How Hair Dryers Work


Wintery morning commutes would be a lot more unpleasant if hair dryers weren't around.
Wintery morning commutes would be a lot more unpleasant if hair dryers weren't around.
Razmaz/Stone+/Getty Images

Many people are familiar with the daily routine of washing, drying, and styling their hair. Although hair will eventually dry on its own if given enough time, most people reach for a hair dryer to speed up the process. While science may have disproven the link between wet heads and catching colds, it's still no fun to sit around with a head full of wet hair, especially in the winter.

Hair dryers, also known as blow dryers, were first sold in the 1920s. At first they were pretty dangerous to use -- hundreds of people were electrocuted when they dropped their hair dryer into water-filled sinks and bathtubs.

That isn't as likely today, however, because of the advent of Ground Fault Circuit Interrupters (GFCI). Since 1991, all portable hair dryers have been required by U.S. federal law to protect you against electrocution should you accidentally drop one in water while it's plugged in [source: CSPC]. This applies whether the hair dryer is on or off. A GFCI is the larger, polarized plug that you'll find on many consumer appliances. When they're plugged in, GFCIs monitor the amount of current that's running from one slot of a wall outlet through an electric circuit and back to the other slot. If they sense a leak in the current, they trip the circuit. (See How does a GFCI outlet work? for details).

What happens to a hair dryer if you drop it in water when it's not plugged in? You don't run the risk of electrocution, since there's no source of current, but you can certainly damage the hair dryer if all of its components get wet. So, plugged in or not, it's a bad idea to throw it in the tub.

Have you ever wondered what those components are inside a hair dryer that lets you blow hot air through your hair without burning your scalp? Keep reading to find out why you won't sizzle.­

 

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Hair Dryer Basics

Inside a hair dryer. Motor-driven fan (left) and heating element (right).
Inside a hair dryer. Motor-driven fan (left) and heating element (right).

You can find a hair dryer like this one in almost any drug or discount store. Basic models have two switches, one to turn them on and off and one to control the rate of airflow. Some models have an extra switch that also lets you regulate the temperature of the airflow.

The hair dryer dries your hair by speeding up the evaporation of water from the hair's surface. The hot air emitted from a hair dryer increases the temperature of the air surrounding each strand of hair. Since warm air can contain more moisture than air at room temperature, more water can move from your hair into the air. The increase in temperature also makes it easier for the individual molecules in a water droplet to overcome their attraction to one another and move from a liquid to a gas state.

Since they were first developed, thousands of patents have been issued for different hair dryer designs, but most of them only tweak the outside packaging of the hairdryer so that it looks more aesthetically appealing to you. Aside from the addition of some safety features, the operating system inside hair dryers hasn't changed too much over the years.

A hair dryer needs only two parts to generate the blast of hot air that dries your hair:

  • a simple motor-driven fan
  • a heating element

Hair dryers use the motor-driven fan and the heating element to transform electric energy into convective heat. The whole mechanism is really simple:

  1. When you plug in the hair dryer and turn the switch to "on," current flows through the hair dryer.
  2. The circuit first supplies power to the heating element. In most hair dryers, this is a bare, coiled wire, but in models that are more expensive there can be fancier materials in action, like a tourmaline-infused ceramic coating.
  3. The current then makes the small electric motor spin, which turns the fan.
  4. The airflow generated by the fan is directed down the barrel of the hairdryer, over and through the heating element.
  5. As the air flows over and through the heated element, the generated heat warms the air by forced convection.
  6. The hot air streams out the end of the barrel.

Now that we've got the heat, read on to find out how the hair dryer gets that heat moving.

Hair Dryer Air Flow

The small black fan sits atop the motor. The motor spins the fan. Air is drawn in through the openings on the side of the hair dryer.
The small black fan sits atop the motor. The motor spins the fan. Air is drawn in through the openings on the side of the hair dryer.

How does a hair dryer generate such a strong gust of air in the first place? This model uses a small fan that looks like a hydraulic turbine (i.e. water wheel). Unlike the water wheel, which harnesses the potential energy of flowing water to generate power, the fan in a hair dryer uses electrical energy to generate airflow. The small motor actually sits inside the fan, which is firmly attached to the tip of the motor. When you supply power to the motor, the motor and the attached fan both spin. The centrifugal movement of the fan blades draws air in through the small round air inlets in the side casing of the hair dryer. These holes are covered by a safety screen that prevents other objects (such as strands of your hair) from being sucked in as well. The air is then blown down the barrel of the hair dryer.

Most hair dryers (including this one) have high and low airflow settings. You'll see this referred to in the manual accompanying the hair dryer as high or low speed, because changing the airflow involves modulating the speed at which the motor is turning. This is accomplished very simply by altering the current flowing through the part of the circuit feeding the motor. When the power supplied is low, the motor and the fan spin slowly. Less air is pushed through the hair dryer. With more power, the motor speeds up. The fan rotates rapidly, drawing in more air and increasing the airflow.

Newer and more expensive hair dryers have systems which don't just deliver hot air, they deliver hot air full of ions in the form of charged particles. The companies utilizing this technology claim it's supposed to help hair dry more quickly, lose the static and become healthier, smoother and shinier. Decreasing static is said to help hair be more manageable and less prone to attracting dirt and dust. Ion generators can manifest in a number of different design schematics and be located in a variety of points within the hair dryer.

Things will really start to heat up on the next page when we take a closer look at how these gusty little appliances actually dry our hair.

Heating a Hair Dryer

The heating element is composed of coiled nichrome wire on an insulating board.

The heating element in most hair dryers is a bare, coiled nichrome wire that's wrapped around insulating mica boards.

Nichrome wire is an alloy of two metals, nickel and chromium. This alloy is used in heating elements in a number of household products, from curling irons to toasters. Nichrome wire has two features that make it a good producer of heat:

  • It's a poor conductor of electricity compared to something like copper wire. This gives the alloy enough resistance to get hot from all of the current flowing into it.
  • It doesn't oxidize when heated. Other metals like iron rust pretty quickly at the temperatures used in toasters and hair dryers.

The airflow generated by the fan is forced through the heating element by the shape of the hair dryer casing. When the air initially enters the barrel, it is much cooler than the nichrome wire, so heat flows from the wire to the air. As the air is pushed along by the fan and convection, it is replaced by cooler air and the cycle is repeated.

How hot the air coming out of the dryer can get depends on:

  • The power supplied to the heating element. The higher the wattage, the more heat is generated by the heating element and transferred to the air. Early hair dryers put out only about 100 watts of heat, but nowadays hair dryers can produce up to about 2,000 watts, drying hair considerably faster [source: Schueller].Hair dryers that offer high heat and low heat settings vary the power supplied in order to modulate the temperature of the airflow. These models are wired so that you can flip a switch and cut off part of the circuit that feeds the heating element.
  • The time the air spends in the barrel of the dryer being heated by the nichrome wire. Most hair dryers limit this to approximately one-half second to prevent the air temperature from getting too high.

Something seen more often these days are hair dryers with a ceramic coating on the heating element. Coming in a variety of different configurations, ceramic-coated heating elements are said to heat more evenly and effectively. It's also popular to infuse the ceramic with materials such as crushed tourmaline, which is said to support the creation of ions and ideal heat flow.

Continue to the next page to learn about all the safety features that are built into hair dryers and how they function to prevent dangerous mishaps like overheating and fires.

Hair Dryer Safety

The heat shield keeps the barrel of the hair dryer from becoming too hot.
The heat shield keeps the barrel of the hair dryer from becoming too hot.

The basic idea behind hair dryers is pretty simple, but producing one for mass consumption requires some hard thinking about safety features. Manufacturers have to predict how their hair dryer might be misused. They then try to design a product that will be safe in the widest variety of conditions.

Besides the ground fault circuit interrupters we learned about on the first page, here are some other safety features hair dryers commonly have:

  • Safety cut-off switch - Your scalp can be burned by temperatures more than 140 degrees Fahrenheit (approximately 60 degrees Celsius) [source: Hardin]. To ensure that the air coming out of the barrel never nears this temperature, hair dryers have some type of heat sensor that trips the circuit and shuts off the motor when the temperature rises too much. This hair dryer and many others rely on a simple bimetallic strip as a cut off switch.
  • Bimetallic strip - Made out of sheets of two metals, both expand when heated but at different rates. When the temperature rises inside the hair dryer, the strip heats up and bends because one metal sheet has grown larger than the other. When it reaches a certain point, it trips a switch that cuts off power to the hair dryer. (For more information on bimetallic strips, see How Thermometers Work).
  • Thermal fuse - For further protection against overheating and catching fire, there is often a thermal fuse included in the heating element circuit. This fuse will blow and break the circuit if the temperature and current are excessively high.
  • Insulation - Without proper insulation, the outside of the hair dryer would become extremely hot to the touch. If you grabbed it by the barrel after using it, it might seriously burn your hand. To prevent this, hair dryers have a heat shield of insulating material that lines the plastic barrel.
  • Protective screens - When air is drawn into the hair dryer as the fan blades turn, other things outside the hair dryer are also pulled toward the air intake. This is why you'll find a wire screen covering the air holes on either side of the dryer. After you've used a hair dryer for a while, you'll find a large amount of lint building up on the outside of the screen. If this were to build up inside the hair dryer, it would be scorched by the heating element or might even clog the motor itself.Even with this screen in place, you'll need to periodically pick lint off the screen. Too much lint can block the airflow into the dryer, and the hair dryer will overheat with less air carrying away the heat generated by the nichrome coil or other type of heating element. Newer hair dryers have incorporated some technology from the clothes dryer: a removable lint screen that's easier to clean.
  • Front grill - The end of the barrel of a hair dryer is covered by a grill made out of material that can withstand the heat coming from the dryer. This screen makes it difficult for small children (or other especially inquisitive people) to stick their fingers or other objects down the barrel of the dryer, where they could be burned by contact with the heating element.

For more information on other household appliances showing their warm, fuzzy side -- whether by drying your clothes, toasting your bread or starting your car -- get your motor going on the next page with a bunch of interesting links.

Related HowStuffWorks Articles

More Great Links

Sources

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