Understanding Pinch and Approach Points

Two important terms, namely, pinch and approach points should be understood by every- one involved in simulation analysis (see Figure 5.2). When gas inlet temperature to a waste heat boiler is low, on the order of 400°C–600°C, pinch and approach points determine the gas–steam temperature profiles and steam generation. Irrespective of surface areas pro- vided, the exit gas temperature (in a single-pressure HRSG) cannot be predetermined. It falls in place depending on steam pressure and pinch and approach points selected in the design mode. In conventional steam generators, the combustion temperature in the furnace may be taken as the gas inlet temperature and hence is very high (1800°C–1900°C), and hence, pinch point or the steam pressure does not govern the temperature profiles; the heat sink in the form of economizer is very huge as a lot of energy is transferred in the furnace, evaporator, and superheater, and hence, the gas is cooled to any level with appropriate siz- ing of the economizer. One can obtain any desired exit gas temperature in steam generators irrespective of steam pressure. However, in low gas temperature heat recovery systems, the steam pressure and pinch point play a crucial role in determining the gas–steam temperature profiles, and the exit gas temperature from the economizer cannot be arbitrarily assumed. A temperature profile analysis must be performed to evaluate the steam generation and exit gas temperature. As a thumb rule, higher the inlet gas temperature, lower the exit gas temperature from the economizer and vice versa in a single-pressure HRSG.

tg1 tso

tg2

Sh. Evap. Eco. tg3

tg4

tw2 Pinch point = tg3–ts Approach point = ts–tw2

tw1

tw1, tw2 are Feed water temperature entering and leaving economizer ts is the Saturation temperature tso is the Superheated steam temperature

tg1, tg2, tg3, tg4 are gas temperatures at superheater inlet and exit, evaporator exit, and economizer exit

FIGURE 5.2

Explanation of pinch and approach points.

HRSG Simulation 267

TABLE 5.1

Suggestions on Pinch and Approach Points

Evaporator Type

Plain Tubes

Finned Tubes

For Both

Gas inlet temp., °C (°F)

Approach,°C (°F) 650–900 (1200–1650)

Pinch point, °C (°F)

Note : Pinch and approach points are differences in temperatures and hence 1.8 × pinch

point in °C = pinch point in °F.

As seen in Figure 5.2, pinch point is the difference between the gas temperature leaving the evaporator and saturation temperature, and approach point is the difference between the saturation temperature and water temperature leaving the economizer. Once the pinch and approach points are selected, the entire HRSG gas–steam temperature profiles, duty of each section, and UA of each section are obtained. Note that we select pinch and approach points only once, in the design mode. The design mode should be the unfired mode at the design ambient temperature or the guarantee point even if the HRSG operates in the fired mode all the time. In off-design cases, the pinch and approach points fall in place. Table 5.1 gives some suggestions on selecting pinch and approach points.

The earlier suggestions are based on author’s experience. When the gas inlet temperature is very high, say, 900°C (incineration plants), a low pinch point is not feasible as it will lead to temperature cross situation (explained later). Also, when plain tubes are used, it is difficult to get a low pinch point as the HRSG will be huge and impractical to build. When we are required to maximize steam generation, we select low pinch and approach points such as 5°C–7°C pinch point in finned tube gas turbine HRSGs. Approach point selection is based on whether the HRSG is likely to operate at low loads and whether economizer steaming is an issue. This is discussed later. If there are multiple evaporator modules and steam generation in the second pressure level is more important and is to be guaranteed, then a high pinch point can be selected for the first evaporator and a low pinch point for the second evaporator.

Many process engineers and consultants are of the view that any desired exit gas temperature can be obtained in single-pressure HRSGs. This is wrong. Gas–steam temperature profile is a function of steam pressure and the exit gas temperature t g4 cannot be preselected. This is explained later.

Understanding Pinch and Approach Points