Purpose of insulating furnaces

[sriram123]

Hello all,

I'm going to ask something that some people may even feel the dumb

I want to know why we insulate furnaces and moulds in plastic processing.

I know that during steady state heat transfer, the heat generated or conducted =heat getting or leaving the body.

And the heat flowing out depends upon the temperature difference between the body and ambient temperature.

So what does an insulator like wool or brick does during steady state heat transfer,

Will it reduce the temperature difference so that only less heat will flow out of the body and the remainder will be used for some application.(like heating something for a furnace)?


Sorry if i am asking something very basic or so.

I'm not an engineer,just a guy with 3 months experience in a foundry

 

[Lsos]

And the heat flowing out depends upon the temperature difference between the body and ambient temperature.

AND the heat transfer coefficient of anything in between, and it's thickness. So if you put an insulator which conducts heat poorly, and you make it thick, then the heat will flow slowly.

 

[CWatters]

If you look at heat conducted through the walls the equation for the power loss is something like...

Power = Constant * Surface Area * (T_in - T_out) / Thickness

Surface Area = the area of the furnace
Thickness = thickness of the insulation.
Constant = Thermal conductivity of the insulation

Different materials have different thermal conductivity. Copper is a good condustor of heat, where as Silica Aerogel is a poor conductor or heat (eg a good insulator).

I want to know why we insulate furnaces and moulds in plastic processing.

I know that during steady state heat transfer, the heat generated or conducted =heat getting or leaving the body.

To maintain a constant temperature in a furnace the heat lost through the walls of the furnace has to be replaced by heating the inside using electricity or gas. Since those aren't free it pays to minimise heat lost by insulating the walls.

Not all molds are insulated. Some are deliberatly cooled. The incoming liquid material carries with it a lot of thermal energy. That has to be removed so that it can solidify. I'm not an expert on molds but I believe a balance has to be found so the mold is maintained at the right temperature.

 

[sriram123]

Thanks for the reply.

But what does this "steady state " exactly mean

It is said at steady state the heat leaving = Heat generated.

Is it like if we do not allow the heat to flow fast then it will increase the temperature of the body , right ?

But if it increases then will the body will be in steady state heat transfer ?




I picked up these concepts from the engineers working with me.I have not actually read any heat transfer book. Pls suggest me some good book with lot of practical examples to start with.

 

[CWatters]

Thanks for the reply.

But what does this "steady state " exactly mean

It is said at steady state the heat leaving = Heat generated.

Imagine you had a factory that was going to start making bricks. Initially materials would arrive at the factory but no bricks would leave the factory. After a certain time bricks would start to leave the factory. Under steady state conditions the amount of material entering would be consistant with the number of bricks leaving. There would be no stock piling or running down of stocks within the factory.

It's the same wth heat. If you increase the amount of power going into something it takes awhile before the amount of heat leaving catches up. A fridge is a good example. Open the door and the contents warms up a bit. When you close the door the pump starts to cool the contents down again. For awhile the power required is more than the steady state condition. Once the fridge is down to it's normal temperature the pump only has to remove an amount equal to that which leaks in through the walls of the fridge.

 

[sriram123]

That helped me lot.I really feel i have understood something now.Thanks a lot