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!

Have you ever wondered what's inside a hair dryer that lets you blow hot air through your hair without burning your scalp? In this edition of, we'll look at how hair dryers work and examine the technology that makes them safe to use.

The Basics
You can find a hair dryer like this one in almost any drug or discount store. This model has two switches, one to turn it 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.

A basic hairdryer

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.

Hair dryers were first sold in the 1920s. Since then, 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 inside of a hair dryer 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:

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

Hair dryers use the motor-driven fan and the heating coil 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 bare, coiled wire of the heating element, which becomes hot.
  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 coil, heat rising from the coil warms the air by forced convection.
  6. The hot air streams out the end of the barrel.

Generating Air Flow
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 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.

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.

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 thus 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.

Two switches regulate whether and how much current flows through the hair dryer circuitry. One (left) regulates the speed of the motor (and thus the airflow) and the other (right) turns the hair dryer on and off.

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

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

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:

  • Nichrome wire is 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.
  • Nichrome does not 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 coil and transferred to the air. The latest models on the market use up to 1875 watts! This concerns some haircare professionals, who worry that the high heat generated by these dryers can damage the hair and the scalp.

    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 coils.

  • 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.

Keeping it Safe
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.

Some common safety features of hair dryers are:

    A bimetallic switch that prevents overheating
  • A safety cut-off switch - Your scalp can be burned by temperatures more than 140 degrees Fahrenheit (approximately 60 degrees Celsius). 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.

    A bimetallic switch is made out of sheets of two metals. Both metals 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).

  • A 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, this hair dryer has a heat shield of insulating material that lines the plastic barrel.

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

    The side screens and front grill keep unwanted objects from getting inside the hair dryer.

  • 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 built 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. 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.

    A GFCI has circuit interrupters that cut off power to the dryer at the outlet should the dryer be dropped in water while it's plugged in.

  • Ground Fault Circuit Interrupt (GFCI) - Since 1991, all portable hair dryers have been required by federal law to protect you against electrocution should you accidentally drop one in water while it's plugged in. 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 are plugged in, GFCIs monitor the amount of current that is 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 question 117 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 is no source of current, but you can certainly damage the hair dryer if all its components get wet. So, plugged in or not, it's a bad idea to throw it in the tub.

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