A minimum permissible temperature difference ΔTmin must be specified, which prohibits any temperature crossing between the hot and cold streams. The value of ΔTmin is determined by the total heat transfer coefficients (U) and the geometry of the heat exchanger. If a higher value of ΔTmin is chosen, the heat recovery in the exchanger decreases and the demand for external supplies increases.
When there are multiple hot and cold streams, the construction of hot and cold composite curves simply involves adding the enthalpy changes of the streams in the respective temperature ranges. In order for heat exchange to take place from the hot stream to the cold stream, the hot stream's cooling curve must lie above the cold stream's heating curve. Due to the "broken" nature of the composite curves, they approach each other most closely at a point defined as the minimum approach temperature (Δ Tmin).
Δ Tmin can be measured directly from the T-H profiles as the minimum vertical difference between the hot and cold curves. Increasing the ΔTmin value results in the curves shifting apart horizontally, resulting in lower process-to-process heat exchange and higher utility requirements. A numerical approach called the "Problem Table Algorithm" (PTA) is also a way to determine the utility needs of a process and the location of the process bottleneck.
The composite curves provide overall energy targets, but do not clearly indicate how much energy must be supplied by different levels of supply.
Grand Composite Curve (GCC)
The method involves shifting (along the temperature [Y] axis) the hot composite curve downwards by ½ ΔTmin and that of the cold composite curve upwards by ½ ΔTmin, i.e. the result is a scale based on the process temperature, where the temperature approach ( ΔTmin) is taken into account. The GCC indicates that we can supply the hot water supply over two temperature levels TH1 (HP steam) and TH2 (LP steam).
In summary, the GCC is one of the most basic tools used in pinch analysis for the selection of appropriate utility levels and for targeting a given set of multiple utility levels. The targeting involves setting the appropriate taxes for the different utility levels by maximizing the least expensive utilities and minimizing taxes on the most expensive utilities. Estimation of minimum energy cost targets: Once the ΔTmin is chosen, the minimum hot and cold utility requirements can be evaluated based on the composite curves.
The GCC provides information on the auxiliary levels selected to meet the requirements for QHmin and QCmin. If the unit cost of each supply is known, the total energy cost can be calculated using the energy equation below.
Estimation of Heat Exchanger Network (HEN) Capital Cost Targets
Area targeting: The calculation of the surface area for a single counterflow heat exchanger is given by the relationship: Area = Q / [U x ΔTLM. Number of units targeting: For the minimum number of heat exchanger units (Nmin) required for EIA (minimum energy requirement or maximum energy recovery), the HEN can be evaluated prior to design by using a simplified form of Euler's graph theorem. HEN Total Capital Cost Targeting: The minimum area (Amin) and number of units (Nmin) targets can be combined together with the heat exchanger cost law to determine the HEN capital cost (CHEN) targets.
The equation used to calculate the total cost of capital and the exchanger cost law is given below. Estimation of the optimal value ΔTmin according to Energy Capital Trade: To reach an optimal value ΔTmin, the total annual cost (sum of the annual total.
Estimation of Optimum Δ T min Value by Energy-Capital Trade Off: To arrive at an optimum Δ T min value, the total annual cost (the sum of total annual
Evaluating practical goals for HEN design: HEN designed based on the estimated optimal ΔTmin value is not always the most suitable design. The importance of peak temperature allows energy targets to be met by designing the appropriate HEN. Pinch separates the process into two separate systems, each of which is in enthalpy equilibrium with the utility.
Violation of any of the above rules results in higher energy requirements than the theoretically possible minimum requirements. By using the pinch rules, it is possible to identify changes in the relevant process parameter that will have a beneficial effect on energy consumption. Decreasing (-) hot stream load below threshold and increasing (+) cold stream load below threshold will result in reduced cold stream target utility.
Appropriate location principles: Apart from the changes in process parameters, proper integration of key equipment in the process with respect to the pinch point should also be considered. The pinch concept of "Appropriate Placement" (integration of operations in such a way as to reduce the utilization requirement of the combined system) is used for this purpose. In addition to the above pinch rules and principles, a wide range of factors must also be considered during the design of HENs.
The essence of the pinch approach is to explore the possibilities of changing the design of the basic process, heat exchangers and utility systems with the ultimate goal of reducing energy and/or capital costs. Heat Exchanger Network Design: Designing a new HEN is best done using the "Pinch Design Method (PDM)". The systematic use of PDM enables the design of a good network that achieves energy goals within practical limits.
This can be achieved by using “tick-off” heuristics to identify the heat load on the pinch exchanger. The squeeze divides the heat exchange system into two thermally independent areas. HENs for both the upper and lower pinch areas are designed separately. After all possible combinations have been made, the two designs above and below the pinch are then brought together and usually refined to further minimize capital costs.
After the network is designed according to the pinch rules, it can be further subjected to energy optimization. The optimization of the network involves both topological and parametric changes of the initial design to minimize the total cost.
