hope someone finds this interesting :-)

What I'm trying to accomplish is:

Having a heating wire getting in contact with an airstream in a ventilation system as much as possible (200mm metal duct). The goal is to 1) heat the air 2) Dry the air 3) Sterilize as much as possible before the air comes in contact with a HEPA filtration system further downline in the ventilation system.

I've built a heating element that does the job somewhat, used it for ½ a year now with success.

The heating element is turned on 10 mins. a time (automatically shuts off) 2x daily, with use of 120v AC that can be regulated up to 240v or down to 0. Right now the design is a simply twisted wire-coil (gauge 28) placed in a ZigZag manner in the 200mm duct using non-cunducting/heat resistant mica sheet with holes to hold the wire. Please see the picture.

Is there any way you could imagine a design that could get more contact between the wire and the air? I have 30 feet of fresh wire and plenty of mica sheet. The air hitting the wires is prefiltered by 2x HEPA vacuum filters, but they do not meet EN2118 filter standards, and we need very clean air. I cannot tell you how hot the wire needs to be, other than it has to be glowing(burning micro particles as it comes in contact with them) with a giving airspeed that also is variating a lot. I monitor the heat to max. hit 70Degree C, measured inside the duct, just 5 cm and 5 cm to the back from the heating wires.

For safety, I have a 120 degree C temp. fuse, nothing that can burn near the installation, mesh wire in front and after the element, I can monitor the temperature inside and further downline in the system. A main emergency power cutt off and a fire extinguisher and fire alarm installed (detects smoke). Automatically shuts off after 10mins runs. Any other safety suggestions are welcome!

Please answer as simple as possible to eliminate misunderstandings, this project came to live due to pretty servire health issues, all help is very appreciated.

enter image description here

  • \$\begingroup\$ Is that a pasta strainer I see in there.... \$\endgroup\$ – Trevor_G Oct 31 '17 at 15:14
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    \$\begingroup\$ It is, it oddly had exactly the right measurements and a net tight enough \$\endgroup\$ – JoeDoe Nov 1 '17 at 13:25

This is more of a mechanical problem than an electrical one.

However, when heating air, the trick is to have the air in contact with the heating surface for as long as you can. As such, having the coils run in line with the airflow is better.

Also, the more turbulent the air is the better. If the air is laminar, only a small percentage of it will collide with the heater coil.

As such, making your coil 3 dimensional, going in and out of the image above, instead of flat, and having a stationary "fan like" plate on the inlet side of the coil to make the forced air spin around inside the heater coil would increase your heat transfer efficiency.

It does however, look like a dangerous contraption, also, a heater alone will not "dry" air, though it may feel less "damp".

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    \$\begingroup\$ In fact, you need a condenser to dry the air thermally, so a cooler below the dew point is what OP wants \$\endgroup\$ – MrGerber Oct 31 '17 at 17:35
  • \$\begingroup\$ Thanks for the good answer Trevor, I will try to make a good design with the coils running with the airflow. \$\endgroup\$ – JoeDoe Nov 1 '17 at 13:25
  • \$\begingroup\$ rest of my comment: Atm. the flow is laminar due to the pre-filters. Can the turbulence-inducing device be a metal propeller, only spun by the airflow passing it, generated by the main centrifugal fan in the system? I know the "drying" is only relative, it seems to help move the moist trapped in the filters. @MrGerber: I've looked into using a proper condenser, but it builds up mold due to the water dripping off the coils even with good pre-filters. \$\endgroup\$ – JoeDoe Nov 1 '17 at 13:37
  • \$\begingroup\$ @JoeDoe Yes a metal prop would be fine. Not driven. It does not need to be able to turn though, since the air itself is being driven. \$\endgroup\$ – Trevor_G Nov 1 '17 at 13:40
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    \$\begingroup\$ @JoeDoe You need to drain off the moisture, then. Do it before you heat it, and make a tray underneath your condenser, from the tray, a drain line with a water trap in it should make a proper air seal. \$\endgroup\$ – MrGerber Nov 1 '17 at 13:56

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