Minimum temperature difference

Figure 16.10 shows another threshold problem that requires only hot utility. This problem is different in characteristic from the one in Fig. 16.9. Now the minimum temperature difference is in the middle of the problem, causing a pseudopinch. The best strategy to deal with this type of threshold problem is to treat it as a pinched problem. For the problem in Fig. 16.10, the problem is divided into two parts at the pseudopinch, and the pinch design method is followed. The only complication in applying the pinch design method for such problems is that one-half of the problem (the cold end in Fig. 16.10) will not feature the flexibility offered by matching against utility.

Example 16.2 A problem table analysis of a petrochemicals process reveals that for a minimum temperature difference of 50°C the process requires 9.2 MW of hot utUity, 6.4 MW of cold utility, and the pinch is located at 550°C... 

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. 

In Figure 3.22, the composite curve for the hot streams and the composite curve for cold streams are drawn with a minimum temperature difference, the displacement between the curves, of 10°C. This implies that in any of the exchangers to be used in the network the temperature difference between the streams will not be less than 10°C. 

In most exchanger networks the minimum temperature difference will occur at only one point. This is termed the pinch. In the problem being considered, the pinch occurs at between 90°C on the hot stream curve and 80°C on the cold stream curve. 

Convert the actual stream temperatures Tact into interval temperatures Tml by subtracting half the minimum temperature difference from the hot stream temperatures, and by adding half to the cold stream temperatures ... 

The use of the interval temperature rather than the actual temperatures allows the minimum temperature difference to be taken into account. ATmm = 10°C for the problem being considered see Table 3.4. 

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. 

So the minimum temperature difference condition, 10°C, will not be violated by this match. 

If the heat capacities of streams are such that it is not possible to make a match at the pinch without violating the minimum temperature difference condition, then the heat capacity can be altered by splitting a stream. Dividing the stream will reduce the mass flow-rates in each leg and hence the heat capacities. This is illustrated in Example 3.16. 

Check that the specified minimum temperature difference ATmm has not been violated, and revise the design as necessary to restore the ATm, . 

For any network there will be a best value for the minimum temperature difference that will give the lowest total annual costs. The effect of changes in the specified ATIllin need to be investigated when optimising a heat recovery system. 

Problems that show the characteristic of requiring only either a hot utility or a cold utility (but not both) over a range of minimum temperature differences, from zero up to a threshold value, are known as threshold problems. A threshold problem is illustrated in Figure 3.29. 

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... 

Cooling water is available with a return temperature of 30°C and a cost of 4.5 kW 1-y 1. Low-pressure steam is available at a temperature of 140°C and a cost of 90kW 1-y 1. Medium-pressure steam is available at a temperature of 200°C and a cost of 135 kW 1-y 1.The minimum temperature difference allowed is 10°C. 

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