|
As the
physical layout of the exchanger cannot be determined until the area is known
the design of an exchanger is of necessity a trial and error procedure. The
steps in a typical design procedure are given below:
1.
Define the duty:
heat transfer rate, fluid flow-rates, temperatures.
2.
Collect together
the fluid physical properties required: density, viscosity, thermal
conductivity.
3.
Decide on the
type of exchanger to be used.
4.
Select a value
for the overall coefficient, U.
5.
Calculate the
mean temperature difference, Δtm .
6.
Calculate the
area required from equation Q = UA Δtm
7.
Decide the
exchanger layout.
8.
Calculate the
individual coefficients.
9.
Calculate the U
and compare it with selected U value. If the calculated value differs
significantly then return to step 6.
10.
Optimize the
design: repeat steps 4 to 10, as necessary, to determine the cheapest exchanger
that will satisfy the duty. Usually that will be one with the smallest area.
Typical
values of the overall heat-transfer coefficient for various types of heat
exchanger are given below:
|
Shell and tube exchangers |
|
Hot
fluid |
Cold
fluid |
U
(W/m2 oC) |
|
Heat
exchangers |
|
Water |
Water |
800-1500 |
|
Steam |
Water |
1500-4000 |
|
Coolers |
|
Organic solvents |
Water |
250-750 |
|
Light oils |
Water |
350-900 |
|
Heavy oils |
Water |
60-300 |
|
Gases |
Water |
20-300 |
|
Condensers |
|
Aqueous vapors |
Water |
1000-1500 |
|
Organic vapors |
Water |
700-1500 |
|
Organics (some non-condensables) |
Water |
500-700 |
|
Vacuum condensers |
Water |
200-500 |
|
Jacketed vessels |
|
Water |
Dilute aqueous sol. |
200-500 |
|
Water |
Light organics |
200-300 |
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Introduction |
Combined heat transfer process |
Heat transfer in cooling tower |
Variables affecting performance of CT heat transfer |
Heat transfer within
cooling system (heat exchanger) |
Types of heat exchanger |
Basic design
procedure and theory |
Designing a test heat exchanger |
Log Mean Temperature
difference | L.M.T.D. Correction factors |
Overall heat transfer coefficient |
Elaborated method for calculating U values |
Effect of scale formation |
Condensation of steam |
Condenser, where the hot fluid temperature varies |
Significance of pressure |
Significance of flow rate |
Methods of checking steam
condenser performance |
Common conversion factors
|
