Temperature pinch point

Pinch Point—This is defined as the differenee between the exhaust gas temperature leaving the evaporator seetion and the saturation temperature of the steam. Ideally, the lower the pineh point, the more heat reeovered, but this ealls for more surfaee area and, eonsequently, inereases the baek-pressure and eost. Also, exeessively low pineh points ean mean inadequate steam produetion if the exhaust gas is low in energy (low mass flow or low exhaust gas temperature). General guidelines eall for a pineh point of 15-40°F (8-22 °C). The final ehoiee is obviously based on eeonomie eonsiderations. 

Another important parameter is the temperature difference between the evaporator outlet temperature on the steam side and on the exhaust gas side. This difference is known as the pinch point. Ideally, the lower the pinch point, the more heat recovered, but this calls for more surface area and, consequently, increases the back pressure and cost. Also, excessively low... 

Some conditions require breaking up the exchanger into multiple parts for the calculations rather than simply using corrected terminal temperatures. For such cases one should always draw the q versus temperature plot to be sure no undesirable pinch points or even intermediate crossovers occur. 

Maximum steam exit temperature Minimum pinch point temperature difference... 

Exhaust gases from the gas turbine are used to raise steam in the lower cycle without the burning of additional fuel (Fig. 7.3) the temperatures of the gas and water/steam flows are as indicated. A limitation on this application lies in the heat recovery system steam generator choice of the evaporation pres.sure (p ) is related to the temperature difference (Tft — T ) at the pinch point as shown in the figure, and a compromise has to be reached between that pressure and the stack temperature of the gases leaving the exchanger, (and the consequent heat loss ). ... 

Newby et al. found that increasing the PO turbine pressure resulted in higher steam flow (for a given pinch point temperature difference in the HRSG), increased PO turbine power and overall plant efficiency. However, at the highest pressure of 100 bar attempts to increase the steam flow further resulted in incomplete combustion in the main combustor and the overall thermal efficiency did not increase substantially at this pressure level. 

Harvey et al. gave a parametric calculation of the thermal efficiency of this plant, as a function of turbine inlet temperature, the reformer pinch point temperature difference and the pressure level in the reformer (the compressor overall pressure ratio, r). 

Unfired cycle This cycle is very similar to the mentary-fired case except there is no added fuel heat input. The approach temperature and pinch point are even more critical, and tend to reduce steam pressures somewhat. Similarly, the gas turbine exhaust temperature imposes further limits on final steam temperature. 

Dual pressure For comparison, a combined cycle scheme with dual pressure is shown in Figure 15.13. In this case, the waste heat recovery boiler also incorporates a low-pressure steam generator, with evaporator and superheater. The LP steam is fed to the turbine at an intermediate stage. As the LP steam boils at a lower temperature than the HP steam, there exists two pinch points between the exhaust gas and the saturated steam temperatures. The addition of the LP circuit gives much higher combined cycle efficiencies with typically 15 per cent more steam turbine output than the single pressure for the same gas turbine. 

As shown in Figure 8.1, this takes place between the points B and H. In fact, point H is defined as the intersection of TB - 30 and the segment DEss the cold composite curve approaches the hot one. Therefore, the pinch point occurs between the temperatures ... 

Remark 1 Since we cannot bring the two composite curves closer, the pinch point represents the bottleneck for further heat recovery. In fact, it partitions the temperature range into two subnetworks, one above the pinch and one below the pinch. Heat flow cannot cross the pinch since there will be violations in the heat exchange driving forces. As a result, we need a hot utility at the subnetwork above the pinch and a cold utility at the subnetwork below the pinch. In other words, having identified the pinch point, we can now apply the first law analysis to each subnetwork separately and determine the hot and cold utility requirements. These can be read from the T - Q diagram since they correspond to the horizontal segments AG and CD, respectively. Hence, for our example we have ... 

Note also that the introduction of hot stream H2 is responsible for the pinch point (i.e., the inlet temperature of H2). 

Remark 4 Model P4 is applied to each subnetwork, that is after decomposition based on the location of the pinch point(s). If, however, we apply model P4 to overall networks without decomposing them into subnetworks, then the quality of lower bound on the nonvertical heat transfer becomes worse. This is due to the fact that the additional variables and constraints have been applied for the overall heat transfer in each match (ij). As a result they do not provide any direction/penalty for local differences, that is, differences between heat exchange loads versus maximum vertical heat transfer loads at each temperature interval k TI. This deficiency can be remedied by introducing the variables Sik,k TI and the parameters Q k corresponding to each temperature interval k, along with the constraints ... 

Remark 1 The problem statement is identical to the problem statement of section 8.5.3.1 for the synthesis of HENs without decomposition (Ciric and Floudas, 1991). Note that as in section 8.5.3.1, there is no specification of any parameters so as to simplify or decompose the original problem into subproblems. In other words, the level of energy recovery (specified by fixing H RAT), the minimum approach temperature (EM AT), and the number of matches are not specified a priori. As a result, there is no decomposition into subnetworks based on the location of the pinch point(s), but instead the pinch point(s) are optimized simultaneously with the matches and the network topology. The approach presented for this problem is from Yee and Grossmann (1990) and is an alternative approach to the one of HEN synthesis without decomposition proposed by Ciric and Floudas (1991), and which was presented in section 8.5.3. 

Conversely, would the GA process be compatible with CEA secondary helium The answer is yes, but the overall production of the HTR coupled plant would be lower. Indeed, one could say that the GA Q(T) curve, as it is, is compatible with the CEA s helium Q(T) curve in both processes, secondary helium leaves Section II at a temperature around 600°C. However, this coupling implies the presence of a pinch point at the end of Section II which precludes increasing the helium flow rate, and hence the hydrogen production rate. Furthermore, setting the helium return temperature to 400°C implies that its heat content between 600°C and 400°C must be used in some way. 

Temperature profiles for the air and water are shown in the figures below. There are two possible situations. In the first case the minimum temperature difference, or "pinch" point occurs at an intermediate location in the exchanger. In the second case, the pinch occurs at one end of the exchanger. There is no way to know a priori which case applies. 

Figure 2.14 illustrates the overall approach by pinch-point analysis. The first step is extraction of stream data from the process synthesis. This step involves the simulation of the material-balance envelope by using appropriate models for the accurate computation of enthalpy. On this basis composite curves are obtained by plotting the temperature T against the cumulative enthalpy H of streams selected for analysis, hot and cold, respectively. Two aspects should be taken into account ... 

Start the analysis of exchangers in the sink and source sections at the pinch point where all temperatures are fixed. 

We plot the sets of temperature versus enthalpy rate values in Figure 4.44. This is a composite diagram for the heat integration problem. It is apparent from the figure that the closest vertical approach of the two curves occurs at an enthalpy change rate of 10,000 kW. This is the pinch point for the two composite curves, and occurs where the temperature of the streams that are to be heated is 120°C and the temperature of the streams that are to be cooled is 153°C. This A7mm of 33°C is simply a consequence of the starting enthalpy rates that were initially chosen. 

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