500KW Electrical heater and heat exchanger calcs

[Mattm86]

I have been tasked with designing an electric water heater for application in a restricted space (long and narrow pipe).

The heater needs to heat water from 18 deg C to 28 Deg C at a flow rate of 60-90 GPM. It will be powered via a 3 phase supply limited to 5KV @ 140 amps.
Im thinking of arrays of 5KW cartridge heaters. What I'm currently stuck on, is how to calculate/design the heatsink/finned area where the water will flow through to be heated.

Any and all advice welcome.

many thanks

Matt

 

[LittleInch]

Care to share your working out? I get about 25kW for that duty which is a looooong way from 500.

The post directly under yours has some data where 20W/ in2 (ignore the unit ambiguity) sounds like a good place to start in terms of heater area. I would forget finning - not worth it for this duty.

Why don't you just get three or four 9kW electric shower units? and run them flat out? They may even be able to control temperature for you as well?

Unless you've made a typo here, you don't need 5kV heaters for this duty.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MFJewell]

Did you bother to look in your heat transfer book?

 

[Mattm86]

Hi,

MFJewell - I don't have one, I'm not familiar with this sort of thing if I'm honest, but if you have one that you would recommend then I would look at obtaining a copy, but this is why I posted on here for a bit of a head start.

LittleInch - the 450 kw number has come from the client who used OLGA software package. My calculations (which is my first foray into heater type calculations) has come up far higher than this based on using some cartridge heater calculations etc. (

https://www.omega.co.uk/heaters/pdf/cartridge_heater_info.pdf

)

The unit needs to heat the fluid as it flows through the heater at a velocity of about 3 fps.

That calcs below pertains to 1 'stage' or heater array, of which there would be 9 stages in line.

Heat chamber flow area 28.7 in2
Length of heat chamber 37.0 in length of heater stage
Volume of fluid 1061.2 in3
Specific weight of water 0.0361 lb/in3
Weight of volume 38.3 lbs

Flow Rate 87 gpm (3000 barrels per day)
Fluid Velocity 3.0 fps 2.3 fps est Q=VA
Time for heatup 1.0 s

Weight of heated fluid (lbs) 38.3 Wt
Specific heat (BTU/ib/F) 1.005 Cp
Temperature Change (F) 1.919 dT (10 degrees C over 9 stages)
Heat up time in hours 0.000282346 H

https://www.omega.co.uk/heaters/pdf/cartridge_heater_info.pdf

Power required 76.69 KW
73.9 Btu's Q=m.Cp.(T2-T1)

 

[LittleInch]

Then like I said, OLGA or otherwise, somewhere the units are wrong, but I confess I got an factor of 10 error before somehow.

87gpm is (assuming this is USGPM) is 5.51 litres per second = 5,5kg/sec. The specific heat of water is 4.2kJ/kg/deg C So a 10C rise is 5.5 x 10 x 4.2 = 230 kW heater.

By the look of it you're trying to do this in a 3" pipe?

forget about heat up time - what you need is steady state dynamics.

How long a 3" pipe do you have?

I don't think cartridge heaters are the way forward and you really need to talk to someone like omega who have done lots of these before.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

Little Inch is right on (though my number was closer to 240kW).
The power density in cold water can be closer 40W/in2.
So even if we used 40 the result is 6,000 sq in of heater surface area (39,000 sq cm).
Which is a lot.
You need a real electric boiler for this application, a device designed to heat water.
I have seen units cascaded in order to get enough power.



= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Mattm86]

Thanks people,

Im wondering if the figure I have from the client is including other heat losses or also gas dissolved in the water?

The annular space where the heaters reside and flow moves, is 7.6" OD and a 2.5" ID (minus the area of 12* 20mm heaters). Using 10 or 11 1m long 5KW heaters (1 x 20mm conduit is used for cables etc). As EdStainless suggested, we would be cascading the units, so I would have 3-9 heater stages in-line. length can be up to 20/30 ft.

Omega did not want to know when I contacted them, so I am discussing with a few other heater companies, but TBH I haven't spoken to anyone who has completely filled me with confidence (actually this thread is already proving more insightful).

Thank you all for the guidance.

 

[LittleInch]

You may be better thinking about using external heat tracing type tape either wrapped around or longitudinal sections with as much external insulation as you can it into your space. This avoids any sealing of cables etc, gives you a larger surface area to work with and if you're going for a realtively small climb of temp per unit should be perfectly feasible.

I'm having difficulty envisaging your space limitations, but look at the external option as well as the internal option.

Another alternative is an annular space and using hot oil or water and having your heating system off line somewhere else where space isn't such as issue. Any explanation of what it is you're doing or background would be very useful.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Also bear in mind that high heat flux rates translate to high tubeskin temperatures at the heating elements, and scaling / fouling of the heating surfaces is more likely at high heat fluxes. Keep within the permissible heat flux rate for the total dissolved solids in this feedwater.