During the operation of heat exchanger networks(HEN)with varying operating conditions

bypass optimal control is considered an effective method to adjust outlet fluid temperature and save energy.However

locations of bypasses are derived without the verification by the control theory.And

it is always very difficult to choose the location of bypasses with the trade-off between controllability and capital investments.Therefore

it is necessary to develop a method to design a HEN control structure based on the control theory.Based on structural controllability

in order to maintain all states controllable and minimize the number of bypasses

a bypass optimal design method with all states controllable(method 1)was proposed.On the other hand

when the number of bypasses was limited

to maximize controllable states an alternative bypass optimal design method with sub-controllable states(method 2)was also presented.The results indicated that with these two methods

the optimal bypass design could be derived for any requirement of controllability and capital investments.A case study of HEN before desalting in an atmospheric/vacuum distillation plant demonstrated the effectiveness of these two methods proposed in this paper.