An air-conditioning system is shown in figure in which air flows over tubes carr

Question

An air-conditioning system is shown in figure in which air flows over tubes carrying Refrigerant 134_a. Air enters with a volumetric flow rate of 50 m^3/min al 328 degree C, I bar and exits at 228 degree C, 0.95 bar. Refrigerant enters the tubes at 5 bar with a quality of 20% and exits at 5 bar, 208 degree C. Ignoring heat transfer at the outer surface of the air conditioner, and neglecting kinetic and potential energy effects, determine at steady state: the mass flow rate of the refrigerant, in kg/min. the rate of heat transfer, in kJ/min, between the air and refrigerant.

Explanation / Answer

solution:

1)for given heta transfer between air to refrigerant we hae following properties for air and R134a

for air

h1=137.922 KJ/kg

density1=.5819 kg/m3

h2=33.653 kj/kg

density2=.6631 kg/m3

here for steady flow heat transfer without workdone we have relation that

Qa=ma'*(h1-h2)

Qa=(V'/60)(density1*h1-density2*h2)=(50/60)(.5819*137.922-.6631*33.653)=48.2845 kj/s=2897.07 kj/min

hence net heat transfer is=Qa=2897.07 kj/min

where heat transfer from air=heat transfer to R134a

Qa=mr'*(h2r-h1r)

from table we get that

h2r=456 kj/kg

h1r=111 kj/kg

2897.07=mr'*(456-111)

mr'=8.3973 kj/min

hencerefrigerant mass flow rate is=mr'=8.3973 kj/min

heat trasnsfer=Qa=2897.07 kj/min

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