Priming a pump

hot_rod

from Meyer, Zoeller, and Little Giant operate around 195F although their curves show performance drop at 200F!

Not hot enough? Zeinth has a molten metal submersible for pumping solder and sulpher at 380F.

Nice to know this stuff is out there.

I think subs would be ideal for solar drainbacks. A clever guy could even adjust the level of the pump in the tank to keep the delta t "on".

Or how about a delta T V/S sub. Maybe add a tekmar VS delta t drive to the motor.

Oh, and add a differential control chip while you are in there.

hot rod

Eric Johnson

Priming a pump

I'm going to be building a non pressurized hot water storage tank per the attached diagram, using a flat plate heat exchanger. Before starting the circulator (probably a Grundfos 20-42), do I need to prime it somehow, or will it get the water flowing on its own without damaging the bearings?

Brad White_9

Any

wet rotor pump needs not only good priming but some residual head on it (Net Positive Suction Head) to keep it from cavitating. This applies to any centrifugal pump but especially so on wet rotor types.

You run the risk of cavitation and pump damage.

How critical is it that you draw off the top of the tank? Understanding you want the hottest water there, would blending the tank (destratifying) hurt you in any way?

What I am getting at is, you could draw off the bottom or further down the tank to maintain your prime (well, your pump prime, we know you yourself are in your prime

Better yet, if you could place your pump below the waterline (come off the bottom/side of the tank) you could maintain good NPSH on the pump suction.

The only types of pumps not prone to cavitation are positive displacement pumps (generally low flow and high head pressures) or peristaltic pumps (not much flow or head for that matter).

I would relocate the pump to deeper water.

hot_rod

Another option

I have been thinking about for solar storage tanks like that... A submersible pump.

Several manufactures build high temperature versions. Zoeller, Danfoss, and Grundfos are a few.

Simple, low maintenance, quiet out of sight and out of mind.

hot rod

Eric Johnson

Mixed tank

I think I'm stuck with a mixed tank any way you slice it, so I don't think it matters where I draw from. When you consider that some of the time the tank will be charging, while other times it will be putting heat back into the system, the heat differential would be wrong on one or the other, even if I could get it to stratify, which I don't think is possible in any event.

My only hesitation in relocating the piping is that the tank liner is a single sheet of rubber and I'd rather make any holes in the lid, which is a smaller sheet of rubber and not in direct contact with the water.

But I can see what you're saying, and so it's probably worth locating the pump below the water line. I can still put the discharge through the lid.

Thanks.

Brad White_9

Hot Rod's Point

on the submersible pump seems to solve all of the issues but this depends on your water temperature of course. Too hot and your pump is cooked.

Drawing off near the bottom makes most sense to me then. You may want to cut off the suction inlet at a 45 degree angle to increase suction area. Over-sizing and putting in a foot valve will aid keeping the line primed and reduce pressure drop.

My thinking re: drawing off the top versus the bottom, was that you may not want to wait for your heat delivery. As you said, the constant circulation from different sources will negate that give you a homogenous temperature you can work with, so all the better.

hot_rod

depends on what the job is

if you want the hottest (stratified water temperature for heating, etc pull from the top.

If you want to use solar or mod cons to reheat the tank pull that from the coldest (bottom) of the tank.

Another idea for a tank is to use ICF blocks to build it. You get 4 or more inches of insulations, easy for DIYers and a nice soft surface for the liner.

A solar contractor in Colorado turned me on to the high temperature submersible idea. He claims to have several up and going somewhere in the wild west.

The pumps I googled had 180F continous and intermitent to 210F.

I'm sure with more dollars spent temperatures could be above that.

The multi coiled tanks I saw at ISH had coils at various elevations depending on what you were doing with the coils.

hot rod

Brad White_9

Hi Temp Submersibles

That is good news, Hot Rod, that you can get submersibles that can run that hot. Whew. No relief for them, is there?

Given that and if the temperatures can be kept at 180 or less predictably, I like your idea better. No priming issues.

Learn something new every day.

Ericjeeper

I am using a submersible.

High dollar unit in my storage tank to feed my woodboiler. It is sitting on the bottom. It has seen temps over 170 degrees but probably on average 140 since december running 24/7. The only issues I have,The flow rate was way more than I needed, so I installed a tee and a valve. I simply return the excess flow to the storage tank. I control the flow by using the valve.
My theory was to have basically a trickle flow rather than full flow. Keep the temps up in the boiler for a hotter firebox.
By the way the pump is a Coleman. 48 bucks at Menards. LOL I figured it had a lifetime gurantee. It said nothing about pumping conditions.
What I would like to do before next year is get a smaller variable speed wet rotor pump(something that does not suck so much electjuicity.) I am using a something like a 5 amp pump and less than an amp would move more than enough.
But it was quick and easy as this was something I just threw together to save a few bucks.