Tuesday, November 29, 2016

Fluid Dynamics











Turns out my low psi on-demand hot water heater is just on the cusp of getting a little too little pressure from the gravity feed on my solar shower tank.  The flow is minimal and the water actually gets too hot even on the low setting.  Although I had already done some research on the matter I thought I would try hosing in two lines from the tank for a little real world experiment that ended up proving the theory.  Thanks to the laws of physics regarding the conservation of energy, and some guy named Daniel Bernoulli - it didn't work.  (I won't hold a grudge against him though because the Bernoulli principle can also be used to calculate the lift force of my paramotor wing.)  As I suspected, doubling the feed in did not double the pressure or increase the flow.  Looks like I will have to add a pump to the mix after all.  Just so happens I have one that I used years ago for a misting system experiment in the greenhouse.  (Note: the inner tube leak on the trike tire is definitely repaired now.)  62,71,37,0,B

9 comments:

Barney The Old Fat Man said...

Roughly 2 feet of water column is 1 psi. You just need a little higher supply.

Don said...

What Barney said, the only way to increase input pressure is increase height of the supply.

Maybe tee the output and inject a larger diameter cold water stream into the hot water flow to cool it off and increase flow?

Don

Deacon75 said...

Could you use a portable air tank to pressurize the column? As the pressure dropped off you could time it in rate of usage...thus as the pressure goes down and temp goes up you could meter your water usage.

Unknown said...

You might want to consider pumping from the reservoir through the heater and then back to the reservoir. Tee off from the pump discharge recycle line to your shower head. As long as you take less from the pump discharge line than the total discharge rate of the pump, you won't starve the pump suction. How do you establish and/or maintain the level in the reservoir ?

Larry G said...

the interesting thing - is when you see one of the large water towers that provides high pressure water to thousands of people - it's fed by a tiny pump.. it's the height that provides the pressure.

Jon P said...

Try mounting it a low as you can to get gravity to help.
How hot is the exhaust? I know it is designed to be used outdoors but I have heard of people mounting indoors. Your thoughts?

John Wells said...

Using it indoors is probably not a good idea - the exhaust is pretty hot. As for mounting it lower - I am pretty sure Bernoulli would suggest while the inlet pressure may increase by locating the heater low - the overall pressure will stay the same since the water would have to be pushed higher to come back up and out of the shower head.
Stay tuned - the next test will be with the on-demand pump I already have.
As for maintaining the level in the tank...there is a sight gauge on the side. (http://thefieldlab.blogspot.com/2014/08/other-plumbing.html) If the tank gets below 3/4, then I use a small transfer pump to fill it quickly. Because I only use about 5 gallons per shower, I usually just top of the tank manually just before each use by pouring it in through a funnel mounted on an inlet on top. http://thefieldlab.blogspot.com/2013/01/getting-behind-catching-up.html

Margery Bills said...

Good news, I did not see any leaks. Is that where the algebra we learned and never used comes in?

Sam Keith said...

If you located the on demand pump (which could also be configured to operate as a recycle pump)close to ground level, perhaps it could be piped up to take suction from either the elevated reservoir ( when operating to supply hot water for the shower ) or from one of your storage tanks ( to fill the reservoir initially ). Might be easier. I follow and applaud your individualism, initiative and ingenuity