# Help with Induction heating for steam

#### jacob.jacob0

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,

#### AndrewT

2quarts (4pints) to steam in 30seconds!
What heat flow rate is that? kW or MW?

#### len_scanlan

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.

#### Geoff

len_scanlan said:
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

#### cuibono

Geoff said:

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

bet my wife could do it

#### ßart West-VL.

And what temperature does the steam need to have?
101°C ... 180°C? ...

Good luck on the project.

#### Eva

Geoff said:

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.

#### ßart West-VL.

Eva said:
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.

#### Eva

What is the difference?

#### len_scanlan

Geoff said:

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.

#### ßart West-VL.

Eva said:
What is the difference?
A cloud is vapor, fog is vapor.
Steam is when the water expands by the heat, in a closed container that would give pressure. vapor does not change the pressure in a container.

Steam is also possible at lower temperatures but then you would have to lower the pressure. High in the mountains you can boil water at about 80°C, from then on it would be steam only because the pressure is lower.
If you can get near vacuum, you can see the water boil at, lets say, 18°C.
It even go's further. (and a bit off topic)
Because the water boils at 18°C, it has to extract the energy from somewhere to keep boiling, the heat of the container or the water itself.
So after a while you get ice, water and vapor all in the same place.

#### thermal runaway

len_scanlan said:

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.

My interpretation is that he wants to heat the water thru transfer from an induction heater. So the induction unit gets hot and the water is circulated around it to transfer the heat.

If pressurised steam is required it will need a huge amount of energy to create as not only the water would need to be heated but the mass of the induction unit too !!

#### AndrewT

AndrewT said:
2quarts (4pints) to steam in 30seconds!
What heat flow rate is that? kW or MW?
are we up to the megawatts guesstimate yet?

#### thermal runaway

Cant be far off !!

#### AndrewT

There may be another way.
Preheat the heater assembly to around 200degC or maybe a lot more.
Then, open the water valve and pass the 4pints through the heat exchanger and out comes 5lbs of steam. But it would need to be a massive heat store and possibly copper rather than aluminium.

#### thermal runaway

Similar technique is used in flash steam model boats !!

Would thermal shock be an issue ?

#### brookly Johaph

Definitely use a single tube. I would use one with vanes on the inside to maximize surface area between the water and the metal.

Get a copy of Davies & Simpson's Induction Heating Handbook through interlibrary-loan.