# Thermal insulation and positive pressure

1. Aug 10, 2011

### Qbomb

I was having a discussion about thermal insulation (high temperature) and I thought that positive pressure (meaning the heat is being blown onto the insulation) would reduce how effective the insulation was.

Say a 1/2" of insulation reduced temperature from 1000 degrees F to 500 degrees F. The question is if the heat was being blown (say with a fan) onto the insulation would it only reduce the insulation to say 600 degrees F?

If this does make sense to someone is there a formula to figure it out?

2. Aug 10, 2011

### cjl

That depends. You're basically talking about forced convection, which will reduce the thermal resistance of the air-surface interface. However, in most insulation, the air-surface interface already can transfer heat much more effectively than the insulation itself (the insulation has a higher thermal resistance than the air-insulation interface). If that is the case, it won't make much of a difference. However, if the air-surface interface has a thermal resistance that is significant relative to the thermal resistance of the insulation, the forced convection will increase the rate of heat transfer and probably the temperature of the opposite side of the insulation. In general, the more effective the insulation is, the less difference the forced convection would make.

3. Aug 10, 2011

### Qbomb

Yes I am talking about forced convection against an insulation. One difference that i should have mentioned, was that in the original discussion I had, we were trying to trap the heat inside and not let it escape. Therefore making the 'cold side' of the insulation the air-surface interface.

So we were discussing if 1/2" of insulation with no forced convection dropped the temp from hot side being 1000 degrees F to cold side being 500 degrees F, what would the change be with forced convection. So say 10 psi of forced convection would make the hot side 1000 degrees F (or possibly hotter?) and the insulation would only reduce the cold side to x degrees F (550 degrees F or something like that).

Any thoughts cjl?...or anyone else out there.

4. Aug 11, 2011

### cjl

As I said, it depends. In most cases, with decent insulation, the change will be pretty minimal. One way to tell would be to measure the temperature of the hot side surface relative to the air. If the surface is 1000F and the air is 1200F (and the cold side is 500F), then forced convection will make a significant difference, since it will make the surface temperature rise to become much closer to the air temperature of 1200F. However, if the air is 1020F and the surface is 1000F, the forced convection really won't make much of a difference.