Pinch point temperature difference

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

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). 

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... 

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 ). ... 

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. 

It is rare for there to be two process pinches in a problem. Multiple pinches usually arise from the introduction of additional utilities causing utility pinches. However, cases such as that shown in Figure 18.19 are not uncommon, where there is strictly speaking, only one pinch (one place where ATmin occurs), but there is a near-pinch. This near-pinch is a point in the process where the temperature difference becomes small enough to be effectively another pinch, even... 

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 ... 

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. 

Since the intermediate temperature difference, THi - TCi is greater than the temperature difference at the end point, TH2 - TC2, the assumption of a pinch at the end is correct. 

Zh is the difference between the heat sources and sinks above the pinch point for pinch candidate p. For hot and cold streams with inlet temperatures given by and t " and outlet temperatures 7 and r° respectively, Zh Cv) is given by... 

Pinch Point Analysis is a systematic process design methodology consisting of a number of concepts and techniques that ensure an optimal use of energy. The Pinch is characterised by a minimum temperature difference between hot and cold streams, and designates the location where the heat recovery is the most constraint. 

In the composite curves (Fig. 58), the cold and hot streams of a production plant are plotted against temperature. When the two curves are shifted horizontally relative to each other until the minimum temperature difference necessary for heat-transfer processes is reached at the point where the curves come closest together (pinch), this gives the theoretical minimum for the heat streams that must be supplied or removed (hot and cold utility targets). 

The appearance of a pinch point leads to an infinitely large heat exchanger area. Therefore, a minimum temperature difference between the hot and cold composite... 

For high thermal efficiency, it is desirable that the cycle should approximate to a reversible one. The cycle can be reversible only if the temperature difference between the primary and secondary coolants is everywhere equal to zero, a condition impossible to attain in practice, since there has to be a finite difference between the two fluids in order to promote heat transfer between them. In the diagram, the smallest temperature difference between the primary and secondary fluids occurs between the points B and 3 this gap is called the pinch point. 

In order to approach a reversible cycle, the temperature difference (AT) at the pinch point should be as small as possible. Since the heat transfer rate in the exchanger is also proportional to the surface area of contact between the two fluids, it is possible to compensate for a smll AT by increasing the size of the heat exchanger. There are, of course, obvious limitations to the practical size of the unit, so that with this type of cycle there will always be an appreciable difference between the average temperatures of the two coolants. 

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