Which is more efficient for heating water, Microwaves, or a heating
coil ?

to be more specific if you were to heat water in a sealed copper
cylinder , which would do the job more energy  efficiently.

A microwave or a traditional heating element.

Will the copper shield the microwaves assuming that there are no,
water, leaks in the cylinder?

The water will be extracted from the cylinder in a continues flow,
using a copper pipe. will the water conduct the microwaves into the
environment , when it leaves the pipe, or will they dissipate after a
few meters inside the copper pipe?

The water is being extracted from the top of  the cylinder .

The cylinder contains about  80 leaders of water.

The magnetron being used is 1000w.

The magnetron is positioned at the top of the cylinder .

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Clarification of Question by martin_sullivan-ga on 12 Mar 2003 05:36 PST

the copper cylinder will have an earth strap on it.  that should
eliminate most electric currents.

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Clarification of Question by martin_sullivan-ga on 12 Mar 2003 06:01 PST

The waveguide end of the magnetron  will be in   side a ceramic 
casing which will be inside the cylinder. the rest of the magnetron,
will be outside the cylinder.  I will use a fan to keep the magnetron
from overheating.

Heating coil.
The plan is to cinvert electrical energy into heat.  Why require that
it first be converted to microwaves?  Unless this step is 100%
efficient, it MUST be less effective than the virtually 100%
efficiency of a heating coil (there are some losses, but they are
minor.
As for the wave propagation characteristics, I can't really say, but
given the above, it may be irrelevant.

If I remember correctly, not only do microwaves reflect off copper
(mostly), they also induce electric currents in it. So the coil is
probably a better option in this case. Still, xarqi, I think your
argument doesn't take everything into account. If the container were
made from a "microwave-transparent" material, it may be better to use
microwaves. The coil would also have to heat the container, whereas
the microwaves would more directly affect the water (which, being
dipolar, are efficiently heated by microwaves). You would also most
likely end up with a more uniform temperature distribution in this
case.

I agree that 'directly heating water molecules' is exactly what
microwave ovens are designed to do but I can't help thinking that a
mere KW microwave source is going to have a hard time heating the
massive 80 litre load of water faster than the heat is lost through
the sides of the coper cylinder (copper being, as it is, a great heat
conductor).  Perhaps we should assume the cylinder is insulated like a
traditional hot water tank?

Jdog:
Typical magnetron efficiency is 75% - heating element efficiency is
100%.
If the container were made of "microwave-transparent" material, there
would be additional losses reducing this further.  The container will
be heated by conduction in either case, so the losses are the same. 
As for evenness of heat distribution - I doubt that the microwaves
would penetrate very far, and in both cases uniformity of temperature
will depend on convection.

izzrd:
Yup - microwaves are great at heating water - first catch your
microwave!  The difference lies in the efficiency of conversion of
electricity to microwave energy - a step not required with a coil. 
Your point about the size of the unit is well made.  As an
approximation, a 1kW heating unit (assuming here 100% efficiency) will
take about 5.6 minutes to heat 80L of water 1 degree Celsius (if my
back of envelope is not lying to me).

The other people are correct. A one KW heating coil will heat the
water just as fast at less cost than 1KW of microwaves which be will
produced at a cost of about 1.5 KW of electricity. The copper
container is ok if it is the resonant cavity for the magnetron,
otherwise it will probably destroy the magnetron. Microwaves a meter
down the copper outlet pipe will probably be negligible, but there is
a slight possibility that the outlet pipe will behave like a wave
guide. The water in the wave guide will disapate the microwaves to
negligible in a few meters. They are also correct that good thermal
insulation is needed and a continous flow of one liter per minute will
hardly be warmed at all unless you use more than one KW.  Neil