Pump Sizing - Misting Systems, How do you pick the right pump?

When you size a pressure pump you have to take several factors into account. First off, pressure pumps are typically rated in gallons per minute. Nozzles on the other hand are rated for lbs. of water per hour.

Lets take an example. Outdoor misting application. You want 10 nozzles on a deck. A good nozzle for a deck is 0.30mm orifice. 0.30mm orifice will produce about 12 lbs. per hour of water vapor. There are 8.34 lbs of water in one gallon of water.

Doing a little math 10 nozzles x 12 lbs.per hour = 120 lbs. per hour. 120 lbs. per hour / 60 minutes = 2 lbs. per min. 2 lbs. per minute / 8.34 lbs. per gallon = 0.2401 gallons per minute.

So you need a pump with at least 0.25 gpm.

Basic rule of thumb is to double the size of the pump. Take your calculations above times 2. My recommendation would be a 0.50 gpm pump. Don't worry about cost. The difference between a 0.25 gpm pump and a 0.50 gpm pump is about $10.00. or a 2% increase in price.

The big decisions is, how do I control this pump?

There are Pumps and there are Pumps

You can buy just a pump and plug it in. However there are a few things you have to know.

A basic pressure pump has a motor and a pressure regulator. That's it. Watch it, some don't even come with a motor. They give you a feeling there is motor with it (pictures) but it's just the pump.

Pump with Motor


Pumps © Misting Solutions.com

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Above pump is far from serviceable. You could say it a raw pressure pump and motor. The pumps itself need several items, Pressure regulator return line, gauge minimum. The motor is designed to be in an enclosure. Open wires would terminate someplace, contactors or terminal blocks.

However, Cat Pumps are quality pumps with good documentation and support. Parts lists, pump breakdowns are shipped with the pump. Service manuals are available online.

Pump Sizer

Below is a pump sizer. It is based on the number of nozzles you are planning on using and the capacity of each nozzle.

Let's look at a second example, a little more complex scenario.

Let's create a pressure pump with some precise control.

Say we want to control the speed of the pump to give us a desired pressure or flow.

How do we calculate pump RPM for our desired output?

Let's look at a scenario.

Nozzles requirements
8 Nozzles, each nozzle has a capacity of 0.75 GPH at 160 PSI.
Totals,  8 nozzles x 0.75 GPH =  6 GPH.  6 GPH x 8.34 #PG = 50 #PH

Analyzing a Pump Selection

Pump #1
Capacity 0.13 GPM x 60 = 7.8 GPH at 1725 RPM
Positive displacement pump are directly porportional RPM to GPM.

6 GPH of Nozzle / 7.8 GPH of Pump = 0.77 x 1725 RPM = 1327 RPM
Pump Frequency = .77 x 60 cycles = 46.2 HZ

Pump #2
Capacity 0.25 GPM x 60 = 15 GPH at 1725 RPM

6 GPH of Nozzle / 15 GPH of Pump = 0.40 x 1725 RPM = 690 RPM
Pump Frequency = 0.40 x 60 cycles = 24 HZ

Pump #3
Capacity 0.50 GPM x 60 = 30 GPH at 1725 RPM

6 GPH Nozzle / 30 GPH of Pump =  0.20 x 1725 RPM = 345 RPM
Pump Frequency = 0.20 x 60 cycles = 12 HZ

Link to Pumps in review.

Now you could say all pressure pumps have bypass pressure regulators.  Why then is a RPM reduction needed?  If pressure or flow control is need then RPM regulation is needed.

If RPM is increased in each of the above scenarios there would be a corresponding increase in pressure and flow.  That increase however is not directly proportional to the corresponding increase in RPM.  All nozzle manufactures have pressure/flow charts for reference.

Pump RPM can be controlled by a Variable Frequency Drive (VFD) and a 3 Phase Electric Motor. The VFD can be programmed for automatic or manual operation through a Programmable Logic Controller (PLC).  The PLC can be controlled through a PC or Touch Screen Display Interface.

The PLC is an excellent Pump System Interface.  It is the heart for pump Control, Monitoring, Alarming and any Logging function needed.

Choosing the right pump.

If the above nozzle requirement is a maximum configuration and the possibility of capacity reduction is needed through reducing the number of nozzles or pressure then Pump #1 would be the better selection.

If a range of performance and control is desired above and below the base nozzle requirements then the proper pump selection would be Pump #2.

If the above nozzle requirement is a minimum configuration and there is a high probability that more nozzles or a much higher pressure will be needed then Pump #3 would be a better selection.

Just exactly how is this Pump controlled?

The above scenerio is overall a small misting system.  Small enough that the only power source needed is standard 120 volt power.  That can be hard wired for safety by an electrican or just plugged into an outlet.

A electrical control box is need to house the controls.  The size of the control box will depend on if zone control is needed.  Typically a 16" x 20" control box will do fine.  The complete package, pump and controls would be approximately 24" x 36".   

The PLC monitors incoming requirements before the pump can start. Incoming water pressure has to be present, minimum 10 psi. And some sort of initial call by an operator, a System ON switch, mechanical or digital.  

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