4gpm (min) circ pump?

Alan(CaliforniaRadiant)Forbes

is just one component of the equation when you're sizing a pump; we also need to know the pressure drop (in feet of head) of what you are trying to pump through.

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stevo_2

What would the horse power be on a 4 gpm (min) circ pump? And how do you calculate HP to GPM with circ pumps?

stevo_2

Its for a one story home with a slab foundation so there will be no feet of head.

jerry scharf

think drag, not height

Stevo,

Alan was talking about the overall resistance to flow of the system that the pump will be pushing fluid through. That resistance is often expressed in feet of head, since all pumps are measured in flow vs head graphs. Pump people started off lifting water (remember James Watt and his steam engine,) so that's how pumps have been measured ever since. If you have the drag at a given flow rate, any units you have can be converted to feet of head very simply.

Also, it's not just about horsepower, as the same motor can produce different results with different pumps attached. It's all about the pump curves and which ones come up with 4 or more GPM when pumping into the piping system you have.

Also, since we're talking about closed pressurized systems (at least I assume we are,) elevation makes no difference here. For every gallon of water going up, there's another one coming down.

hope that helped a bit,
jerry

Tom Meyer

A pump curve stated as GPM vs foot head is a power statement. At any point on the curve, the power put into the water (water horse power) is the water energy input rate. This a foot head (foot pound per pound) X GPM (converted to pounds per minute) and expressed as water Horsepower (WHP).

Water HP = GPM x Head x fluid specific gravity divided by 3960.

As the other fellows said, it's not just related to GPM (flow).

If you need more information, give me a shout and I'll try to help. I didn't know how deep you wanted to get into this.

Tom Meyer
Senior Designer/Trainer
Precision Hydronics Corp
www.precisionhydronics.com

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stevo_2

feet of head?

so how do I calculate feet of head? I thought this is done by measuring the hieght and dividing by 2.33. How do I calculate this for my situation. Do I also have to find the resistance of the pipe runs? If so do you know what it would be per foot of pipe and elbow?

Tom Meyer

Static head

Static head is computed based on vertical rise. Static head doesn't really figure into pump sizing on a CLOSED loop system.

The head you need to be concerned about is the the friction forces through the pump's system. Think of it this way. Take four lengths of pipe and four elbows, solder them together. If you stand the rectangle of pipe straight up in the air, or lay it down flat on a table, the pump head is the same.

Pump head is determined by the distance, pipe size, restrictions to flow, etc.

Remember when I said "the pump's system"? When you do primary-secondary, the pump's universe is only that part of the system it sees. The primary circulator "sees" only the primary loop (assuming closely spaced tees separate it from the distribution system). The secondary circulator only "sees" the head in the secondary circuit. And to make matters even more complex, your calculation is based on the loop with the greatest resistance (sometimes referred to as the "longest loop" or the "index loop").

Tom Meyer
Senior Designer/Trainer
Precision Hydronics Corp
www.precisionhydronics.com

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ALH_3

Head Loss

Friction head loss is related to many things: Viscosity of the fluid, velocity in the pipe, diameter of the pipe, the inside surface of the pipe, and the length of the pipe.

Minor losses due to fittings and valves are related to velocity, and a minor loss coefficient (also a function of viscosity) which is related to the type of fitting or valve, whether it is threaded or soldered, etc.

In parallel pipes the head loss is equal. The velocity automatically compensates for differing lengths, and therefore the flow rate is not equal. The longest loop in the parallel pipe system will have the least velocity and therefore the least flow (flow=velocity*cross-sectional area). It is important to keep your loop lengths close to avoid shorter loops stealing flow from longer loops, especially if they are the same zone.

Better system designs have low pressure drop (shorter loop lengths) to allow the use of smaller pumps.

Head(ft)/2.31=psi

GPM required= HeatLoss(btu/h)/(500*temperature drop)

Use a chart like the one below to approximate your head loss from the flow rate.

Once you know your head loss you look it up on Grundfos or Taco's pump curves and select the pump you need to deliver the flow desired at the head loss you have in your system.

ALH_3

Chart

Here's the chart.