Why there is no outside insulation on fired heaters shell/box ? One of my colleague asked me this question and I was wondering why. Can anybody explain ?
I don't know for sure, but many people worry a lot about enhanced corrosion under insulation due to the heat and the thermal losses are acceptable to them. Many are insulated for personnel protection.
sometimes heaters would need high temperature insulation.
Depends on the location and the designer / client
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There is insulation on the inside to protect the steel structure from high temperatures, which would weaken or damage it. Insulation on the outside would simply increase the temperature of the structural steel and require more insulation inside, which would be pointless. Steel is painted for corrosion protection, so it is usually the paint that actually controls the maximum allowed temperature.
Very true. I was thinking more water bath heaters rather than fired heaters. The internal for brick lining will take the heat but you don't want to cook the steelwork.
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Thanks LittleInch and Compositepro for your replies. The quaestion of outside insulation on fired heaters arose for the purpose of energy saving by reducing heat loss to atomsphere through the heater wall. Even after refractory is applied inside the heater walls, skin temperature of a fired heater is considerably high (100 - 200 degC depending upon the operating temperature). Considering atmosphere temp in the range of 35 - 40 degC, there is scope for reduction of heat loss.
Yes, but if you put insulation on it the temperature of the steel could rise to 400+ causing significant structural issues. Be very careful about retrofitting anything which causes the skin temperature to increase compared to the design assumptions.
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While the insulation outside the steel structure for energy saving, the internal refractory is used to maintain the heat energy inside the heater as well as the mechanical strength of the outside structure. The structure steel becomes weak if the surface temperature goes up.
If covered with insulation outside, it may block the inspection of the hot spot from outside as the refractory damaged. As to reduce the surface temperature of the steel, one may increase the refractory thickness as if the cost effective.
Hot spots may appear on parts of the fired heater shell if there are gaps in the internal brickwork. Check at next shutdown and ask the fired heater vendor to patch up.
Thank you LittleInch, mk3223, georgeverghese for your replies. I will sum up here -
1. Refractory lining thickness inside the fired heater may be increased in order to reduce heat loss through heater wall.
2. Outside insulation of fired heater to reduce heat loss is not recommended because it will lead to increase in metal temperature thereby weakening its strength. Also hot spots due to defective refractory lining can easily be detected on a naked wall while on a insulated wall the same may not detected immediately which in turn may pose a greater safety threat.
