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Before equation (eq – 1.7) can be used to
determine the heat transfer area required for a given duty, an estimate of the
mean temperature difference Δtm must be made. This will normally be calculated
from the terminal temperature differences: the difference in the fluid
temperatures at the inlet and outlet of the exchanger. The well-known
“logarithmic mean” temperature difference (LMTD) is only applicable to sensible
heat transfer in true co-current or counter current flow. For counter flow the
LMTD is given by:
In a heat exchanger the temperatures of the hot and cold fluids keep on changing
from point to point along the length of the exchanger. The question therefore
arises what value of the temperature difference should be used to compute the
rate of heat flow. The need of mean temperature difference which, when
multiplied by the overall coefficient of heat transfer and the appropriate area,
will give the correct heat flow, originated. An expression for the mean
temperature difference
Where ΔT m = log mean temperature difference,
T1 = inlet shell side fluid temperature,
T2 = outlet shell side fluid temperature,
t1 = inlet tube side temperature,
t2 = outlet tube-side temperature,
The equation is the same for co-current flow, but the terminal temperature
differences will be (t1 – t1) and (T2 – t2). Strictly, equation (eq – 1.9) will
only apply when there is no change in the specific heats, the overall
heat-transfer coefficient is constant, and there are no heat losses.
In most shell and tube exchangers the flow will be a mixture of co current,
counter current and cross flow. Fig 1.3 show typical temperature profiles for an
exchanger with one shell pass and two tube passes (a 1: 2 exchanger).
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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
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