Page 140 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

5.6 Design algorithm 137

Fig. 5.9D shows the construction when we introduce intermediate utilities, namely LP steam and
cooling water (CW). A horizontal line at the LP steam temperature level starting from the vertical
(shifted temperature) axis and touching the grand steam composite curve denotes the LP steam
quantity. HP steam satisfies the balance heating duty. This maximises LP steam consumption prior to
use of HP steam and thus minimises the total utilities cost. Similar construction can be performed
below the pinch to maximise the use of cooling water for minimum use of refrigeration.
It is important to note that the horizontal lines indicating different utility levels should be drawn at
the appropriate shifted temperature levels and not at their actual temperatures, i.e. a hot utility is
indicated by a horizontal line at its temperature reduced by ðDT min =2Þ and a cold utility at its tem-
perature increased by ðDT min =2Þ.
Thus the construction of the grand composite curve automatically ensures a minimum temperature
difference ðDT min Þ between the hot and cold process streams (at the process pinch marked in the
figure) as well as between utilities and process streams.
“Utility Pinches” are the points where the LP and CW levels touch the grand composite curve in
Fig. 5.9D. Cross-utility pinch heat flow results in reducing heat load from a
cheaper utility level and adding it to a more expensive utility level. As discussed
earlier, violation of a process pinch also results in heat load penalty for the
Utility Pinch
utilities.
A grid diagram involving multiple utilities should include all utilities so as to
have a network that is balanced with respect to enthalpy.

5.6 Design algorithm

Input data
• Process stream heating and cooling information
- Process stream start and target temperature TS ð CÞ & TTð CÞ

- Stream enthalpy change ðDHÞ or product of mass flow rate and heat capacity (CP)
- Stream heat transfer coefficients for heating and cooling streams (h i )
• Utility system information
- Available utility (e.g. refrigerant, CW or steam system)
- Existing utility levels (e.g. steam header pressures, refrigeration level temperatures, etc.)
- Existing utility loads in case of retrofit problems e to assess potential savings
- Utility system constraints (such as maximum or minimum permissible flows)
- Plans regarding future investment in the utility system (e.g. plan for a new gas turbine or
cogeneration)
• Cost information to identify economically attractive projects.
- Fuel and power tariff structure
- Cost of various utility levels e.g. cost of LP steam, HP steam, etc.
- Cost laws for heat exchangers (cost as a function of heat transfer area)
- Investment criterion (e.g. minimum acceptable pay-back period, etc.)