0
\$\begingroup\$

I'm working on a project that requires heating of a copper pipe with water running through it. I'm aware that this is a common issue by itself but as there will be people working around the system and the pipe has condensation running down it I am looking for some safety guidelines to avoid shocking people working in the lab.

I calculated that I'd like to run the wire at around 48 VDC with 5.3Amps resulting in ~260W. I've been reading around the internet that when water is introduced that it substantially increases the danger with regard to fatal human shock (as opposed to dry hands touching lowish voltages).

How can I make the setup safe to work around, specifically are there any circuit design considerations and board assembly tricks (conformal coat to some extent I imagine), or is this more of a mechanical placement/guarding issue?

Issues I'm worried about: accidental contact with wire/pipe, accidental water dripping around control box or down wires into control box, plugging and unplugging heater from wall with moist hands (maybe that's just a common sense issue).

Thanks for the help.

\$\endgroup\$
3
  • \$\begingroup\$ An explanation of how they make electric water kettles safe for use by the average Joe would probably be very helpful in this instance. \$\endgroup\$
    – user2152277
    Dec 27, 2014 at 2:46
  • \$\begingroup\$ Welcome to EE.SE. There are several ways of addressing this safety concern. Here are just a few $0.02 thoughts/questions. [1] Does this heater have to run off 48V? 24V presents less shock hazard. 24V is commonly used in wet environments where things can go wrong (e.g. mines). [2] Do you have to heat with DC? If you heat with AC, you could use GFCI. \$\endgroup\$
    – Nick Alexeev
    Dec 27, 2014 at 3:13
  • \$\begingroup\$ Yeah, now that I look back at my calculations there doesnt seem to be a discernible reason to have to use 48V. Just have to get different length/diameter NiChrome wire. I was going for DC mostly because I'm familiar with DC and have little to no experience with AC. I was also going to try using PWM via a microcontroller and FET switch with PID to get some temperature control and with my inexperience with AC I don't know how I would vary the frequency (which I assume with be AC equivalent of PWM). \$\endgroup\$
    – user2152277
    Dec 27, 2014 at 5:24

1 Answer 1

Reset to default
0
\$\begingroup\$

Cheap(ish), safe, off the shelf ..."

You can get a really really really safe result by buying commercially made water heater elements that operate from eg 30VDC and which have eg 3 x 300 Watt "spears" which can be used in series or in parallel or with 1 or 2 only operating. These screw into the standard fitting used by domestic hot water tanks and allow you to isolate the DC from the water. These are commonly used in alternative energy / solar type applications.

Example below is 12V rated. Higher voltage units are available.
Lower wattages can be obtained using PWM and/or lower than rated voltages. eg the 600W unit below would give about 260 W at 8V.
Power under PWM is essentially proportional to on time %.

A number for sale on ebay - from about $US20

eg $US19.98 buy now Mansfield Missouri.

12 volt 600 watt dc Low Voltage Submersible Water Heater element 4 wind or solar

enter image description here

DIY with Nichrome wire:

48 VDC would be considered ELV (extra low voltage) in most cases and not directly covered by safety regulations that apply to the next level up - LV. 48V can certainly kill you if you let it, but it takes a fair bit of permission on your part.
Reducing that to 24VDC helps appreciable and, if you really cared, it would be very easy to use eg +/- 12V with centre earthed so no exposed voltage is more than 12V above (or "below") local ground.
You can even die using 12VDC but it's extremely hard to do and is "safe" in most cases. Standing in salt water and grabbing 12V wrt the water is not at all recommended. I have a still alive friend who can tell you how unpleasant that can be.

260 W / 12V ~= 22A
you can buy 12V and 15V 250+ Watt power supplies that would do it all at 12V.
Or half that in two halves as above.

Or you could operate a number of lower wattage 12V say Nichrome windings in parallel with wire or eg copper pipe return.

TELL US WHAT YOU WANT AND WE'LL TELL YOU WHAT YOU NEED:
Some more information on whether you really want to heat the tube or the water or .... would help. eg do you have a copper tube and you want hot water to come out and what is heated or how is not directly relevant, or is what gets hot when and where of importance in itself. And knowing how long the tube is, water flow rate etc does not apparently address your core question but may help with answers. Knowing the application in more detail can also help. Also the environment - university research lab / private company prototype / science fair or private project / ... .

\$\endgroup\$
1
  • \$\begingroup\$ Thanks for the response, it's very helpful. At the moment it's more of a personal project so I'm trying to keep cost at a minimum and am a little taken aback by the ~$100 160W+ power supplies on Digikey (wall adapters specifically). Do you know any alternatives to getting that kind of power safely without breaking the bank (the open frame and chassis mount seem dangerous around water)? I'm interested in temperature controlling (heating) the water flowing through the tube which is a closed system of ~1-2kg of water. The temperature difference from ambient is ~20C. \$\endgroup\$
    – user2152277
    Dec 27, 2014 at 15:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.