Watt density on electric bundle heaters

[XL83NL]

Dear all, were appyling an electric heater used for evaporating propylene in a pressure vessel. If the heat input from the heater bundle is too high, concern is that decomposition of the prop. will initiate. Im looking for both reference data (i.e. literature references) and values on the maximum allowable watt density for evaporating propylene, which can be used as an input variable for design of the electric heating bundle.

 

[Compositepro]

You really should target using the lowest watt-density you can get away with economically rather than the highest, unless your only goal is keeping initial cost and volume low. Low watt density will have the highest reliability, longevity, and least possibility of degrading your product. With propylene you probably have to worry about polymerization first. Once you get any polymerization on the heater that will quickly turn to coke.

There is no precise way to determine a max. density, although heaters designed for heating water will be at the high end, which I think is about one watt/cm2 for standard heaters. Of course, special heaters can be much higher that that. The actual decomposition temperature of your product and how close the bulk fluid temperature is to that limit are probably the most important factors. The problem is that once coking or scale build-up starts to occur at even a tiny hot-spot, it will accelerate very rapidly, depending on the watt density. Coking often plugs and fouls the whole piping system before the heater burns-out.

Chromalox and Watlow have good technical literature on how to select heaters.

 

[XL83NL]

Thanks. We're aiming at using a low watt density, there certainly is awareness about the issues that may occur of having a too high heat transfer rate. Having data of what can be considered low makes it easier to do the assessment, even though reference values on comparable media may help (a bit). Hence any references to literature for propylene would be useful.

 

[georgeverghese]

Commercial grades of propylene may contain small fractions of C4= (isomers are 1-butene, 2-butene, isobutene). If this is applicable to your case, would suggest we look up thermal decomposition temps for the these trace components and select the lowest of these to be the max permissible skin temp for these heater elements when operating at high tubeskin temperature trip. Heater vendors know how to translate this max permissible skin temp limit to the corresponding electrical W/m2 heat output.

Presuming there are no solids that will lower decomposition temp catalytically. Thermal decomposition temp for C3= is stated to be 580degC minimum from articles on the net.

 

[moltenmetal]

In addition to keeping watt densities low, make the assembly as long and thin as practical. For gas heaters we typically specify two bundles, as long as you can make them, each inserted into either end of a flanged spool. Sometimes you even need to go with multiple shells in series to keep the diameter small enough. Higher velocity means better gas film coefficients and lower operating element sheath temperatures.

 

[racookpe1978]

Natural convection only for the fluid, or is it a freely-flowing (continuous flow exchange) HX, or do you have a mixer in a static tank/PV?

 

[XL83NL]

The propylene is liquid, and the vessel is continuously fed with new liquid propylene. Above the (horizontal) liquid level is a vertical part where vapours rise up and exit the vessel (it's like an upside big T-piece).

 

[EmmanuelTop]

Gulley web site has empirical equations for calculating critical heat flux for Propane vaporizers, as a function of shell bundle diameter:

http://www.gulleyassociates.com/newtips.htm

(find "critical heat flux" in the page).

For conventional reboilers I am familiar with, the design critical heat flux is limited to ~37,000 W/m2 but in reality you will rarely see figures above 25,000 W/m2.
Also look at

http://www.cambchemtech.com/files/9112/8984/5743/Understanding_Heat_Flux_Limitations_CCTI_2010.pdf

Dejan IVANOVIC
Process Engineer, MSChE

 

[XL83NL]

Thanks. I believe the reference to Gulley's website is for gaseous propane rather than liquid propene (liquid propene @ 35 bar(g)).

 

[EmmanuelTop]

It is actually for boiling Propane (critical heat flux has no meaning for vapor phase). I wouldn't expect any significant difference between Propane and Propylene, with regards to maximum heat flux. They are very similar components.


Dejan IVANOVIC
Process Engineer, MSChE

 

[moltenmetal]

Propylene is thermolabile (reactive) whereas propane isn't nearly so. Indeed I would expect a difference...but it's a good check.

It should be easy enough to estimate the h expected from boiling, assume uniform heating on the bundle at x W/in2 as a first pass, then estimate the metal temperature of the element sheath. Below 200 C? You're likely totally in the clear. 200-300 C? Get a little worried. Above 300 C? Get even more worried, and probably drop the watt density and try again.

 

[georgeverghese]

Process electric heaters are also prone to high element skin temp trip due to an operating scenario where inlet stream flow drops suddenly and the heater keeps firing at the rate prior to the drop in flow. This trip is the result of one or both of these 2 factors

a) A TIC with a TT which has a large response lag
b) Heater with high w/m2 flux.

Response lags in the TT are mostly due to the thick walled thermowell and poor contact of the TE to the TW, so this is something to be improved on also.

 

[georgeverghese]

Oops, this is an electric heater in pool boiling service for a single component, so (a) doesnt apply from a temperature excursion point of view. But it could lead to excess C3= vapor production ?

 

[racookpe1978]

Does this problem not need to be analyzed like the nuclear power "saturated/nucleate/film" boiling problem around fuel rods?

That is, what "kind of boiling" begins to destroy the fluid (carbonize, burn, or disassociate, or clog) or stop the process quality? Once the fluid requirements are known, THEN you go backwards and read the chart to find out the maximum heat rate that can come FROM the electric heater tubes that can be tolerated without fouling the fluid.

The problem statement seems to ask "What is the maximum amount of heat transferred if I have a electric power of so-and-so into a boiling liquid?"? First, you have to decide if you can tolerate boiling at all.

 

[crshears]

Dumb question, maybe, as I know little of thermal dissociation, but is forced circulation ever used in such applications to ensure constant and thorough sweeping of all heating surfaces so as to minimize if not completely preclude any possibility of spot heating and product decomposition? I could envision this being useful where the delta T between the required temp and the temp of dissociation is small.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Compositepro]

crshears, it is very common to place immersion heaters into high velocity fluid flow to improve the heat transfer and to allow for higher watt density to be used. Such systems are more prone to failure due to loss of flow, and thus often have flow switches to protect the heaters. Also, temperature sensors are often placed very close to the heater surfaces to be able to respond to overheating more rapidly.

 

[crshears]

Thanks, Compo; I did not know that, especially the more prone to failure part.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[moltenmetal]

An overtemperature thermocouple, or more often several of these, are typically installed physically attached to the heating elements to detect loss of flow- or in this case, loss of level.

 

[MJCronin]

Why would you want to place the electric heater inside of the pressure vessel ?

This makes heater maintenance and eventual replacement difficult and dangerous, IMHO

Propylene has many of the same characteristics as propane ...

Electric heaters (vaporizers) for propane are a commodity item and are typically located on the adjoining piping system.

Locating the heater outside of the vessel has many advantages, including protecting the vessel from overpressure and thermocouple maintenance. A spare heater can even be kept in stock for ultimate system reliability.

I would talk with the designers of these heaters

http://algas-sdi.com/2015-11-05-20-28-34/torrexx-electric-lpg-vaporizer

MJCronin
Sr. Process Engineer

 

[Compositepro]

How are those Algas units different from a "pressure vessel with a heater inside"? They are just prepackaged for a specific application. It is possible that they have some desirable design features. They say their heaters are cast into aluminum. The purpose of doing that would be to lower the watt density of the heater surface.