Environmental Engineering Reference
In-Depth Information
identified. The task is then becomes to design the heat exchanger network. In a little
elaboration define the HEN streams matching, duty of each heat exchanger, inlet and outlet
temperatures of each heat exchanger and with given overall heat transfer coeffiecients
calculate the surface area of each heat exchanger. In the section below we will be defining the
topology, duty and temperatures information of the HEN.
The Pinch Design Method
The best design for an energy efficient heat exchange network will often result in a
tradeoff between the equipment and operating cost. This is dependent on the choice of the
DTmin for the process. The lower the DTmin chosen, the lower the energy costs, but
conversely the higher the heat exchanger capital costs, as lower temperature driving forces in
the network will result in the need for greater area. A large DTmin on the other hand will
mean increased energy costs due to less overall heat recovery, but the required capital cost
will be less. This is true most of the time but not all of the time. Designers may find that a
very large DTmin might lead to high levels of heat duties available to be handled by process
to process heat exchangers and process to utiltities ones resulting in high surface area needs
and consequently both capital cost and energy cost will be increased.
Any way early in this chapter we introduced how to set energy and area targets for the
process before considering the HEN design because in the early days of pinch technology this
technique was important to help make the trade-off between the HEN capital cost and operating
cost quickly and without any heavy calculations. However, nowadays lot of software is
available to make a preliminary synthesis of any large size HEN and estimate its capital cost
directly and then automatically make the trade-off between the operating cost and the capital
cost for the HEN in order to determine the optimal Dtmin for the HEN to be designed.
Now let us first estimate the minimum number of units in a HEN (
N
units ) using the
following formula:[1]
N
units =
S - 1
Where,
S =
number of streams (hot and cold) including utilities
It is important to keep in mind that using the above formula for HEN minimum number of
units calculation will only be giving an estimate and in some cases designers can come up
with designs which exhibit less number of units due to some perfect matches in temperature
range and load among hot and cold process streams.
Having calculated an estimate to HEN minimum number of units we can then use the
composite curves again used before to determine the energy targets for a given value of
DTmin to determine another estimate to the minimum heat transfer area required to achieve
the desired energy targets ahead of HEN design.
