# evap fan speed while iced up

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• 02-16-2013, 08:34 PM
mark beiser
Quote:

Originally Posted by Core_d
I have to disagree with you saying air density and static pressure are completely different things. Static pressure is pressure applyed in all directions, a pressurized tank would infact have static pressure.

Just to be clear, I'm assuming that in your pressurized tank vs vaccum tank example, that the fan assembly, including any duct inlets/outlets, are completely contained within the hypothetical tank. Basically simulating extremes of the ambient pressure the fan system is operating in.

It would have static pressure pushing out on the walls, but that pressure does not work directly against the fan paced within the tank.
The increased density of the air in your hypothetical pressurized tank is what would cause the fan to operate under a higher load than the same fan placed in your hypothetical vacuum tank.

To simplify things for better understanding of the air density thing, imagine that your standard motor type fan is a device made for throwing boxes of air, 1 cubic foot in size.
If the air in the 1 cubic foot boxes is very dense, it will weigh more than if the inside of the boxes were in a vacuum.
Since the fan has to work harder to throw the heavier boxes of very dense air, it will have higher amp draw than it does throwing the lighter boxes that are in a vacuum.

The static pressure the fan is moving air against is a measure of the flow resistance of the fan assembly, and all associated components it has to move air through.
• 02-16-2013, 09:50 PM
jpsmith1cm
Quote:

Originally Posted by Core_d
I dont own a tachometer but i did compare amprage on two like units one evap clogged killing air flow and one not cloged both on the same fan speed. My unit pulled 2.8A and the clogged one pulled i think 2.5 slightly less. Looking at a fan curve chart it says a fan at 1.5"W.C with 800 RPM will have 900RPM at 2"W.C I dont know if thats accurate or just an example, its in my school book.

With that being said im picturing two extreams of the situation a squirel cage in a vacum and a squirl cage in a pressurized tank. It seems to me the one in the vacum would have very little load compared to the other one. Basically I DONT GET IT, would anyone care to explain the phsyics of this to me?

You're also making an apples to oranges comparison.

Since we're in the Refrigeration and Ice Making forum, we're most likely talking about Prop type fans which will exhibit the exact opposite behavior that you're describing.

Squirrel cage, centrifugal type blowers will do exactly what you described.
• 02-16-2013, 10:08 PM
ECtofix
Quote:

Originally Posted by jpsmith1cm
You're also making an apples to oranges comparison.

Speaking of fruit, I know it's harder to SIP a Sonic slush drink when bits of fruit clog the other end of my straw.

Well..maybe different physics there compared to a fan motor.
• 02-16-2013, 11:27 PM
Core_d
Quote:

Originally Posted by jpsmith1cm
You're also making an apples to oranges comparison.

Ooops didnt see that. I guess that answers the OPs question. Thanks for the reply mark i have a little better prespective now. IM gonna have to think on this for a while and probabley do a little experimenting befor i can say i actually understand it. I run the scenarios by my wife squirel cage,propeler/restricted,not restricted and how the fans would work in comparison. She was right every time, Oh well lol
• 02-16-2013, 11:48 PM
mark beiser
Quote:

Originally Posted by Core_d
Ooops didnt see that. I guess that answers the OPs question. Thanks for the reply mark i have a little better prespective now. IM gonna have to think on this for a while and probabley do a little experimenting befor i can say i actually understand it. I run the scenarios by my wife squirel cage,propeler/restricted,not restricted and how the fans would work in comparison. She was right every time, Oh well lol

Now if you really want to be confused, we can jump to variable speed motor driven fans. :D
• 02-17-2013, 12:07 AM
MicahWes
If you consider that all types of fans (radial/centrifugal and axial) are airfoils, and you consider how airfoils create thrust by creating pressure differences, and you also consider that air as a fluid cannot be pulled, only pushed...Then you should be able to derive most of your answers to the original question.
• 02-17-2013, 01:08 AM
Core_d
Quote:

Originally Posted by MicahWes
consider that air as a fluid cannot be pulled, only pushed...

Thank you this spoke volumes to me. I will add their is one thing that can pull air... Gravity, and even suction is a push created by the pull of gravity.
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