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1.8
Finned Magnetron
The increased power demands on magnetrons,
and their use in airborne equipment
has created a need for a light weight,
economical cooling system. Forced air
cooling, using properly designed cooling
fins, is described here as a logical solution
PUNCHING DETAIL
SECTION A -A
RIB DETAIL
SECTION 8 -8
attached by heat conducting paths.
These must be highly conductive or
the large temperature drop from
the component to the exchanger
surface will destroy the gains obtained
by the large hS value of the
heat exchanger, h being the average
heat transfer coefficient and S
the heat transfer surface area.
Fins, which increase the heat transfer
surface area by the addition of
a secondary surface area, are an
example of a simple type of heat
exchanger.
The surface temperature (ta) of
the electronic part is fixed by the
allowable component temperature,
generally a hot spot tmperature;
the fluid or coolant temperature
(te) is given by whatever may be
available for the cooling system,
and the heat dissipated (q) depends
upon the component dissipation.
Consequently, knowing values for
these quantities, one may define an
overall hS product required of the
particular cooling system.
q
(11í') = - (I)
required (t - te)
The hS value available is a function
of the coolant mass rate of flow
W and the cooling system. For any
particular system a plot can be
made of (hS)neeil;,,,, vs. W (see
Fig. 1). In addition, a plot of Ap
vs. W can be made (see Fig. 1).
The curve in Fig. 1 applies only for
the particular coolant density involved;
however this can be easily
corrected for other values of density,
assuming incompressible flow.
Entering the (hS) n, eiiai,re curve with
the value obtained from Eq. 1, the
Dr. M. MARK, Consulting Engr., 1384 Mass.
Ave., Cambridge 38, Mass. (The work described
in this article was performed while
the author was employed at Raytheon Mfg.
Co., Newton, Mass.)
A --
0 A U
a B
a
Description
Table I
Flow, cubic
ft.,min.
:1p, in.
of water
Ideal Air
horsepower
Original fin (Fig. 3) 37.5 8.0 .0473
Deflector insert (Fig. 5) 29 2.8 .0128
Streamlined exit (Fig. 5) 27.5 2.1 .0091
Punchings and ribs (Fig. 6) .... 18 2.2 .0062
Punchings, ribs and streamlined
exit ( Fig. 7) 18 1.9 .0054
Addition of finned output, and fins
of Fig. 7 (see Fig. 8) 15
I .0043
Data for hS = 6.25 w. / °C -(t, ,
PUNCHING DETAIL
RIB DETAIL
SECTION A -A SECTION 68
W required can be obtained. With
this value of W, the necessary .1p
can be determined, and finally the
horsepower required for cooling.
For a magnetron with a given
cooling system, hS can be defined in
terms of the hot spot temperature
t, 111,,, the inlet coolant temperature
tr,1n1 ,, and the heat dissipated q,
from Eq. 1.
q = hS (t,.,, - tr.leel,.,) (2)
To obtain the curve for (hS)n,e,la,,ie
for a particular cooling system, the
quantities q and (ta .1aax tr.151et) are
measured at various values of flow
i
Fig. 6: Fin with punch
inns and ribs
Fig. 7: Fin with punch -
ings, ribs and streamlined
exit
and the quantity hS calculated using
Eq. 2. Where q is a constant, it is
not too important which quantity,
(hS)nrnilahl or (ts.max tf.iniet), is
plotted as a function of the flow.
The electrical designer prefers
(ts.maz tr.inl t) as a function of the
flow; consequently, this form is
commonly used in electrical work to
present heat transfer results where
a constant heat dissipating element
is involved. However, it is not as
general a form for the results as
the hS vs. W plot.
(Continued on page 174)
° , - tr.)°LL) = 40 °C -Inlet Air Temperature, 30 °C
% of originel
air horsepowar
100
27
19
13
11
9
Tele -Tech & ELECTRONIC INDUSTRIES
J une 1956
101