A hair dryer, or blow dryer, is a device that uses electric heat and forced convection to apply hot air to a mass of hair (usually on a human head) for the purposes of quickly drying it. Hair holds moisture well and takes a long time to dry on its own, unless assisted with a towel or a hair dryer.
Hair dryers are available in two basic designs with a great deal of variation seen in each one. The first, and older, design is a slightly cone-shaped hood attached to the back of a chair with a hinge or pivot, designed to fit over someone's head without making contact. Typically they will have a level switch, such as a mercury switch, which turns on the heater and fan when it is lowered over the head. These large and expensive machines are rarely seen outside of professional beauty salons, and are increasingly rare even there. The handheld design is much more popular.
The handheld hair dryer uses the same basic concept, an electric heater and fan, in a much smaller, cheaper, lightweight package. Although it does not dry the entire head at once, this is its major appeal. Using a smaller nozzle gives more control over the drying area, and varying the distance at which it is used helps control the temperature and force of the drying air. Also, by not covering the head, it allows access to the hair for brushing and shaping during the drying process. The standard design uses a pistol grip with one or two switches on the handle to control temperature and fan speed.
Regardless of design, the basic principle is the same. Electricity is passed through a NiChrome wire, which heats the wire and thus the air around it. A fan behind the heater blows the hot air across the hair to facilitate drying, aiding evaporation. Brushing or combing the hair during this process also helps by increasing the affected surface area.
The heating wire has a low resistance so that a high current passes through it, which causes the heating, rated in Watts. The lower the wire's resistance, the higher the current and the more power is heating the wire. A 120 Volt, 1875 Watt hair dryer uses a 7.5 Ohm NiChrome wire to pull 15.5 Amps of current – enough to trip a standard 15A circuit breaker. The National Electrical Code requires 20A circuit breakers to be installed in bathrooms, where hair dryers are most likely to be used. The typical range for hair dryers available today is about 1200 Watts to 1875 Watts, although there is concern that higher powered hair dryers could cause damage to the hair and scalp. Temperature can be controlled with a switch which adds or removes more resistive wires to the circuit.
The chair mounted hair dryers are large and self-supporting, so they can use heavy AC motors to drive their fans. Handheld hair dryers must be lighter and use much smaller DC motors. Very small DC motors, however, complicate the process in two ways: the hair dryer is powered by a wall outlet, which is AC power and at a much higher voltage than the small DC motor requires. To reduce the voltage, the motor is put in the circuit with the resistive heating elements, which bleed off much of the voltage before it gets to the motor. Small diodes provide the AC to DC conversion. It should be noted that since the motor is in the circuit with the heating elements, adding more resistance to lower the temperature also lowers the available voltage to the fan motor, slowing it down.
Hair dryers can be dangerous because of the very hot resistive heating elements, the fan blades, and the high levels of current they require to operate, so a number of safety devices are built in to modern designs to make them safer. The heating wires are protected by a screen of some sort that will allow air to pass out but keep foreign objects, such as fingers, from coming into contact with the heating elements. Additionally, a thermal fuse is installed with the heating elements that will automatically cut off the power if the temperature gets too high. The fan motor which blows the hot air out of the hair dryer is protected by a screen or filter to prevent anything from clogging or becoming entangled in the fan blades. Finally, the latest models have a ground fault circuit interruptor unit built right in to the power cable so that any ground faults, such as those caused by dropping the hair dryer into the bathtub, will quickly shut off the power.
Even with these safety features, it is best not to press your luck. I understand that an episode of the popular Discovery Channel series MythBusters recently confirmed that a hair dryer is capable of killing a person if dropped into a bathtub if the GFCI protection is missing, not working properly, or slow to cut off the power. Never use a hair dryer in or near standing water.
A hair dryer is very similar to a heat gun, except that a heat gun operates at a dangerously high temperature and is not designed to be used on a person. Heat guns are typically used for heat shrinking things such as heatshrink tubing.