Heat exchangers minimum temperature approach

Cooler/Process Heat Exchangers Minimum approach temperature =... 

The heat exchanger area can be calculated in the hypothesis of counter-current 1-2 heat exchangers, as expressed by the relation 10.6. Then the cost of capital can be computed by considering a cost law. The number of shells is found straightforward by assuming a maximum area per unit of 500 m. The optimisation variable is the minimum temperature approach. 

Heuristic 26 Near-optimal minimum temperature approaches in heat exchangers depend on the temperature level as follows ... 

As discussed in detail in Section 13.1, the minimum approach temperature in a countercurrent-flow heat exchanger may occur at an intermediate location rather than at one of the two ends when one of the two streams is both cooled and condensed. If the minimum temperature approach is assumed to occur at one of the two ends of the heat exchanger, a smaller approach ora temperature crossover that violates the second law of thermodynamics may occur at an intemie-diate location. To avoid this situation, the following heuristic should be applied ... 

Minimum Temperature Approach AT For a feasible heat transfer between the hot and cold composite streams, a minimum temperature approach must be specified, which corresponds to the closest temperature difference between the two composite curves on the T H axis. This minimum temperature approach is termed as the network temperature approach and defined as AT an-Maximal Process Heat Recovery The overlap between the hot and cold composite curves represents the maximal amount of heat recovery for a given AT r - In other words, the heat available from the hot streams in the hot composite curve can be heat-exchanged with the cold streams in the cold composite curve in the overlap region. 

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. 

In the ultimate version of the reheated and intercooled reversible cycle [CICICIC- HTHTHT- XJr, both the compression and expansion are divided into a large number of small processes, and a heat exchanger is also used (Fig. 3.6). Then the efficiency approaches that of a Carnot cycle since all the heat is supplied at the maximum temperature Tr = T ax and all the heat is rejected at the minimum temperature = r,nin. 

Determine the pinch temperature and the minimum utility requirements for the process set out below. Take the minimum approach temperature as 15 °C. Devise a heat exchanger network to achieve maximum energy recovery. 

A Tmm or EM AT, Minimum approach temperature, specifies the minimum temperature difference between two streams exchanging heat within an exchanger. 

To design the heat exchanger, the heuristic is used that the minimum approach temperature differential ATH = Tont — TCout is 25 K, which is reasonable for the temperature level in this process. This pinch temperature differential occurs at the hot end of the heat exchanger. An overall heat transfer coefficient U = 0.142 kJ s 1 m-2 K-1 is used in this gas-gas system. 

A process has four streams with the characteristics given below. Devise a heat-exchange network to maximize the annual savings as compared to no heat exchange. Use a minimum approach temperature ATmin = WC. 

AT ,n Minimum temperature difference (minimum approach) in heat exchanger e... 

At what value of the minimum approach temperature does the problem in Example 3.16 become a threshold problem Design a heat exchanger network for the resulting threshold problem. What insights does this give into the design proposed in Example 3.16 ... 

Wen, Y. and Shonnard, D. R (2003). Environmental and economic assessments of heat exchanger networks for optimum minimum approach temperature, Comput. Chem. Eng., 27, pp. 1577-1590. 

Optimisation in flowsheeting implies by principle several variables, because a mono-variable optimisation can be solved easily by a sensitivity study. A flowsheeting optimisation problem is always constraint. Firstly, there are equality constraints, as for example the material and heat balances, but also phase equilibrium conditions, as the equality of component fligacities. Secondly, there are inequality constraints. Usually these consist of bounds on temperatures, pressures, flow rates, and concentrations, but they can express also performance limits, as minimum reflux ratio, temperature approach in heat exchangers, etc. 

---