This case study models a vapour compression refrigeration system using R22 refrigerant with Flownex software. It includes steady-state and transient simulations to analyze system performance. The steady-state results serve as initial conditions for the transient simulation, which examines dynamic responses to temperature changes. Key parameters like condenser heat transfer, compressor power, and refrigerant mass flow are evaluated, demonstrating Flownex's capability in refrigeration system analysis.
CHALLENGE
The primary objective of the simulation is to determine the steady-state performance of the system given the design of all the components together with appropriate boundary conditions.
The secondary objective is to investigate the dynamic performance of the system under varying operating conditions.
BENEFITS
This case study demonstrates the modeling of a vapor compression refrigeration system using R22 as refrigerant. Both a steady-state and a transient simulation will be considered.
SOLUTION
Flownex was used to simulate a vapor compression refrigeration system using R22 as refrigerant. First a steady-state simulation was done and the results of this simulation were used as initial conditions for a transient simulation.
INTRODUCTION
This case study demonstrates the modeling of a vapor compression refrigeration system using R22 as refrigerant. Both a steady-state and a transient simulation will be considered.
SYSTEM DESCRIPTION
The lay-out of the system is shown in Figure 1.
Refrigerant (R22) vapor enters a positive displacement compressor at a slightly superheated state (1) and is then compressed to state 2. From 2 the high pressure high temperature vapor is first sensibly cooled to saturation, then condensed and ultimately sub-cooled to state 3. From 3 to 4 the liquid refrigerant flows through a tube to the inlet of the expansion valve in an adiabatic process. The expansion valve is a restrictor that causes a large pressure drop with an accompanying adiabatic temperature drop. From 5 to 6 all the liquid is evaporated in the evaporator and then slightly superheated to state 6. From there the vapor refrigerant flows through a tube to the inlet of the compressor (1) to close the cycle.
Figure 2 shows the process on a temperature entropy diagram.
OBJECTIVE OF SIMULATION
The primary objective of the simulation is to determine the steady-state performance of the system given the design of all the components together with appropriate boundary conditions.
The secondary objective is to investigate the dynamic performance of the system under varying operating condition.
DESCRIPTION OF SIMULATION
The Flownex model of the system is shown in Figure 3. The compressor is modeled with Flownex’s Positive Displacement Compressor (PDC) element while the evaporator and condenser is modeled with Flownex’s discretized parallel or counter flow heat exchanger element. Each heat exchanger element is subdivided in 20 increments. Flownex’s Restrictor with Discharge (RD) element is used to model the expansion valve. This type of element use accurate relationships to model choking.
Two simulations are done. The first is a steady-state simulation and the second is the transient simulation that uses the steady-state results of the first case as initial conditions. The condition that is varied in the transient simulation is the temperature at node 9.
The following boundary conditions are specified for the steady-state case:
Table 1: Boundary conditions specified for steady-state case.
| Node number | Boundary condition |
| 4 | Quality = -0.03 (to make sure that the refrigerant is in the sub-cooled liquid region) |
| 6 | Quality = 1.03 (to make sure that the refrigerant is in the superheated vapour region) |
| 7 | P = 100 kPa, T = 15 ºC |
| 8 | Mass flow = 3.09 kg/s |
| 9 | P = 100 kPa, T = 30 ºC |
| 10 | Mass flow = 1.7 kg/s |
Flownex’s Designer capability is used to determine the diameter of the restrictor (element 8) as well as the pressures at nodes 1 and 4 so as to ensure that the quality conditions at nodes 4 and 5 is satisfied as well as continuity at node 4.
In the transient case the temperature at node 9 is reduced from 30 ºC to 20 ºC during a 10 second period. At the start of the transient simulation the boundary conditions are set such that the total mass of refrigerant in the system is kept constant. The time increment for the transient simulation is 50 ms.
RESULTS: STEADY-STATE CASE
Some important results of the steady-state case are summarized in the following table.
Table 2: Summarized steady-state results.
| Parameter | Value |
Condenser heat transfer | 118.13 [kW] |
Evapourator heat transfer | 94.04 [kW] |
Compressor power | 24.12 [kW] |
Refrigerant mass flow | 0.616 [kg/s] |
Pressure at node 1 | 512 [kPa] |
Pressure at node 2 | 1959 [kPa] |
Temperature at node 2 | 84.4 [ºC] |
RESULTS: TRANSIENT CASE
Figure 4 to Figure 7 show some results for the transient case. From Figure 4 it can be seen that the mass flow increases during the transient. This is due to the decrease in compressor discharge pressure. Both compressor power and quality to the inlet of the compressor also decrease during the transient.
CONCLUSION
Flownex was used to simulate a vapor compression refrigeration system using R22 as refrigerant. First a steady-state simulation was done and the results of this simulation were used as initial conditions for a transient simulation.