Student questions asked in live office hours about this problem
OH 85
Q: When using Table 9.4.1 for heat loss from bare steel pipe, my results differ significantly (88% error) from the natural convection equation approach. Should I use the table when problem conditions align with it, or is there a way to know which method to use?
A: Natural convection is very sensitive to inputs, so reasonable error can be much larger than 15%. If the table was generated from empirical data (likely from ASHRAE Fundamentals), it becomes the golden source; use the table when problem conditions align exactly with the table's stated conditions, otherwise use theoretical calculations.
OH 96
Q: When is it reasonable to use the ASME handbook's typical value of H = 1.2 versus deriving it from equations, and should kinematic viscosity and density be evaluated at the same temperature or different temperatures?
A: Use the handbook's default value only when you lack sufficient information to calculate H directly from the problem; otherwise, use actual data for confidence. For viscosity and density, check if property variations across your temperature range are significant (5-10% is typically acceptable), and if they differ more substantially, use values at the exact operating temperature from tables.