Minimum temperature difference heat exchangers

In Figure 3.20 the stream temperatures are plotted on the y-axis and the enthalpy change in each stream on the x-axis. For heat to be exchanged a minimum temperature difference must be maintained between the two streams. This is shown as Armin on the diagram. The practical minimum temperature difference in a heat exchanger will usually be between 10 and 20°C see Chapter 12. 

It can be seen by comparing Figure 3.20a and b that the amount of heating and cooling needed will depend on the minimum temperature difference. Decreasing ATmm will increase the amount of heat exchanged between the two streams and so decrease the consumption of the hot and cold utilities. 

Below the pinch the procedure is the same the aim being to bring the cold streams to the pinch temperature by exchange with the hot streams. For streams adjacent to the pinch the criterion for matching streams is that the heat capacity of the cold stream must be equal to or greater than the hot stream, to avoid breaking the minimum temperature difference condition. 

Determine the pinch temperatures and the minimum utility requirements for the streams set out in the table below, for a minimum temperature difference between the streams of 20°C. Devise a heat exchanger network to achieve the maximum energy recovery. 

A Trim Minimum temperature difference in heat exchanger 9... 

While the shell-and-tube heat exchanger is the most commonly used in the process industries, it has the disadvantages that the flow is not truly countercurrent, which limits the minimum temperature difference that can be accommodated, and the area density is relatively low. Commonly used alternatives for shell-and-tube heat exchangers are ... 

So far, it has been assumed that the minimum temperature difference for a heat exchanger network applies globally between all streams in the network. However, there are occasions when nonglobal minimum temperature differences might be required. For example, suppose a heat... 

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

This means that for a given reaction characterized by its thermo-kinetic constants (fc0, E, Of) processed in a given reactor, and characterized by its heat exchange parameters (U, A, T0), there is a minimum temperature difference required for stable performance of the reactor ... 

During polymerization, a certain amount of polymer evaporates. One design option giving additional cooling capacity is to condense, in an external heat exchanger, the polymer from the vapor phase and to return the liquid to the reactor bulk. We assume that, under the most demanding conditions, 30% of the heat generated by the polymerization reaction can be removed in the condenser. Under these conditions, the minimum temperature difference becomes ... 

A vapor-compression refrigeration system is conventional except that a countercurrent exchanger is installed to subcool the liquid from the condenser by heat exchange with the v stream from the evaporator. The minimum temperature difference for heat transfer is 10(°F). Amm is the refrigerant, evaporating at 22(°F) and condensing at 80(°F). The heat load on the evapo is 2,000(Btu)(s) . If the compressor efficiency is 75 percent, what is the power requirement ... 

Since we are concerned only with changes in enthalpy, we can treat the enthalpy axis as a relative scale and slide either the hot stream or the cold stream horizontally. As we do so, we change the minimum temperature difference between the streams, ATminj and also the amount of heat exchanged and the amounts of hot and cold utilities required. 

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

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