Flow and Pressure

rlingineni

I am an amateur in engineering and new to this forum, so please excuse me if I am posting this in the wrong location. I am doing a little project outside of school that deals with water generation.

The project consists of a coolant that travels through a network of pipes and cools a coil to a desired temperature. For my coolant I am using dry ice(solid Carbon Dioxide), because it directly sublimates from a solid into a gas.I ran into a little problem during the execution of my project. Basically, what happened was, when I placed the dry ice within my containment chamber( which by the way is a homemade flask that consists of a water bottle placed within a 2 liter bottle with the gaps filled in by insulation foam), the rate of sublimation was too slow to spread through the entire network of tubes and cool the coil.

As a result, I need to somehow make make the cool air travel through the tubes. I have also considered the application of a fan, but it seems impractical on a large scale to use a battery and its not really applicable for an experiment that is meant to be low cost. So I explored the idea of adding air via a bicycle pump into the chamber and "pressurizing" it to force the cool air into the tubes and cool the coils. That didn't work(BTW I am also using tire valve stems), in fact, I ended up overheating the bicycle pump, and exploding the tube off.

With that in mind, I tried the same experiment in a more smaller environment, where I tried it with a simple water bottle and I observed that the bottle expands when the air is pumped but then contracts when the cylinder within the pump moves up. Essentially, I was expecting the bottle to work like a pressurized inner tube releasing air at a high force.

As a result, I retried the experiment but with one little change, I filled the bottle half with water and pumped air into it, and pressure began to build to the extent where the bottle would eventually burst. I want to know why this happened(water vs empty bottle) and also can anyone suggest to me how I may circulate air within the tubes without the use of a fan?

*Sorry I wrote a lot, Thank you in advance

Simon Bridge

It sounds like the bottle lets the air out slower than the pump supplies it, but the pump only supplies air on the downward stroke.

I don't understand - how was this different? What were you hoping the water would do?You can cool pressurized air .... keep the air in a large sturdy pressure vessel. I have used a 40L drum for this myself. You can pressurize it with a bicycle pump and then release air slowly through a valve when you need it. Or you could just pump your existing setup more slowly.

rlingineni

Well, I added water because I read online this is how they make pressure rockets:
http://www.wikihow.com/Make-a-Powerf...ressure-Rocket

Since they make it like so, I was thinking that if water is what causes the build-up of pressure, perhaps if I add water to a water bottle, then maybe it would result in some pressure build-up.

Simon Bridge

Water is used in bottle-rockets for reaction mass.
http://en.m.wikipedia.org/wiki/Water_rocket

All it is doing in your case is occupying volume - so the higher pressure builds up in fewer strokes of the pump. Do you understand how pressure forms when you pump air?

I still don't understand what the difference between the two cases was.

rlingineni

No, I think that was the difference, "higher pressure builds up in fewer strokes of the pump" with the water in place. Ok, I understand.

Thank You

Simon Bridge

... because the water takes up some of the volume in the chamber. Put the same amount of air into a smaller volume and you get a higher pressure.

(Just making sure.)