2 replies to this topic

[Elango Balu]

Hello All ,

i currently use a water bubbler to introduce water vapour in my experimental setup.I have no problem using it at room temperature and when i want to increase the watervapour content i try to heat the bubbler to a set point using a temperature controller and a heating coil.

but thwe problem is since the glass has a lot of thermal inertia ,even though the controller switches off the heating coil after sensing the water temperature at say 50 °C , because of the constant heat dissipation from the heated glass the temperature keeps rising and i am having a hard time able to stabilise the temperature of the water using this setup.Hence i cant control the amount of water vapor more accurately in the gas stream that is bubbled through.

I would like to know if anyone had tried such methods and come up with any tricks to maintain the water temperature considering the thermal inertia of the glass.I have attached the pictures showing my setup.

any help would be greatly appreciated.

Thanks ,

Elango

Attached Files

•  181.JPG   1.94MB   5 downloads
•  218.JPG   1.5MB   5 downloads

[Art Montemayor]

Elango:

I strongly doubt that what you are describing is what really is taking place. There is NO "thermal inertia" that is transmitted to the glass. Allow me to explain:

You are heating water in a glass vessel – what seems to be a flask – with an immersed electric resistance. You are also bubbling presumably dry gas into the hot water using a copper tubing. This is a conventional, normal humidification technique.

If you heat the water to 50 °C, your glass flask should actually be slightly cooler than the 50 °C temperature. If your temperature instruments are accurate, they should be reflecting this. The flow of heat is outward, not inward. It has to be outward due to the natural heat sink that is formed with the surrounding atmosphere being cooler than 50 °C and in keeping with the law of thermodynamics: heat flows from hot to cold. In other words, the driving force is outward, not inward. And because of this thermodynamic mandate, there can be no increase of the water temperature once the electrical current is turned off by the thermostat on the electric resistance heater.

What CAN happen (and probably what IS happening) is that the electric controls on the heater are not working properly or quick enough. You are continuing to heat the water after the temperature set point on the heater has been surpassed.

There is no such “thermal inertia”. We do the same thing in industry day in and day out, and it works with the proper instrument calibration and actuation. You should be able to heat the water to 50 °C or any temperature you desire - and maintain it there. If you have the proper temperature sensor/indicator and temperature switch, this operation should work without any problems and yield the constant gas temperature/humidity that you seek.

[Elango Balu]

Hello ,

I am sorry for confusing with my second pic that has the heater coils removed.The brown chord is actually the water resistant thermocouple to avoid any corrosion issues that measures the water temperature and in turn is connected to the controller.

The first pic shows the bubbler placed inside the sheet metal jacket with the heating coils wrapped around on the outsied and glass wool for insulation.So the heat is now flowing from outside the glass to inside.

I apologize for any confusion.