Watt density on electric bundle heaters
Watt density on electric bundle heaters
(OP)
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.





RE: Watt density on electric bundle heaters
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.
RE: Watt density on electric bundle heaters
RE: Watt density on electric bundle heaters
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.
RE: Watt density on electric bundle heaters
RE: Watt density on electric bundle heaters
RE: Watt density on electric bundle heaters
RE: Watt density on electric bundle heaters
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/U...
Dejan IVANOVIC
Process Engineer, MSChE
RE: Watt density on electric bundle heaters
RE: Watt density on electric bundle heaters
Dejan IVANOVIC
Process Engineer, MSChE
RE: Watt density on electric bundle heaters
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.
RE: Watt density on electric bundle heaters
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.
RE: Watt density on electric bundle heaters
RE: Watt density on electric bundle heaters
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.
RE: Watt density on electric bundle heaters
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: Watt density on electric bundle heaters
RE: Watt density on electric bundle heaters
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: Watt density on electric bundle heaters
RE: Watt density on electric bundle heaters
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-e...
MJCronin
Sr. Process Engineer
RE: Watt density on electric bundle heaters
RE: Watt density on electric bundle heaters
This makes heater maintenance and eventual replacement difficult and dangerous, IMHO/quote]
Flanged electric immersion heaters are used for heating and vapourizing flammables all the time. There's nothing inherently more hazardous about using or maintaining them than any other type except perhaps a FIRED heater, which of course is far more hazardous- but those are used for flammables to, on a gigantic scale.
If your choice is between building a new heating utility (steam, hot oil, Dowtherm vapour etc.) for one or two users or using electric directly, and the duty is modest so the energy consumption isn't a huge cost issue, the selection is a fairly easy one.
You can put the heating element on the outside of the heater shell if you only need a tiny amount of surface area and you don't care about surface metal temperatures. With a propylene vapourizer, that's not likely to be the case.
If you're really worried about heater surface temperatures, and your energy source is ultimately electricity, then you need an intermediate fluid- something like hot oil or Dowtherm vapour.