Pressure -vs- Flow rate ?

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iamjcl

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Stemming from another thread on geothermal HVAC units:

I'm not understanding the basics here.

You have a 1/2" pipe.

You have 60psi of pressure on this pipe.

You WANT 2 GPM flow rate.

By definition, its going to take 1 minute to get 2 gallons through the pipe.

How does increasing or decreasing the pressure change anything if the flow rate is all you are concerned with?

I've been told that 10psi is all that is needed for a geothermal HVAC application, yet my well pressure tank setup is providing (or set to) around 60psi. Good for sprinklers, bad for HVAC.

Would letting the current 2hp pump continue to feed the pressure tank (set to 60psi), and then that feed the sprinklers, and another line T off of the feed (before the pressure tank), with a CSV set to 10-20 psi, then feed the HVAC run ?

So I assume under this arrangement, the pressure tank fills, and nothing else happens there until you call for irrigation. (very little of what my well is used for).

The HVAC units coming on and off will be drawing water straight from the pump, via a CSV, and supposedly drawing less power than feeding both the irrigation system and the HVAC system from the pressure tank at 60 psi ?

What if all HVAC units shut off, which equals no water demand - what does the CSV do ? What tells the pump to turn off ?

I guess I'm just a little slow.
 

Valveman

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If you have a 30 GPM, 2 HP pump, it is able to produce 30 GPM while lifting from 120' and producing 40 PSI. Your sprinklers need 40 PSI, the heat pump does not. All the heat pump cares about is having enough flow to cool the coils. You pump only needs to build enough pressure to overcome the friction loss of the heat pump coil and the pipe going to it. I am guessing you only need 5 PSI to push 3 GPM through the heat pump.

A 1 HP pump can produce 30 GPM at 5 PSI after lifting 120'. This will give 10 units, 3 GPM each, and only use 1 HP worth of power. As less heat pump zones are running, the reduction in flow will make the pressure on the pump increase as the flow rate decreases. When only a single 3 GPM zones is running, you will have about 20 PSI, and the 1 HP pump will only be using about ½ HP worth of electricity. There is no way to make a 2 HP pump pull less than 1 HP worth of power, even when pumping 3 GPM. The excess pressure produced by the 2 HP is just wasted horse power, as the pressure is just dumped out of the heat pump discharge.

If you use a 1 HP, 30 GPM pump, it will still push 30 GPM through the heat pump, just at 5 PSI. Then when you want to irrigate, you have to use an additional pump to pick up this water at 5 PSI and boost it to 50 PSI for the sprinklers.

It is basically impossible to operate a pressure tank and pressure switch at really low pressure, say 5 to 15 PSI. Even then the pump would cycle itself to death anytime you were using less than about 20 GPM. A Cycle Stop Valve would keep the pump from cycling but, adds a little more friction loss. You want to keep the friction loss and the pressure produced as low as possible to save energy. So I would not use a pressure tank or CSV. Just use a relay at each heat pump, that starts the well pump when any or all of the heat pumps are running. Each heat pump closes it's own relay, which in turn closes the circuit to start the well pump.

Then you need a relay on the irrigation system with two set of points. One set of points starts the well pump, and the other set of points starts the booster pump.

It only takes 1.55 PSI to push 2 GPM through a 100' of ½" pipe. You need to find out the actual friction loss across the heat pump coil. You also need to know what size and how long the pipe to the heat pump is. These two things added together is all the pressure the pump needs to build. A pump that builds more pressure than you need, is using more energy as well. Best case scenario is cutting the pumping cost in half, because you are cutting the pressure produced in half.
 

iamjcl

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Thanks for bearing with the slow understanding, Valveman. It is slowly sinking in I think.

Related to your example of needing only 1 or 2 PSI to push 2 gpm through 100' of 1/2" pipe, I presume that means you could push 10 gpm through the same 100' of 1/2" pipe with, say, 20 psi ?

So increased pressure can overcome longer piping runs, or smaller diameter piping, and still achieve flow rates that are needed ?
 

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Friction loss also depends on the type of pipe you have. I was looking at a chart for sch 40 PVC, which will have 30 PSI loss for 100' if you are trying to push 10 GPM through it. ½" type L copper pipe would have 67 PSI loss in 100', with a flow of 10 GPM.

There is really no way to make your 2 HP pump more efficient than it is now, because it is designed to deliver water at 50 PSI. It would even be worth spending a little more on electricity to keep the pump from cycling, so the pump will last much longer. You never did say how much that 2 HP cost. Divide that cost by about 36 months to see how much per month the equipment will cost, to let it cycle like it is now. Then divide that cost by 120 months to see how much per month you would save if the equipment lasted 10 years. You may find that the cost of electricity is not as important as making the pump last longer.

You can also cut the electric pumping cost in half, to see how much you would save to install a smaller well pump, with a booster pump for the sprinklers. If the savings does not pay for the new pumps in less than 5 to 7 years, it may not be worth the trouble.
 

hj

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flow

Pressure and volume are two sides of the same coin and are interrelated. Increasing the pressure WILL increase the flow but not linearly, and only up to the point where friction loss cancels out any further increase in volume.
 

NHmaster

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HJ, I hope the guy on the plumbing thread asking about his 3/4" shower valve doesn't read that, it'll confuse hell out of him. :D

There is a lovely chart in the appendix of the IPC that graphs the pressure\volume\friction loss curves. Very handy indeed.
 

iamjcl

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You never did say how much that 2 HP cost. Divide that cost by about 36 months to see how much per month the equipment will cost, to let it cycle like it is now.

Well, I understand the point about cycling causing shortened pump / motor life, but I have the "old" pump / motor that was taken out when the new 2HP went in (old was 2hp also). The date code is 1991 - that is when the well was originally done.

So, maybe I've been lucky, but I've no complaints on the lifespan of the pump / motor, which was grundfos / franklin. In fact, I'm not sure the guy sold me a new pump / motor / controller box when I suspect just the control box might have been bad. He mumbled something about the motor drawing too much current ("almost 12 amps" - I didn't see the meter when he checked). The controller box was smoking / shutting the pump off. I don't know if the capacitors were just bad, or the pump was drawing so much it made it bad. The pump was working, however, at the time of replacement. The new one draws about 10.6 amps, by my measure.

The HVAC system and irrigation has been the same since 1991. So, with probably only little variation, that old pump had the same amount of cycling that the current one will have (Franklin motor, goulds pump).
 

Valveman

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I think you will find that the new motor is a few inches shorter than the old motor. The Goulds pump is also plastic, where the Grundfos had Stainless Steel impellers. I also believe you were just lucky to have gotten 17 years life, when the average is only 7 years. I hope you didn't use up all your luck on the first one. Guess you have given up on trying to reduce the electric bill? I do lots of pump systems for heat pumps. A lot of people tell me that the electric bill is not as important as having the pump last. Hope you get 17 years out of this one but, I will bet you won't get half that much.
 

iamjcl

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Don't know if the guy who put it in had any clue, but he made a big deal about the goulds wet-end having a brass impeller. Don't know if matters even, fwiw.
 

Valveman

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Apparently he hasn't got a clue but, that is not unusual. The Goulds pump can have a brass discharge adapter, which is the part you can see. However, there hasn't been a small pump of any kind with brass impellers since the 70's. The internals are all plastic, and the motor is shorter than before. Now they just hope they have the design right, so it cycles itself to death in an average of 7 years. Usually the systems that cycle a lot only last 2 or 3 years, and the systems that don't cycle much last 14 to 15 years. This gives an overall average of 7 years, and also means that most pumps, which cycle an average amount, last exactly 7 years.
 

iamjcl

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Thanks, Valveman

So I guess its the motor that fails, not so much the wet-end?

It seems that the wet-end can pump continuously for 15 years, so I guess impeller wear is not a factor, pretty much.

Maybe I can work a deal with the local well man to give me some credit for an almost-new 2hp motor and pump combo, and re-install a 1/2 HP pump or whatever works, in conjunction with a jetpump / booster for sprinkler use.

I just a few days ago put an hour meter on my well control box, and see that yesterday (no sprinklers yesterday, not even all that hot) the pump ran about 19 hours in a 24 hour period. Lots of cycling in that, also.

I calculate that at about 30 cents per hour:

(.12 / KW hour rate, 10.6 amps * 235 volts = 2,491 watts, or 2.49 KW/Hour * .12)

Yesterday, I spent $6 on well pump power alone.

So that means summer useage of at least $180 / month - and around here, summer is a solid 4 months +.

So if I could cut JUST the Summer useage (never mind the rest of the year - my heat comes from geothermal also) by 50%, that would save me $90 / month * 4 months, or at LEAST $360 in one summer.

If I could cut my well power use by 50%, it would be worth $1,500 easy to make some changes - esp. if it makes the new equipment last 10 years plus, instead of 3.

Thanks for all your comments - I'm learning quite a bit, and appreciate your time.
 

Valveman

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If that pump ran 19 out of 24 hours, you are going to need to stay with a pump that delivers the same volume, just lower the pressure. A 1 HP 30 GPM pump can deliver the same volume as the 2 HP. The 1 HP can only give you about 5 PSI, where the 2 HP is producing 50 PSI. To save $1500 per year, it would certainly be worth changing out the pump to a 1 HP, and adding a 3/4 HP jet pump to boost the pressure when needed for irrigation. You will probably have a hard time selling the slightly used 2 HP back to the pump man but, you may can sell it to someone else yourself. I would sure be interested in how many cycles per day that 2 HP is doing while running 19 hours.
 

iamjcl

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I would sure be interested in how many cycles per day that 2 HP is doing while running 19 hours.

Well I'll tell you exactly - I ordered an electronic counter (to go with my hour-meter). As soon as I get it wired in, I'll post back.
 
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