When is an unfired steam boiler not a heat exchanger?

[Stephen Max]

In the preamble to Section I, there is this verbage:

"Unfired pressure vessels in which steam is generated shall be classed as unfired steam boilers with the following exceptions:
(a) vessels known as evaporators or heat exchangers"

What is the difference between a device classified as an evaporator or heat exchanger and an unfired steam boiler?

Thx.

 

[metengr]

Section I has not been deliberate in defining unfired boilers. The committee has issued prior interpretations in an attempt to further clarify differences, I-81-01.

A unfired boiler using waste heat creating steam for external use is a Power Boiler, especially if duct burners are installed. As a member of the BPV I, I view unfired boilers as being able to be constructed to Section VIII, Div 1 (following more stringent requirements) or Section I. Heat exchangers and evaporators have been singled out by Section I to be Section VIII, Div 1 items as stated in the Preamble because it has been this way for many years and from petrochemical applications. It may be that looking at how waste heat was produced could be the determining factor. For example, waste heat generated by combustion.

To answer your question more directly, there are no clear cut definitions with unfired boilers. You may seek more information from the location that regulates boilers where they may have more deliberate definitions regarding the above.

 

[racookpe1978]

Obviously, all boilers of any kind are "heat exchangers" , but that is a not a complete term for engineering.

A heat exchanger is almost always thought of as a fixed device that exchanges energy between fluids, doesn't mix the two product hot and cold streams (fluids or gasses) and doesn't usually require or expect phase changes in either side. The fluid motion is outside of the heat exchanger - you don't put a pump inside the HX.

An unfired boiler requires a phase change obviously, but doesn't burn the fuel directly to create the hot gasses that are one side of the heat exchanger (boiler) tubes. A classic example is the waste heat recovery boilers at the ends of gas turbine power plants that boiler water.

An evaporator expects (requires) a phase change in the heated fluid. But the usual intent of the evaporator is the production of the evaporating vapors, not the heat exchange between hot and cold fluids: A refrigerator, for example, is designed to cool food or products, but the phase change going on inside the coils as they chill and evaporate as they expand from high pressure to low pressure is merely a part of the total reaction.

 

[XL83NL]

Im not really familiar with boilers, its designs and the applicable design codes. However, the PED treats boilers differently from pressure vessels (and thus heat exchangers), when it comes to classification. The idea behind this different classification is (I believe) based on the old Stoomwezen rules, the first Dutch set of rules pertaining to design of pressure equipment. The rules where set up when the industry started using steam as a means for power in the late 19th, early 20th century.

I dont have detailed information at hand why this classification was treated differently, but from what I can (best) guess, the reason may be that the risks involved in boiler designs are different from ordinary pressure vessels. The PED looks at pressure equipment from a risk point of view. The higher the risks (usualy due to volume and pressure), the higher the assessment class. Boilers have a risk of overheating the tubes by the hot flue gases, when there's no or too little product feed. This may push the surface temperature beyond the design temperature or acceptable skin temeprature for the material involved. Such risks are normally not applicable to pressure vessels.

Sure, the rules in ASME are different from PED: one being a design code, the other being a directive with no design rules (at all). Just my 2 cents, not sure if it contriibutes anything to this discussion.

 

[Stephen Max]

Thank you, gentlemen, for your replies.

I have a device that heats water in a tube by means of external electric resistance heating and heat conduction through a metal matrix surrounding the tube (and not by having the heaters directly immersed in the water) to generate steam. The temperature and pressure involved is 550C at 100 bar.

In your opinion(s), is this considered an "unfired pressure vessel in which steam is generated", per the wording of Section I?

 

[metengr]

It is a power boiler. External heat is being applied to generate steam.

 

[chicopee]

I would classify it as an electric boiler in the class of power boilers.

 

[GenB]

Per ASME this is not an electric boiler. There are 2 types of electric boilers, Immersed resistance element and electrode.
The above post mentions external heat therefore is is a Fired steam boiler and requires full power boiler certification and most jurisdictions (surely US&Canada)will require permit to operate regardless of physical or input size. Regards.


General Blr. CA,USA