1. Heat Exchange Networks

2. Outline Heat Integration Design Procedure for MUMNE
Temperature interval diagram
Cascade diagram
Temperature-Enthalpy diagram
Minimum number of exchangers
Design above and below pinch

3. These limits often واضح manifest themselves as mechanical constraints.
Pinch Technology
INTRODUCTION
Whenever design of a system is considered, limits exist that constrain design.
These limits often واضح manifest themselves as mechanical constraints.

4. For example, a distillation of two components that requires 400 equilibrium stages & a tower with a diameter of 20m would not be attempted because construction of such a tower would be virtually impossible with current manufacturing techniques.
A combination of towers in series & parallel might be considered but would be very expensive.

5. Whenever driving forces for heat or mass exchange are small, equipment needed for transfer becomes large & we say that design has a “قرصة Pinch.”

6. When considering systems of many heat or exchange devices (called exchanger networks), there will exist somewhere in system a point where driving force for energy or mass exchange is a minimum.

7. This represents a pinch or pinch point.
The successful design of these networks involves defining where pinch exists & using information at pinch point to design whole network.
We define this design process as pinch technology.

8. The concepts of pinch technology can be applied to a wide variety of problems in heat transfer. The focus of this chapter on implementation of pinch technology to new processes for both heat exchanger exchanger networks.
Retrofitting an existing process for heat conservation is an important but more complicated problem.

9. The approach followed in remainder of this chapter consists of establishing an algorithm for designing a heat exchanger network, (HEN) that consumes minimum amount of utilities & requires minimum number of exchangers (MUMNE).
Although this network may not be optimal in an economic sense, it does represent a feasible solution & will often be close to optimum.

10. Heat exchange networks
Heat Integration
Heat exchange networks
It saves money to match streams rather than pay to heat one & pay to cool another

11. Even in a preliminary design, some form of heat integration is usually employed.
Heat integration has been around in one form or another. Its early use in process industries was most apparent in crude preheat trains used in oil refining.

12. In refineries, thermal energy contained in various product streams is used to preheat crude prior to final heating in fired heater, upstream of atmospheric column.

13. The formalization of theory of heat integration & pinch technology has been attributed to several researchers, Linhoff & Flower, Hohmann, & Umeda et al.

14. If feed is to be reacted at an elevated temperature, then it must be heated. Likewise, after reaction has taken place, reactor effluent stream must be purified, which usually requires cooling stream, & possibly condensing it, prior to separating it.

15. The concept of heat integration, in its simplest form, is to find matches between heat additions & heat removals within process.
In this way, total utilities that are used to perform these energy transfers can be minimized or rather optimized.

19. Heat Integration There is a rigorous methodology
We will learn MUMNE (Minimum Utility, Minimum Number of Exchangers) method
Not necessarily (and unlikely to be) economic optimum

20. It is important to remember that object of this exercise is to obtain a heat exchanger network that exchanges minimum amount of energy between process streams & utilities & uses minimum number of heat exchangers to accomplish this.
This network is almost never optimum economic design. However, it does represent a good starting point for further study & optimization.

21. Design Procedure
Complete energy balance on all streams to determine all temperatures, values, and heat flows.
Choose minimum approach temperature. Typically, this is between 5°C and 20°C, but any positive number is valid.
Complete temperature interval diagram, Each stream is drawn and labeled. The heat flow in each interval is calculated.

22. Design Procedure
Complete the cascade diagram. The energy excess or deficit is calculated for each interval on the temperature interval diagram.
Find the minimum hot and cold utility requirements and identify the pinch temperature.
Complete the composite temperature enthalpy diagram. This is a T-Q diagram for the entire process.

23. Design the heat exchanger network.
Design Procedure
Determine the minimum number of heat exchangers required above and below the pinch.
Design the heat exchanger network.

24. Each of last steps given above is considered in detail using an example problem that is outlined below.

26. It should be noted that overall heat balance for these streams yields a net enthalpy change of 50 kW.
This does not mean that a heat exchanger network can be designed to exchange heat between hot & cold streams by receiving only 50 kW from a hot utility.

27. This is because above analysis takes into account, only first law of thermodynamics, that is, an enthalpy balance.
In order to design a viable heat exchanger network, it is also necessary to consider second law of thermodynamics, which requires that thermal energy (heat) only flow from hot to cold bodies.

28. As a consequence, utility loads will, in general, be significantly higher than those predicted by a simple overall enthalpy balance.

32. تتالي

36. To minimize hot & cold utility requirements, energy should not be transferred across pinch

37. At this point, it is important to note that not all problems have a pinch condition.
The cascade diagrams for two situations that do not have a pinch are illustrated in Figure In Figure 13.4(a), heat is only cascaded downward rejected to cold utility.

38. The conditions of hot & cold streams are such that after cascading energy downward, there is either an excess of energy or energy is exactly balanced in every temperature interval.
In this situation, there is no need to supply energy from hot utility to process.

40. In Figure 13.4(b), opposite situation from that described above exists. In this case, heat is only cascaded downward or supplied from hot utility.
The conditions of hot & cold streams are such that after cascading energy downward, there is either an energy deficit, or energy is exactly balanced in every temperature interval.

41. In this situation, there is no need to reject energy from process to cold utility. Although much of remaining discussion focuses on processes that have a pinch, approach for designing MUMNE network remains essentially same.

43. Above Pinch
The easiest way to evaluate minimum number of heat exchangers required is to draw boxes representing energy in hot & cold process streams & hot utility as shown at top of Figure 13.5.

45. We now transfer energy from hot steams & hot utility to cold streams.
These energy transfers are indicated by lines with amount of energy transferred shown to side of lines.

46. Clearly, all energy in hot streams & utilities must be transferred to cold streams.
For each line that we draw, we require one heat exchanger; thus, by minimizing number of lines we minimize number of heat exchangers.

47. It should be pointed out that although number of heat exchangers equals number of connecting lines, lines drawn at this stage may not represent actual heat exchangers. The actual design of exchanger network is covered in step 5.

48. Below Pinch
We use same method to calculate minimum number of exchangers below pinch. The diagrams for above & below pinch are shown in Figure 13.5, & from this we see that five exchangers are required above pinch & three below pinch or a total of eight heat exchangers for entire network.

50. From Figure 13.5, it can be seen that above pinch, problem is split into two subproblems. This split is possible because energy in two of hot streams (Streams 2 & 3) exactly matches energy requirement of one of cold streams (Stream 6). Below pinch, such a partition of problem is not possible.

51. Such exact matches between groups of hot & cold streams are often not possible, & if such matches do not exist, then for a given problem, above or below pinch, following relationship can be written:

52. Min. No. of exchangers =
No. of hot streams +
No. of cold streams +
No. of Utilities -1

53. The result in above Equation is consistent with result in Figure 13
The result in above Equation is consistent with result in Figure 13.5 for below pinch.

54. Step 5: Design Heat Exchanger Network
Again, consider systems above & below pinch separately

55. Design Above Pinch
To start, draw temperature interval diagram above pinch, Figure The algorithm we use to design network starts by matching hot & cold streams at pinch & then moving away from pinch.

57. Design At Pinch
For streams at pinch, match streams such that mcp,hot≤ mcp,cold. Using this criterion, we ensure that ∆Tmin from Step 1 is not violated.
From Fig. 13.6(a), see that we can match Stream 2 or 3 with Stream 5 or 6. Note, for this step, we consider only streams that are present at pinch temperature

59. Next step is to transfer heat from hot streams to cold streams by placing heat exchangers in temperature diagram. This step is shown in Figure 13.6(b).
There are two possibilities when matching streams at this point: exchange heat between Streams 2 & 5 & Streams 3 & 6 or exchange heat between Streams 2 & 6 & Streams 3 & 5.

60. Only first combination is shown in Figure 13. 6(b)
Only first combination is shown in Figure 13.6(b). A heat exchanger is represented by two circles connected by a solid line; each circle represents a side (shell or tube) of exchanger. It is important to exchange as much heat behtween streams as possible.

61. Temperatures of streams entering & leaving exchangers are calculated from an enthalpy balance. For example, consider enthalpy change of Stream 5 as it passes through Exchanger 1. The total heat transferred is Q1= 100 kW & mcp,5= 8; therefore, temperature change for Stream 5, ∆T5 = 100/ 8 = 12.5°C. Thus, temperature change for Stream 5 through Exchanger 1 is 90°C to 102.5°C, as shown in Figure 13.6(b).

63. DESIGN AWAY FROM PINCH
The next step is to move away from pinch & look at remaining hot & cold streams. There are several ways in which we may exchange heat from Stream 1 (only remaining hot stream) & three cold Streams 4, 5, & 6.

64. The criterion for matching streams at pinch does not necessarily hold away from pinch; however, we should make sure that when network is designed following constraints are not violated:

65. 1. A minimum approach temperature of 10°C, set in Step 1, is used throughout design.
2. Only five exchangers are used for design above pinch, as calculated in Step 3.
3. Heat is added from coolest possible source

66. The matching of streams & final design of network are shown in Figures 13.6(c) & 13.6(d). From these figures, it is clear that we have a design with five heat exchangers, minimum approach temperature is nowhere less than 10°C, & that we add heat to process at lowest temperature consistent with this system, that is, 190°C.

68. Design Below Pinch
The approach for design below pinch is similar to that described above for above pinch. We start at pinch & match streams & then move away from pinch & match remaining streams. The temperature interval diagram below pinch is shown in Figure 13.7(a).

70. DESIGN AT PINCH
For streams at pinch, match streams such that mcp,cold≤ mcp,hot. Using this criterion, ensure that ∆Tmin from Step 1 is not violated.
From Fig. 13.7(a), see that for three streams at pinch this criterion cannot be met.

71. This problem can be overcome by splitting cold Stream 6 into two separate streams. However, before consider this, let us see what happens if we try to match streams that violate above condition. In Figure 13.7(b), Stream 2 is matched with Stream 6.

72. The net result is impossible because it would cause a temperature cross in exchanger that violates second law of thermodynamics. In order to maintain minimum temperature approach set in Step 1, split Stream 6 into two equal streams, each having an mcp = 2.0

73. We can now match these split streams with hot Streams 2 & 3 without violating criterion above. The net result is shown in Fig. 13.7(c), from which it can be seen that minimum temperature difference is always greater than or equal to 10°C. It should be noted that Stream 6 could be split in a number of ways to yield a viable solution. For example, it could be split into streams with values of 3 & 1.

75. DESIGN AWAY FROM PINCH
The final step is shown in Fig. 13.7(d), where third exchanger is added to transfer excess heat to cold utility.
The final heat exchanger network is shown in Fig The exchangers are represented by single circles, with fluid flowing through both sides.

78. Example Problem Stream Properties for Example Problems Stream Tin Tout
kW/°C Q kW 1
200
120 3
240 2
140
100 5
170
-210 4
110
190
-160
Net heat flow 70

79. Example Problem
The value of Q might not be given in above table, or Q is given and is missing. These are calculated from the energy balance. The sign convention is positive for heat available from a stream and negative for heat needed by a stream.
Choose the minimum approach temperature. For this problem, it is 10°C.

80. Example Problem
Draw and label the temperature interval diagram. Label the intervals beginning with “A” for the highest temperature interval. The heat flow for each interval is calculated from, , where the sum is over all streams existing in that interval.

83. Example Problem
Draw the cascade diagram. This represents the cascade of heat flowing down from high to low temperatures. Add utilities where needed. Label the heat flows. The net utility flow should agree with the net heat flow on the earlier table.
On the cascade diagram, there will be a location where the heat-flow cascade is not continuous. This represents the pinch temperature

85. Example Problem
Construct the composite temperature enthalpy diagram. This provides useful information, but it is not required to solve the problem.

86. Example Problem interval T (°C) Q (kW) E 100 90 D 110 50 C 120 30 B
Hot Cold
interval
T (°C)
Q (kW) E
100 90 D
110 50 C
120 30 B
140
260
130 A
180
380
170
330
200
440
190
370

87. Example Problem
In the table, the temperature shown is at the lower end of the interval. The Q values are obtained by summing all existing on the interval and adding it to the previous interval. The temperature difference is for that interval. The value is the sum of all existing streams on that interval.

88. Example Problem
The hot and cold stream lines are plotted, as shown on the following figure. Clearly, there is a temperature cross, so the cold stream line is shifted to the right until the minimum approach temperature of 10°C exists at one point. (It could exist at more than one point by coincidence.) For this problem, all Q values for the cold stream must be increased by 130 kW, as shown in the figure. Note how the hot and cold utility requirements are apparent from the diagram.

90. Example Problem
By representing the heat available in each stream and from the utilities both above and below the pinch, the minimum number of heat exchangers can be determined. This identifies the minimum number, but not necessarily the correct stream matches. The correct number of heat exchangers is the number of process streams + the number of utility streams – 1.

92. Example Problem
Note that if a “direct match” is found, i.e., where sets of two streams match heat flows exactly, one fewer exchanger may appear to be possible. However, be careful, the minimum approach temperature may be violated.

93. Example Problem
Design the heat exchange network. There may not be unique streams here. The design is started at the pinch and you work away from the pinch. Above the pinch, for any streams that exist at the pinch, streams can only be matched such that

94. Example Problem
When dealing with streams away from the pinch, this criterion is no longer needed. Any streams can be matched as long as the temperatures are valid. If the criterion at the pinch appears impossible to satisfy, streams can be split to satisfy the criterion.

99. Example Problem
The same procedure is done below the pinch, except that the criterion is
Streams are matched and heat exchangers are added until all required heat transfer is accomplished. The entire network, both above and below the pinch, can then be represented on one diagram.

103. In-class Example Problem
Stream Properties for In-class Example Problem
Stream
Tin
Tout
kg/s Cp
kJ/kg°C 1
250
100 2
280
120 3
200 4
230 5

104. In-class Example Problem
Determine (minimum approach T = 20°C)
minimum hot and cold utility consumption
pinch temperatures
minimum number of heat exchangers required above and below the pinch
design of heat exchange network above and below the pinch

112. Summary Heat Exchange Networks Well-established procedure
Not necessarily (and unlikely to be) economic optimum but a very good starting point
Straight forward, but must be careful when matching streams at pinch
Different correct answers possible

113. COMPOSITE TEMPERATURE ENTHALPY DIAGRAM
Physically, pinch zone, mentioned above, represents a point in heat exchanger network at which at least one heat exchanger, or two streams, will have minimum approach temperature set in Step 1.

114. This point is perhaps more clearly shown in a composite enthalpy-temperature iagram. This diagram is essentially the same as the combination of all the T -Q diagrams for all the exchangers in the network.

115. Such a diagram for this example is shown in Figure 13
Such a diagram for this example is shown in Figure 13.9 & is constructed by plotting the enthalpy of all the hot streams & all the cold stream as a function of temperature, as shown below: