try to solve the two problems The T-s diagram of two-stage compression refrigera
ID: 1859251 • Letter: T
Question
try to solve the two problems
The T-s diagram of two-stage compression refrigeration system with a flash is shown in figure below. The enthalpies of the refrigerant at several states arc determined from the refrigerant tables to be: h1=239.16 kJ/kg; h2=255.90 kJ/kg; h3=251.88 kJ/kg; h4=278.94 kJ/kg; h5=107.32 kJ/kg; h6=107.32 kJ/kg; h7=55.16 kJ/kg; h8=55.16 kJ/kg; h9=254.84 kJ/kg Determine: The fraction of the refrigerant that evaporates as it is throttled to the flash chamber The amount of heat removed from the refrigerated space The compressor work per unit mass of refrigerant flowing through the condenser The coefficient of performance. Consider a two-stage cascade refrigeration cycle and a two-stage compression refrigeration cycle with a flash chamber. Both cycles operate between the same pressure limits and use the same refrigerant. Which system would you favor? Why?Explanation / Answer
a) h6-h7/(h3-h6)
f = 0.3
b) Ql = h1-h8 = 239.16-55.16 = 184 kJ/kg
c) Wc = h2-h1 + h4-h9 = 36.84 kJ/kg
d)COP = Ql/Wc = 184/36.84 = 4.994 ~ 5
e)
Though multi-stage systems have been very successful, they have certain
limitations. These are:
a) Since only one refrigerant is used throughout the system, the refrigerant used
should have high critical temperature and low freezing point.
b) The operating pressures with a single refrigerant may become too high or too low.
Generally only R12, R22 and NH3 systems have been used in multi-stage systems as
other conventional working fluids may operate in vacuum at very low evaporator
temperatures. Operation in vacuum leads to leakages into the system and large
compressor displacement due to high specific volume.
c) Possibility of migration of lubricating oil from one compressor to other leading to
compressor break-down.
The above limitations can be overcome by using cascade systems.
So i would use two stage cascade systems
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