ALL ABOUT TEMP-PLATE® - Paul Mueller Company

16
SOLVING HEAT TRANSFER PROBLEMS
What Is Heat Transfer?
In general, heat transfer is the flow of heat from one
substance to another. The flow of heat is always from the
higher temperature to the lower temperature.
The general formula for heat transfer is Q = A x U x ▲t
(see Glossary of Symbols, page 15). Expressed in more
U= 1 or U = 1
1/ho + x2/k + 1/hi Summation of
resistance to heat flow
exact terms, with 1/ho equal to film resistance on one
side of a wall and 1/hi the film resistance on the other
side and x2/k the wall resistance between the films. It
should be noted that in most cases the film resistance will
be higher than the wall resistance.
Liquid being heated
Metallic wall (x2)
Film (ho)
Film (hi)
Although it is possible to compute the “U” factor, it is an
involved process and the results are only accurate when
all of the controlling factors are considered. The usual
method to obtain a given “U” factor is to test the given
materials under normal conditions encountered in the
industry. Through extensive testing, the “U” factors given
in table (see page 20) are recommended for Temp-Plate
as safe design coefficients (i.e., scale deposits, film
factors, and vapor film coefficients have been taken into
account as well as a safety factor added).
Solving Heat Transfer Problems
Generally, in industrial applications, the equation Q = A
x U x ▲t is rewritten to establish “A”, the area required
to perform a given heat transfer job, and assumes the
form:
A = Q
U x ▲t
This equation can generally be solved in four steps, as
follows:
1. Determine “Q” where Q = V x (62.4 x sp. gr.)* x cp x t.d.:
V = Volume of vessel in cubic feet
cp = Specific heat of product
t.d. = t1 –t2 = Temperature change
*Can be expressed in pounds per cubic foot.
2. Determine “U” from Table on page 20.
3. Determine effective temperature difference “▲t.” In
actual practice and in order to obtain accuracy, in
place of “▲t” we use the log mean temperature
difference, ▲Tm. This can be computed by applying
the equation:
▲Tm = ▲t1 – ▲t2
In ▲t1
▲t2
but for simplicity, the ▲Tm (see page 21) can be used
for Temp-Plate, and by interpolation reasonable
accuracy can be obtained.
4. Solve equation A = Q
U x (▲Tm)
Note: The entire heat transfer solution as determined
in the preceding heat transfer steps can be solved
simultaneously by combining the entire formulas
from steps 1, 2, and 3 and solving for:
A = W x cp x t.d.
HRS x U x (▲Tm)
or if the area is known, the equation can be solved
for the time required. See Examples 1, 2, and 3.
HRS = W x cp x t.d.
A x U x (▲Tm)