Heat transfer and surface area of a pipe

[alonion]

So a friend of mine is living in an apartment with central heating but the radiators are removed, so there are just the pipes.
He seems to think that if you cut aluminium jar caps and cover the pipes with them, it will increase the surface area of the pipe and therefore heat the room more. The same general idea applies to anything from aluminium, like covering the pipes with beer cans.
This is a popular myth around here and I believe that it should either not work at all or the increase would be extremely small, like in the sub 5% range. Can you help me understand the problem and show me how to do the calculations ?

 

[CWatters]

A simple approach might be to say that the heat loss (from the pipe into the room) is proportional to the surface area. By what percentage would adding caps increase the surface area?

 

[alonion]

I don't really know how much but let's say a 100% increase in surface area.
So if we have a 100% increase in surface area we have a 100% increase in heat transfer from the pipe to the room ?

 

[billy_joule]

In general yes. But fins are a bit different. In your case the contact resistance of the fin on the pipe will likely play a large role.
Thermal conductivity of the fin itself and fin spacing also matters.
Cooling Fins can be found in many heat transfer applications (cars, computers & AC should be familiar applications). Their use is so widespread many heat transfer textbooks devote at least a chapter to them.