Refrigeration system (MECH 324)

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  • Carnot Heat Engine
  • Heat added and Heat Rejected
  • Cascade System A. Closed type cascade condenser
  • B. Direct contact type cascade condenser
  • Refrigeration system (MECH 324)

    1. 1. REFRIGERATION SYSTEM By: Engr. Yuri G. Melliza
    2. 2. Refrigeration : Is that area of engineering that deals with the different mechanism involved in maintaining a temperature of a space or material below that of the immediate surroundings. Uses of Refrigeration: 1. Ice making 2. Cold Storage 3. Air conditioning 4. Food preservation 5. Other industrial processes that uses refrigeration
    3. 3. Carnot Cycle: A. Carnot Engine Processes: 1 to 2 - Heat Addition (T = C) 2 to 3 - Expansion (S = C) 3 to 4 - Heat Rejection (T = C) 4 to 1 - Compression (S = C) T S 1 2 34 TH TL QA QR
    4. 4. Heat Added (T = C) QA = TH(S2 - S1) → 1 S2 - S1 = S3 – S4 = ∆S QA = TH ∆S → 2 Heat Rejected (T = C) QR = TL(S3 – S4) → 3 S2 - S1 = S3 – S4 = ∆S QR = TL ∆S → 4 Net Work W = ΣQ ; W = QA - QR W = ∆S (TH – TL) → 4 Where: TH – high temperature, °K TL – low temperature, °K
    5. 5. Thermal Efficiency 100%x Q W e A = 100%x Q Q-Q e A RA = 100%x Q Q e A R       −= 1 100%x T T-T e H LH       = 100%x T T e H L       −= 1 → 5 → 6 → 7 → 9 → 8
    6. 6. B. Carnot Refrigerator Processes 1 to 2 - Compression (S = C) 2 to 3 - Heat Rejection (T = C) 3 to 4 - Expansion (S = C) 4 to 1 - Heat Addition (T = C)TH T TL S 1 23 4 QA QR
    7. 7. Heat Added (T = C) QA = TL(S1 – S4) → 10 S2 – S3 = S1 – S4 = ∆S QA = TL ∆S → 11 Heat Rejected (T = C) QR = TH(S2 – S3) → 12 S2 – S3 = S1 – S4 = ∆S QR = TH ∆S → 13 Net Work W = ΣQ ; W = QR - QA W = ∆S (TH – TL) → 14
    8. 8. Coefficient of Performance: It is the ratio of the refrigerating capacity to the net cycle work. W Q COP A = Q-Q Q COP AR A = T-T T COP LH L = → 15 → 16 → 17
    9. 9. C. Carnot Heat Pump: A heat pump uses the same components as the refrigerator but its main purpose is to reject heat at high thermal energy level. Heat Added (T = C) QA = TL(S1 – S4) → 20 S2 – S3 = S1 – S4 = ∆S QA = TL ∆S → 21 Heat Rejected (T = C) QR = TH(S2 – S3) → 22 S2 – S3 = S1 – S4 = ∆S QR = TH ∆S → 23
    10. 10. Performance Factor Net Work W = ΣQ ; W = QR - QA W = ∆S (TH – TL) → 24 W Q PF R = Q-Q Q PF AR R = T-T T PF LH H = 1COPPF += → 25 → 26 → 27 → 28
    11. 11. Vapor Compression Cycle Processes: 1 to 2 - Compression (S = C) 2 to 3 - Heat Rejection (P = C) 3 to 4 - Expansion (h = C) 4 to 1 - Heat Addition (P = C) Basic Components: 1. Gas Compressor 2. Condenser 3. Expansion Valve 4. Evaporator
    12. 12. Schematic Diagram Evaporator Condenser 4 1 23 QA (Heat Added) QR (Heat Rejected) W (Work) Compressor Expansion Valve
    13. 13. Ph and TS Diagram 1 23 4 S = C P h 1 2 3 4 h = C T S
    14. 14. Compressor W = m(h2 – h1) KW For Isentropic Compression (PVk = C) k 1k 1 2 1 2 P P T T −       =         −      − = − 1 P P 1k kmRT1 W k 1k 1 2 P1V1’ = mRT1
    15. 15. Where: m – mass flow rate in kg/sec V1’ – volume flow rate in m3 /sec P1 – suction pressure in KPa P2 – discharge pressure in KPa T1 – suction temp. in °K T2 – discharge temp. in °K
    16. 16. 100%x P P c-c1η 100%x V V η k 1k 1 2 v D 1' v                 += = − Volumetric Efficiency:
    17. 17. Where: V1’ - volume flow rate measured at intake,m3 /sec VD -displacement volume, m3 /sec Displacement Volume: a. For Single acting m3/sec 4(60) Nn'LD =V 2 D π b. For Double acting (without considering piston rod) m3/sec 4(60) Nn'LD2 =V 2 D π
    18. 18. c. For Double acting (considering piston rod) [ ] m3/secd-2D 4(60) LNn' =V 22 D π Piston Speed: PS = 2LN m/min Where: L - length of stroke, m D - diameter of bore, m d - piston rod diameter, m N - no. of RPM n’ no. of cylinders
    19. 19. Compressor Efficiencies: 100%x WorkIndicated WorkIdeal cn =η a. Compression Efficiency 100%x WorkShaftorBrake WorkIndicated m =η b. Mechanical Efficiency 100%x WorkShaftorBrake WorkIdeal c mcnc =η ηηη = c. Compressor Efficiency
    20. 20. Condenser: QR = m(h2 – h3) KJ/sec For an air cooled condenser QR = m(h2 – h3) = mCPa(ta2 – ta1) KJ/sec For water cooled condenser QR = m(h2 – h3) = mwCPw(tw2 – tw1) KJ/sec Where: a – refers to air w – refers to water 1 – inlet condition 2 – exit condtition Cpa = 1.0045 KJ/kg-°C CPw = 4.187 KJ/kg- -°C
    21. 21. Expansion Valve: h3 = h4 %100x h hh x fg4 f44 4 − = Where: x - quality Evaporator: QA = m(h1 – h4) KJ/sec or KW QA = 60 m(h1 – h4) KJ/min 1 TR = 211 KJ/min TR – tons of refrigeration
    22. 22. Coefficient of Performance W Q COP A = where QA – refrigerating effect or Refrigerating capacity, KW W – compressor work, KW
    23. 23. Wet compression 1 1 2 2 33 4 4 P h S T
    24. 24. Subcooling the refrigerant 1 1 2 2 3 3 4 4 P h S T
    25. 25. Superheating the suction vapor 1 1 2 2 33 4 4 P h S T
    26. 26. Effects of Operating Conditions Effects of Increasing the vaporizing temperature: a. The refrigerating effect per unit mass increases. b. The mass flow rate per ton decreases c. The volume flow rate per ton decreases. d. The COP increases. e. The work per ton decreases. f. The heat rejected at the condenser per ton decreases.
    27. 27. Effects of Increasing the condensing temperature: a. The refrigerating effect per unit mass decreases. b. The mass flow rate per ton increases c. The volume flow rate per ton increases. d. The COP decreases. e. The work per ton increases. f. The heat rejected at the condenser per ton increases.
    28. 28. Effects of superheating the suction vapor A. When superheating produces useful cooling: a. The refrigerating effect per unit mass increases. b. The mass flow rate per ton decreases c. The volume flow rate per ton decreases. d. The COP increases. e. The work per ton decreases. B. When superheating occurs without useful cooling: a. The refrigerating effect per unit mass remains the same. b. The mass flow rate per ton remains the same. c. The volume flow rate per ton increases. d. The COP decreases.
    29. 29. e. The work per ton decreases. f. The heat rejected at the condenser per ton increases. Effects of subcooling the liquid: a. The refrigerating effect per unit mass increases. b. The mass flow rate per ton decreases c. The volume flow rate per ton decreases. d. The COP increases. e. The work per ton decreases. f. The heat rejected at the condenser per ton decreases.
    30. 30. Liquid – Suction Heat Exchanger The function of the heat exchanger are: 1. To ensure that no liquid enter the compressor 2. To subcool the liquid from the condenser to prevent bubbles of vapor from impeding the flow of refrigerant through the expansion valve.
    31. 31. Actual vapor compression cycle: As the refrigerant flows through the system there will be pressure drops in the condenser, evaporator and piping. Heat loses or heat gains will occur depending on the temperature difference between the refrigerant and the surroundings. Compression will be polytropic with friction and heat transfer instead of isentropic.
    32. 32. Condenser Compressor Evaporator Heat exchanger
    33. 33. Multipressure System A multipressure system is a refrigeration system that has two or more low-side pressure. The low-side Pressure is the pressure of the refrigerant between the expansion valve and the intake of the compressor. Removal of Flash gas: The flash gas that develops during the throttling process between the condenser and evaporator was removed and recompressed before complete expansion. With flash gas removal a savings in power requirement will occur.
    34. 34. Intercooling Intercooling between two stages of compression redu- ces the work of compression per kg of vapor. Intercoo- ling in a refrigeration system can be accomplished with a watercooled heat exchanger or by using refrigerant. The watercooled intercooler may be satisfactory for two stage air compression, but for refrigerant compression The water is not cold enough. The alternate method uses liquid refrigerant from the condenser to do the intercooling. Discharge gas from the low stage com- pressor bubbles through the liquid in the intercooler. Refrigerant leaves the intercooler as saturated vapor at the intercooler pressure.
    35. 35. Two evaporators and one compressor 1 23 4 5 6 7 8 compressor Pressure-reducing valve condenser HP evaporator LP evaporator
    36. 36. condenser evaporator Flash tank and Intercooler LP compressor HP compressor Two compressors and one evaporator
    37. 37. condenser LP evaporator Flash tank and Intercooler LP compressor HP evaporator Two compressors and two evaporators HP compressor
    38. 38. Optimum Intercooler or Inter-stage pressure 41i PPP = Where: Pi – optimum interstage or intercooler pressure in KPa P1 – suction pressure of LP compressor, KPa P4 – discharge pressure of HP compressor, KPa
    39. 39. Cascade System Condenser Cascade Condenser Evaporator HP Compressor LP Compressor A. Closed cascade condenser
    40. 40. Condenser Cascade Condenser Evaporator HP Compressor LP Compressor B. Direct Contact type cascade condenser
    41. 41. Air Cycle Refrigeration A. Closed or Dense - Air System Cooler Expander Compressor Refrigerator
    42. 42. Cooler Expander Compressor Refrigerator B. Open - Air System P V 1 23 4 T S 1 2 3 4
    43. 43. Compressor Work: 111 k 1k 1 21 C mRTVP 1 P P 1k kmRT W =         −      − = − Cooler: )T(TmCQ 32pR −= Expander: 333 k 1k 3 43 E mRTVP 1 P P 1k kmRT W =         −      − = −
    44. 44. Refrigerator: )T(TmCQ 41pA −= Network W = Wc – WE W = QR - QA
    45. 45. PRODUCT LOAD Product Load – is the total amount of heat removed from a product in a refrigerated space. m m m t1 t2tf Q1 Q 2 Q 3 Q = Q1 + Q2 + Q3 + Q4 CP1 CP2
    46. 46. Where: Q1 – sensible heat in cooling the product from t1 to tf Q2 – latent heat of fusion (freezing) of the product at tf Q3 – sensible heat in cooling further the product from tf to the final temperature t2 Q4 – heat losses or other heat gains from the products Q1 = mCP1(t1 – tf) KJ/hr
    47. 47. Q2 = m(hL) KJ/hr Q3 = mCP2(tf – t2) Q4 = Q – (Q1 + Q2 + Q3) Where: m – mass of product, kg/hr Cp1 – specific heat of product below freezing, KJ/kg-C or KJ/kg-K Cp2 - specific heat of product above freezing, KJ/kg- °C or KJ/kg- °K t1 – initial temperature, °C tf – freezing point temperature, °C t2 – final temperature, °C hL – latent heat of freezing, KJ/kg

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