**Exercise Set 4-13 Heat Loss from Pipe Insulation
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The answer is 3. If there is insulation, heat loss should mainly be due to heat conduction; how could it be radiation or convection heat transfer?
The heat loss simply shifts from the pipe surface to the surface of the insulation material. It’s still thermal radiation and thermal convection, only the transfer coefficient is much smaller. Simply put, it is the balance of heat retention and temperature resulting from pipe heat exchange, which can be regarded as a single entity.
Choose C here; both radiation and convection need to be taken into account. The amount of heat transfer by radiation is primarily related to the temperature difference between the outer surface and the surrounding environment, while convection is related to both the temperature difference and wind speed; its magnitude is not fixed, and the calculation methods differ as well.