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    Unhappy Guidance for refrigeration question



    hey guys, i ve bumped into some problems dealing with a refrigeration qn, thermofluid based. maybe some kind and experienced souls can help me out with it. Here it goes :

    A cold storage facility uses a two stage ammonia cooling system to
    supply refrigeration to a blast freezer. The system is designed for air in the blast
    freezer at –35oC, and ambient conditions of 40oC. Ammonia enters the low stage
    compressor superheated by 10oC, enters the high stage compressor saturated, and
    leaves the condenser subcooled by 5oC. Both evaporator and condenser require a
    temperature difference of 5oC to transfer heat.
    Intercooler pressure is set at the point where one-sixth of the mass of the liquid
    from the condenser has flashed off (in reaching the intercooler pressure). Liquid is
    then fed from the intercooler to the evaporator. Isentropic efficiency for each
    compressor stage is 90%, and expansion is achieved via throttling.

    Qn: What is the intercooler temperature (to the nearest C), and what are the
    condenser, intercooler and evaporator pressures?

    Calculate the cooling effect (qrefrig).

    120 kW of refrigeration is required. What is the refrigerant mass flow rate
    through the condenser?

    appreciate it!!



  2. #2
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    Re: Guidance for refrigeration question

    Don't have detailed tables for Enthalpy and Sensible Heat in SI but:

    Tc is 5-over ambient = 45 deg. C.
    Liq subcooled 5 before being throttled:

    hL entering Throttle Device Condenser to Intercooler is -571.3 kJ/kg

    First rough on intermediate:
    At 0-deg C. hfg=1261.93 kJ/kg

    to make 16% vapour: Flash Gas represents removal of 201.9 kJ/kg

    Cp through temperature range 40 deg C to 0 deg C averages about 4.7 kJ/kg.

    Liq phase temperature drop due to flashing @ 201.9/4.7 =43.

    At (40-43) = -3 Deg. C.: Hg=495.9; hl=-776.8 and (solving for Vapor Fraction based on adiabatic expansion

    x=(-571.3-776.8)/(495.9+776.8)/
    = .161 QED;
    Sooooo....

    Ti-sat=-3 deg. C.

    Pc=1.7775 MPa
    Pe=0.07154 MPa
    Pi= 0.353 Mpa

    Second Step of Throttling:

    hli=-776.8 kJ/kg; hv=445.7 and hlsat=-943.1
    hgx=445.7+(2.15*5) for 5-deg C of useful Superheat: (compressor superheat says 10 but we only know that target temperature is 5-deg greater than Te)...

    Flash at thottling device=.12 (entering mass flow)

    Refrigerating Effect: per kg Vap through Low Stage Comp
    =.88*((445.7+(2.15*5))-(-943.1))+.12*(2.15*5)
    =1232.9 kJ/kg refrig effect at the evap...
    Per Kg Condensed:
    =(684-497)/.90 for LS compressor Power Input

    High Stage compressor flow is LSEVAP/.88/.84/(1-(205.7/1339))











    Low Pressure Cooling effect per kg condensed:

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