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Old 31st January 2008, 11:21 AM   #1
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Post Help with Induction heating for steam

Hi,

I'm working on a project that would require the ability to heat water into steam via electric induction, preferably DC or via AC converter if necessary.

I'm thinking a monotube design with an induction coil surrounding it that could produce steam from 2 quarts of water in say 30 seconds or less.

Anyone here have experience with that kind of design?

Thanks,
:)
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Old 31st January 2008, 11:27 AM   #2
AndrewT is offline AndrewT  Scotland
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2quarts (4pints) to steam in 30seconds!
What heat flow rate is that? kW or MW?
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Old 31st January 2008, 11:45 AM   #3
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OK lets have a go...

Specific heat capacity of water is 4.2 joules per gram per degree C, or putting it another way 4.2 watts will heat 1 gram of water by 1 degree C in 1 second.

So 4 quarts = 2200g of water
Assume the water starts at +20C and we heat it to +100C, an increase of +80C

So energy to heat the water to +100C is 4.2 x 2200 x 80 = 739200 joules.

Over 30 seconds that equals (739200 / 30) = 24640 watts, or about 25KW.

You'd just about be able to do it with a 240V / 32A 3-phase feed.
Here in the UK you can install 10KW electric shower units which operate off a single-phase feed, so it could be done as long as you can get the power into your building.

Cheers,

Len.
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Old 31st January 2008, 01:43 PM   #4
Geoff is offline Geoff  United Kingdom
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Quote:
Originally posted by len_scanlan
OK lets have a go...
Len

Sorry to disagree with you but...

You've got the water to boiling point but not to steam. To convert water at 100degC to steam at 100degC you need to supply further energy (the latent heat of vapourisation).

The latent heat of vapourisation for water is 2260 kJ/kg so in addition to the 739kJ needed to raise the 2.2kg of water from 20degC to 100degC a further 4972kJ will be needed to convert it to steam.

This gives a total input requirement of 5711kJ which, over 30 seconds, equates to 190kW.

Geoff
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Old 31st January 2008, 01:53 PM   #5
cuibono is offline cuibono  United States
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Quote:
Originally posted by Geoff


This gives a total input requirement of 5711kJ which, over 30 seconds, equates to 190kW.

bet my wife could do it

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Old 31st January 2008, 01:53 PM   #6
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And what temperature does the steam need to have?
101░C ... 180░C? ...

Good luck on the project.
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Old 31st January 2008, 05:23 PM   #7
Eva is offline Eva  Spain
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Quote:
Originally posted by Geoff


Len

Sorry to disagree with you but...

You've got the water to boiling point but not to steam. To convert water at 100degC to steam at 100degC you need to supply further energy (the latent heat of vapourisation).

The latent heat of vapourisation for water is 2260 kJ/kg so in addition to the 739kJ needed to raise the 2.2kg of water from 20degC to 100degC a further 4972kJ will be needed to convert it to steam.

This gives a total input requirement of 5711kJ which, over 30 seconds, equates to 190kW.

Geoff
This isn't completely true either. Some steam will start to be produced above 50 degrees or so, and part of the water will be already steam at, say, 75 degrees, not to mention at 90 degrees... The energy required to get to these temperatures will also be higher than expected because the phase change is already taking place.

These theoretical energy figures are true for converting up to the last gram of water into steam, but water is a funny substance with a gradual vapourization process that starts well below the boiling point.

So yes, you can get "some" steam after 30 seconds with a practical energy supply, although vapourizing all the water takes much more time...

Pressure may make a difference too.
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Old 31st January 2008, 05:28 PM   #8
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Quote:
Originally posted by Eva
This isn't completely true either. Some steam will start to be produced above 50 degrees or so, and part of the water will be already steam at, say, 75 degrees, not to mention at 90 degrees...
You're confusing 'steam' with 'vapor', those are two different things.
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Old 31st January 2008, 05:35 PM   #9
Eva is offline Eva  Spain
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What is the difference?
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Old 31st January 2008, 08:06 PM   #10
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Quote:
Originally posted by Geoff


Len

Sorry to disagree with you but...

You've got the water to boiling point but not to steam. To convert water at 100degC to steam at 100degC you need to supply further energy (the latent heat of vapourisation).

The latent heat of vapourisation for water is 2260 kJ/kg so in addition to the 739kJ needed to raise the 2.2kg of water from 20degC to 100degC a further 4972kJ will be needed to convert it to steam.

This gives a total input requirement of 5711kJ which, over 30 seconds, equates to 190kW.

Geoff

Cheers Geoff,

I knew I'd forgotten something - of course steam burns you much more than hot water, its the additional energy to vaporise the water.

Going back to the original post, I don't know if induction heating would be suitable for directly heating water? As far as I know it requires some conductivity in the material being heated. Fresh water would have too high a resistance to get any appreciable heating effect? Salt water maybe... Ovens with induction heating heat the metal pot, not the contents

Len.
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