Best steels for high temp heat exchanger w/ possible H2S exposure? 1

[coteesh]

I'm designing a custom heat exchanger that is to be embedded in the walls of a large high temperature "oven" of sorts. The purpose is for heat extraction into a heat-transfer fluid (such as glycol), the heat energy is to be used as a power source. I need some recommendations/material specs for some steels that might suit this application. Here are some data pertaining to this particular device:

- Tubing (OD ~ 2 inches) with a wall thickness that is relatively thin for low pressures, tubing is to be bent into a spiral shape with spiral diameter of approximately 8 ft. Total length of tubing may be very long (hundreds of ft.).

- Tubing is embedded into a brick material that is poured and therefore the tubing is sheltered from weathering (wind, rain etc). However cracks could develop in the solid brick later during operation and fluids could seep in and touch the surface of the tubing. One possibility is H2S coming into contact with the OD of the tubing, in small areas. Other than H2S, perhaps water and air could get through any cracks in the brick and come in contact with surface of tubing.

- A glycol mixture is to be circulated in the tubing and heated up from the surrounding hot brick by conduction through the tubing (during operation of this device the brick is heated by a flame and hot gasses in the oven).

- Glycol pressure in the tubing is very low (only enough to overcome friction pressure drop in tubing when pumping it through).

- Minimal cost of material for tubing is ideal.

- Estimated temperatures of the brick in contact with the tubing is expected to be as high as 1000 deg.C (1800 F), but could be higher or lower depending on how much brick we pour around the tubing. We would like to choose a steel that can withstand as high a temperature as possible, but perhaps we could settle for a steel that is good up to 700 deg.C (1300 F). It would be ideal to find some steel that sustains even higher temperature and can take some H2S exposure, we can minimize the amount of brick required if the steel can sustain higher temperatures.

If anyone can recommend some steels it would be very helpful, thank you very much!

 

[metengr]

In reviewing the conceptual scope of your project, I would presume that the design of this heat exchanger will be in accordance with the ASME Boiler and Pressure Vessel Code for unfired pressure vessels (Section VIII, Div 1) or some other applicable European code or standard for unfired pressure vessels - correct?

If not, I would strongly advise that you contact a reputable A/E firm that specializes in the design of heat exchangers. Your selection of materials will be based on many design conditions that you need to evaluate, and optimize for this type of project. The overall list of candidate heat exchanger materials will ultimately be limited to those approved or endorsed by the applicable code.

 

[NickelMet]

I don't think you are going to find a cheap solution to this problem. High temperature attacks at 1800°F (or even the lower 1300°F you suggest) will rule out the carbon steel realm I believe. (Oxidation limit of ~1050°F for plain carbon steel and ~1300°F for 9Cr low alloy carbon steel.) You are probably going to have to look at stainless steels, maybe even nickel-based alloys.

Another thing you're going to have to remember is thermal cycling (of both your brickwork and metal). And, remember that the refractory may contain some small amounts of contaminants that could affect the metal. (For example, if you mix the brick with chloride-containing water, you introduce the possibility of chloride attack during cooldowns, especially if water were allowed to get to the tubes as you state as a possibility.)

Like Metengr said (paraphrasing), you need to be very aware of your design conditions and their applicable codes. Also, be aware of how the environment is going to operate.

Doesn't sound like you have an easy project.

~NiM