Heat exchangers mean temperature difference

ATgw temperature difference from saturated vapor to wall K, °F AT, heat exchanger inlet temperature difference K, °F ATlm log mean temperature difference K, °F AT shell-side to tube-side exit temperature difference K, °F ATml wall-minus-saturation temperature difference K, °F U overall heat transfer coefficient W/(m2-K), Btu/(h ft2 oF)... 

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. 

ARm log—mean temperature difference for a given heat exchange... 

Mean Temperature Differenee The temperature difference between the two fluids in the heat exchanger vm, in general, vaiy from point to point. The mean temperature difference (AT, or MTD) can be calculated from the terminal temperatures of the two streams if the following assumptions are valid ... 

There are two basic approaches to heat-exchanger design for low temperatures (1) the effec tiveness-NTU approach and (2) the log-mean-temperature-difference (LMTD) approach. The LMTD approach is used most frequently when all the required mass flows are known and the area of the exchanger is to be determined. The effec-... 

In the field of heat transfer, a good example of this category of shortcut design method is the famous F correction factor to correct the log mean temperature difference of shell and tube heat exchangers for deviations from true countercurrent flow. For multipass heat exchangers, the assumptions are ... 

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. 

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 ... 

In the basic heat transfer equation it is necessary to use the log mean temperature difference. In Equation 2-4 it was assumed that the two fluids are flowing counter-current to each other. Depending upon the configuration of the exchanger, this may not be true. That is, the way in which the fluid flows through the exchanger affects LMTD. The correction factor is a function of the number of tube passes and the number of shell passes. 

For heat exchangers in true counter-current (fluids flowing in opposite directions inside or outside a tube) or true co-current (fluids flowing inside and outside of a tube, parallel to each other in direction), with essentially constant heat capacities of the respective fluids and constant heat transfer coefficients, the log mean temperature difference may be appropriately applied, see Figure 10-33. ... 

Log mean temperature difference is not used because the distribution of the exchanger area varies through the unit, due to changing heat load. 

LMTD = log mean temperature difference, °F M = mass flow rate, Ib/hr Ntu = number of heat transfer units, dimensionless N = number tubes/row in direction of air flow n = number tubes/row, per ft of exchanger width, 1 /ft Q = total exchanger heat load (duty), Btu/hr R = = heat capacity ratio, dimensionless... 

The radiant section of an industrial boiler may typically contain only 10 per cent of the total heating surface, yet, because of the large temperature difference, it can absorb 30-50 per cent of the total heat exchange. The mean temperature difference available for heat transfer in the convective section is much smaller. To achieve a thermally efficient yet commercially viable design it is necessary to make full use of forced convection within the constraint of acceptable pressure drop. 

The effective mean temperature difference will usually be higher with the plate heat exchanger. 

A further advantage of the plate heat exchanger is that the effective mean temperature difference is usually higher than with the tubular unit. Since the tubular is always a mixture of cross and contra-flow in multi-pass arrangements, substantial correction factors have to be applied to the log mean temperature difference (LMTD). In the plate... 

The effectiveness—NTU method is a procedure for evaluating the performance of heat exchangers, which has the advantage that it does not require the evaluation of the mean temperature differences. NTU stands for the Number of Transfer Units, and is analogous with the use of transfer units in mass transfer see Chapter 11. 

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 ... 

Though these conditions will not be strictly satisfied in practical heat exchangers, the Ft values obtained from the curves will give an estimate of the true mean temperature difference that is sufficiently accurate for most designs. Mueller (1973) discusses these... 

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

The major things to specify for heat exchangers are the materials of construction and the heat-transfer area required. Generally, streams containing materials that can precipitate out or form a scale are placed on the tube side. If this is not a factor, it is generally best to place the stream flowing at the highest velocity on the tube side. Usually a 20% improvement in the corrected mean temperature difference can be realized if this is done.2... 

The hot oil gives off sensible heat only (heat capacity = 0.5 Btu/lb F, density = 4.58 Ib gal). The heat transfer area in the exchanger is C52 ft, Assume the temperature on the tube side of the heat exchanger stays constant at 200 F and the inlet hot oil temperature stays constant at 400°F. A log mean temperature difference must be used. 

The heat exchanger is operated in countercurrent configuration in order to improve the properties of the heat transfer between fluids. As a consequence, the mean temperature differences can be modeled, within the domain of the physically realizable temperature x G [52], by... 

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... 

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