Help with Induction heating for steam

[Ross_ross]

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,

[Bigglez]

Greetings Ross^2,

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.

Induction only works with conductive lossy materials,
I doubt tap water would be heated by an RF field.
Perhaps the water is not pure? Or the water is in a
lossy (steel) vessel that in turn is heated by induction?

There are a couple of companies that make industrial
induction heaters. LePEL is one that I'm familiar with.
These are self-oscillation power tubes running between
500kHz and about 1MHz.
There's also a forumfor the induction heating topic.

Comments Welcome!

[bodgy]

Greetings Ross^2,

I doubt tap water would be heated by an RF field.

Isn't a microwave oven a form of RF field ? - 2G4Hz does eventually boil water - though I agree not quite the same as an induction heater.


Colin

[Externet]

...2 quarts of water into steam in 30 seconds... that is like flash evaporation...

From a few kilowatts to too many kilowatts depending on the initial water temperature.
What temperature is the water fed to the steamer stage ? That makes a huuuuuge difference.
Miguel

[jwax]

Welcome Ross_ross! What's a "monotube"?
We'd need more info than provided to attempt an answer. Starting water temp, pressure, water purity etc.
What's the final application, other than to consume huge amounts of power?
John

[electroken]

The first person who replied to this question was correct and the water will boil etc but not unless it is conductive and/or the pipe will heat it indirectly. We assume he is going to use power from the normal power grid and that is 60hz and not a high enough frequency to be anything like a microwave which are in the megaherz range as the other poster said.
Also it would take a huge amount of power to do the job, but the amount of heat necessary to get the water to boiling from any temp at which it is water, is not going to be significant compared to the heat of vaproization. I can't recall the figure for that but it is something like 50 times what it takes to get the water from 32F to 212F.
A monotube is simply a single tube and not a length of tubing formed into a loop or where multiple passes of the water will occur thru the heater.

[Bigglez]

Greetings (No Name Supplied),

The first person who replied to this question was correct.

That would be me. Peter, or Bigglez, either will do.

We assume he is going to use power from the normal power grid and that is 60hz and not a high enough frequency to be anything like a microwave which are in the megaherz range as the other poster said.

Actually, a monotube self-oscillating amplifier can be
any frequency. As I stated earlier, many industrial
induction heaters operate in the 500kHz to 1MHz
range.

A monotube is simply a single tube and not a length of tubing formed into a loop or where multiple passes of the water will occur thru the heater.

Not true. A monotube is
a single vacuum tube amplifier.

Comments Welcome!

[jwax]

I was hoping the OP would enlighten us with his meaning of a "monotube", along with other details.

[Externet]

I goofed inverting the calories for heating and the calories for vaporization; the difference is not huge depending from the initial water temperature as electroken correctly pointed.

The 38 KiloWatts during 30 seconds won't differ much by the preheating.
Miguel

[charistian]

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.

See also: Induction Heating

[MrAl]

Hi there,

As others have alluded to, water isnt very magnetically active,
and to get something to heat with induction (directly) the material
needs to be able to respond to a magnetic field. Since water doesnt
respond much it's not going to heat up with ordinary ac frequencies,
however that doesnt mean you cant insert something into the water
to heat the water indirectly. That is, the material placed into contact
with the water could be some metal and the heating of that metal by
induction causes the water to heat up indirectly.

I would think a metal pipe would do this, but the diameter of the pipe is going
to play an important role in the ability to heat the water fast enough.
Too large a diameter and the water wont heat fast enough. Of course
a smaller diameter restricts the flow to rates that are probably too slow
to be useful, so an idea would be to construct a device that uses several
small metal tubes where the water can run through each one and
the total flow is equal to the flow of one tube times the number of tubes.

One other thing i'd like to add is that this is going to take quite a bit
of power no matter how it's heated, induction or simple resistive heating.
The amount of power required will be determined by the volumetric
flow rate, which of course depends on how much water you need and
how fast you need it. Since you said 30 seconds, you need to be able to
heat the total volume in the tubes in 30 seconds so you can calculate
the power needed by the volume and the required temperature
increase and the time of 30 seconds.

[haklesup]

MrAl has put a finger on most of the important engineering concerns I was about to talk about so I'll spare us from that.

The way I see it is that you would need to preheat the tube (inductively) to a reasonably high temp (200C perhaps) without melting this tube so you will need some sort of power regulator with temp feedback.

Once you start applying water the pipe will cool acording to the flow and initial water temp. I might try to mitigate that heavy heatsinking effect by atomizing the water and spraying in the pipe. This will give you better control over the flow rate and avoid wicking heat into the part of the water stream not yet in contact with the heating surface. You chould be able to find a balance where you can maintain sufficient temp and flow at a constant rate.

From there ist all thermodynamics. (Jules of heat in to the induction heater X induction heater efficiency - ml of water X energy needed to vaporize.)/ second or Watts in = Watts out.

Preheating will help, every watt gets used somewhere but the heat of vaporization usually trumps the heat required to rise to 100C. Starting with hot water can't hurt.

[rshayes]

About 1980, Westinghouse showed a demonstration at a trade show using induction heating of a section of iron pipe as a source of hot air to pop popcorn. The induction heater used a couple of their newly introduced bipolar transistors, which were rated somewhere around 400 volts at 100 amps (the transistor die was about the size and shape of a quarter). The frequency was probably in the several hundred kilohertz range.

Using water may be a little trickier than air. The water flow would have to be fast enough to remove bubbles from the heat transfer surface, or the efficiency of the heat transfer would be reduced.

Some high powered transmitting tubes used boiling water as a method of cooling. The high heat of vaporization of water allowed a great deal of heat to be carried away as the water changed to steam. Eimac published some information on this in book form, but I don't know where this might be currently available.

[rshayes]

2 quarts x .946 liters/qt x 1000 mL/L x 1 gm/mL = 1,892 gm

Specific heat of water:
@25 C = 4.1796 joule/gm C
@100 C =4.2160 joule/gm C
average specific heat = 4.198 J/gm C

To raise water from 25 C to 100 C:
75 C x 1,892 gm x 4.198 J/gm C = 5.957 x 10^5 J

Specific heat of vaporization = 539.55 cal/gm x 4.1868 J/cal = 2,259 J/gm

To vaporize water:
1,892 gm x 2,259 J/gm = 4.274 x 10^6 J

Total energy:
5.957 x 10^5 J + 4.274 x 10^6 J = 4.870 x 10^6 J

Power required;
4.870 x 10^6 J / 30 sec = 1.62 x 10^5 watts = 162 KW

Someone else should double check this figure.

Otherwise, "Don't try this at home".

[dyarker]

Just to boil a couple quarts in a pan on stove top (about a KW) is 5 to 10 minutes. So 162KW for steam in 30 seconds is certainly in the ball park.

Not a rigorous check of your calculations, but a pretty good sanity check.

C U L -