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Advanced Refrigeration Systems Lecture by Prof. Lee Carkner

Learn about refrigeration cycles, cascade systems, multistage compression, gas refrigeration, heat pumps, reversible heat pump, and more in this informative lecture by Professor Lee Carkner.

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Advanced Refrigeration Systems Lecture by Prof. Lee Carkner

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  1. Throttling Thermodynamics Professor Lee Carkner Lecture 22

  2. PAL #21 Refrigeration • Refrigerator cycle where P = 120 kPa and x = 0.3 before the evaporator and 60 C after the compressor • Start at point 4, P4 = 120 kPa, x = 0.3, look up h4 = • h3 = h4 = 86.83, for a saturated liquid this means P3 = • Since P2 = P3 and T2 = 60 , look up h for superheated vapor, h2 = • At point 1, P1 = P4 = 120 kPa, saturated vapor, h4 =

  3. PAL #21 Refrigeration • Mass flow rate if W’in = 0.45 kW • W’in = m’(h2-h1) • m’ = (0.45)/(298.87-236.97) = • Find COP from W’in and Q’L • Q’L = m’(h1-h4) = (0.000727)(236.97-86.83) = 1.091 kW • COP = Q’L/W’in = (1.091)/(0.45) =

  4. Cascade Systems • For larger commercial systems, efficiency becomes more important • e.g., deep freeze • Called cascade cycles

  5. Two-Stage Cascade

  6. Cascade Efficiency • The condenser of cycle B (points 1-4) is connected to the evaporator of cycle A (points 5-8) • m’A(h5-h8) = m’B(h2-h3) • COPCascde = m’B(h1-h4)/[m’A(h6-h5)+m’B(h2-h1)]

  7. Multistage Compression • Some fluid is vaporized and is sent back to the high pressure compressor • Can also use just one compressor and multiple throttle valves and evaporators for multiple temperatures

  8. Gas Refrigeration • We can also us a reverse Brayton cycle • Isentropic compression • Isentropic expansion in turbine

  9. Reversed Brayton Cycle

  10. Gas Refrigeration Efficiency • wnet,in = wcomp – wturb = (h2-h1)-(h3-h4) • COP = qL/wnet,in = (h1-h4) / [(h2-h1)-(h3-h4)]

  11. Heat Pumps COPHP = QH/Wnet,in = QH / (QH – QL) COPHP,Carnot = 1 / (1 – TL/TH) • Often designed as dual heat pump/air conditioners • Low COP if the outside temperature is very cold • Can also push the heat extraction underground

  12. Reversible Heat Pump

  13. Joule-Thompson Expansion • Can be achieved by a pump circulating fluid through a pipe with an expansion valve in the middle • We know that in this case, hi = hf • What will be the final properties of the fluid?

  14. Isenthalpic Curve • If the apparatus is changed a little, a new Pf and Tf are produced • The curve represents possible beginning and ending points for a throttling process • A series of isenthalpic curves can be produced for a substance

  15. Inversion Curve • Each curve has two regions • Tf >Ti • Tf < Ti • In between, the slope, or Joule-Thompson coefficient (m), is zero: • For a series of isenthalpic curves, a curve connecting m=0 points is the inversion curve

  16. Liquefying Gasses • In order to cool a gas, its temperature must start below the maximum inversion temperature • TM.I. is near room temperature for many gasses • Some gasses have to be pre-cooled

  17. Heat Exchanger • How is gas liquefied? • Throttled and cooled • Cold gas runs back through the heat exchanger cooling the incoming gas • Cycle starts over

  18. Next Time • Read: 12.1-12.3 • Homework: Ch 11, P: 35, 56, Ch 12, P: 8, 27

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