Supplying power to a small heat exchanger

Hello,

As part of my senior project, I am designing a hand-held battery powered smoke generator to be used to identify air leaks within a house.

Essentially the system works by pumping "fog juice" through a stainless steel tube that is heated to the point where the fluid vaporizes and thus generates smoke. It is basically the same principle as the big fog machines but just at a much smaller scale.

Right now I am looking at coming up with the power supply for the stainless steel tube and am looking for advice as to where to go with it.

The tube is a 22ga hypodermic needle that has a resistance of 1.65 to 1.80 Ω and from playing around with everything, it looks like I need to pass anywhere from 2.0A to 2.5A through the tube to get it to heat properly. I would like to keep the temperature of the tube relatively constant so I think I need an adjustable power supply where I can use a microcontroller to vary the voltage across the tube based off of the change of the resistance of the tube and the fluid in the tube reducing the temperature.

I'm just not sure where to go with how to generate the voltage for the tube. Would a DC-DC converter that is adjustable over the range of 2.5V to 5.0V be the best solution for this type of problem? Is there a good way to use a microcontroller to vary the voltage from the DC-DC converter to heat the tube to a specific temperature profile? I have seen some solutions that use a digital pot to change the voltage sent back to the feedback pin of a voltage controller.

Thank you,
matt

 

First a stupid question: Are you sure you want to heat the tube itself, as opposed to surrounding the tube with a thermal mass of some sort? Fluid passing through the low-mass tube will cool it a lot and diminish its ability to vaporize the fluid. But a block of metal with the needle running thru it would be much steadier.

Then you could mount a thermometer IC (e.g. LM35) into or onto the block. Heat might be applied to the block using a TEC (peltier device) using a thermostat (comparator and a MOSFET) responding to the LM35.

There are lots of other options for thermostatic control, such a bi-metallic strip like in a toaster. Or you could even forget the thermometer and just set the current to the heater manually base on experience. But all of this is so much easier if your "heater" is not fluctuating so much in temperature.

Another stupid question: Why not just use the larger units already available?

 

Thanks so much for the reply.

As to why not use the larger units already available, the main reason is that they produce much more smoke than is needed to evaluate the air leaks around a window. The second reason is I am looking to make this battery powered and roughly the size of a flashlight.

I have not really investiagted using a TEC. Just after a quick look, I'm wondering if TEC could get hot enough to vaporize the fluid which needs to be around 300F. Are those temperatures something a TEC could reach?

I was definitely looking to set the current to the heater manually as I have a INA219 current sensor inline with the heater and was thinking I could adjust the current and temperature based off of how much current the heater draws based on the supplied voltage. The microcontroller also controls the piezo pump which pumps the fluid into the heat exchanger so that can be varied depending on the temperature.

Right now, I have everything running from the benchtop power supply for testing and am looking for the best way to move this to being battery powered.

-matt

 

a variable voltage really isn't needed.. Once you find the "sweet spot" you just set it and forget it..
Check out how electronic cigarettes/vaporizers are made for ideas.. Its basically exactly what you are making minus a fan..

 

I basically agree, if the fluid flow rate is constant. I missed the use of a micro controller.

 

I think one of the issues I may be running into is that as the temperature of the heated tube changes, the resistance of the tube changes. With that, I am thinking that I need to then change the voltage applied to the system to regulate the temperature of the tube. One of the ways that this would differ from the electronic cigarette is how long the system is actually generating smoke. For example it could take thirty seconds of applying smoke to actually outline a window to see where possible air leaks are.

 

as the temperature of the heated tube changes, the resistance of the tube changes.

This is an argument that suggests controlling the current is the right method.
I do this for my day job, and I use a cigarette. Unfortunately, cigarettes don't use electricity.

 

No, a TEC will not be able to handle the high temperature. Not certain how the connections to the tube will handle it either. 300ºF is high enough for some plastics to soften/melt. Even electrical connection could be tricky. I wonder if one of those "instant soldering irons" would work.

 

You know, beekeepers use handheld smokers to pacify the bees. Might be something to look into.