Furnace Fan Efficiency??

[miningman]

I am the first to admit that this is a rather trivial query, but does anyone know the typical electrical fan efficiency of a regular North American furnace fan. ?? I am not seeking anything on the thermal efficiency of the furnace system , just the blower fan. Since these are relatively low cost units, I suspect it is low, and I am well aware that the operating point on the fan curve .... and therefore efficiency.... will vary from one system to another but surely there is a normal expected range.

 

[DRWeig]

Good numbers for the fractional horsepower motor in the Baldor book. It says 68 percent for a 1/2 HP motor and worse below that mark. That'll be the biggest differenct in efficiency. I'm used to 90++ percent in larger motors.

The little forward-curved fan in the poor mounting position might not hit the 60-70 percent mechanical efficiency we see in the big 'uns where the ME selects based on the system curve and BEP. But I bet it isn't a whole lot lower.

http://www.baldor.com/pdf/501_catalog/CA501.pdf

Best to you,

Goober Dave

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[willard3]

50-60% for forward curved fractional hp fans.

 

[miningman]

Thanks guys, 60% maximum expected efficiency doesnt contradict my gut feel and is accurate enough for my immediate purposes.

 

[chicopee]

"....regular North American furnace fan..." what do you mean by afurnace fan since the term furnace can apply either to a residential steam or hot water boiler, or a hot air furnace.

 

[miningman]

Chico, I think youve just answered your own question. My original query made zero reference to a steam or hot water boiler, therefore de facto I was referring to a hot air furnace. If I had a query about boilers, I would have used that term.

 

[chicopee]

I would go more aroun 80% with squirrel cage fan motors.

 

[ScottyUK]

If it is a shaded pole motor rather than a squirrel cage machine then the efficiency will be very low - shaded pole machines are used because they are simple and therefore cheap, not because they are good. Squirrel cage machines are rare below about 0.1HP, so the answer to your original question is 'it depends'!

 

[DRWeig]

Scotty & Chicopee: Are you using the term "squirrel cage" to describe a motor or a fan? I have only ever heard that term used as a slang for a small centrifugal fan.

Anyway, just to make certain we're all thinking about the same thing, I'll ask miningman: We're talking about something like the attached spec sheet shows, right?

That's what I assumed (shame on me for assuming).

Best to you,

Goober Dave

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[miningman]

Yep thats exactly the type of unit I was referring to...... whether its a squirrel cage fan or motor is beyond my limited electrical knowledge.

 

[DRWeig]

Thanks. In that case, the less-than-60 percent number will be good. The smaller, the lower.

Best to you,

Goober Dave

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[ScottyUK]

Goober Dave,

No those are industry terms for our motors.

A 'squirrel cage' motor is the most common type of motor. It gets its name from the rotor design, which is a 'cage' akin to a hamster's wheel formed (typically) from straight copper rods or bars with the ends of the rods being welded or brazed to copper rings. Very large induced currents (hence 'induction motor') flow in the copper cage, resulting a corresponding magnetic field. The rotor field interacts with the stator field causing torque to be produced. A quick look on Google found this:

http://www.youtube.com/watch?v=3MbP4t920Is,

or alternatvely here's the Wiki entry:

http://en.wikipedia.org/wiki/Squirrel-cage_rotor

A shaded pole motor uses a copper loop to distort the magnetic field in an iron yoke forming the stator. Induced currents in the rotor - which is similar to that described above - interact with the distorted stator field and produce torque. The copper loop dissipates a lot of power. They are limited to small sizes because they make poor use of the magnetic materials and have dreadful efficiency; ratings greater than a few tens of watts are uncommon. To drive a circulator fan for a small furnace - which looks like a kiln to me - a few tens of watts would be more than enough. Here's the wiki entry which shows the basic design:

http://en.wikipedia.org/wiki/Shaded-pole_motor

 

[DRWeig]

Thank you Scotty!

Shaded pole is something I've never investigated. I'm not much of a motor guy. The last time I worked a problem with a motor was when taking my PE exam in 1984.

Everything I see in the US for the fractional-HP residential furnaces is permanent split capacitor. Many are being sold with variable-speed drives now.

Best to you,

Goober Dave

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