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Logarithmic mean temperature exchangers

The LMTD, ie, logarithmic mean temperature difference, is an effective overall temperature difference between the two fluids for heat transfer and is a function of the terminal temperature differences at both ends of the heat exchanger. [Pg.486]

The rate of temperature drop of a fluid as it flows along the length of a heat exchanger is not constant. In order to take account of this nonlinear relationship, the logarithmic mean temperature difference (EMTD) is used. If the inlet and outlet temperatures do not differ widely, an arithmetic mean can be used, because the relationship is considered to be linear. [Pg.694]

The logarithmic mean temperature difference is the same as the temperature difference at the entrance and exit of the heat exchanger, i.e., AT, = AT, = AT ... [Pg.696]

Figure 9.71. Correction for logarithmic mean temperature difference for single shell pass exchanger... Figure 9.71. Correction for logarithmic mean temperature difference for single shell pass exchanger...
Before equation 12.1 can be used to determine the heat transfer area required for a given duty, an estimate of the mean temperature difference A Tm 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 (see Volume 1, Chapter 9) is only applicable to sensible heat transfer in true co-current or counter-current flow (linear temperature-enthalpy curves). For counter-current flow, Figure 12.18a, the logarithmic mean temperature is given by ... [Pg.655]

The usual practice in the design of shell and tube exchangers is to estimate the true temperature difference from the logarithmic mean temperature by applying a correction factor to allow for the departure from true counter-current flow ... [Pg.655]

Q = heat exchanger duty U = overall heat transfer coefficient A Tlm = logarithmic mean temperature difference... [Pg.417]

As can be seen from the nonlinear temperature profiles, the temperature difference between the fluids varies from one end of the heat exchanger to the other. To find an effective temperature difference between the two fluids, a logarithmic mean temperature difference (LMTD) is defined as... [Pg.356]

Obtain a relation for the logarithmic mean temperature difference for use in the LMTD melhfld, and modify it (or different types of heat exchangers using the correction factor,... [Pg.625]

The temperature difference between the two fluids decreases from AT, at the inlet to AT-i at the outlet. Thus, it is tempting to use the arithmetic mean temperature AT = (AT, + AT2) as the average icmperalure difference. The logarithmic mean temperature difference ATj is obtained by tracing the actual temperature profile of the fluids along the heat exchanger and is an exact representation of the average temperature difference between the hot and cold fluids. It truly reflects the exponential decay of the local temperature difference. [Pg.640]

Note that ATj is always less than AT. Therefore, using AT in calcula rions instead of AT will overestimate the rate of heat transfer in a heat exchanger between the two fluids. When AT, differs from AT) by no more than 40 percent, the error in using the arithmetic mean temperature difference is less than 1 percent. But the error increases to undesirable levels when AT, differs from A7) by greater amounts. Therefore, we should always use the logarithmic mean temperature difference when determining the rate of heat transfer in a heal exchanger. [Pg.640]

Knowing the inlet and outlet temperatures of both fluids, the logarithmic mean temperature difference for this counter-flov/ heat exchanger becomes... [Pg.644]

SC Can the logarithmic mean temperature difference AFia of a heat exchanger be a negative quantity Explain. [Pg.664]

It was seen from the discussion of heat exchangers that the fluid streams are not strictly countercurrent. Baffles on the shell side induce crossflow, and in a two-tube-pass heat exchanger both countercurrent and cocurrent flow occur. To account for deviations from countercurrent flow, the logarithmic-mean temperature difference is multiplied by a correction factor, F. Thus,... [Pg.163]

Because the cost of a heat exchanger depends on its size, and because its size will depend on the heat-transfer rate, a rate equation must be introduced. The rate equation is given by Equation 4.4.3. The logarithmic-mean temperature difference in Equation 4.4.3 is given by Equation 4.4.4. Because perfect countercurrent flow can never be achieved in an actual heat exchanger, the logarithmic-mean temperature difference correction factor, F, is needed. For simplicity, Equation 4.10, discussed earlier, is expressed as Equation 4.4.5, which states that F depends only on the terminal temperatures, once a particular heat exchanger is selected. [Pg.171]

For isothermal condensation, the logarithmic-mean temperature difference correction factor, F, equals one. Therefore, from Equation 4.7.3 for the existing heat exchanger, the available overall heat-transfer coefficient,... [Pg.181]

So A m is also the logarithmic mean temperature difference at both ends of the heat exchanger in cocurrent flow. [Pg.53]

Figure 5. Logarithmic mean temperature difference in a counterflow heat exchanger with no phase changes. (Luwa Corporation)... Figure 5. Logarithmic mean temperature difference in a counterflow heat exchanger with no phase changes. (Luwa Corporation)...
INTEGRATION OVER TOTAL SURFACE LOGARITHMIC MEAN TEMPERATURE DIFFERENCE. To apply Eq. (11.9) to the entire area of a heat exchanger, the equation must be integrated. This can be done formally where certain simplifying assumptions are accepted. The assumptions are (1) the overall coefiR-cient U is constant, (2) the specific heats of the hot and cold fluids are constant,... [Pg.316]

EFFECTIVE TEMPERATURE DIFFERENCE iN BAFFLED HEAT EXCHANGER Zitm LOGARITHMIC MEAN TEMPERATURE DIFFERENCE... [Pg.34]


See other pages where Logarithmic mean temperature exchangers is mentioned: [Pg.258]    [Pg.57]    [Pg.284]    [Pg.354]    [Pg.149]    [Pg.390]    [Pg.315]    [Pg.589]    [Pg.183]    [Pg.662]    [Pg.589]    [Pg.166]    [Pg.126]    [Pg.553]    [Pg.62]    [Pg.458]    [Pg.632]   


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