Posts Tagged ‘centrifugal pump’

Centrifugal Pumps

Sunday, May 16th, 2010

     Last week we focused on various types of positive displacement pumps.  Today we’ll take a look at centrifugal pumps.  See Figure 1.

Figure 1 – A Centrifugal Pump

     Just like the positive displacement pumps we talked about last week, centrifugal pumps have rotating parts as well, but that’s where their similarities end.  Unlike positive displacement pumps that take “bites” out of liquid before trapping it between moving parts, centrifugal pumps rely on kinetic energy to move liquid in a continuous stream.  Kinetic energy is the energy of motion, and in the case of the centrifugal pump kinetic energy is developed by rotating parts within the pump and transferred to the liquid contained within the pump.  In other words, the liquid is moved through the pump by means of centrifugal force.

     To illustrate this concept, we can tie a rope to the handle of a bucket that has a small hole punched in the bottom.  Now, you know what will happen if you fill the bucket with water…  There’s a hole in the bucket, Dear Liza, Dear Liza…  That’s right, the water will just dribble out of the hole, thanks to gravity.  But before we fix the hole as Liza suggests, let’s do an experiment.  Pick up the rope and spin the bucket around as fast as you can in a circle.  You’ll notice that this rapid spinning creates centrifugal force, resulting in a rather powerful stream of water shooting from the hole.  The faster you spin the bucket, the stronger the stream.

     When it comes to centrifugal pumps, the idea is basically the same.  The objective is to forcefully spin water around in a circle, thus ejecting it from the pump.  This is accomplished with a rotating part called an impeller.  See Figure 2.

Figure 2 – Cutaway View of a Centrifugal Pump    

 

     In our illustration the impeller is attached to a shaft that’s powered by some source of mechanical energy, such as an electric motor.  The water enters the pump at the center of the rotating impeller, referred to as the “eye.”  The water then slides over the face of the impeller, moving from the center to its edge due to the action of centrifugal force.  That force pushes it off the impeller and into the pump housing.  You’ll note that the housing has a special shape, called a “volute.”  This volute looks a lot like a spiraled snail shell.  The shape of the volute helps direct the water coming off the impeller into an opening in the side of the pump where it is discharged.  The faster the pump impeller rotates, the more kinetic energy the water picks up from the impeller.

     This ends our discussion on pumps.  Next time, we’ll move on to a new topic of discussion, braking systems.

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A Pump By Any Other Name…

Monday, May 10th, 2010

     Pumps are all around us.  They keep our drinking water flowing, the cooling water circulating in your car’s engine, and even your blood flowing.  They’re essential in many aspects of our lives, but most of us don’t think too much about them.  For our discussion let’s put them into two categories:  positive displacement pumps and centrifugal pumps.  This week, we’ll focus on positive displacement pumps.

     Positive displacement pumps, as their name implies, displace a quantity of liquid with each complete cycle of movement.  This takes place when moving parts of the pump take “bites” out of the liquid at the inlet, then force them to exit through the outlet.  A familiar example of a positive displacement pump is the type of hand operated water pump that’s commonly found in campgrounds.  See Figure 1.

 

Figure 1 – A Positive Displacement Pump

     This type of pump is known as a reciprocating positive displacement pump.  By reciprocating, I mean that the moving parts travel back and forth in a straight line during its operation.  Let’s see how it works by referring to the cutaway view in Figure 2.  

 

Figure 2 – Cutaway View of the Pump Shown in Figure 1 

     In the cutaway view, the pump’s piston and internal check valve are shown, and there’s another check valve in the bottom of the pump housing.  When you pull up on the handle, the piston moves down into the water in the pump housing, and the pressure caused by this movement forces the check valve in the bottom to slam closed, while the check valve above is forced open.  This causes water movement to flood through the open check valve and fill up the space above the piston.  When you push down on the handle, the opposite happens.  The piston is made to move upward.  The upward acceleration of the water above the piston causes the check valve on the piston to slam shut, and this traps the water above it.  As the piston moves back up, a suction is created below, which causes the check valve in the bottom of the housing to pop open and more water is drawn up into the space below the piston.  Eventually, when the piston gets high enough, the water trapped on top of it will flow out of the spigot.

    Another type of positive displacement pump is represented by a rotary pump.  These pumps operate in a circular motion to move a volume of liquid with each revolution of the pump shaft.  This is done by trapping liquid between moving parts, such as gears, lobes, vanes, or screws, and the stationary pump housing itself.  

     To show how this works, refer to the gear pump shown in Figure 3.  Its gear teeth mesh together in the middle of the pump, blocking the flow from going straight through and trapping it within the spaces formed by rotating gear teeth and the pump housing.  It’s like the water is being forced through a turnstile.

Figure 3 – A Cutaway View of a Gear Pump

 
     Next week, we’ll talk about centrifugal pumps and how they move liquids along using centrifugal force.
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