Multi-Stage Refrigeration System

Consider a two-stage compression refrigeration system that uses refrigerant-134a as its working fluid and operating between the pressure limits of 0.8 MPa and 0.14 MPa.  The refrigerant leaving the condenser as saturated liquid is throttled to 0.32 MPa prior to entering a flash-chamber.  Part of the refrigerant evaporates during the flash process and this vapor is mixed with the super-heated refrigerant leaving the low-pressure compressor and subsequently compressed to the condenser pressure by the high-pressure compressor.  The liquid from the flash-chamber is throttled to the evaporator pressure where it cools the refrigerated space as it passes through the evaporator.  Assuming the refrigerant entering the low-pressure compressor as saturated vapor and assuming ideal compressors (100% isentropic efficiency) determine:

  • Evaporate mass fraction in flash chamber
  • Heat removal from refrigerated space
  • COP of the cycle

 

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Start ThermoFlo by double-clicking its icon.

Create new model:

  • Click on Create Model in “Create New” section of home screen:
  • Type filename “Multi-Stage Refrigerator” and click Save

 

Model the Components and Connectors:

  • Click on Thermodynamic folder in “Library Components Window”
  • Drag and Drop “Compressor”, “Condenser”, “Valve”, “Flash-Chamber”, another “Valve”, “Evaporator”, another “Compressor” and a “Mixer” onto the Canvas, as shown above.
  • Connect the outlet of each component to the inlet of the next by first clicking on the “Connect” button on the “Home” tab and then using the mouse to connect the nodes.

 

Set Working Fluid and Loop Properties:

  • Right-click on the top “Compressor” inlet node and choose “Set Loop Fluid” from the options (note that this be done using any node in the loop).
  • Double-click on the fluid icon to open “Set Fluid Form”; choose “Library Fluids”/”Refrigerants”/”R134A”.
  • Click Select Fluid.
  • Now right-click on the same “connection node” near the compressor inlet and set Mass Flow Rate (from the Set Properties Menu) to 1 kg/s.
  • Not let’s set the loop pressure limits.  Right-click on the lower “Compressor” inlet node and Set Pressure to 0.14 MPa.  Similarly, right-click at the top compressor outlet and the flash-chamber inlet to set the high and mid pressures to 0.8 MPa and 0.32 MPa.
  • Now we need to set the refrigerants phase at the inlet of the “low-pressure compressor and the outlet of the condenser.  To do this, right-click at the Connection Node near the lower compressor and set the quality to “1” (saturated vapor) from Set Properties Menu.  Repeat this procedure to set the quality to “0” at the condenser outlet node.

 

Set Component Properties:

  • Double-click on the top compressor and change its name to “HP Comp”.  Note that the isentropic efficiency is set by default to 100% (Ideal Compressor).
  • Repeat the procedure and change the name of the lower compressor to “LP Comp”.
  • Note that the pressure losses in all other components are left by default to “0” to indicate ideal cases.
  • Note that you can Line up the components and create additional “connection nodes” to set the display cleanly.  Your canvas should look similar to the screen shown below.

 

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Run Model and View Results:

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