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Thread: Blower rotation question
08-05-2011, 03:19 AM #1
Blower rotation question
This is a squirrel cage type blower....as viewed in this photo, it rotates in a clockwise direction. The intake of air is in the center of the wheel, and the discharge of the air flow is in the direction of the blue arrow.
That said, can someone explain how that works? It seems the curvature of the vanes is wrong for that type of flow path?
08-05-2011, 04:03 AM #2
What does the housing look like a picture of it would really help, I've repaired blowers with double squirrel cages and and found that some one had installed it incorrectly.
08-05-2011, 04:56 AM #3
Blade configurations for blowers come as forward curved, backward curved and straight. All have pro's and cons regarding noise, susceptibility to particles, flow and pressure.
Centrifugal fan - Wikipedia, the free encyclopedia
Maybe a sufficient explanation?
08-05-2011, 04:59 AM #4
08-05-2011, 05:12 AM #5
Years ago, when I was a mechanical inspector at the Kaneohe Marine Corps Air Station, I had one of the machinists in the machine shop overhaul a large air handling unit on top of a dynamometer room. After completing the overhaul and starting it up, he called me over to the jobsite and said that there was a great reduction of airflow since the overhaul. I opened the air handler's door and quickly noted that the squirrel cage impeller was installed backwards.
I mentioned this to the machinist and he argued with me saying that the impeller on all of the waterpumps that he had overhauled are installed that way.
I concurred with him on the installation of waterpump impellers, but not on air handling units. I finally convinced him to reverse the impeller. He was amazed at the increase of airflow.
As an air conditioning mechanic for 20 some years, I probably have overhauled a hundred air handlers. That's why I'm familiar with them.
08-05-2011, 05:21 AM #6
"As an air conditioning mechanic for 20 some years, I probably have overhauled a hundred air handlers. That's why I'm familiar with them."
Ok, but what's your take on the OP's example?
08-05-2011, 08:10 AM #7
The inner portion of the blade starts the air rotating near the same velocity as the blade. With a straight blade, the air would flow straight out, following what is usually called centrifugal force, although in a strict sense, centrifugal force does not exist. What we call centrifugal force is actually the centripetal force required to pull the air around in a circle instead of letting it go straight. Newton and all that. Anyway, the air starts to flow radially. As it flows outward along the blade, it must speed up because of the slope of the blade. The result is that it exits going faster than the velocity of the rotor. Think of a jai alai basket for air. In a highly developed blower, there are fixed scroll vanes outside of the rotor that enhance the effect, translating the outward motion into forward velocity toward the outlet.
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08-05-2011, 08:40 AM #8
This is the housing. From this view, the rotation of the wheel is still clockwise and the air flow path is in the direction of the red arrow.
There is no chance I have the wrong wheel or rotation.
It seems to me the outer edge of the vane would 'bite' into the air and send it toward the center of the wheel. Or, if spun backwards, the inner edge of the vane would bite the air and send it outwards.
But both are wrong as I know waht happens in reality is the air flows in the direction of the arrow.
I just don't undertand why!
08-05-2011, 09:59 AM #9
I opened up a coolant pump, one time, that had the impeller on backwards. Probably from the factory. The strange thing is...I was working at a place that made centrifugal pumps. THEIRS were similar, except in size AND...location of the "discharge" and "suction". Otherwise known as intake and outlet. THAT, at least in liquid pumps, made the difference as to which way the vanes were curved.
08-05-2011, 10:39 AM #10
That is a forward curve blower wheel, characteristics are high volume air movement at lower wheel speeds, quiet operation, good static performance through the 2-3 inch W.C. range. Drawbacks are the the curves or paddles that cup the air are susceptible to debris accumulation.
Think of it as a paddle wheel on a riverboat. Is that the answer you were looking for?
Other models are straight paddle and reverse incline, then you get into the propeller types, crossaxials, tubeaxials, vaneaxials .. etc.
08-05-2011, 10:52 AM #11
08-05-2011, 01:37 PM #12
Let's see if I can thoroughly confuse everyone. To get an idea of why the air velocity varies with vane shape, consider flatbed truck going down the highway at 100 mph. On the bed of the truck is a baseball pitcher and an observer. There is also an observer on the ground watching the truck as it passes. There is no wind resistance.
Case 1. The pitcher fires a fastball at 100 mph in the direction the truck is going and at an angle to the horizontal of 30 degrees. To the observer on the truck the ball appears to travel at 100 mph at an angle of 30 degrees. To the observer on the ground, because he sees not only the ball moving, but also the truck, the ball appears to be moving at 193 mph at an angle of 15 degrees. This is because of the vector sums of the two velocities. This corresponds to a forward curved blade fan. The speed of the truck is the wheel velocity and thrown ball is the velocity of the the air leaving the tip of the blade.
Case 2. The pitcher turns around and throws in a direction opposite the direction of the truck otherwise same as before. Again to the observer on the truck sees the ball appear to be thrown at 100 mph at an angle of 30 degrees. To the observer on the ground, the velocity appears to be only 52 mph at an angle of 75 degrees to the horizontal (almost straight up!). This is the backward curved blade.
Seems strange but that is way vectors work.
The air leaving the edge of the blade always leaves in the a direction tangent to the blade tip. The wheel tip velocity is the product of the wheel rotation speed and the radius.
08-05-2011, 02:06 PM #13
Yep it's gonna take a lot more than that for most of us. Help us out. I see HUGE centrifugal fans, ID and FD fans in power plants. Don't recall anything like that.
08-05-2011, 02:25 PM #14
08-05-2011, 02:39 PM #15
And not only that, when you block the outlet of the blower, increasing the back pressure, why does the current drawn by the motor decrease? Answer tomorrow.
08-05-2011, 02:54 PM #16
08-05-2011, 03:05 PM #17
As shown, the fan spins clockwise. It fits into the housing also as shown. In other words, when it is installed, you will see the side of the housing you currently see. You will see the side of the wheel you currently see. It will be spinning clockwise.
08-05-2011, 03:43 PM #18
A fan doesn't "push" air....it creates a low pressure area behind the blade as it moves and sucks air, creating forward velocity in the air. The laws of physics state that an object moving in a circle wants to move straight instead, some other opposing force must keep it going in a circle.
In the blower above, the shape of the blade creates an increasingly low pressure as you go from the center out, thus the delta of force gets higher, and velocity increases.
08-05-2011, 04:20 PM #19
"And not only that, when you block the outlet of the blower, increasing the back pressure, why does the current drawn by the motor decrease? Answer tomorrow"
Oh! Oh! I know! It is NOT because you have obstructed an air hose. It is because the "blower", consider a ShopVac, is no longer having to move 14 pounds of air, and it is NOT shrieking in agony, it is sighing with relief..
By the same token, a centrifugal pump that is air bound will draw less current than one that is pumping water. It will get HOT, but it will draw running current, not "full load". Centrifugal with 100 ft head will be .433+ PSi, and depending on the capacity, do the calcs and learn what HP is needed for your application. 100 feet up, you have NO delivery. Simply peters out.
Last edited by gmatov; 08-06-2011 at 12:39 PM.
08-05-2011, 07:23 PM #20
I understand the OP's question, it could be intuited that the forward curved blades could "scoop" air inward or at the very least make the fan operate very poorly. In fact, have encountered the same question before.
"annoying" offers an example of it restated by declaration that only the opposite is a reasonable way for it to operate.
Many of the previous posters have offered valid explainations and descriptions of why and how it works. It's counterintuitive for some but that's how our Newtonian world works. Air flies outward from a fan such as yours no matter which way the blades are curved. I'll spare the physics and math.
However, without understanding the basic physics it might seem like a parlor trick when Bill tells us the answer to motor current draw. Never been able to convince anyone how such machines work without some physics background.
The flow, pressure and power characteristics of such fans are manipulated by blade design. The forward curve blades of your fan are optimized for low pressure and high flow.