Heat Transfer Area Required

FIGURE 7B.24 Block diagram for heat transfer design. 

Chilled water available at 20°C serves as the cooling medium. The reactor is to be operated isothermally at 36°C. Assuming an overall heat transfer coefficient of 250 W/mVK, 

The external cylindrical area of the vessel= rx rx7/-28 m. This is five times more than the heat transfer area prescribed for the duty, and therefore, a safe and stable operation is assured. Indeed, cooling tower water available at 32°C and leaving the jacket at 34°C can also be used since it requires approximately 20 m of heat transfer area in the jacket. However, (i) seasonal variations in humidity, (ii) possibility of malfunctioning of the cooling tower, and (iii) the narrow temperature window for viability of animal cells dictate that chilled water should be used. 

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Now consider the heat transfer area required by enthalpy interval k, in which the overall heat transfer coefficient is allowed to vary... 

A low temperature of approach for the network reduces utihties but raises heat-transfer area requirements. Research has shown that for most of the pubhshed problems, utility costs are normally more important than annualized capital costs. For this reason, AI is chosen eady in the network design as part of the first tier of the solution. The temperature of approach, AI, for the network is not necessarily the same as the minimum temperature of approach, AT that should be used for individual exchangers. This difference is significant for industrial problems in which multiple shells may be necessary to exchange the heat requited for a given match (5). The economic choice for AT depends on whether the process environment is heater- or refrigeration-dependent and on the shape of the composite curves, ie, whether approximately parallel or severely pinched. In cmde-oil units, the range of AI is usually 10—20°C. By definition, AT A AT. The best relative value of these temperature differences depends on the particular problem under study. 

The pitch of the coils and the area covered can be selected to provide the heat transfer area required. Standard pipe sizes from 60 imn to 120 mm outside diameter area are often used. Half-pipe construction can produce a jacket capable of withstanding a higher pressure than conventional jacket design. 

Heat transfer equipment has a great variation in heat transfer area per unit of material volume. Table 4.4 compares the surface compactness of a variety of heat exchanger types. Falling film evaporators and wiped film heat exchangers also reduce the inventory of material on the tube side. Process inventory can be minimized by using heat exchangers with the minimum volume of hazardous process fluid for the heat transfer area required. 

This same concept incorporating the TEMA charts can be used to (1) determine the fti heat transfer area required for an exchanger and (2) determine flow rate and outlet temperature of the fluids (shell or tube side) . 

Estimate the heat transfer area required for the system considered in Examples 9.1 and 9.36. assuming that no data on the overall coefficient of heat transfer are available. 

Study the effects of varying flow rates of both hot and cold fluids on the heat transfer area required. 

In a heat exchanger, the heat-transfer area required to transfer a specified heat load is inversely proportional to the temperature difference between the streams see Chapter 12. 

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 ... 

If the degree of superheat is large, it will be necessary to divide the temperature profile into sections and determine the mean temperature difference and heat-transfer coefficient separately for each section. If the tube wall temperature is below the dew point of the vapour, liquid will condense directly from the vapour on to the tubes. In these circumstances it has been found that the heat-transfer coefficient in the superheating section is close to the value for condensation and can be taken as the same. So, where the amount of superheating is not too excessive, say less than 25 per cent of the latent heat load, and the outlet coolant temperature is well below the vapour dew point, the sensible heat load for desuperheating can be lumped with the latent heat load. The total heat-transfer area required can then be calculated using a mean temperature difference based on the saturation temperature (not the superheat temperature) and the estimated condensate film heat-transfer coefficient. 

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 determination of the heat-transfer area required for a given situation is independent of when the calculation is made, but the cost of purchasing and installing that exchanger depends on when the order is placed. It is also affected by the number purchased, and whether it is a standard or a specialty item. 

In the equations considered in Section 14.4.1, various simplifying assumptions have been made which are now considered further in the calculation of a multiple-effect system. In particular, the temperature distribution in such a system and the heat transfer area required in each effect are determined. The method illustrated in Example 14.2 is essentially based on that of Hausbrand(12). ... 

A low temperature of approach for the network reduces utilities but raises heat-transfer area requirements. Research has shown that for most of the published problems, utility costs are normally more important than annualized capital costs. For this reason, AT is chosen early in the network design as part of the first tier of the solution. The temperature of approach, A 7, for the network is not necessarily the same as the minimum temperature of... 

The design is completed by finding the heat transfer area required in the condenser... 

It is important to remember that with all these options, the heat transfer area required is significantly greater than that attainable with just jacket cooling. 

Figure 7. Energy balance on a hypothetical 100 boiler and the in-bed heat transfer area required to maintain the bed temperature at 1116 K, e.g., 6 rows of 50 mm O.D. tube with a horizontal pitch of 150 mm and a vertical pitch of 125 mm.

Comparison between Co-Current-Flow and Counter-Current-Flow Heat Exchangers. The worked examples reported below will demonstrate the different efficiencies of the two heat exchanger configurations in terms of the heat transfer area required in both cases for the same U. Let us consider two fluids between which heat is being... 

For a given duty, q, and assuming that U is the same for parallel and counter-flow exchangers the heat transfer area required is given by Equation (42) ... 

The ratio between the heat transfer areas required for parallel and counter flow shows that the heat transfer area required with the parallel... 

Distilled water at 34 °C is cooled to 30 °C by a raw-water feed at 23 °C flowing to an evaporator. Estimate the heat-transfer area required to cool 79,500 kg/h (8.16x10 Ib/h) of distilled water using a 1-2 heat exchanger. 

Calculation of the rate of heat transfer required does not of itself determine the heat transfer area required or the configuration thereof. The configuration (tubes, plates, etc.) is typically chosen first, by rules of thumb and experience, depending on the liquor to be processed. 

Figure 3.18b shows the same streams plotted with a lower value of AT in- The amount of heat exchanged is increased and the utility requirements have been reduced. The temperature driving force for heat transfer has also been reduced, so the heat exchanger has both a larger duty and a smaller log-mean temperature difference. This leads to an increase in the heat transfer area required and in the capital cost of the exchanger. The capital cost increase is partially offset by capital cost savings in the heater and cooler, which both become smaller, as well as by savings in the costs of hot and cold utilities. In general, there will be an optimum value of ATmin, as illustrated in Figure 3.19. This optimum is usually rather flat over the range 10°C to 30°C.

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