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There are a couple of issues here, 1st the water pressure in the building is based on the difference in height between the top of the water and the outlet of the faucet. So the closer you are to the tank the lower the pressure. This would lead to very complex plumbing system. |
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Many municipal systems are fed by reservoiurs(sp) that are located higher than the building in town so in many cases it doesnt cost a penny to pump the water to the top of the building. in areas where its not that way I would guess that the multiple pumps required to feed the different tanks would be less efficient than one larger more efficient pump pumping the water to the top of the building. As 90% of the water used in any given area is at or near ground level. Im not sure if there is really a need or a benefit for this. |
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as i understand it, those water tanks at the top of buildings are actually for fire protection and do not actually connect to the indoor plumbing system. |
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No no you've got it all wrong, the water is on the roof to heat up from the sunlight! |
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[jhomrighaus], a little short-sighted. Why do you need multiple pumps? Just use ball valves to close off the water supply to the full tanks. Use restrictors upstream of the ball valves if need be. Route the overflow into the next tank down. |
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If you have one pump powerful enough to pump to the top of the building then there is no need for all the extra tanks. |
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The multiple pumps would seem the logical extension here as each would be smaller and sized to the amount of head for each respective tank, this also simplifies the plumbing and controls as each tank would be only controling its own pump, otherwise a complex system that monitors all the tanks would be required to control the pump, but again, if the pump can pump to the top, it is very simple to get it where needed from there, At a significantly reduced cost of operation and mainteneance. |
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The other advantage of one big tank on roof is that water can be pumped into the tank at night(when power cost and demand is low) or during periods of low demand during the day, thus ensuring a full and uninterupted flow of water for all the residence. |
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But the power required is a function of the head required and the desired flowrate. If you're not taking all the water up to the roof, then you don't need such a big pump (less work is being done). |
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If you all are greatly in favor of the one huge tank theory, one could also take this to the logical extreme and make an enormous water tower, higher than any building in the city, and an enormous pump to get the water up there. This would have the benefit of having an enormous tank, that would really be huge. |
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One could put a restaurant on top of it, or even a church, though of course they would have to haul their water up in buckets from the reservoir below. |
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Nah, just put a tank on the restaurant's roof and pump the water up. |
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The point of storing the water on the roof is exactly to address the size of the pump. It is more than possible to run a building on a pressure demand type pump that will maintain constant pressure in the building. The pump must be sized so that it can maintain constant flow at a given pressure. With the big tank on the roof, a MUCH smaller pump works 24/7 to maintain the water level in the tank. |
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A 10GPM pump can supply over 14000 gallons of water in a 24 hour period. If each person in a building used 10 gallons of water during the day you could supply the needs of over 1400 people with that 1 small pump. Meanwhile your max flow is limited only by the diameter of the plumbing. With pressure feed you would need a much larger pump. If 20 people filled the bathtub at the same time at a flow rate of 5gallons a minute you would need a 100GPM pump, add in 20 toilets flushing, 10 dishwashers, 10 washing machines, 10 sinks all at similar flow rate and you now need a 350GPM pump. Thats a REALLY big pump, and I would venture to guess that if you had a building with 1400 people in it those would be VERY conserviative estimates of the peak usage. For the gravity system this kind of Peak usage is no problem at all. |
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Yes you will expend additional energy raising all the water to the roof, but by the same token, a 10GPM pump running 24hours a day will use far less energy than a 500GPM pump Running for 12 hours. |
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I agree with [jhomrighaus]. But to be fair to the idea, it does not propose a pressure feed system but a multitude of gravity feed systems.
In my view, what derails the idea is the economics of finding space for the extra tanks and the much-increased operational costs of having many pumps. |
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But the idea here is neither constant pressure, nor the flow rate.
This idea is all about saving energy/work done. |
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which it really does not do. |
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A pump is specified in terms of the head it can achieve and the flowrate it can manage at that head. The lower the head, the higher the flowrate. Hence a pump to take all the water up to the roof will need to be more powerful than one which takes some to the roof, some to one storey below, etc. |
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I'm not sure whether or not this is a good idea for other reasons - complexity being one. However, one thing this would allow is a smaller pump. |
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To use a simple illustration, consider just one flat. If it's at roof level, the water supply to it will need to be driven by a bigger pump than for a flat at ground level. Two flats at roof level = twice the pumping power required for just one flat. Now if we have one at roof level and one at ground level, we need to supply the power requirement of the roof level flat, and the lesser power requirement of the ground level flat. This total is less than that for two flats at roof level. |
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however the efficiency of the Pumps and the amount of water they need to pump is where the difference comes in. To feed numerous smaller tanks you will need numerous smaller pumps. Electric motors incur the greatest load when they start up, by a whole lot. Starting and stopping many pumps will use more power than starting and stoping one big pump. also the bigger the pump it is generally more efficient per gallon than 2 smaller pumps. Finally you have mainteneance considerations. |
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On the surface this idea seems logical, but look closer and it really doesnt make as much sense. |
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To feed many small tanks, you still only need one pump. I've suggested above, use ball valves to fill tanks that are being used. All the pump needs to know is that there is a pressure drop somewhere above it. |
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For that to work you need a pressure feed system that runs constantly(or nearly so) to maintain pressure. |
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Could you use a displacement-type pump in this application? Given a reasonably constant demand for water, it would run continuously, but if it detected that no water was required, a brake on the driveshaft would allow the pump to be stopped without the water draining back through the pump. |
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Or thinking about it, er, a one-way valve would achieve that nicely. Oops. |
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