The engineering of a portable hair dyer.
The problem statement:
Do we need portable hair dryers?
We do not need hair dryers the way we need food, clothes, shelter. We can do without, and people did do without before portable hair dryers came on the market. I have one but I seldom use it.
Is there a problem to be solved? No, no in the same sense as there may be a problem with swing sets tipping, or a problem with garbage thrown on streets before municipal garbage removal was introduced.
What we have here is a response to an opportunity, essentially a business opportunity. The driving force in the development of portable hair dryers has to be the opportunity presented to the entrepreneur who could produce one that people would buy. Why would they buy? For the convenience provided, i.e. life with a hair dryer is easier than without one. Much the same is true of many of our household gadgets and appliances, though in some cases there is more than convenience involved. For instance, the refrigerator and freezer solved the problem of storing perishable food for considerable periods of time, which was only possible to a limited extent before that technology was developed. However, in the case of the portable hair dryer people managed quite well without it before it was developed. Therefor we are dealing here essentially with an opportunity.
What are the general criteria a successful hair dryer has to have as a business opportunity?
It must make hair drying a more convenient and quicker operation than the alternatives.
It must be affordable by the largest number of potential customers.
It must be attractive to the potential customers.
It must be safe to use.
What technical criteria must a hair dryer have to meet these general criteria? i.e. there has to be translation of the opportunity criteria into technical terms.
Convenient hair drying
Compared with letting hair dry naturally in a typical room, we have to produce air at a higher temperature than room temperature and blow it across the hair to remove moisture quicker than natural convection, because warm air can contain more moisture than cold air, or warm air is dryer than cold air, even with the same water content. So an obvious technical requirement is to raise the temperature of the air.
It is not enough to produce air at a certain temperature, it has to move over and through the hair to provide effective removal of moisture. The physics of drying is fairly complex, but we need not concern ourselves here with it. Clearly the faster warm dry air moves over a wet surface the quicker it removes moisture. If we are dealing with a flow of air, i.e. so many cubic metres of air to heat per second, we can work out the rate at which heat has be delivered to it to obtain a certain temperature rise, say from 30 C to 60 C, .the highest water temperature the hand can stand
It is a fairly simple matter to calculate the temperature rise in a volume of air for a given amount of heat put into it.
How would you do this?
This is basically physics and has to do with the specific heat, C, of the air. Specific heat is the amount of heat needed to raise the temperature of a unit mass of the substance by one degree. Heat, mass, and temperature units used will determine the units of specific heat., i.e
temperature change = heat/( mass ×sp.heat)
or mass = heat/(temperature change ×sp.heat)
We will probably need to calculate in terms of rate of flow of air, J, in units of volume per unit time.
As mass = density ×flow rate × time
we have flow rate = mass/(density ×time)
With heat = heating rate × time, and heating rate = power, W, dissipated by the heater, heat = power × time
Q = Wt
flow rate = (power× time)/(temperature change× sp heat × density × time)
= power/(temperature change × sp ht × density)
This allows us to estimate the flow rate we will need to keep the temperate change within limits for a given rate of supply of electrical power. The lower the temperature rise we can tolerate, the higher the air flow rate must be, and the higher the wattage the higher the air velocity must be.
We could also design in terms of a maximum flow rate available and determine the maximum power which can be handled.
The specific heat of air is about 1 J/(g.degC) = 1000 J/(kg.degC)
The density of air depends on the temperature and pressure. We could use the ideal gas equation PV = RT for one mole of a substance to get an estimate of the mass per unit volume of air. The volume of a given mass in inversely proportional to the absolute pressure and proportional to the absolute temperature. Air is a mixture of gases, which complicates the estimation of density.
The pressure of the atmosphere at sea level is about 101 kPa. The temperature we are concerned with is around 60 C, a rise of about 40 degC from room temperature.
An equation for the density is
density in kg/m3 = 1.29P/(101×(1+0.00367 T)) where P is in kPa and T is in C.
At 60C this works out to 1.057 kg/m3
If we want to make full use of the power available from an ordinary household outlet, that is not much more than 1500W. The minimum flow rate the dryer fan must produce to keep the average air temperature below 60C works out to be about 0.047 m3/s or 2.8 m3/min., i.e a volume of air measuring 1m × 1m × 2.8m.
Check this calculation out!
The earliest hand-held hair dryers appeared in 1925, produced no more than 100W heat and were made of steel or zinc, and weighed about 1kg. By 1940 the maximum power was up to 300W, and remained at less than 500W until the 1960's. Why was the power so low? Most likely the capability of the available small electric motors was the limiting technology, rather than the ability to produce heat in a small space. As motor development took place providing greater and greater flow rates, the more heat could be supplied, and competition between manufacturers of hair dryers could have encouraged motor development. We have reached the stage now that the maximum power available from a conventional outlet, 1500W, can be used by a hair dryer, so increasing power or motor speeds is not longer a big issue.
Obviously, a hand held hair dryer must be light enough to be held in the hand and readily directed at all parts of the head from all directions by the person with the wet hair.
Affordability is a relative term. Currently dryers cost from $10-$50. There might well have been a market for $100 dryers at one time. I have no idea at what price the first portable dryers had a sufficient market for commercial production. That threshold may well have been above $100 at today's values, but once a market is established, competition drives development towards ever lower production costs and prices.
I am not going to discuss appearance here, but fashion is important in consumer goods, and changes in time like all other fashion items. Just think of changing styles in radio and electronic cabinets. Not too long ago the preference was for dull chrome, now it is black for cabinets to sit in a living room. Pastel colours were popular for hair dryers, now bolder colours and pure white are probably more popular.
Safety has at least two aspects: Safe as far as the hair and skin is concerned and safe to hold.
Hair and skin safety depends on the air temperature as already mentioned. We have to be concerned not only with the temperature when everything is operating as intended, but also with the temperature reached when unusual situations apply, such as the dryer held so close to the skin that the air flow rate is reduced.
Safe to hold is safety from electrical shock and from heat. Safety from electrical shock comes from using appropriate insulation, and eliminating the possibility of misuse or accident producing a situation in which insulation is destroyed or damaged, or the electrical components break.
Safety from heat means ensuring the housing stays comfortable to hold, doesn't melt, catch fire, or char. All criteria evolve, changing with time and safety criteria will become more stringent as we learn from accidents.
Concern with safety leads to designing so that normal operating conditions are safe, it also means designing so that any unusual conditions are safe. Furthermore, it can mean providing a way of cutting of the power when abnormal conditions occur.
If air is blowing out of a dryer it is also being sucked into it. Therefore we have to guard against hair being sucked in and also consider a possible blockage of the inlet.
We can summarize the task criteria in a technical sense as follows. Some of the numbers are guesses merely used for illustration.
The weight of the hair dryer must not exceed a certain value, say 500g.
The motor and heater cannot use more than 1500 W electrical power.
Include a heater and a fan driven by an electric motor so that the dryer produces a flow of air at a temperature no more than 60 C (or some such temperature). The air flow rate should be as high as possible and the heater power consistent with this.
If the fan slows down or stalls or the air flow is obstructed for any reason the heater power must automatically be cut off.
There must be no possibility of the user comint into electrical contact with the heater or other wiring even when there is a break in the wiring, or a connection is loose.
There must be no likelihood of the housing being damaged. or being uncomfortable to hold due to heat.
The housing must be attractive.
The cost to the consumer must be low. The actual cost to aim for will change over time, but translates into a production cost which may be as low a $10. This can only be achieved through mass production with a great deal of attention to detail, including the ease of assembly. Over time the number of separate parts and fasteners used in a hair dryer has decreased in line with this concern.
Examination of a hair dryer.
My hair dryer consists of a plastic housing. The heater element is a coil of bare wire in the conical outlet portion through which an electric fan or blower delivers the air. There is a simple white heat shield which goes between the heater and the plastic housing. You can see the switch which controls both the heater and the fan, low heat goes with a low fan speed, and high heat with a high fan speed.
There are also safety devices which are not easy to see, primarily a bi-metallic switch to cut the power if the air gets too hot.
The basic choice in portable hair dryer design seems to have been mainly between putting some kind of hood over the head, or just blowing warm air at it. Both techniques have been used in hair dressing salons and have also been employed for hair dryers to be used at home. The blower without a hood seems to be by far the most common now.
There does not seem to have been much in the way of choice in the basic technical principles employed in a hair dryer. I cannot think of another way of achieving the desired result, other than using an electric motor to power a fan to blow air over an electrically heated element. The alternatives a designer faces are in the selection of the components and their arrangment.
The options in the technology employed to meet the needs with the stated criteria obviously include the following:
The heating element and the wattage used.
The blower type and the air flow rate produced.
The housing material, shape and colour.
Heat and blower controls and switches and safety features.
In each of these there were choices to be made and choices on the production methods for each of the three parts and the assembly of the whole, and the choices in the previous items depends on the feasible production methods. Production technology is undoubtedly a key consideration, along with external styling.
Heater and blower
The choice and design of heating element and blower are closely linked to each other. There are different kinds of heater elements used in technology and different kinds of electric motors and different designs of fan blades. There are different forms of electric heating element available. Bare coiled wire, wire embedded in ceramic insulator in a metal sheath, heating elements made from metallic oxide compounds. Here we have a basic form, a length of bare uncoated coiled wire, supported on a insulating frame and an axial flow fan which operates at a very high speed. The design of all this is electrical engineering.
The motor does not have to a constant speed, just a fast speed, and has to be reliable for a considerable length of time without attention. The motor probably is made with ceramic magnets, which are very powerful, so this motor produces much more power than the earlier motors and hence can blow more air. The availability of mass-produced small motors of this type is probably the most important key technical development contributing to the modern portable hair dryer.
The temperature of the heating element is another concern. Most heating element wires become brittle when they become hot. This is obviously not desirable in a hair dryer which may be dropped or shaken. It seems to me that the element of a hair dryer does not heat up to much above 300oC, if that much, because it does not glow in the dark. The effective removal of hat from the element is also a factor in the reliability of the hair dryer.
Dryers now have a plastic housing, a product of materials engineering. Plastic provides good thermal and electrical insulation, provided it does not get hot. Some plastics have high impact strength. That plastic can be used so close to a 1500W heat source is the result of good electrical, Mechanical and materials design.
The choice of plastic is also related to the method of manufacture of the housing, most likely some kind of moulding process. Moulded plastic parts can be mass produced at a high rate quite cheaply, a feature of production engineering. The design of the housing had evolved over time, in both appearance and ease of assembly. earlier driers had many more fasteners, i.e. screws or bolt than the present ones. This one has two screws, earlier designs employed as many a eight fasteners.
Heater and fan switches
There has to be a means of switching the heat and air on and off, possibly different settings for both heater and blower. Different models provider different choices for these controls. Providing a reliable small switch to handle up to 15 A of current is not a trivial matter.
Safety cut-off switch
How do you use a rise in air temperature to activate a switch to cut off power. Among the alternatives might be a circuit relying on a semiconductor sensor known as a thermistor. The resistance of a thermistor drops a great deal as its temperature rises. So the change in resistance could be used with suitable electronics. However, a much simpler and often used device is the bimetallic strip. Sheets of two different metals with different coefficients of expansion are bonded together. When the temperature of this bimetallic sheet changes it will curl one way of the other as the two metals try to expand or contract to differing amounts. The bending of a bimetallic strip can therefore be used directly to operate an electrical switch. The choice between differing models may depend on size and reliability.
Examples of constraints are the availability of parts, materials, personnel and facilities. The race organizers also place constraints on the designs through their specifications.
With the hair dryer what are, or were, the constraints? The key limitations to label "constraint" are the limitations on the designer and producer. With the solar car the student team is both designer and producer. With the hair dryer the designer will likely operate separately from the production facility.
We all have a picture of inventors working in their basements developing ideas for new useful gadgets. It is true that there are many such individuals and some are quite successful in developing and patenting new ideas, but probably most ideas for industrial products and their development occurs in industrial companies. In either case, similar constraints of the availability of parts, materials, personnel and facilities apply, giving the industrial facility the edge since usually industry will have more resources than private individuals to try out ideas and construct prototypes.
The situation is, however, quite different in the two cases. In the development of a consumer product a company can afford to place resources at the hands of it designers. The constraints that apply to the final product will be the limit to the cost to the consumer that will ensure that the dryer will sell.
We have seen that the hair dryer depends on the use of existing technology for all its parts. In earlier versions limitations to the power of small fans restricted the hair dryer to low heat outputs. Now it is the limit to the current that can be drawn from a standard outlet that limits the heat output.
Production technology is probably the key constraint now. There is probably little point in considering hair dryer manufacture unless your production facilities will allow you to mass produce the dryers at a very low unit cost. This may mean quite costly large-scale machinery capable of processing the parts at very high rates. If there is much labour involved it would imply that low wage countries have the advantage. This is a consideration for the fan and motor units, the heater, and the housing, as well as for the process through which these and other parts are assembled to form the dryer.