Two engineers are having an argument about the efficiency of a tube-side multipass heat exchanger compared to a similar exchanger with a single tube-side pass. Mr. Smith claims that for a given number of tubes are rate of heat transfer, more area is required in a two-pass exchanger than in a one-pass, because the effective temperature difference is less. Mr. Jones, on the other hand, claims that because the tube-side velocity and hence coefficient is higher, less area is required in a two-pass exchanger. With the conditions given below, which is correct? Which case would you recommend, of what changes in the exchanger would you recommend?

Exchanger specifications 

– 200 tube passes total

– 1 inch O.D copper tubes, 16 B.W.G.

Tube-side fluid

Water entering at 16°C, leaving at 28°C, with a rate of 225,000 kg/h

Shell-side fluid

Mobiltherm 600, entering at 50°C, leaving at 33°C.

Shell side coefficient = 1700 W/(m² K)

GIVEN

• Tube and shell heat exchanger - water in tubes, Mobiltherm 600 in shell
• Number of tube passes (N_p) = 200
• Tubes are 1 in copper 16 B.W.G.
• Water flow rate (m_w) = 225,000 kg/h = 62.5 kg/s
• Water temperatures
• T_w,in = 16°C
• T_w,out = 28°
• Mobiltherm temperatures
• T_m,in = 50°C
• T_m,out = 33°C
• Shell side heat transfer coefficient (h_o) = 1700 W/(m² K)

ASSUMPTIONS

• Thermal resistance of copper tube wall is negligible

Moblitherm Mobiltherm Water + Water Case (a) Case (b)