Question

d. Answer is not among the choices 24. *A heat exchanger is used to heat cold water at 15°C entering at a rate of 2 kg/s by a hot air at 100°C entering at a rate of 3 kg/s. The heat exchanger is losing heat at a heat rate of 40 kJ/s. If the exit temperature of hot air is 20°C, the exit temperature of the cold water is °C. Neglect changes in potential and kinetic energies. 39 49 b. d. Answer is not among the choices 25. A heat engine receives heat from a source at 1000 and reiects the waste heat to rin at 500 h a. c. 44

Answer 1

As per detailed answer solved below, option 'b' i.e 44℃ is correct answer which is exit temperature of cold water.

- cold water Tei = entering temp = 15°c. Tee = exit temps ? Mass flow rate= me = 2 kgls Specific heat of water = cpw = 4.186 kg / kg k Hot air Thi= entering temp = 100°c The = exit temp= 20°c. Mass flow rate = m n = 3 kg 15 Specifcc heat of alrs cpa= 1.005 kg 1 kg k → Temp. distribution in heat exchanger. Thi leoge Hot air. 3 kgls The = 20°c. Teea? 2kg|5 Tei= 1500 cold water counter How HE we know- Heat given by hat fluid = Heat gained by cold fluid mn-epa (Thi The) Me' cpw ( Tee- Tei). 3 x 1.005 (100-20) = 2x4186 C Tee - 15) 8.372 (Tce-15) 241.2 15 - Tee 8.32 Tee = 43.810 44°c. exit temperature of cold water 241-2 (Ang)

Answer 2

thats NOT the answer, it is 39°C because you neglected the heat loss of 40kJ/s which is NOT neglectable
MCp*deltaT of water is NOT equal to MCp*deltaT of air, but its equal to the loss:
. Q= - 40 kJ/s=(MCp*[Tf-Ti]) of water + (MCp*[Tf-Ti]) of air