water heater element

Yep.
Close. 1 BTU is about a kilowatt-second. Heating water from 77 deg F to 212 deg F is a difference of 135 deg F, times 2.2 lbs is 297 BTUs. Boiling to steam is 965 BTUs/lb or 2123 BTUs for 2.2 lbs. So 2420 BTUs total or 2550 kilowatt-seconds. So with 1.380 kilowatts of heat you need 1850 seconds or about 31 minutes to make a kilo of schteam from room-temp water.

Sounds about right - but you probably need to feed it 240V to get enough steam to work a bender box.
Make DARNED sure it doesn't run dry, I'd have a float safety to make sure the water level doesn't drop too far. And a float valve fill of some sort, you want to dump the cold water right in front of the heater inlet.
I would suggest putting the element in a 1-1/2" pipe stub (to allow clearance around the element) with a 1' x 1-1/2" x 1-1/2" tee on the feed end (the 1" for the element mount threads), then run a pump to force rapid water circulation around the element - they are designed to be under pressure in a water heater and not have active steam boiling off the element.
It's going to have a shorter life from the cavitation bubbles, but if you keep the water moving swiftly as it goes over the heater element it should minimize this. One of the "Rough Service" or "Low Watt Density" (double-folded) elements will help too. And get a blade-style flow switch to keep the element off till the water is moving.
Let the hot water get carried along by the velocity out into the main tank and /then/ go "Oh, I'm supposed to boil now..."
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Element is 60 inches long, double folded.

none of the comments so far address what seems to be a critical error in your calculation. Heat losses. You are going to loose heat to ambient, and it will get worse as the temp increases. Also don't let the element get above the water level. Getting your steam box to drain back into your pot would also be a good idea.
snipped-for-privacy@snyder.>I picked up a 4500 watt 208 water heater element today cheap. I need jk

The double-folded is /usually/ the low watt density style, but you need to cross the part number in the book to know for certain. And read the footnotes, make sure that the sheath material is compatible with any additives you will put in the water.
And if it isn't the 1" pipe thread screw in style, get another heater element - you'll play hell duplicating that four bolt flange. And the elements are easier to get.
They sell adapters for going the other way - a four bolt flange with the 1" threaded bung in the center.
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I found out the hard way that heater element threads are straight o- ring (STOR) rather than tapered pipe (NPT). They may not even be exactly STOR because they use a squared ring. At least the straight thread was easier to cut on a lathe.

I helped a cabinet-maker friend who lives off-grid build his steamer out of 2" galvanized pipe, with steam from a pressure cooker on a propane barbecue burner, which made plenty of steam once we had insulated every bit of the plumbing thoroughly.
The trickiest part was knowing when the wood was ready and removing it quickly. We slanted the pipe so condensate drained out the bottom until the pipe was up to temperature, then we could see it become steam and start timing. He had oven gloves to unscrew the HOT end fitting and pull out the wood. Any pressure would have scalded him, condensate would have soaked through the glove.

Yes, but I didn't mention that because it isn't critical to the use. If you are going to build a production machine around it, then you machine the straight threads with the gasket pocket.
For a low-pressure one-off like this it should work fine into NPT.
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Fully aware of both points. The energy requirements were only for the boiling off - not the pre-heating and basically I was looking mostly at the power requirements of the element at various voltages - and my numbers were a few percent >

4 bolt flange mounting (I can easily make the flange) and there will be no additives in the water. Elements are copper or copper coated (Chromalox 20200 860 d80) Cost me $5 brand new - and there are 3 more in stock at the same price that I will buy if it works out (for spares down the road).

Watch any additives in the water, anything alkaline will eat through the elements. We got 3 weeks before failure on a wash tank.