Guidance for refrigeration question

[hanwei_86]

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!!:confused:

[sterl]

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: