evap fan speed while iced up

[cool whiz]

I have to ask. In theory, I always thought that when an evaporator coil is iced up, the evap fan motor speed would increase due to no load. Why am I seeing the opposite? Same thing with condenser fan motors and plugged up condensers. Fan speed decreases with blocked airflow.
Shed some light on an old-timer.

[trippintl0]

When the coil is frozen, the fan tries to move air anyways, creating a pressure or a vacuum (depending on air flow direction) in the space between the fans and the ice block. This force is fighting the normal airflow that the fan would produce, which causes it to slow down. Like a "brake" for the fan. I am imagining how you can blow on a fan that is not energized, and this causes it to turn one direction. So the force of the air can affect the fan.

*Disclaimer - this is just a SWAG *

* EDIT in fact, now that I think of it more, it's not that the fan is no longer doing any work. It's just that the work has a counter force acting against it. Doing no work would be if the blade fell off and it was just a motor spinning. Then it might increase.

[mark beiser]

Prop fans load up when the static pressure is increased.
Squirrel cage fans unload when the static pressure goes up.

[cool whiz]

Interesting.
So propellers react the opposite of centrifugal or squirrel cage blowers with a rise in static pressure. I get it. Thanks.

[jpsmith1cm]

Prop fans load up when the static pressure is increased.
Squirrel cage fans unload when the static pressure goes up.

Now, is this enough to cause a significant and measurable change in fan speed?

I've never taken the time to measure it, myself.

Hmmm... Maybe I should

[ECtofix]

Now, is this enough to cause a significant and measurable change in fan speed?

I've never taken the time to measure it, myself.

Hmmm... Maybe I should

4-5 years ago I worked on a frozen up W/I cooler that had GE motors (I think) that were running so slow, I thought they were defective and had CAUSED the freeze up. I hadn't ever seen those particular motors before and had only Fasco equivalents I purchased locally. When took the GE motors out and got a better read on the data sticker, I discovered that they had an internal overload that would kick the motor down into a MUCH reduced RPM. The motors had time to cool down while I de-iced the coils. Sure enough, those motors attained a normal RPM by the time I was done, so I subsequently let them be.

That experience revealed to me that the motors must be much more labored when airflow is blocked.

Oh...the freeze up was solved by some attentive tweaking of the thermostat.

[mark beiser]

Now, is this enough to cause a significant and measurable change in fan speed?

I've never taken the time to measure it, myself.

Hmmm... Maybe I should

Results will vary with different motor and fan combos, but in general the RPM will change because the motor will have more slip under load than with reduced load.
A shaded pole motor would likely have a lot more slip under a higher load, where a 3 phase motor would have very little increase in slip if it is not being over loaded.

[jpsmith1cm]

Results will vary with different motor and fan combos, but in general the RPM will change because the motor will have more slip under load than with reduced load.
A shaded pole motor would likely have a lot more slip under a higher load, where a 3 phase motor would have very little increase in slip if it is not being over loaded.

Of course, I understand the theory behind it.

I'm just curious, now, as to the actual field application of it.

I think I'm going to start taking before and after tach readings and see...

[Core_d]

Now, is this enough to cause a significant and measurable change in fan speed?

I've never taken the time to measure it, myself.

Hmmm... Maybe I should

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?

[ryan1088]

You guys pay that much attention to how fast the fans are spinning when you walk in and see a big ball of ice? Usually I'm cursing all the way to the truck to get the torch......

[mark beiser]

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?

Your example simulates extremes of air density, not of the static pressure the blower works against, completely different things.

There is an easy example that you have probably seen many times. Look at the amp draw and RPM of a blower motor in an air handler with the access panel off, vs the amp draw and RPM with the panel on.
When you pull the panel off, the motor is working against a lower ESP, so it loads up, causing the amp draw to go up, and the blower to slows down.

[Core_d]

Your example simulates extremes of air density, not of the static pressure the blower works against, completely different things.

There is an easy example that you have probably seen many times. Look at the amp draw and RPM of a blower motor in an air handler with the access panel off, vs the amp draw and RPM with the panel on.
When you pull the panel off, the motor is working against a lower ESP, so it loads up, causing the amp draw to go up, and the blower to slows down.

I realize to much return air is a bad thing because you need a pressure differential. I was gonna mention that fact to the OP maybe that has somthing to do with him seeing the opposite effect. 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. I assume since static pressure+velocity pressure=total pressure a clogged coil would stiffel velocity pressure thus decreasing total pressure. In that since I do get it picturing in my head is a different story. Oh well, not the first time reality and my mind werent on the same page. Thanks for your input if im way off here please elaborate.

[Core_d]

You guys pay that much attention to how fast the fans are spinning when you walk in and see a big ball of ice? Usually I'm cursing all the way to the truck to get the torch......

lol ive got nothing better to do.

[mark beiser]

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.

[jpsmith1cm]

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.

[ECtofix]

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.

[Core_d]

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

[mark beiser]

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.

[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.

[Core_d]

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.