Liquid water with a specific heat of 4.18?kJ/kg•K is heated in a circular tube by an electrical heater at 10?kW. The water enters the tube at 10°C with a mass flow rate of 10?g/s. The inner surface of the tube is lined with polyvinylidene chloride (PVDC) lining. According to the ASME Code for Process Piping (ASME B31.3-2014, A323), the recommended maximum temperature for PVDC lining is 79°C. Determine whether the water exiting the tube would be in compliance with the ASME Code for Process Piping. What is the minimum mass flow rate of the water needed to keep the exit temperature from exceeding the recommended maximum temperature for PVDC lining?

Question

What will be an ideal response?

Answer 1

Liquid water entering at 10°C and flowing at 10 g/s(0.01 kg/s) is heated in a circular tube by an electrical heater at 10 kW. Determine whether the water exit temperature would be below 79°C and comply with the ASME Code for Process Piping, and the minimum mass flow rate to keep the exit temperature below 79°C.

Assumptions1 Water is an incompressible substance with constant properties. 2 Heat loss from the tube is negligible. 3 Steady operating conditions.

Properties The specific heat of water is given as 4.18 kJ/kg•K.

AnalysisThe tube is taken as a control volume. For steady state flow, the mass flow rate at the inlet is equal to the mass flow rate at the exit:



The energy balance for the control volume is



Solving for the exit temperature,



Thus, the exit temperature is not in compliance with the ASME Code for Process Piping for PVDC lining.

The minimum mass flow rate needed to keep the water exit temperature from exceeding 79°C is



The higher the value of mass flow rate, the lower the water exit temperature is achieved.

Discussion If the desire is to have higher exit temperature, then a different thermoplastic lining should be used. Polytetrafluoroethylene (PTFE) lining has a recommended maximum temperature of 260°C by the ASME Code for Process Piping (ASME B31.3-2014, A323).