Does a Hairdryer blow harder with heat

[Jedi_Sawyer]

Does a Hairdryer blow harder with heat on than it does with heat off ? and why?

My feeling is the blowing force, or pressure that the air exhaust will exert on a surface that opposes it will stay the same whether the hairdryer is blowing hot or cold air. That the difference will be that the hot blown air will not be as dense as cold blown air. That would be because the heater which is in the air stream will be slowing down the air coming at it thereby making the air stream being dammed up a little, or raising the air pressure. Not as much air will get past the damn but the air that does get past the heater will have more Kinetic Energy. So since the heat radiates in all direction it lowers the mass/sec output of the hairdryer but the mass that comes out is more energetic.

 

[Drakkith]

Hot air does not have more kinetic energy than cold air. Its temperature is simply higher.
Are you thinking that the heating coil isn't in the way of the airstream when the coil is off?

 

[NascentOxygen]

Does a Hairdryer blow harder with heat on than it does with heat off ? and why?

Probably true. You get about twice as much air out as is sucked in when the drier is switched to HEAT.

Have you tried measuring the force, to see whether the air blast pushes harder with heating?

 

[Jedi_Sawyer]

Hot does mean more kinetic energy

Hot air does mean more kinetic energy than cold air. Heat is energy and the PV = nrT. The only way to have the same blowing force out of a heat gun for cold air as for warm air, is for the cold air to be more dense.

 

[Drakkith]

Probably true. You get about twice as much air out as is sucked in when the drier is switched to HEAT.

If there is any difference to the amount of air blown through the dryer I'd guess it's either because of the power requirements for the heating elements or because it's designed to.

Hot air does mean more kinetic energy than cold air. Heat is energy and the PV = nrT. The only way to have the same blowing force out of a heat gun for cold air as for warm air, is for the cold air to be more dense.

I don't see how the ideal gas law applies here. You're talking about the amount of air getting blown out of a hair dryer, not the kinetic energy of the molecules of the gas.

Honestly I can barely understand your original post. You talk about the heating element being in the way, yet I don't see how that makes any difference between the cold and hot air. I also don't see how radiating heat affects the airstream.

 

[256bits]

Probably true. You get about twice as much air out as is sucked in when the drier is switched to HEAT.

Have you tried measuring the force, to see whether the air blast pushes harder with heating?

Is the output air that hot? I would have thought about 20 to 30% more.

In any case with enough hair dryers and a long enough cord, one should then be able to propel a mini car mimicking propulsion as seen on your regular passenger jet aircraft.

 

[sophiecentaur]

The speed of the air will probably be greater after it leaves the machine if it has been heated by the element by, say 40K. Its volume will have increased by a factor of 340/300 (gas laws) or 13%. The nozzle diameter is the same so this will mean an increase in speed of 13%, or an increase of 28% in Kinetic Energy. This is not a massive difference but would be detectable.
I have noticed a (subjective) air speed increase when I have used a fan heater / hair dryer. It would be easy enough to register an increase by blowing it against a suspended, light object.
Also, the note of the motor always drops when the heater is turned on - whilst this could, in part, be due to a resistive voltage drop in the power lead, I would suggest it's more likely to be because of the extra power being used to supply the extra KE of the blast. (The fan does more work)

 

[kb7wg]

Off. I believe the voltage drop across the blower motor would effect air volume more than air density. I would assume the heating element draws several times the current of the blower motor. The voltage drop across the power cord will increase and voltage to motor will decrease. The blower will slow down.

 

[jbriggs444]

Also, the note of the motor always drops when the heater is turned on - whilst this could, in part, be due to a resistive voltage drop in the power lead, I would suggest it's more likely to be because of the extra power being used to supply the extra KE of the blast. (The fan does more work)

This is almost certainly incorrect. If you increase the pressure load on a fan, the motor does not slow down. It speeds up. This is because it passes a lower volume of air. You can hear this effect very clearly if you block the end of your vacuum cleaner hose.

 

[sophiecentaur]

Off. I believe the voltage drop across the blower motor would effect air volume more than air density. I would assume the heating element draws several times the current of the blower motor. The voltage drop across the power cord will increase and voltage to motor will decrease. The blower will slow down.

Density = Mass/Volume
How could you affect one without affecting the other, if the mass is constant?

It would not be difficult to measure any voltage change. I might just do that.
I think my 13% figure for 40K temperature increase must be correct so the motor will certainly have a greater mechanical load on it.

Whenever you block the input flow of air to those shaded pole motors, they always speed up noticeably, so they are very load sensitive (i.e. with the inlet blocked, they can't be shifting as much air. What they actually do is a bit counter intuitive, I know, but you should try it.

 

[sophiecentaur]

This is almost certainly incorrect. If you increase the pressure load on a fan, the motor does not slow down. It speeds up. This is because it passes a lower volume of air. You can hear this effect very clearly if you block the end of your vacuum cleaner hose.

See my post above. The air flow through fan is different from a normal 'braking load'. If you take a fan motor and give it an extra mechanical load, it will slow down (as do all motors) and not speed up. But, by starving a fan of air to work on, it cannot do as much work at its normal speed and so it speeds up to a new equilibrium situation. You will notice that you get no rise in pitch of you try to block the output of a fan because there is air available for the fan to work on.
Not intuitive, I know, but an experiment will prove it to be right.

 

[jbriggs444]

See my post above. The air flow through fan is different from a normal 'braking load'. If you take a fan motor and give it an extra mechanical load, it will slow down (as do all motors) and not speed up. But, by starving a fan of air to work on, it cannot do as much work at its normal speed and so it speeds up to a new equilibrium situation. You will notice that you get no rise in pitch of you try to block the output of a fan because there is air available for the fan to work on.
Not intuitive, I know, but an experiment will prove it to be right.

I understand that air flow thruogh a fan is different from a normal braking load. That is rather the point. Starving a fan of input air (decreasing the pressure of its input) is identical in this respect to increasing the pressure of its output. The relevant effect is to increase the pressure gradient against which the fan works.

As we both agree, if you increase this pressure gradient, you reduce the air flow and increase the fan blade speed.

Accordingly, I predict that you will get an increase in pitch when you block the output.

 

[sophiecentaur]

I understand that air flow thruogh a fan is different from a normal braking load. That is rather the point. Starving a fan of input air (decreasing the pressure of its input) is identical in this respect to increasing the pressure of its output. The relevant effect is to increase the pressure gradient against which the fan works.

As we both agree, if you increase this pressure gradient, you reduce the air flow and increase the fan blade speed.

Accordingly, I predict that you will get an increase in pitch when you block the output.

If it were as simple as that, the greater the pressure difference, the more work the fan would do and it would slow down - as it will with an applied braking load. So it cannot be as simple as that. There is some sophisticated fluid flow going on and it's not like a simple piston pump, working between two reservoirs at different pressures.
My point is that the air which gets to the fan needs to be flowing past it at an appropriate speed and direction for the fan to deliver the appropriate amount of energy to it. When you block the inlet, air circulates round, from exit side to inlet side because new air cannot get there. I guess this will mean that the air will follow a helical / circular path, going faster than it would usually. Under normal circumstances, the fresh air would be arriving parallel to the axis and would need to be accelerated by the fan, taking more power.

In any case, I would need a lot of convincing that the motor actually increases its speed under a load.

And my point about the greater speed for hot air exiting has to be correct, I think. There's just more volume passing through per second.

I blocked both input and output of our vacuum cleaner and the inlet block made the predicted speed increase. Not so, the outlet. This would be because the work situation is different and the outlet air flow is actually directed by the blades but the flow to the back of the blades is just due to a general pressure difference. There's no reason why the situation would be symmetrical.

 

[jbriggs444]

If it were as simple as that, the greater the pressure difference, the more work the fan would do and it would slow down - as it will with an applied braking load. So it cannot be as simple as that.

You are arguing with a straw man.

The minimum power requirements are given by pressure gradient multiplied by flow rate. If you reduce the flow rate by increasing the pressure gradient, the effect on the flow rate may result in either an increase or a decrease in power.

We agree on this.

There is some sophisticated fluid flow going on and it's not like a simple piston pump, working between two reservoirs at different pressures.

Again, you are arguing with a straw man. We are dealing with a shrouded fan. A shrouded fan, as you say, does not behave like a piston pump.

We agree on this.

My point is that the air which gets to the fan needs to be flowing past it at an appropriate speed and direction for the fan to deliver the appropriate amount of energy to it. When you block the inlet, air circulates round, from exit side to inlet side because new air cannot get there.

Yes. If the shroud on the fan is not tight, you get ambient pressure outlet air leaking back to the inlet.

For a given pressure gradient, this bypass path will work to increase fan load and reduce motor speed. The tighter the shroud, the less bypass, the less resulting load and the higher the motor speed until, in the limit, the fan is simply free-wheeling, spinning some air in place with essentially zero power requirement.

This fact works against the point that I thought you were trying to make. So perhaps I have misunderstood what you were trying to explain.

In any case, I would need a lot of convincing that the motor actually increases its speed under a load.

Another straw man. The motor increases its speed as its load decreases. The load presented to the motor by the fan decreases as the air flow rate decreases. The air flow rate decreases when you put your hand over the inlet.

We agree on this.

I claim that the flow rate decreases because of the increased pressure gradient that results from your hand. Possibly you disagree with that.

I blocked both input and output of our vacuum cleaner and the inlet block made the predicted speed increase. Not so, the outlet. This would be because the work situation is different and the outlet air flow is actually directed by the blades but the flow to the back of the blades is just due to a general pressure difference. There's no reason why the situation would be symmetrical.

For an incompressible fluid (and air is largely incompressible at the pressure ranges we are considering) the situation is indeed symmetrical. Can you explain better what asymmetries you see in light of this apparent symmetry?

 

[kb7wg]

Air temperature and air density have nothing to do with this question. The heating element is after the fan blower. The air that the fan is moving is at constant temp. and pressure. The only thing that can effect fan speed is motor voltage. Anyone whose has worked around electric motors very long can tell. When you turn high heat on...you can hear the speed go down.

 

[Jedi_Sawyer]

And my point about the greater speed for hot air exiting has to be correct, I think. There's just more volume passing through per second..

I agree that the hot air is traveling faster from a hair dryer than cold air for any given fan setting. The part about more volume/sec passing through is mysterious as to what that means. I think he is saying that the hair dryer should blow harder when it is pushing hot air than cold air and I don't think it does. The experimental evidence is that it does not blow harder see You Tube video

I blocked both input and output of our vacuum cleaner and the inlet block made the predicted speed increase. Not so, the outlet. This would be because the work situation is different and the outlet air flow is actually directed by the blades but the flow to the back of the blades is just due to a general pressure difference. There's no reason why the situation would be symmetrical.

I believe that this is a correct analysis for the vacuum situation. However the hairdryer changing pitch is different. I think that the change in pitch adding heat is due to two effects. First there is line loss and it is not much. I measured 119.5VAC no heat to 117.9VAC with heat. The heater fan itself is DC and is 12 or 24 V depending on speed setting and it would also be subject to the AC changing unless they do expensive regulation I am sure they don't. The other more important effect is that the load increased due to the air damming effect of heat being added I suspetct

 

[sophiecentaur]

I agree that the hot air is traveling faster from a hair dryer than cold air for any given fan setting. The part about more volume/sec passing through is mysterious as to what that means. I think he is saying that the hair dryer should blow harder when it is pushing hot air than cold air and I don't think it does. The experimental evidence is that it does not blow harder see You Tube video



I believe that this is a correct analysis for the vacuum situation. However the hairdryer changing pitch is different. I think that the change in pitch adding heat is due to two effects. First there is line loss and it is not much. I measured 119.5VAC no heat to 117.9VAC with heat. The heater fan itself is DC and is 12 or 24 V depending on speed setting and it would also be subject to the AC changing unless they do expensive regulation I am sure they don't. The other more important effect is that the load increased due to the air damming effect of heat being added I suspetct

Yes. The position of the fan vs the element is not too relevant. The air expands as it goes over the element and that will increase the pressure that the fan has to work into, one way or another - hence the load.

Perhaps we should be talking in terms of momentum when considering the effect in the movie. The same mass of air is being ejected per second and the fan could be imparting the same amount of momentum per second (well - I think so) so the same amount of momentum is expended on the sheet in the movie per second. That sort of explains that result.
However, when you blow a hairdryer into your face and turn the heating on, it 'feels' stronger. (Whatever that means in objective terms.)

 

[nsaspook]

If there is any difference to the amount of air blown through the dryer I'd guess it's either because of the power requirements for the heating elements or because it's designed to.

Yes, the device is usually designed to increase the fan speed with increasing heat to prevent over-temperature.

A typical example:
http://img.docstoccdn.com/thumb/orig/41771342.png

 

[sophiecentaur]

For many decades, blown heaters have had independent switching. The only design feature has been to ensure the fan is on whilst an element is in operation. That attachment does not describe common practice.

 

[256bits]

Not the usual design, maybe for a higher priced model
Many and most models have temperature control separate from speed control.

( I see Sophiecentaur has already explained common practice )

 

[256bits]

The other more important effect is that the load increased due to the air damming effect of heat being added I suspetct

What is an air damming effect?

The blow dryer operates on basic principles of the First Law - Energy In = Energy Out, and the ideal gas equation - PV = nRT.

If the temperature of the air is increased then either the pressure or the volume must increase, or both. What do you think is happening here, considering that the heat input is adding to the internal energy of the air?

PS. We should also add change in momentum.

 

[nsaspook]

For many decades, blown heaters have had independent switching. The only design feature has been to ensure the fan is on whilst an element is in operation. That attachment does not describe common practice.

Well, the OP was talking about generic 'hairdryers' not industrial heat guns. There are UL limits to the exit and body temperatures of a 'hairdryer' so the 'typical' design is to increase airflow with increasing heater power to help maintain the safe temperature range with the fan speed decreasing with lower power to maintain proper drying temperature. This is usually done by putting the fan motor in series with the heater element in some way on cheaper models or with a thermostat/TC and electronics (microcontroller) in fancy ones. The 'no heat' fan speed could be set to just about any usable value.

From UL 859

37.5.1 A hand-supported hair dryer is to be tested as follows. With the adjustable temperature control, if
any, set for the most severe condition of use, the dryer is to be operated continuously until stabilized
conditions are achieved. During this operating time the dryer is to be supported in the position
representing the most severe conditions of use, first without any attachment on the heated air outlet
nozzle, and then, if the dryer is provided with one or more attachments for the heated air outlet, in turn
with each attachment in place, as intended. During each of these tests the plane of the thermocouple grid
specified in 37.5.2 is to be positioned 1 inch (25.4 mm) from the plane of the heated air outlet of:
a) The dryer nozzle or
b) The attachment nozzle.
The center of the air stream is to be directed at the center of the grid. Temperatures are to be measured
throughout the test. There shall not be a temperature rise greater than the limits specified in Tables 37.1
and 37.2, nor greater than 100°C (180°F) for the average of the five highest thermocouple readings on
the grid described in 37.5.2.

 

[256bits]

Well, the OP was talking about generic 'hairdryers' not industrial heat guns. There are UL limits to the exit and body temperatures of a 'hairdryer' so the 'typical' design is to increase airflow with increasing heater power to help maintain the safe temperature range with the fan speed decreasing with lower power to maintain proper drying temperature. This is usually done by putting the fan motor in series with the heater element in some way on cheaper models or with a thermostat/TC and electronics (microcontroller) in fancy ones. The 'no heat' fan speed could be set to just about any usable value.

From UL 859

The exit temperature is limited to about 60C so the scalp does not burn. A bimetal strip in the exit stream will cutoff the electrical supply after a certain temperature is reached. In addition, for added protection, a thermal fuse is included. That satisfies any UL stipulations.

One does not put the motor in series with the heating element.

 

[256bits]

the following is a typical hair dryer circuit

http://www.tutorsglobe.com/CMSImages/2045_hair%20Dryer%20Homework%20Help%204.jpg

http://www.tutorsglobe.com/CMSImages/2045_hair%20Dryer%20Homework%20Help%204.jpg

 

[nsaspook]

The exit temperature is limited to about 60C so the scalp does not burn. A bimetal strip in the exit stream will cutoff the electrical supply after a certain temperature is reached. In addition, for added protection, a thermal fuse is included. That satisfies any UL stipulations.

One does not put the motor in series with the heating element.

It depends on the design. With the consumer market DC motor design commonly seen today some of the heater element is used as a voltage divider for the DC motor in either a series or series parallel element with a tap. How much or if the speed changes with heating depends on the design. Some use a low power element string to supply the motor, some tap off the whole element at a low resistance point.

https://www.youtube.com/watch?v=Vq7EOmvU1eQ&feature=player_embedded#t=547
http://uk.answers.yahoo.com/question/index?qid=20090327055413AAwA9sF

My point is that speed and heat may interact in ways other than ON/OFF in even a simple 'hairdryer'.

 

[256bits]

nsaspook,
OK, Now I understand what you are getting at with the series and parallel taps.
I had something other envisioned in error.

Original inquiry would have to be about just adding heat to an airflow.

 

[Jedi_Sawyer]

There has been a lot of speculation about what the temperature of the hairdryer is doing so I thought I would make measurements for the modified hairdryer mentioned in post 16 of this thread I measured the temperature by placing a thermocouple about a quarter inch away from outlet grate of the hairdryer and this is what I measured.
Modified Conair hairdryer low speed Low fan Temp ≈578 F 303 C
High fan Temp ≈315 F 157 C

I did the same test on a new unmodified hairdryer to see what it did. It was obviously doing some sort of temperature control and the temperature was jumping all over the place. Throwing away outrider indications these are my measurements.

Conair 1875 Low fan high heat 156 – 208 F 69 – 98 C
High Fan high heat 128-285 F
Low Fan low heat 102 -123 F
High Fan low heat 130 – 180 F

 

[Jedi_Sawyer]

What is the air damming effect

What is an air damming effect?

The blow dryer operates on basic principles of the First Law - Energy In = Energy Out, and the ideal gas equation - PV = nRT.

If the temperature of the air is increased then either the pressure or the volume must increase, or both. What do you think is happening here, considering that the heat input is adding to the internal energy of the air?

PS. We should also add change in momentum.

This post is getting to the heart of the question of this thread. The air damming effect basically means that turning the heat on in the hair dryer causes the pressure to build up in front of the fan and causes the air mass transfer to decrease for the fan. That is the heater is blocking the airflow and the air that is being reflected back at the fan is more energetic when the heater element is on, so it will cause more momentum from the fan to be canceled out.

I wanted to explain why heating the blowing air was not creating more momentum for the hairdryers heated air output which is what I experimentally got when I tested for it, see http://api.viglink.com/api/click?fo...=P_ZcpC2Tlk8&jsonp=vglnk_jsonp_13906829808369

Your comment that momentum should increase also was what I thought when I was trying to do another much more ambitious experiment, and was surprised when it does not seem to be true.

 

[klimatos]

Hot air does not have more kinetic energy than cold air. Its temperature is simply higher.

This is true on a "per unit of volume" basis but not on a "per unit of mass basis". The mean kinetic energy of translation of gas molecules at gas temperature 65°C is higher than that of the same gas molecules at a gas temperature of 25°.

Having read your posts in the past, I'm sure that you agree.

 

[sophiecentaur]

This post is getting to the heart of the question of this thread. The air damming effect basically means that turning the heat on in the hair dryer causes the pressure to build up in front of the fan and causes the air mass transfer to decrease for the fan. That is the heater is blocking the airflow and the air that is being reflected back at the fan is more energetic when the heater element is on, so it will cause more momentum from the fan to be canceled out.

I wanted to explain why heating the blowing air was not creating more momentum for the hairdryers heated air output which is what I experimentally got when I tested for it, see http://api.viglink.com/api/click?fo...=P_ZcpC2Tlk8&jsonp=vglnk_jsonp_13906829808369

Your comment that momentum should increase also was what I thought when I was trying to do another much more ambitious experiment, and was surprised when it does not seem to be true.

I was in the car today and I came to that conclusion too. It makes sense and accounts for the change of tone as the fan does more work against the back pressure of the heated air. *
I think we can forget about 'advanced' dryers with fancy temperature control as this effect happened with ancient two speed / two heat models.
So that takes care of the motor speed change. The heater must increase pressure and volume, in some relative proportion but I still don't see why, if a bigger volume of air is leaving the tube, why that doesn't correspond to greater velocity and, hence, more momentum.
* Jet engines work the same way - only the effect is more extreme. In that case, the added energy (chemical burning) definitely produces more thrust than the compressor would, on its own.
I think we are definitely getting somewhere with this now.

 

[Jedi_Sawyer]

klimatos- I think you are wrong. That is the basic definition of temperature, a measure of energy? When it comes to gases, the kinetic energy, 3/2 MV^2, is all the sensible heat you can measure, so for a unit volume if the air feels warmer it means the kinetic energy is higher, period.

 

[Jedi_Sawyer]

So that takes care of the motor speed change. The heater must increase pressure and volume, in some relative proportion but I still don't see why, if a bigger volume of air is leaving the tube, why that doesn't correspond to greater velocity and, hence, more momentum.
* Jet engines work the same way - only the effect is more extreme. In that case, the added energy (chemical burning) definitely produces more thrust than the compressor would, on its own.
I think we are definitely getting somewhere with this now.

I think we should be talking about air molecules leaving the tube/per second, and the velocity of those molecules. So we are talking about the momentum of the hairdryer's exhaust. If we increase the back pressure against the fan, as we assume we are doing when the heater is on, that means the fan is not pushing as much air through the hair dryer as it did when the heater is off. The air that is pushed through the heater is heated up meaning, the kinetic energy of that air was increased so it has a higher velocity. So the exhaust of the hairdryer is (less air) x (traveling faster) = momentum = pressure you can feel = approximately the same momentum as you had before you heated the air.

I am surprised myself how the two effects seem to cancel each other out for momentum from a hairdryer.

 

[sophiecentaur]

Except that the load / speed characteristic of the motor would affect the actual change in mass flow of the air.
You have two engines involved. The Fan drives the air and then the heater is a further energy source so you have a heat engine - it's like a very feeble jet engine in more or less every respect.

I think that the hair dryer result they got is only a special case and it 'just happens' that there was no detectable difference. That could be confusing the wider issue.

 

[256bits]

This post is getting to the heart of the question of this thread. The air damming effect basically means that turning the heat on in the hair dryer causes the pressure to build up in front of the fan and causes the air mass transfer to decrease for the fan. That is the heater is blocking the airflow and the air that is being reflected back at the fan is more energetic when the heater element is on, so it will cause more momentum from the fan to be canceled out.

I wanted to explain why heating the blowing air was not creating more momentum for the hairdryers heated air output which is what I experimentally got when I tested for it, see http://api.viglink.com/api/click?fo...=P_ZcpC2Tlk8&jsonp=vglnk_jsonp_13906829808369

Your comment that momentum should increase also was what I thought when I was trying to do another much more ambitious experiment, and was surprised when it does not seem to be true.

No. I never said that momentum increases as a whole.

Since we seem on the same page.
The air stream is moving to the right, let's say. We then add heat. The volume increases. The exit pressure still is at atmospheric and the pressure from is the fan is also the same. So the stream expands in both directions, towards the exit and towards the fan. So while it has gained momentum towards the exit, gaining velocity, but it has lost momentum towards the fan loosing velocity. The expansion towards the fan gives a slight rise in pressure on the fan side, which becomes what you are calling the the damming effect. The mass flow should then decrease slightly. The lower mass flow together with the higher exit velocity will give the same exit momentum as previously with the heat off.

That is my desciption in layman terms what is happening, except I am not sure sure about the last statement of the exit momentum being the same with heat on and with heat off. I would have thought it would have increased slightly.

 

[nsaspook]

I was in the car today and I came to that conclusion too. It makes sense and accounts for the change of tone as the fan does more work against the back pressure of the heated air. *
I think we can forget about 'advanced' dryers with fancy temperature control as this effect happened with ancient two speed / two heat models.

I just think you have to careful about what's really happening in systems designed to limit or maintain the exit temperature for human comfort when changing fan speeds. Even ancient car two speed / two heat models have air-blending systems adjusting ducting and mixing that could the change back pressure independent of fan speed settings while limiting temperature. The physics of an independent fan and ducted heater system without designed limits is really the subject being discussed (very nicely by others than me) here.

The hairdryer is a good example of engineering using basic physics to increase the heating power:
http://www.engr.mun.ca/~molgaard/courses/1000/dryerlcth.htm