet programme. The properties of the refrigerant and humid air were derived from the EES software, while the properties of water-glycol mixture were determined by using the equations from M. Conde Engineering (2002) as described in Appendix B. The energy performance parameters of the refrigeration system were calculated which include refrigeration capacity, power consumption and coefficient of performance (COP). The calculations also involved determination of the circulation ratio and uncertainty analyses of the calculations.
5.2.5.1 Calculation of the refrigeration capacity
The refrigeration capacity of the evaporators of the display cabinets was calculated using the enthalpy difference across the coil and the mass flow rate of refrigerant. For the LT DX evaporator the expansion of the refrigerant was assumed to be isenthalpic.
The enthalpy of the refrigerant liquid entering the evaporator, Hr,i,LT , and the refrigerant vapour leaving the evaporator, Hr,o,LT , were determined from measurements of temperature and pressure of the refrigerant at inlet of the expansion valve and outlet of the evaporator respectively. For the MT flooded evaporator, the enthalpy of the refrigerant entering the evaporator, Hr,i,MT , was determined from the MT evaporating temperature and pressure, while the enthalpy of the refrigerant leaving the evaporator, Hr,o,MT , was determined from the evaporating temperature and vapour quality of the refrigerant at the outlet of the evaporator (xo), where xo = 1/CR. The calculation of the circulation ratio (CR) is detailed in Section 5.2.5.4. For the LT DX additional load, the refrigeration capacity was calculated from the energy balance between the refrigerant and the water-glycol sides assuming adiabatic heat transfer. The refrigeration capacities of the MT and LT systems were calculated from:
)
QLT,cab and QLT,add are the refrigeration capacities of the evaporator of the frozen food display cabinet and additional load respectively; mwis the water-glycol mass flow rate.
5.2.5.2 Power consumption
The total power consumption of the system is the sum of the power consumption of the LT compressor and the power consumption of the CO2 pump. The power consumption of the LT compressor (Wcomp) was determined by recording the power when only the LT system was in operation, whereas the power consumption of the CO2 pump was
determined when only the MT system was in operation. Because the refrigerant flow rate from the pump was partially bypassed to the liquid receiver, the pumping power (WCO2,pump) used in the COP calculation was assumed to be proportional to the measured refrigerant mass flow rate through the MT evaporator coil.
5.2.5.3 Calculation of the COP
The coefficient of performance (COP) of the LT, MT and overall refrigeration system were calculated from the equations described in Chapter 3: (3.15), (3.16) and (3.17) respectively. The COP of the LT system was also compared to the reversed Carnot cycle calculated from:
LT evap LT cond
LT evap
Carnot T T
COP T
, ,
,
(5.5)
where evaporating and condensing temperatures Tevap,LT and Tcond,LT respectively are in Kelvin (K).
5.2.5.4 Circulation ratio
The flow of CO2 refrigerant in the MT evaporator coil starts as liquid, gradually evaporates along the coil pipe and then exits the evaporator coil at certain quality (xo) which is the inverse value of the circulation ratio (CR). The circulation ratio, as defined in Section 3.2.1.3 (Chapter 3) can be determined from:
vap MT r
m CR m
,
(5.6)
where mr,MT= total refrigerant mass flow rate in MT evaporator, m = refrigerant vap vapour mass flow rate, and CR = 1 occurs when mr,MT mvap.
In the test system, CR = 1 can be established by adjusting the regulator and bypass valves upstream of the MT evaporator coil while observing the quality of the liquid refrigerant flowing through a sight glass fitted at the outlet of the coil. In order to make the adjustment of the refrigerant flow rate easier, the regulator valve should be installed adjacent to the sight glass as shown in Figure 5.1 and the adjustment should be started from overfeed flow rate so that the liquid refrigerant can be clearly seen in the sight
glass. With the bypass valve slightly open, the regulator valve can be gradually closed until the liquid refrigerant just disappears from the sight glass. The supplied mass flow rate to the evaporator (mr,MT), can then be measured from the flow meter which is fitted on the liquid line upstream of the MT evaporator coil. The mass flow of refrigerant vapour (m ) at a particular evaporating temperature (Tvap evap,MT) can be calculated from:
fg vap MT
H
m Q (5.7)
where Hfg is the enthalpy of evaporation of the CO2 refrigerant at evaporating temperature (Tevap,MT). QMT is the refrigeration load of the MT cabinet which is equivalent to the heat absorbed from the air crossing the MT coil and can be determined from:
) ( a,ON a,OFF
a
MT m H H
Q (5.8)
ma is the mass flow rate of the air flowing through the evaporator coil as a function of air velocity and cross sectional area of the air passage in the cabinet; Ha,ON and Ha,OFF
are the enthalpies of the air entering and leaving the coil respectively.
In a real supermarket application, the circulation ratio can be controlled by integrating the cabinet controller with a motorized valve installed upstream of the evaporator coil and with the speed controller of the CO2 pump.
5.2.5.5 Uncertainty in the calculation of CR and COP
Considering the uncertainty of the measured variables which include air mass flow rate, air temperature and relative humidity, refrigerant temperatures, pressures, power and refrigerant mass flowrate and assuming that the individual measurements are uncorrelated and random, the uncertainty in the calculation of CR was determined, using the Engineering Equation Solver (EES) software, to be ± 10.8%. The uncertainty in the calculations of the COPLT, COPMT, COPOverall and COPCarnot were found to be ± 0.2%
± 10.8%, ± 3.6% and ± 0.6% respectively. The uncertainty of the COPMT is relatively high because its calculation involved the circulation ratio. Detailed explanation of the uncertainty analysis is given in Appendix J.
5.5.33 TTeesstt rreessuullttss
5.3.1 Thermodynamic cycle of the refrigeration system
Figure 5.3 shows the thermodynamic cycle of the CO2 refrigeration system obtained from the test results. The cycle refers to the schematic diagram Figure 3.4 (Chapter 3).
Pressure drop of MT circuit (9-10-11) was found to be 37.5 kPa which corresponds to 0.5 K temperature drop. The pumping process (9-10) and heat extraction process in the MT cabinet (10-11) can be assumed to be at constant temperature and pressure.
Figure 5.3 Pressure-enthalpy diagram of the CO2 refrigeration cycle based on the test results
Figure 5.3 also shows the LT circuit (1-2-3-4-5-6-7-8) which was integrated with the heat rejection loop of the MT system (12-3-4). The circuit was equipped with an internal heat exchanger (IHX) which exchanged heat from cold stream (8-1) to the hot stream (5-6) of the circuit to provide sub-cooling of around 2.5 K on the CO2 liquid before being expanded into liquid-vapour mixture through the EXV. This exchanged heat increased the superheat of the CO2 gas from the LT evaporator (of 5 K superheating at point 8) to be around 10 K superheating (at point 1) before entering the LT compressor. The expansion process in the EXV (6-7) was assumed to be isenthalpic.
The compression process (1-2) utilised a semi hermetic reciprocating compressor of isentropic efficiency of around 0.69. This efficiency is discussed further in Section 5.4.
Pressure (kPa) is absolute pressure Enthalpy (kJ/kg)
-32oC -7oC
8oC
Pressure (kPa)
0.2 0.4 0.6 0.8 6 4,5,9
10
11
12 3 2
1 8 7
-500 -400 -300 -200 -100 0 100 500
1000 10000 20000
42oC R-744
2870 1350
∆Tsc = 2.5 K
∆Tsh = 10 K