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I'm trying to find the canonical method for transferring heat from a 70–80 °C source to heat up some air in a box to 45–50 °C. My understanding of heat pipes is that the liquid inside has to evaporate in order for heat transfer to occur. Many heat pipes use water, but water doesn't boil at 50 °C.
What is the canonical/best way for transferring heat at 70 °C?
Here's a simple sketch:
where the black rod is the proposed heat pipe insulated with some rubber (green).
The advantage of a heat pipe is that it is a passive way of getting a high rate of heat transfer between two points using a single, compact component.
However a heat pipe is only one part of a heat transfer system and you also need heat exchangers of some sort at the source and sink to effectively transfer heat from one medium to another.
The most common way to transfer heat to air is to use a radiator (they actually work by convection not radiation bu that's what they are called). Essentially you pump hot water through a network of pipes with a large surface area exposed to the air. Air has a low coefficient of thermal conductivity so the surface area has a big impact on its performance.
The situation you describe is broadly similar to a domestic central heating system or water cooling in an engine.
Whether you use a heat pipe or pumped coolant is largely down to what is most convenient for your particular installation. Heat pipes tend to be more attractive for small compact systems whereas pumps will tend to give more performance in large scale and more demanding applications and also give more flexibility in the relative positions of the source and sink as you can make the pipe runs as long as you want as long as it is well insulated.
Pumps also have the advantage that you can use the flow-rate to control the rate of heat transfer.
Yes, many heat pipes use water as a working fluid but the pipe is evacuated of air before the water is introduced so they will work fine at 50 °C.
When I was first presented with a heat pipe, I went into the bathroom and put it under the hot water to feel it conduct the heat.
Like you suggest, a heat pipe is a good solution, and most of them do not use water but application-specific heat-transfer fluids (some examples) that can evaporate at temperatures below 100 °C. Those are used in processor cooling in computers for example.
