Network pinch

Figure 18.35 The network pinch limits the energy recovery in the existing heat exchanger network. (From Asante NDK and Zhu XX, 1997, Trans IChemE, 75A 349, reproduced by permission of the Institution of Chemical Engineers.)...

In practice, if the network pinch is being identified in a design study, then a practical minimum temperature... 

Consider now how the network pinch might be overcome. There are four ways in which the network pinch can be overcome and the performance of the existing heat exchanger network improved beyond that for the pinched condition14. 

Again, it should be emphasized that in practice a finite practical A Tmin should be used rather than 0°C. However, any assumption of A Tmin to identify, and then overcome, the network pinch does not guarantee... 

It is not trivial to determine the best modification options that could feature minimal capital cost and the smallest effect on existing infrastructure and thus achieve maximum energy savings. The search for such options could be very time consuming for a complex heat recovery system. The incentives to find practical yet optimal modifications have been discussed by many other researchers and practitioners. The network pinch method developed by Zhu and Asante (1999) has been proven successful for practical applications, which will be discussed in detail in this chapter. 

In the past, the retrofit methods based on the proeess pineh concept was widely used (Tjoe and Linnhoff, 1986 Policy etal., 1990 Shokoya and Kotjabasakis, 1991). However, the process pinch is developed for grassroots design and is fundamentally irrelevant to the retrofit scenario. The method (Zhu and Asante, 1999) to be discussed here is a two-stage approach identify the network pinch, the true bottleneck of an existing heat recovery system, and determine modihcations with minimum capital costs to overcome the network pinch— that is the reason why this method is called the network pinch method. 

NETWORK PINCH—THE BOTTLENECK OF EXISTING HEAT RECOVERY SYSTEM... 

Difference Between the Process Pinch and the Network Pinch... 

It must be noted that the process pinch is different from the network pinch. For the network in Figure 10.2, the process pinch occurs at a hot pinch temperature of 158 °F and cold pinch temperature of 133 °F, respectively, as shown in Figure 9.8, while the network pinch occurs at a hot pinch temperature of 111 °F and cold pinch temperature of 86 °F, respectively, as displayed in Figure 10.2. The reason for the difference is that the process pinch is defined by the process conditions such as temperatures... 

The network pinch identifies the heat recovery limit inherent in an existing heat exchanger network and therefore indicates the requirement of structural changes to the HEN. In other words, when a network reaches its network pinch limit, the only way to overcome it or increase heat recovery is to make stmctural changes if no process changes are considered. 

However, not all network stmctural changes are able to overcome the network pinch, but only those that can move heat from below to above the network pinch can. This principle provides a valuable guideline for screening out nonbeneficial modification options, which will be discussed next. 

Having identified the network pinch or the bottleneck, what can we do about it The answer is that only changes to the network stmcture can overcome the bottleneck. In the stage of selecting stmctural changes, the maximization of heat recovery is used as the selection criteria. Four types of modifications can be considered for the heat exchanger network, which are ... 

In most cases, many stmctural changes capable of overcoming the network pinch can be identified, and usually they produce different energy savings and capital costs. To select the best modification(s), among many alternatives, it is necessary to define a measure of optimality. Ideally, the cost-based objective should be employed. 

The rationale of the network pinch method is based on the effort and difficulty required for each assessment task. The effort for conducting a revamp study is demanding as it could involve weeks or up to months depending on the complexity of the process. During the revamp evaluation stage, not only itemized costs need to be determined, but effects on piping, pressure drops, and operation will be evaluated as well. To reduce the manpower effort, the network pinch method has been implemented into mathematical... 

CASE STUDIES FOR APPLYING THE NETWORK PINCH RETROFIT APPROACH... 

CASE STUDIES EOR APPEYING THE NETWORK PINCH RETROEIT APPROACH... 

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