Page 122 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

CHAPTER

5
Heat exchanger network analysis

5.1 Introduction
In a process, there are streams that are required to be cooled. These are hot streams. Similarly there are
cold streams that are required to be heated. It is technically possible to meet all the heating and cooling
requirements in a plant by employing only hot utilities for the cold and cold utility for the hot streams.
Hot and cold utilities are the external sources of heating and cooling in a process plant and these
constitute a sizeable component of the operating cost. Hot utilities include steam at various pressures,
hot oil, fuel fired or electrical heating furnaces, etc. Common cold utilities are cooling water, ambient
air used in fin fan coolers, refrigerated brine and generation of steam in waste heat boilers, to name a
few. To reduce the utility costs and their requirement, the hot process streams may be used to heat the
cold process streams, wherever possible. One may note that the amount of heat thus exchanged be-
tween the hot and the cold streams reduces the requirement of both hot as well as cold utility by the
same amount.
A typical example illustrating the saving in energy by mutual heat exchange between process
streams is shown in Fig. 5.1. It presents a simple continuous distillation process which has hot streams
and cold streams that require cooling and heating, respectively. In Fig. 5.1A, heat is added to the feed
preheat exchanger (E1, 840 kW) and reboiler (E3, 8.7 kW) using steam as hot utility and heat is
removed from overhead condenser (E2, 2 kW), bottom product cooler (E4, 189 kW) and the distillate
cooler (E5, 87 kW) using cooling water as cold utility. Total hot and cold utility loads in this case are
848.7 kW and 278 kW, respectively. Keeping the process same, the heat exchange scheme is slightly
altered and presented in Fig. 5.1B. Here, the available heat from the column bottom hot stream at
105 C is utilised to preheat the cold feed stream from 30 Cto70 C by introducing a feed vs bottom

product exchanger (E6, 189 kW). The interchange of 189 kW heat within the process steams in E6
increases the inlet temperature of E1 from 30 to 70 C and there is a consequential reduction of its heat

load from 840 to 651 kW. Also the exchanger E4 becomes redundant as the heat removed by the same
is now utilised for heating the feed stream. An audit of the hot and cold utility load for this new scheme
shows the figures to be (8.7 þ 651 ¼ ) 659.7 kW and (87 þ 2 ¼ ) 89 kW, respectively. Thus the
requirement of both hot and cold utility in this scheme is lower by 189 kW, the quantity of heat ex-
change between the hot and cold streams in the process. The new heat exchange scheme is a network of
heat exchangers whose basic types and designs are covered in Chapters 2 to 4. Compared to the
previous scheme (Fig. 5.1A), this new scheme (Fig. 5.1B) has a lower operating cost due to lower
utility consumption but the net effect on capital cost needs to be assessed. It increases due to incor-
poration of E6, the new heat exchanger and reduces due to lower heat load of E1 and elimination of E4.
Process Equipment and Plant Design. https://doi.org/10.1016/B978-0-12-814885-3.00005-1 119
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