Network evolution: Heat load loops and heat load paths

After the network has been designed according to the pinch design rules it can be subjected to further simplification and capital-energy optimisation. The figures below illustrate the degrees of freedom available for network evolution after the pinch design method.

A heat load loop

The figure above shows a degree of freedom called the "Heat Load Loop" [2, 3]. The loop is shown by the solid purple line marking a circuit between matches 2 and 4. Heat duties can be shifted around a loop without affecting the heat duties imposed on other units in the network which are not part of the loop. This may result in reducing the heat load on a unit to zero (e.g. exchanger 4). However such a change will affect the temperature driving forces in the network. The temperature driving force will become less than DTmin and in some cases it may result in temperature infeasibility. The cost saving in installation of a unit needs to be compared with increase in the surface area due to the reduction in temperature difference.

Such an optimisation can be carried out in the SuperTarget PROCESS Module. PinchExpress can also be used to shift heat loads around loops. In many situations using heat load loops can reduce both the number of exchanger units and the overall network cost.

A heat load path

The Figure above illustrates the second degree of freedom for network evolution called the "Heat Load Path" [2, 3]. A path provides a continuous connection between two utilities and therefore allows the transfer of heat loads between heat exchanger units and the utilities. In the figure the path connects heater, exchangers 2, 4 and the cooler. By changing the heat loads of exchangers 2 or 4, the heater and cooler heat loads will change by the same amount but in an opposite manner. This is demonstrated below. The heating duty can be eliminated if the duty on exchanger 2 is increased by the corresponding amount. Then the cooling duty must also be decreased by this amount to maintain the energy balance on the hot stream. Heat load paths exploit capital-energy trade-off in specific parts of the network. SuperTarget

Pinch Technology Introduction

PROCESS and PinchExpress also have the facility to optimise the network performance using heat load paths.

Using a path to reduce utility use

Heat load loops and paths provide additional degrees of freedom for the designer in order to further simplify the network or reduce the overall network cost.

Network design for multiple utilities

The example for network design as demonstrated previously uses one hot and one cold utility. For processes involving multiple utilities, all the utilities are incorporated in the grid diagram as streams. The network is therefore "balanced" in enthalpy terms. The grid diagram also incorporates all the utility as well as process pinches.

The network design for multiple utilities is more complex due to the occurrence of multiple pinches in the problem. References [2, 3] are appropriate for further background on network design involving multiple utilities. PinchExpress and SuperTarget PROCESS incorporate features for the placement of multiple utilities.

Summary: New heat exchanger network design

The steps to be followed in network design for a grass roots project can be summarised as follows:

a)Develop a Minimum Energy Requirement (MER) network

•Divide problem at the Pinch

•Start at the pinch and move away

•Start with biggest stream "IN"

•Observe CPOUT ≥ CPIN, splitting streams where necessary

•Place all pinch matches first

•Maximise loads on all pinch matches to minimise number of units (the "tick-off" rule)

•Fill in the rest

•Merge above and below the pinch designs

b)Evolve the MER network for network simplicity and capital-energy trade-off