Question

A compressor receives 0.2 kg/s of R-134a at 250 kPa,-10${}^{\circ }C$ (h=393 kJ/kg) and delivers it at 1100 kPa. 90${}^{\circ }C$n (h=418 kJ/kg). The power input is measured to be 8 kW. The compressor has heat transfer to air at 100 kPa coming in at 20 ${}^{\circ }C$ and leaving at ${26}^{\circ }C$. The mass flow rate of air is _____ kg/s.

• A. $4.975kg/s$
• B. $0.4975kg/s$
• C. $0.6975kg/s$
• D. $2.4975kg/s$

Answer 1

The correct option is B $0.4975kg/s$

From energy balance for compressor

${\dot{m}}_1{h}_1+W_{in}+{\dot{m}}_{air}{h}_3={\dot{m}}_2{h}_2+{\dot{m}}_{air}{h}_4$



Treating air as ideal gas

$h_4-h_3=\left(C_P{)}_{air}\right(T_4-T_3)$

${\dot{m}}_{air}=\frac{{\dot{m}}_1(h_1-h_2)+W_{in}}{\left(C_P{)}_{air}\right(T_4-T_3)}=\frac{0.2(393-418)+8}{1.005(26-20)}$

$=0.4975kg/s$