THE DESIGN OF A COMBINATION ROOM HEATER AND HAIR DRYER
by
Eleanor L. Semple
Submitted in partial fulfillment of the requirements for the degree of Bachelor of Science in Mechanical Engin-eering at the Massachusetts Institute of Technology.
(1951)
Signature of author EE icuslied rm
Signature of thesis advisor Signature [dace
120 Bay State Road
Boston 15, Massachusetts
May 18, 1951
Prof. John E. Arnold
Department of Mechanical Engineering
Massachusetts Institute of Technology
Cambridge 39, Massachusetts
Dear Prof. Arnold:
In accordance with the regulations of the faculty, 1 herewith submit a thesis entitled "The Design of a Combination Room Heater and Hair Dryer," in partial fulfillment of the requirements for the degree of Bachelor of Science in Mechanical Engineering.
Respectfully yours,
Signature redacted
ACKNOWLEDGMENTS
The author wishes to acknowledge gratefully the aid and assistance rendered to her by the following: Prof. John E.
Arnold, Department of Mechanical Engineering, as thesis advisor; Mr. Gordon Florian, Industrial Designer, whose suggestions on the appearance of the product were invaluable; and those mem-bers of the faculty who so willingly advised the author with
ABSTRACT
The purpose of the design of a combination room heater and
hair dryer is to fulfill the need for a small portable electric
heater to supply warmth in a localised area of the home, and at the same time, to incorporate the additional feature of a hair
drying unit.
In such a design it is necessary to choose the several
ele=-ments which constitute the mechanism, i.e., the heating coil, fan,
motor, and hair drying attachment. It is also necessary to
en-close these components in a case which will withstand the wear
conditions to which the product will be subjected and will have
an attractive appearance.
The heating element, fan, and motor were chosen from standard
parts by considering the approximate operating conditions to be
met by the unit, and by varying these conditions slightly until
the individual parts would meet them.
A nozzle was employed for the hair drying attachment to
in-crease the velocity of the air and to provide a more limited area of exit. Provision was made to invert the nozzle for use sas a
diffuser when the unit is applied as a heater.
The case was designed for a low-carbon cold-rolled steel stamping with a baked enamel finish.
TABLE OF CONTENTS
Acknowledgments
-Abstract ii
I Introduction 1
II Engineering Design e
IIT Product Design 11
IV Design Drawings
a. Machine Drawings 15
b. Rendering 17
V Appendix
a. Graphs 18 b. Cost and Weight Analysis 22 ¢. Bibliography 23
INTRODUCTION
The present design was first undertaken in the form of a
small hand hair dryer. This project was discarded for two reasons: l. it has been demonstrated that restyling the present models of hair dryers into more modern-appearing products would not increase
thelr sales appeal, and 2. since the net sales of hair dryers in the United States represents a relatively small business, an in-vestment in new tools and dies for a complete redesign would not be warranted,
There has been a demand for auxiliary room heaters for
warm-ing and drywarm-ing purposes to supplement the regular heatwarm-ing system
In the bathroom, nursery, playroom, etc., of many homes and
apartments... Therefore it was proposed to design a small electric heater to satisfy these applications and to include the
attrac-tion of a hair dryer unit as an integral part of the heater,
HAIR DRYER ATTACHMENT
It was deemed desirable for several reasons to attach the hair dryer unit to the heater so that it need not be removed; a
separate hair dryer unit would require storage space and might
be mislaid. Since a halr dryer requires a greater velocity of
alr flow in a more concentrated area, a nozzle provided the best solution to the problem of a suitable attachment. The nozzle could be reversed and used as a diffuser, thus integrating the two parts into a compact unit.
No definite data could be found on design of nozzles operat-ing at subsonic speeds such as would be encountered in a hair dryer, It was suggested that any smooth curve connecting the in-let and outin-let areas of the nozzle would prove adequate, and that the shape of the curve would be largely dependent on the
appear-ance. %*
THE HEATING ELEMENT
There are three types of heating elements which would be directly applicable to an electric heater: a simple coil of re-sistance wire, a finstrip element, or a tubular element. Of the three available the finstrip element was chosen because it is stronger than the resistance wire, may be quite easily bent to a desired shape, and gives more heating surface than the tubular element.
About 1500 watts power may be drawn from an ordinary house circuit at one time. Thus, to avoid any possibility of overload-ing a circuit, a value slightly lower than 1500 watts should be * Personal communication from Prof. M. S. Silberstein
selected.
The Chromalox Koilfin element, type KSEF, with a steel sheath was adaptable to the product because of 1ts circular shape. l. #* A desirsble power demand for the heater would be 1350 watts on a 115 volt house circuit. As may be seen from the Chromalox cata-log, the standard heaters avallable are too large for this appli-cation. Since a special order would be required, the size of the heater may be adjusted within reasonable limits. From the dimen-sions and ratings given for finstrip heaters, an outside diameter of 8 inches would be a sensible choice for the element.
The only available date correlating power demand for a fin=-strip heating element and volume rate of flow of the air with different temperature rises was found in the Westinghouse cata-log. 2. It 1s desirable to have a temperature rise of the air of 100° P or slightly less. From fig. 1, an enlargement of the lower part of the graph found in the Westinghouse catalog, a flow of 55 cublc feet of air per minute was chosen. With a 1350 watt power demand, a temperature rise of about 90° F will be effected.
AIR VELOCITY
In order to determine the air velocity in both the heater and the halr dryer, approximate values for the size of the heater and
its entrance and exit orifices had to be found. It was considered desireble to keep the size of the case to a minimum so that it would fit easily into a confined space in small rooms. An esti-mate of the sizes of the parts gave an idea of the necessary case
size.
# Numbers refer to the appendix.
The dimensions of a fractional horsepower motor such as would be used to drive a small fan would be about 23 inches in diameter by 2 inches in length! the fan to be used would probably not be greater than 8 inches in diameter and 1 inch deep; thé heating element would be a coil about 8 inches in diameter and would be 2 inches deep. An air space of about 3 inch should be left between the heating element and the front opening of the heater for safety when in use. The total depth required would be about 7 inches when separations between the various components
and mountings for them are considered. A dlameter of 3 inches for the entrance and exit orifices would be reasonable on the basis of the diameter of the heating coll. It was decided to
use a cross section of the case tending toward a square rather than a circle since a square appears to have better balance and
to require less space to most women, and the cholce is primarily one of sales appeal. For the 8 inch front and back openings al-ready suggested, a 12 inch square would provide sufficient space
to give a balanced design. Thus the size of the case was approxi-mated as 12 inches by 12 inches by 7 inches with 8 inch front and back circular orifices.
Velocity Through the Heater
Since the design 1s based on approximations and will ultimate-ly depend on performance when a first model is built, the
continu-ity equation was used inthe following form, without considering
differences in specific volume due to temperature changes: Q 8 AV
where Q® volume rate of flow of air, cubic feet per minute
A= area of orifice, square feet Vz velocity of alr, feet per minute
From this equation the value of the velocity of alr through the orifices of the heater was determined.
V=Q/A
55
a
4 144
V=158 feet per minute
A velocity of from 100 to 200 feet per minute 1s satisfactory for a room heater; the values chosen, therefore, are suitable for
the design.
Ultimate Nozzle Design
It was decided on the basis of personal market research of the hair dryers manufactured by the Standard Products Corporation,
Whitman, Massachusetts and the Master Bppliance Manufacturing
Company, Racine, Wisconsin to use a velocity of about 1000 feet per minute for the hair dryer attachment,
Again applying the continuity equation with the substitution
of several values of diameter for the nozzle, the best combination of alr velocity and diameter of the nozzle was found.
Q= AV
V,: Q/A.,
Q Vo or careiibtmmimrinsi n EW dn* 4° 1494where d,=diameter of nozzle, inches
Vv... ‘0 teoNT em
da (inches) | V (feet per minute)
: 2525
2e5 1617
2 1122T.5 82h
The optimum values were fixed at 3 inches for the dlameter of the nozzle and 1122 feet per minute for the velocity of the air. The shape of the nozzle can now be more clearly defined. The entrance diameter was chosen as 8 inches; the exit diameter
was found to be 3 inches. A depth of 3 inches was chosen pri-marily for appearance considerations so that the nozzle would be well balanced, As stated before, the outside surface of the
nozzle will be a smooth curve connecting the inner and outer
edges.
FAN REQUIREMENTS
In choosing the fan it was necessary to determine the pres-sure hedd required to fulfill the flow conditions already imposed on the design.
The total pressure head required is equal to the sum of the pressure drops through the inlet orifice, over the motor housing, over the heating unit, and through the exit nozzle, The
assump-tion was made that the pressure drop through the nozzle, rather than that through the diffuser, would be the limiting condition.
The following calculations were used to find the pressure
drops through the orifice and nozzle, applying incompressible
flow equations. 3. Agaln the calculations are approximate and do not take into account variations in temperature or the
pressibllity of the air.
Pressure Drop Through the Inlet Orifice
A= 1.97x1073 slugs per cubic foot at 1700 F
Mx 11.323x10~7 pound seconds per square foot
: at 1709 B®
d= 8 inches
Assume the inside area of the heater case is
nn i circular with D= 12 inches.
edo Shs oly,
Q = AV
V=Q/A
2 iD semi
Ir. A 60
V=,0175 feet per second
Vv x 107%, « }
et ys tas
Lr + ,18x10-3
Cs.72
Qz A 26ep
F
14.6x10k Ap = 3.65
1h.6x10l ap = 13.3
5 p= 9.115105 pat.
Ap= 0.00252 inches of water
Pressure Drop Through the Nozzle
Vv, D, Oo and gu have the same values as in the
previous calculation.
d #3 inchesLtr 0.0625
APD 1.18103
At. Cc =.,88Q = if2a;
P
1 .6x10k Ape 21.2)
1.6x10%a ps i151
A pz 30.9x10~4 psi.
Ap » .0856 inches of waterThe previous assumption of pressure drop through the nozzle being the limiting condition seems to be justified in the
pre-ceding calculations.
Other Pressure Drops
No method could be found to determine the pressure drops over the motor housing and heating coil, but these factors are probably negligible compared with the pressure drop through the nozzle. Evidence to support this theory may be seen in the negligible pressure drop found in the calculaiion for the entrance orifice.
Since there are small losses other than the nozzle loss, it seemed logical to assume a necessary pressure head for the fan of
el inches of water.
Choice of Fan
It was declded to use one of the Torrington Manufacturing Company's "One Plece" Series because these fans are adequate for the purpose and are relatively inexpensive.
Since a lowfan speed is desirable to eliminate vibrations it was found necessary to extrapolate the data given in tabular form in the Torrington Catalog to find the fan to fit the design
conditions, lj. Fig. 2 shows the extrapolation of the data.
On the basis of this curve it was seen that the fan # O-621-li was large enough to give a flow of 55 cubic feet of air per minute at a sufficiently low speed, about 1725 RPM. This fan is 6 inches in diameter, is 21 pitch, and has l blades.
CHOICE OF MOTOR
The motor required to run the fan was chosen from the same tables after extrapolating to the curve found in fig. 3. The value tsken from the curve is about .001 horsepower; however,
the accuracy of the curve is doubtful, and it was deemed
ad=-visable to specify a .005 horsepower motor to run at 1725 RPM.
The motor which met these conditions best was the Barcol motor # CYAE }78, a reversible AC motor requiring 115 V on a
60 cycle circuit. Fig. lL. shows the torque-speed characteristics
of the motor plotted with the torque-speed characteristics of the fan. From this graph it may be seen that the speed will be about
2250 RPM with the .,0055 horsepower motor. The increase in speed
will increase the rate of air flow slightly over the chosen value, but the design will probably meet the approximate conditions ime posed on it.
THE SWITCH
A three-position rotary switch was chosen with the wiring to be done. in the following manner so that it would be possible to
run the fan and heater at the same time, or the fan alone:
Cpr ampere
No provision was made to operate the heating coil alone since there would be danger of overheating the whole case. The switch chosen was a Mallory Rotary switch of the non-shorting type.
SHAPE OF THE CASE
The approximate shape of the case already specified was 12 inches by 12 inches by 7 inches with 8 inch orifices. To add interest to the square cross-section, a taper was introduced to form a trapezoidal section. By trial and error scale drawings, final dimensions of 12 inches at the bottom and 8 inches at the
top with a 12 inch height were chosen.
The shape of the case suggested a stamping as the easiest and most inexpensive method of fabrication. In the interest of
economy, it was decided to stamp the case in two identical halves, thus using the same die for the whole case.
In the design of stamped or drawn parts it is necessary to
have a corner radius of at least 5 times the thickness of the mat-srial used. 5. From appearance considerations a radius of 13
inches was chosen; this radius is at least 5 times as large as
the thickness of a very heavy gage material.
Upon drawing the case outline to scale, it was discovered that a slight outward bow in the sides would improve 1% still
further. Thus the final shape as seen on the engineering
draw=-ings wes determined.
MATERIAL OF THE CASE
Plastics, aluminum, and steel were investigated as possible
materials for the case,
Plastics have many desirable characteristics, but no single plastic would have the strength, durability, or heat resistance
of a metal. Also plastics have unfortunately become assoclated -]l]le
with leck of quality. On this basis plastics as a possible case
material were discarded.
Aluminum, although light and strong, is more expensive than
steel. At the present time it is impossible to obtain aluminum for peacetime production, and this condition may prevail for some
time to come. Therefore steel was chosen as the case material.
A low-carbon steel, about SAE 1020, in 20 gage sheets will be
sufficiently strong and may be easily stamped. Cold rolled steel
should be specified to give a better surface finish. 5. COATING THE CASE
A colored enamel coating was decided upon as the most
in-expensive and most attractive finish for the case.
A primer coat 1s necessary on the base metal. A baking type
primer of lead chromate in a resin or varnish vehicle which will
dry by polwmerization will suit the application. 6.
A surfacer which has the same composition as the primer coat with additional pigment sould be applied next to cover minor
irregularities.
A baking type enamel will provide sufficient wear, abresion, and chemical resistance, and will be cheaper and easier to apply than a porcelain enamel which must be baked at very high
tempera-tures. One of the common baked enamels which finds wide use on
refrigerators and stoves is composed of a vehicle of short oil
glyceryl phthalate in combination with melamine resin to improve
stability in heat, chemical resistance,and drying time. The
enamel 1s baked at from 200 to 350° F for about 10 minutes.
It was decided to specify three different color finishes for
the case: light green, cream, and ebony black. These three
colors would all fit into most rooms - bathroom, nursery,
liv-ing room, sun porch, or bedroom. The followliv-ing are samplesof the
colors suggested:
Z
PRODUCING THE NOZZLE
The nozzle may be spun from thin sheet, about .025 inches thick. 7. Chrome plating the nozzle would protect the base metal, giving it good wear and corrosion resistant properties. It was suggested that a satin finish be used rather than bright chrome which would overemphasize the nozzle in comparison with
the case.
OTHER TRIM
To hold the two halves of the case together, it was de-cided to use a band of steel formed to fit over the Joining
edges, and fastened at the bottom. This strip would have the
same satin finish as the nozzle.
Runners were chosen to support the case. They were
de-signed to be fabricated of a zinc die casting with the satin finish to harmonize with the other accessories. Wood Wlocks
would be used inside the hollow castings through which the case and runners would be fastenéd together to aid in holding the
# Personal communication from Prof. John E. Arnold
-strip.
The rolled rims around the orifices and that which is used to hold the diffuser are also to finished with the satin finish,
as are the front and back grilles.
The knob on the switch is to be made of plastic with a gray
metallic finish.
A handle will be fashioned into the back grille by leaving a space to grip the case and holding the edges of the grille in
a2 thin rin,
DESIGN DRAWINGS
a. Machine Drawings
APPENDIX
a. Graphs
b. Cost and Weight Analysis ¢. Bibliography
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Part Approximate Costs Approximate Weight
Heater $l. 00 2.00 pounds
Fan «20 «1l
Motor }i.00 3.75
Switch ae -——Case 1.00 3.00
Nozzle 1.00 o13
Trim «25 ————Runners WE +50
Wire. Le20 ——Total $11.65 0.52 pounds
roBIBLIOGRAPHY
l. Chromalox Electric Heating Units and Equipment, Industrial
Catalog # CS-500.
z. Westinghouse Industrial Electric Hesting Units and Controls,
Catalog # 28-020, Sept. 1948.
Hunsaker, J. C., and Rightmire, B., G., Engineering Applica~
tions of Fluid Mechanics, McGraw-Hill Book Co., Inc., New
York, 1947.
Alristocrat Quiet Propeller Fan Blades, The Torrington
Manu-facturing Company.
>. Chase, Herbert, Handbook on Designing for Quantity Production,
McGraw-Hill Book Co., Inc., New York, 19.
O. Dumond, T. C., Engineering Materials Manual, Reinhold
