Shell and tube heat exchanger maximum temperature and pressure 2

[doctorsundar]

Would it be possible to heat CO2 which is at 20 - 40 psia pressure to a final temperature of 400 deg C, in a heat exchanger of the shell and tube type? I suppose that the heating medium (hot air, for example) should have an entry temperature of 500 deg C.

I seem to recall reading somewhere that 500 deg C is the maximum temperature for a shell-tube HX, in current engineering practice. But what pressures would be possible at that temperature.

What kind of steel alloys would be used in such a heat exchanger? Any costing information available? Perry's Handbook does have cost info, but one needs to know the type of alloy used.

Thanks in advance.

Sundar

 

[djack77494]

Sundar,
There is no reason you could not perform this function in a shell & tube heat exchanger. In fact, S&T exchangers are probably the most versatile of all heat exchange equipment. Neither the temperature nor the pressure are at the limits of the technology, although the temperature would qualify to be called severe in my opinion.

I do not know why you have speculated that you might perform the heating using a hot air stream. Heat transfer between two gases is the most difficult type of transfer. I would be inclined to use a hot oil (or maybe steam-I'd have to check on that) as the heating medium. I also believe you could accomplish your goal with the heating medium at a lower temperature. Pressure should not be a problem at all for such a system.

You could quite possibly get by with some low alloy steels (Cr-Mo alloys) or upgrade to stainless steel. I am not sufficiently versed in metallurgy to go beyond this, and certainly do not have any costing information.

Good luck,
Doug

 

[metengr]

What kind of steel alloys would be used in such a heat exchanger? Any costing information available? Perry's Handbook does have cost info, but one needs to know the type of alloy used.

After you have decided on the maximum design temperature, the selection of suitable alloys starts with ASME B&PV Code, Section II, Part D. At 500 deg C max service temperature, most certainly Cr-Mo steels like Grade 11 (1.25% Cr - 1/2% Mo or even Grade 22 (2.25% Cr- 1% Mo).

 

[moltenmetal]

djack: the reason doctorsundar is thinking about hot air as the heating medium is because hot oil and even synthetic heat transfer media are not useful beyond his exit temperature of 400 C- in fact, most of these fluids aren't useful above about 350 C. To achieve 500 C with a liquid heat transfer medium you're looking at a molten salt system, liquid metals etc.- or a gas.

As to whether or not you can build a shell and tube exchanger for these conditions, the answer is yes. The challenge is to specify one which will last long-term under the combined thermal and pressure stresses it will experience. As to materials of construction, that's an optimization exercise. There are plenty of choices of suitable materials for heat exchange ~ 500 C, each with its pros and cons in terms of alloy cost and ease/difficulty of fabrication.

doctorsundar: what's your source of heat energy? If it's electric, you're better off to simply use an electric heater and forget about the air. You can buy flange-type elements which are essentially a shell and tube unit with electrically heated tubes.

 

[unclesyd]

Here is a discussion of the practical limit for heat exchangers from one of the best fabricators.

http://www.yuba.com/flash/ebrochure/hemilok.pdf

We used to have a process that operated a 454°C that was heated by circulating combustion gases. We had 18 units and keep a crew to reset the explosion doors on the gas heaters.

We have a process now that operates at the same temperature but is cooled by circulating molten salt. The salt is used to generate steam and preheat boiler feed water.