Why would the heat exchange tubes be behind a liner and not exposed to the flame?

[haff99]

This is a curiosity question that I've been thinking about. My old pellet insert used to have the heat exchange tubes exposed directly to the flame. So when you looked into the stove, you could see the heat exchange tube and the flame would make direct contact with them. My new insert, a lopi agp, has a top liner(and back and 2 sides) that keeps the tubes hidden from view, thus the flame never makes contact with the tubes. When you remove the top liner to clean the stove, then you can see the tubes, so it looks like this liner is acting like a heat shield.

So my question is why wouldn't you want the heat exchange tubes exposed directly to the flame? WOuldn't that be more efficient as they would get even hotter? I'm guessing the hotter they get, the hotter the air coming out of them would be. I would assume the liners prevent them from getting as hot as they would with a direct flame. I don't know how many stoves have the heat exchange tubes exposed/not exposed, but there has to be a reason for it. Just curious to why that would be.

 

[Don2222]

The hot air from the fire is still drawn around the heat tube on it's way to the exhaust blower which heats the tubes more evenly and may help them stay cleaner.

 

[bogieb]

My Harman has exposed tubes, the Hastings is covered by a baffle. the baffle is the biggest pain in the hind end to get out and back in (correctly anyway - I've had it heating up and the baffle fall down - user error, I know, but shouldn't be that hard). Can't see any difference in how dirty the tubes get in the Harman versus the St. Croix.

 

[Bioburner]

Harman has a accordion exchanger that allows flame to touch all exposed part not making any cold to surface to want to warp and pull out tubes like what happens on Santa Fe?

 

[haff99]

My Harman has exposed tubes, the Hastings is covered by a baffle. the baffle is the biggest pain in the hind end to get out and back in (correctly anyway - I've had it heating up and the baffle fall down - user error, I know, but shouldn't be that hard). Can't see any difference in how dirty the tubes get in the Harman versus the St. Croix.[/quote

The first time I replaced the liners it must have taken me 40 minutes to get them back now. Now that i done it a few times, I got the trick down and only takes a few minutes. So I know the giant pain they can be to get back on.

Just a note, the air coming out of the tubes is HOTTER now then on my other insert, and I'm using the same pellets. Still seems odd that that it wouldn't be more efficient with the tubes exposed.