I'd need a formula

[JJ]

Hi!

Suppose I have 1 litre of water in a plastic tank, its
temperature is ambient temperature, i.e. 20 C.
Now I want it to reach 50 C in, say, 10 minutes, using an
electrical resistor as heater.

How many watts does the resistor need to be? Is the above
a good way to warm up a liquid (talking about cheap/simple
ways, of course)?

I'd like to build myself a heater for my ferric perchloride
tank and for the sur-tin one, because otherwise the former
will be very slow and the latter won't work at all..

By the way, will silicone be adequate to isolate the wires?
Even if no electrolysis took place, the acid will corrode
the leads.. so I need to isolate them.

Thanks! It's cold here, you know!
John

 

[Garrett Mace]

"JJ" <johnospam@invalid.com> wrote in message
news:bp0p83$1ir98o$1@ID-50260.news.uni-berlin.de...

Hi!

Suppose I have 1 litre of water in a plastic tank, its
temperature is ambient temperature, i.e. 20 C.
Now I want it to reach 50 C in, say, 10 minutes, using an
electrical resistor as heater.

How many watts does the resistor need to be? Is the above
a good way to warm up a liquid (talking about cheap/simple
ways, of course)?

I'd like to build myself a heater for my ferric perchloride
tank and for the sur-tin one, because otherwise the former
will be very slow and the latter won't work at all..

By the way, will silicone be adequate to isolate the wires?
Even if no electrolysis took place, the acid will corrode
the leads.. so I need to isolate them.

Thanks! It's cold here, you know!
John

Hmm, back to basic science...

A calorie raises 1 gram of water by one degree Celsius; you've got 1000
grams to raise by 30C so you need 30,000 calories. That's 125600 joules or
watt-seconds, 2093.33 watt-minutes, so the required wattage for desired
temperature rise in 10 minutes is 209.333 watts. That doesn't include any
losses during the heating period, and there will be some through the walls
of your tank unless you choose to insulate it.

I'd suggest looking for a hot plate or some similar heating element; it'll
be lot cheaper than buying a 209 watt power resistor.

 

[Watson A.Name - "Watt Sun]

In article <bp0p83$1ir98o$1@ID-50260.news.uni-berlin.de>,
johnospam@invalid.com mentioned...

Hi!

Suppose I have 1 litre of water in a plastic tank, its
temperature is ambient temperature, i.e. 20 C.
Now I want it to reach 50 C in, say, 10 minutes, using an
electrical resistor as heater.
[snip]

Thanks! It's cold here, you know!

That's a very important point. The ambient temperature, and the
container insulation make a great difference. If the container is
foam plastic, the liquid's going to get warmer faster and with less
power than if the container is glass, for example.

John

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[MikeM]

Garrett Mace wrote:

"JJ" <johnospam@invalid.com> wrote in message
news:bp0p83$1ir98o$1@ID-50260.news.uni-berlin.de...
Hi!

Suppose I have 1 litre of water in a plastic tank, its
temperature is ambient temperature, i.e. 20 C.
Now I want it to reach 50 C in, say, 10 minutes, using an
electrical resistor as heater.
. . .
I'd suggest looking for a hot plate or some similar heating element; it'll
be lot cheaper than buying a 209 watt power resistor.

Actually, if a power resistor is immersed in water, then even a 10W
resistor
is sufficient. As long as the resistor is surrounded by liquid water,
then
the case of the resistor cannot be hotter than 100C!

I have used this trick when I wanted to test a 20A 12V Power Supply
under load.
I used a 10W aluminum jacketed resistor suspended in a beaker of water
as a dummy load. The 240W of dissipation didn't harm the resistor at
all...

MikeM

 

[Garrett Mace]

Actually, if a power resistor is immersed in water, then even a 10W
resistor
is sufficient. As long as the resistor is surrounded by liquid water,
then
the case of the resistor cannot be hotter than 100C!

I have used this trick when I wanted to test a 20A 12V Power Supply
under load.
I used a 10W aluminum jacketed resistor suspended in a beaker of water
as a dummy load. The 240W of dissipation didn't harm the resistor at
all...

That's a great point, but unfortunately the original poster is dealing with
corrosive fluids. Short of some elaborate heat-exchanger setup, I don't
think using a resistor for immersion heating is going to work out too well.

Also, the case of the resistor *can* get hotter than 100C...once the
contacting water reaches vapor phase, a bubble of steam will form around the
resistor and you might get it to run away from you. I still stick to the
plan of a mundane hotplate element under the tank, and maybe not being so
ambitious about the time required to reach temperature. Household resistive
heating elements are in everything and cost pennies at any thrift store.