Cabin A/C air filter replacement a necessity or a nice to have?

Maintenance/Repairs
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81

FOR PITY’S SAKE!!!

82

Now I’m glad everyone seems to have ignored my explanations.

83

No, the force vector of pressure differential on the blades or vanes is normal (perpendicular) to their surface. This vector has a component in the direction the blade/vane moves.

84

No, the force vector of pressure differential is perpendicular to the surface of the cylinder formed by the squirrel cage and has no component in the direction the blade moves. What if the vanes were positioned radially (pointing straight away from the axis) as is common on many (non-auto) centrifugal blowers. What say ye then?

85

Circuitsmith, I appreciated and agree with all of your explanations.

86

You guys have been hitting yourselves over your heads for some time now

One or more of you guys is going to have a concussion soon

And the winner will be the last guy standing

87

Except for a minor friction component, the force of the air on a fan blade is normal to the surface of the fan blade. Radial blades, as in a house table fan, are typically tilted.
The resulting force will have a component pointing upstream, and a component pointing against the direction of rotation.

88

Someone please drink the kool-ade already !!!

89

melott, do you understand that a table fan is not a centrifugal fan? A centrifugal fan (blower) can have blades of various orientation (see http://en.wikipedia.org/wiki/Centrifugal_fan ).

90

And in your diagram, as the fan turns, there is low pressure behind the blades if there is a blocked filter, and normal pressure in front of them. So, there will be a force normal to the fan blade surface which will have a component opposite the direction of motion.

91

So why does a vacuum cleaner fan speed up when you block the suction hose?

92

I asked for proof of that statement, and no one ever provided it. I do not accept the change in pitch as evidence.

93

I just realized something else. A vacuum cleaner is a different setup. The filter is after the fan, so there is considerable resistance when there is air flow. In a car, it’s filter first, then fan, then the heat or ac exchanger. Not much resistance there. So it’s not the same.

94

“I do not accept a change in pitch as evidence”

You must have a tough time driving a clutch without a tach…higher engine pitch? Slower? Faster? WHO KNOWS…

95

All my shop-vacs are filter first, then fan and, of course, speed up when the hose is blocked. Here’s a different take from an HVAC expert’s web site concerning vacuum cleaners:

"Let’s clear up one more myth about vacuums. When you block off the suction, the motor speeds up in the machine. Most people think the motor is working harder then. Just the opposite is true. The ‘fan’ in a vacuum cleaner is technically called a “centrifugal blower”. The blades on the blower wheel “grab” a chunk of air, then spin it around in a circle at high speed. Centrifugal force then causes the air to fly outward, away from center and towards the outer edge of the blower wheel. The suction is created by new air entering the inner part of the blower wheel to replace the air that was thrown outward. When the machine is running normally, this process happens continuously. The “new” air entering the blower wheel is not spinning until the blades grab it. It takes energy from the motor to get this new air spinning, which slows the motor down. But when you don’t allow new air to enter the blower, such as blocking the hose, any air that remains in the blower wheel simply spins around continuously, and the motor is not working hard to get new air spinning, so it speeds up.

But running the vacuum with the hose blocked can still damage the machine. Many vacuums depend on that air moving through the blower to cool the motor, so the motor could overheat. Also, when the same air is allowed to spin continuously inside the blower, the air gets quite warm from friction. This could cause a ‘meltdown’ of plastic blower parts.

As long as we’re talking about “centrifugal” blowers, the one in your furnace is that type too. If your furnace filters become plugged, less air will move through the furnace. Although this can drastically cut the efficiency of the system, the blower itself will actually use less power, due to the principals explained above. The same goes for centrifugal pumps, like a sump-pump or a well pump. The more the flow is restricted, the less power it will use, and the less water it will pump."

I couldn’t agree more.

96

Here’s some eye candy, guys . . .

https://www.google.com/search?q=blower+motor+picture&tbm=isch&source=iu&imgil=I4fVZ4gc2NWnKM%3A%3Bhttps%3A%2F%2Fencrypted-tbn1.gstatic.com%2Fimages%3Fq%3Dtbn%3AANd9GcTEUO7x-A3BuV-5XWD1jugCTBp8Z8eF0R5ikNVgjAG_y6opONK2%3B350%3B350%3BmUOLFhZExK3BvM%3Bhttp%253A%252F%252Fwww.autopartswarehouse.com%252Fshop_brands%252F4-seasons.html&sa=X&ei=uRC5UvysIcvvoATSwYHICA&ved=0CDoQ9QEwAg&biw=1241&bih=583#facrc=&imgdii=&imgrc=I4fVZ4gc2NWnKM%3A%3BmUOLFhZExK3BvM%3Bhttp%253A%252F%252Fstatic1.apwcontent.com%252Fext%252Flp%252Fbrand_store%252F4-seasons%252Fimages%252Fproducts%252FAC%252520Blower%252520Motor.jpg%3Bhttp%253A%252F%252Fwww.autopartswarehouse.com%252Fshop_brands%252F4-seasons.html%3B350%3B350

97

insightful, just a bit of a point here, in the cars blower motor, there is a tube that takes air from the vent side of the fan and ducts it to the motor. If the filter clogs up, then the pressure is lower inside the squirrel cage so the pressure difference from one side of the motor to the other is greater, therefore, unlike a vacuum cleaner, more air will flow through the motor helping it to stay cool.

Lets also keep in mind that the filter will only slow down airflow, it won’t block it completely and the blower assembly in the vehicles HVAC is an air mover, not an air pump. It won’t create very much of a pressure differential like an air pump would.

98

kieth, good point. With a restricted filter, the airflow through the motor increases; however, that cooling air will be warmer. I’ll defer to Nevada-545’s expertise and agree that plugging filters have little real-world effect on blower motor life.

99

I think this argument has run its course, I don’t think anyone will change their positions here so I’d like to end it on a lighter note, and I plead guilty on this as well. Andy would be proud of us. We have done such a good job of proving his thesis once again.

100

Kieth, here’s a few more:

http://www.alibaba.com/product-gs/1489422397/Heater_Blower_motor_for_GM_DC12V.html

http://www.alibaba.com/product-gs/1489243003/Heater_Blower_motor_for_FORD_DC12V.html

http://www.alibaba.com/product-gs/527260464/6441_K0_Car_Blower_Motor_For.html

http://www.alibaba.com/product-gs/568599792/6441_74_Auto_Heater_Blower_Motor.html

http://www.alibaba.com/product-gs/582807405/Car_heater_blower_NISSAN_ATLAS_blower.html

http://www.alibaba.com/product-gs/536584054/Car_heater_blower_Car_blower_for.html

Tester, as others have pointed out, there is “windage” or turbulence in a blocked blower and although the power required is less, it is nowhere near zero and significant amps will still be drawn.