An absoloute fan question

[DanMax&Belle]

Hi All,

A happy new year to you all!

Simple one from me this morning, i need to set up some hot air distribution within my off mooring, CC'ing boat. I plan to run some air ducting from behind the stove to the other side of the bulkhead in a room that struggles to benefit from the heat. I am concerned about drain on the battery so want the absoloute lowest wattage computer fan out there, I am assuming I want a 12v fan (the boat is 12v) rather than a lesser voltage fan and some voltage reducer between the battery and the fan because of inefficiencies (although please do let me know if a 5V fan is the way to go).

Thanks in advance!

[MtB]

4 minutes ago, DanMax&Belle said:

Hi All,

A happy new year to you all!

Simple one from me this morning, i need to set up some hot air distribution within my off mooring, CC'ing boat. I plan to run some air ducting from behind the stove to the other side of the bulkhead in a room that struggles to benefit from the heat. I am concerned about drain on the battery so want the absoloute lowest wattage computer fan out there, I am assuming I want a 12v fan (the boat is 12v) rather than a lesser voltage fan and some voltage reducer between the battery and the fan because of inefficiencies (although please do let me know if a 5V fan is the way to go).

Thanks in advance!

 

This never really works particularly well, which is why it is not standard practice in narrow boats.

 

I would say the warmest air comes from above the stove at ceiling level rather from behind it, if you are determined to install some ducting. 

 

Ebay is the place to find low wattage computa fans. The lower the wattage, the less air it will move. This is technical limitation.

 

 

[DanMax&Belle]

1 minute ago, Mike the Boilerman said:

 

This never really works particularly well, which is why it is not standard practice in narrow boats.

 

I would say the warmest air comes from above the stove at ceiling level rather from behind it, if you are determined to install some ducting. 

 

Ebay is the place to find low wattage computa fans. The lower the wattage, the less air it will move. This is technical limitation.

 

 

Thanks Mike,

Will follow your advice on the height of the inlet.  i have found lots of fans on ebay most of them dont state wattage, would be good to know optimum size and any other features to look for. I want the very lowest wattage and am willing to accept this will only provide a trickle of warm air - on the basis a trickle all night will make a difference to the current 8 degree differential between the 2 rooms.

[Alan de Enfield]

1 minute ago, DanMax&Belle said:

Thanks Mike,

Will follow your advice on the height of the inlet.  i have found lots of fans on ebay most of them dont state wattage, would be good to know optimum size and any other features to look for. I want the very lowest wattage and am willing to accept this will only provide a trickle of warm air - on the basis a trickle all night will make a difference to the current 8 degree differential between the 2 rooms.

 

This one moves 41 cubic feet per minute and is only 1.8 watts

 

https://www.digikey.co.uk/product-detail/en/sunon-fans/EE80252S1-000U-A99/259-1883-ND/2037295?utm_adgroup=&utm_source=google&utm_medium=cpc&utm_campaign=Google Shopping_Fans%2C Thermal Management&utm_term=&mkwid=s945U0Wvq&pcrid=337578057584&pkw=&pmt=&pdv=c&productid=2037295&slid=&gclid=CjwKCAiA6bvwBRBbEiwAUER6JYJbNFx5z_TICwcjHzdAc5ajI158aRvkFgmBa-WbOJjIKY__ZneUsBoCw5IQAvD_BwE

[DanMax&Belle]

1 minute ago, Alan de Enfield said:

 

This one moves 41 cubic feet per minute and is only 1.8 watts

 

https://www.digikey.co.uk/product-detail/en/sunon-fans/EE80252S1-000U-A99/259-1883-ND/2037295?utm_adgroup=&utm_source=google&utm_medium=cpc&utm_campaign=Google Shopping_Fans%2C Thermal Management&utm_term=&mkwid=s945U0Wvq&pcrid=337578057584&pkw=&pmt=&pdv=c&productid=2037295&slid=&gclid=CjwKCAiA6bvwBRBbEiwAUER6JYJbNFx5z_TICwcjHzdAc5ajI158aRvkFgmBa-WbOJjIKY__ZneUsBoCw5IQAvD_BwE

Nice one Alan!

Is there a 12v option (it's 24) or is it ok to connect this to a 12v supply and get less performance, 20 cubif feet per min would be fine by me esp if it was less than a watt draw

Cheers

[mrsmelly]

Eco fan ?

[Alan de Enfield]

1 minute ago, DanMax&Belle said:

Nice one Alan!

Is there a 12v option (it's 24) or is it ok to connect this to a 12v supply and get less performance, 20 cubif feet per min would be fine by me esp if it was less than a watt draw

Cheers

The specification states :

 

Current Rating (Amps)                                                                                                                     0.073A

Voltage Range	10 ~ 27.6VDC

Edited January 3, 2020 by Alan de Enfield

[DanMax&Belle]

Just now, mrsmelly said:

Eco fan ?

erm ok

Just now, Alan de Enfield said:

The specification states :

 

Voltage Range	10 ~ 27.6VDC

lovely stuff - so would we be safe to assume it would draw less than the 1.8 watts and deliver less than 41 CF on a 12v supply? 

[Alan de Enfield]

2 minutes ago, DanMax&Belle said:

erm ok

lovely stuff - so would we be safe to assume it would draw less than the 1.8 watts and deliver less than 41 CF on a 12v supply? 

Watts is a 'fixed' number.

 

W = A x V

 

So for example (to make things easy) 

at 12v, 12 watts draws 1 amp

at 24v, 12 watts draws 0.5 amps

 

So running a 24v appliance at 12v will double the current but the watts remain the same.

The spec says it is suitable to run at 12 v so motor heat generation should not be a problem.

 

 

[DanMax&Belle]

1 minute ago, Alan de Enfield said:

Watts is a 'fixed' number.

 

W = A x V

 

So for example (to make things easy) 

at 12v, 12 watts draws 1 amp

at 24v, 12 watts draws 0.5 amps

 

So running a 24v appliance at 12v will double the current but the watts remain the same.

The spec says it is suitable to run at 12 v so motor heat generation should not be a problem.

 

 

hitting new highs Alan - thanks again. Dont suppose you know if this model is sold on ebay? Digikey wants to charge additional delivery etc, not used them before so trying to avoid setting up a new account etc....

I did actually find it on ebay but it's coming from Germany so prob makes sense to order it from Digikey...

Cheers again

 

https://www.ebay.co.uk/itm/Fan-Dc-Axial-24VDC-80x80x25mm-69-7m3-H-33dBA-Sliding-EE80252S1-000U-A99-Dc/383338364613?hash=item5940bf3ec5:m:mg4XdvwAj-8cbFFfPWOXlxA

 

[MtB]

14 minutes ago, Alan de Enfield said:

So running a 24v appliance at 12v will double the current but the watts remain the same.

 

No it won't.

 

The resistance of the appliance  will remain the same so with half the voltage, the current will halve too. And as W = V x A, the wattage running it at 12V will reduce to 25% of the wattage running it at 24V.

 

 

[Detling]

The air flow resistance of the ducting is the bad boy here, the figures for air flow quoted by fan makers are in free air with no restriction. 6 meters of ducting will probably reduce it by a factor of 4.

[MtB]

9 minutes ago, Detling said:

The air flow resistance of the ducting is the bad boy here, the figures for air flow quoted by fan makers are in free air with no restriction. 6 meters of ducting will probably reduce it by a factor of 4.

I was leaving that for later! 
 

I’d say more like a factor of 40.... 

[dmr]

1 hour ago, Mike the Boilerman said:

 

No it won't.

 

The resistance of the appliance  will remain the same so with half the voltage, the current will halve too. And as W = V x A, the wattage running it at 12V will reduce to 25% of the wattage running it at 24V.

 

 

Computer fans are brushless and many have a speed control wire so this infers that they have slectronicky bits inside them. They may well run at a constant speed regardless of voltage, but unless you can infer something from the data sheet the only option is to buy one and see how it performs.

And doing a quick experiment with a bit of ducting before drilling big holes in bulkheads would be a good idea.  Computer fans are pathetic air-flow devices designed to move a very small amount of air with not too much noise.

 

...............Dave

[Boater Sam]

1 hour ago, Alan de Enfield said:

 

 

So running a 24v appliance at 12v will double the current but the watts remain the same.

Sorry, that's where it goes wrong, running at half the voltage on a resistive load will halve the current passed, it will be slightly different for an inductive load.

It was all OK up to there.

  Computer fans are stepper motors, should run at the same speed within the voltage band specified but the power may be reduced by the ducting resistance.

 

Crossed with  Dave, same comment.

Edited January 3, 2020 by Boater Sam

[nicknorman]

2 minutes ago, Boater Sam said:

Sorry, that's where it goes wrong, running at half the voltage on a resistive load will halve the current passed, it will be slightly different for an inductive load.

It was all OK up to there.

But it probably isn’t a resistive load, nor an inductive load. It is likely to be a brushless motor and thus the speed is controlled by electronics. So quite probably it is a constant power device (constant power within the specified voltage range, which is quite wide). As per DMR’s post.

[Boater Sam]

1 minute ago, nicknorman said:

But it probably isn’t a resistive load, nor an inductive load. It is likely to be a brushless motor and thus the speed is controlled by electronics. So quite probably it is a constant power device (constant power within the specified voltage range, which is quite wide). As per DMR’s post.

I said that in edit. Resistive or inductive, its one or the other or both!

[dmr]

Even on a bog standard DC motor its not a case of simple resistance or inductance. The motor will run at a speed mostly dictated by the voltage. The motor is like a generator and generates an internal voltage proportional to speed. The difference between the applied voltage and this internal "back emf" dictates the current taken. At increased voltage the speed is higher but the current draw will reflect the extra energy required to handle the increased air flow.

 

..............Dave

[nicknorman]

4 minutes ago, Boater Sam said:

I said that in edit. Resistive or inductive, its one or the other or both!

Do you know how a brushless motor works? Resistive /inductive (or even capacitive) is all about the power factor. But you can’t model the effects of the electronics switching the various windings on and off, by means of a simple power factored model. It is non-linear and time-variant.

[Boater Sam]

2 minutes ago, dmr said:

Even on a bog standard DC motor its not a case of simple resistance or inductance. The motor will run at a speed mostly dictated by the voltage. The motor is like a generator and generates an internal voltage proportional to speed. The difference between the applied voltage and this internal "back emf" dictates the current taken. At increased voltage the speed is higher but the current draw will reflect the extra energy required to handle the increased air flow.

 

..............Dave

You are correct. The back EMF is a function of speed and resistance of the windings in a DC motor. The back EMF reduces the current flowing and reaches equilibrium at a certain speed and load.

These stepper motors are a bit different, its a sequential switching of the DC voltage to successive windings on the stator, the rotor is a permanent magnet. As the voltage is switched by semiconductor junctions, the back EMF has no return current path hence the speed is controlled by the speed of switching alone.

 

This is why cranking an engine on a flat battery actually cases more current to flow in the starter motor and can burn it out, a point not understood by most. (especially farmers!)

[Bod]

1 hour ago, Mike the Boilerman said:

I was leaving that for later! 
 

I’d say more like a factor of 40.... 

On this idea of ducting air about, would it be best to have the fan pushing air into the ducting, or pulling it out?

 

Bod

[Alan de Enfield]

10 minutes ago, Bod said:

On this idea of ducting air about, would it be best to have the fan pushing air into the ducting, or pulling it out?

 

Bod

I have an 8Kw 'blown-air' system with 6 outlet vents. It is certainly 'blown' air rather than 'sucked' air, but the 'problem' is that it is 115w 

 

 

 

Edited January 3, 2020 by Alan de Enfield

[nicknorman]

53 minutes ago, Bod said:

On this idea of ducting air about, would it be best to have the fan pushing air into the ducting, or pulling it out?

 

Bod

Pushing I think. If you pull (suck) you can cause a pressure drop which reduces the air density near the fan inlet and thus the fan moves less mass of air.

[blackrose]

 

I think the whole fans and ducting idea might be a waste of time. What you want is a stove with a backboiler and rads set up with a thermocycling CH system. More efficient heat distribution and no continuous load on the batteries at all.