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Throttling

Throttling. Thermodynamics Professor Lee Carkner Lecture 22. 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, P 4 = 120 kPa, x = 0.3, look up h 4 =

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Throttling

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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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