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Hierarchy of Decisions
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HEAT EXCHANGER NETWORK (HEN)
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SUCCESSFUL APPLICATIONS
O ICI ---- Linnhoff, B. and Turner, J. A., Chem. Eng., Nov. 2, 1981 Energy savings Capital Cost Available Expenditure Process Facility* k$/yr or Saving, k$ Organic Bulk Chemical New same Specialty Chemical New saving Crude Unit Mod saving Inorganic Bulk Chemical New saving Specialty Chemical Mod New saving General Bulk Chemical New unclear Inorganic Bulk Chemical New to unclear Future Plant New to 40 % % saving Specialty Chemical New Unspecified Mod New saving General Chemical New unclear Petrochemical Mod Phase I Phase II *New means new plant; Mod means plant modification.
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Linnhoff and Vredeveld, CEP, July, 1984
SUCCESSFUL APPLICATIONS Table 1. First results of applying the pinch technology in Union Carbide Project Energy Cost Installed Payback Process Type Reduction $/yr Capital Cost $ Months Petro-Chemical Mod ,050, , Specialty Chemical Mod , , Specialty Chemical Mod , , Licensing Package New ,300, Savings Petro-Chemical Mod , Yet Unclear ? Organic Bulk Mod ,000, , Chemical Organic Bulk Mod ,243, ,835, Specialty Chemical Mod , , Organic Bulk Mod ,000, , Linnhoff and Vredeveld, CEP, July, 1984
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SUCESSFUL APPLICATIONS
Fluor --- IChE Symp. Ser., No. 74, 1982, P.19 --- CEP, July, 1983, P.33 FMC (Marine Colloid Division, Rockland, ME)
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CONCLUSION separate and distinct task in process design
HEN/MEN synthesis can be identified as a separate and distinct task in process design
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IDENTIFY HEAT RECOVERY AS A SEPARATE AND DISTINCT
TASK IN PROCESS DESIGN. 9.60 200C 18.2 bar 1.089 36C 16 bar RECYCLE REACTION 7.841 126C 18.7 bar TO COLUMN D 201 1.614 0 179 200C PURGE CW 180C 153C 35C 7 703 FLASH 141C 40C 115.5C 17.3 bar 120C bar FEED 5C 19.5 bar 114C Figure Flowsheet for “front end” of specialty chemicals process
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FOR EACH STREAM: TINITIAL, TFINAL, H = f(T). 126C
Reactor 35C 200C RECYCLE TOPS Product Purge Reactor 35C 5C FEED FOR EACH STREAM: TINITIAL, TFINAL, H = f(T). PRODUCT 126C Figure 2.6-Specialty chemicals process-heat exchange duties
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。 。 = 1722 = 654 a ) DESIGN AS USUAL H 6 UNITS REACTOR C STEAM
= 1722 = 654 a ) DESIGN AS USUAL H 6 UNITS REACTOR C STEAM RECYCLE 70 1 。 。 STEAM 1652 3 2 654 COOLING WATER FEED PRODUCT
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。 。 。 。 = 1068 = 0 b ) DESIGN WITH TARGETS H 4 UNITS REACTOR C
= 1068 = 0 b ) DESIGN WITH TARGETS H 4 UNITS REACTOR C STEAM 。 RECYCLE 1068 。 。 1 。 2 3 FEED PRODUCT
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SUGGESTED PROCEDURE FOR THE DESIGN OF NEW HEAT EXCHANGER NETWORKS
1. Determine Targets. Energy Target -maximum recoverable energy Capital Target -minimum number of heat transfer units. -minimum total heat transfer area 2. Generate Alternatives to Achieve Those Targets. 3. Modify the Alternatives Based on Practical Considerations. 4. Equipment Design and Costing for Each Alternative. 5. Select the Most Attractive Candidate.
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STEP ONE Determine the Targets
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§ ENERGY TARGETS (TWO STREAM HEAT EXCHANGE) T/H DIAGRAM
Q =CP(TT-TS) TT TS H H Figure Representation of process streams in the T/H diagram
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TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM
200 UTILITY HEATING 140 135 115 100 70 UTILITY COOLING 350 300 400 H (KW) TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM
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TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM
200 UTILITY HEATING 130 135 T 120 100 70 UTILITY COOLING 350 300 400 H (KW) = = =300 TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM
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( ) ( ) FACTS 1. Total Utility Load Increa se Increa se
in = in Hot Utility Cold Utility ( ) ( )
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§ENERGY TARGETS (MANY HOT AND COLD STREAMS) COMPOSITE CURVES
(T1-T2) (B) (T2-T3) (A+B+C) (T3-T4) (A+C) (T4-T5) (A) CP=B CP=A CP=C H
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§ENERGY TARGETS (MANY HOT AND COLD STREAMS) COMPOSITE CURVES
T1 T2 T3 T4 T5 H
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PINCH POINT Minimum T hot utility “PINCH” minimum cold utility H
Energy targets and “the Pinch” with Composite Curves
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m hot Qin Streams n cold Streams Qout - Qin = H Qout or
Heat Exchange System n cold Streams Qout - Qin = H Qout or
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The “Problem Table” Algorithm - A Targeting Approach
---Linnhoff and Flower, AIChE J. (1978) Stream No CP TS TT and Type (KW/C) (C) (C) (C) (C) (1) Cold T6 T3 (2) Hot T1 T5 (3) Cold T4 T2 (4) Hot (T2) (T6) Tmin = 10C
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Subsystem # CPHot - CPcold TK HK T1* = 165C T2* = 145C T3* = 140C T4* = 85C T5* = 55C T6* = 25C 2 4 3 1
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90C 145C Heat Exchange Subsystem (3) 80C 135C from subsys #2
hot streams 145C Heat Exchange Subsystem (3) . . . . . Cold streams 80C 135C To subsys #4
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T1* = 165C -------------------------- ( 0 )------
FROM HOT UTILITY minimum hot utility 20 H1 = 60 H2 = 2.5 H3 = -82.5 Pinch H4 = 75 H5 = -15 minimum cold utility 60 TO COLD UTILITY
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§ “PROBLEM TABLE” ALFORITHM
SUBSYSTEM TM TC=T 0 (T0) 1 (T1) 2 (T2) TP Tmin Hh2Hc2 Hh1 Hc1
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§ “PROBLEM TABLE” ALFORITHM
ENTHALPY BALANCE OF SUBSYSTEM As T = T1 - T2 0
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5. The Grand Composite Curve
80 60 40 20 -20 Q(KW) CU Qc,min “Pinch” HU Qh,min T6* T5* T4* T3*T2* T1*
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SIGNIFICANCE OF THE PINCH POINT
1. Do not transfer heat across the pinch 2. Do not use cold utility above 3. Do not use hot utility below
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Q Qh Qh HU Qc,min CU Qh,min Tc Tp Th T Qh Qh,min Qc Qc,min
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Q CU Qc,min Qh,min HU Tc Tp T1 Th T
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Q Qc CU2 Qh HU Qc,min CU1 Qh,min Tc Tp Th T
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Q Qh,min HU Qc,min CU Tc Tp T1 Th T
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Q Qh,min HU2 Qc,min Q1 CU Q2 HU1 Tc Tp T1 Tp’ Th T
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H=27MW H= -30MW FEED 2 140 PRODUCT2 230 REACTOR 2 200 80 H=32MW FEED 1 20 REACTOR 1 180 250 OFF GAS 40 H= -31.5MW 40 PRODUCT1 40 Figure A simple flowsheet with two hot streams and two cold streams.
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Supply Target capacity temp. temp. H flow rate CP
TABLE 6.2 Heat Exchange Stream Data for the Flowsheet in Fig. 6.2 Heat Supply Target capacity temp temp H flow rate CP Stream Type TS (C) TT (C) (MW) (MW C-1) 1. Reactor 1 feed Cold 2. Reactor 1 product Hot 3. Reactor 2 feed Cold 4. Reactor 2 product Hot
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HOT UTILITY HOT UTILITY H= -1.5 H= -1.5 H= 6.0 H= 6.0 H= -1.0
(a) HOT UTILITY (b) HOT UTILITY 245C MW MW H= -1.5 H= -1.5 235C MW MW H= 6.0 H= 6.0 195C MW MW H= -1.0 H= -1.0 185C MW MW H= 4.0 H= 4.0 145C MW MW H= -14.0 H= -14.0 75C MW MW H= 2.0 H= 2.0 35C MW MW H= 2.0 H= 2.0 25C MW MW COLD UTILITY COLD UTILITY Figure The problem table cascade.
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Figure The grand composite curve shows the utility requirements in both enthalpy and temperature terms.
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(a) BOILER T* HP Steam LP Steam pinch CW H
Process HP Stream Process Fuel Boiler Feedwater (Desuperheat) BOILER LP Stream Condensate T* HP Steam LP Steam pinch CW H Figure The grand composite curve allows alternative utility mixes to be evaluated.
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Hot Oil (b) T* pinch CW H
Hot Oil Return Fuel FURNACE Process Hot Oil Flow T* Hot Oil pinch CW H Figure The grand composite curve allows alternative utility mixes to be evaluated.
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(a) TC 300 250 200 150 100 50 0 5 10 15 H(MW) HP Steam LP Steam
HP Steam LP Steam H(MW) Figure Alternative utility mixes for the process in Fig. 6.2.
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(b) TC 300 250 200 150 100 50 Hot Oil H(MW) Figure Alternative utility mixes for the process in Fig. 6.2.
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T*TFT Theoretical Flame Temperature T*O T*STACK QHmin Flue Gas Air
Fuel T*STACK T*O Ambient Temperature Stack Loss ambient temp. QHmin H Fuel Figure Simple furnace model.
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T*’TFT T*TFT Flue Gas T*STACK T*O Stack Loss T*
Figure Increasing the theoretical flame temperature by reducing excess air or combusion air preheat reduces the stack loss. T*STACK T*O Stack Loss H
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T* T* away from the pinch
T*TFT T* T*TFT T*ACID DEW T*PINCH T*C T*ACID DEW T*PINCH T*C (a)Stack temperature limited by acid dew point (b)Stack temperature limited by process away from the pinch Figure Furnace stack temperature can be limited by other factors than pinch temperature.
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T* 1800 1750 Flue Gas 300 250 200 150 100 50 H(MW) Figure Flue gas matched against the grand composite curve of the process in Fig. 6.2
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SOME RESULTS IN GRAPH THEORY
1 ) A graph is any connection of points, some pairs of which are connected by lines. 2 ) If a graph has p points and q lines, it is called a (p,q) graph. points process and utility streams lines heat exchangers 3 ) A path is a sequence of distinct lines, each are starting where the previous are ends, e.g. AECGD in Fig. A. A B C D Figure A Figure B E F G H A B C D E F G H
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SOME RESULTS IN GRAPH THEORY
4 ) A graph is connected if any two points can be joined by a path, e. g. Fig. A 5 ) Points which are connected to some fired point by paths are said to form a component, e. g. Fig A has one component. Fig B has two components. 6 ) A cycle is a path which begins and ends at the same point, e. g. CGDHC in Fig. A. 7 ) The maximum number of independent cycles is called the cycle rank of the graph. 8 ) The cycle rank of a (p,q) graph with k components is q - p + k
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A Result Based on Graph Theory
U = N+L-S Where, N = the total number of process and utility streams L = the number of independent loops S = the number of separate component in a network U = the number of heat exchanger services
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U = N+L-S U = N-1 = 5 U = N-2 = 4 U = N+1-1 = N = 6 30 70 90 ST H1 H2
ST H1 H2 U = N-1 = 5 U = N-2 = 4 U = N+1-1 = N = 6 C1 C2 CW ST H1 H2 C1 C2 CW ST H1 H2 X X 30-X X C1 C2 CW
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CAPITAL TARGET Umin = N - 1 where,
Umin = the minimum number of services N = the total number of process and utility streams Note, U = N + L - S
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§ PINCH DESIGN METHOD RULE 1: THE “TICK-OFF” HEURISTIC UMIN = N-1 - THE EQUATION IS SATISFIED IF EVERY MATCH BRINGS ONE STREAM TO ITS TARGET TEMPERATURE OR EXHAUSTS A UTILITY. - FEASIBILITY CONSTRAINTS : ENERGY BALANCE TMIN
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Example 1 Stream No TS TF CP Heat Load and Type (F) (F) BTU/hr F Q BTU/hr (1) Cold (2) Cold (3) Hot (4) Cold (5) Hot (6) Cold (7) Hot Tmin = 20F Qhmin = 104 BTU/hr Qcmin = 0
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Hot streams CP Q 1.32 2.624 590 471 419 533 400 430 400 280 3 5 505.6 7 1 416 2 505.6 4 341.1 6 341.1 Cold streams
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CP Q 1.557 4.128 590 574 471 400 430 400 3 86.3 5 254 1 86.3 2 412.8 4 412.8
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CP Q 590 400 430 3 1 H 2 22.4
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CP Q 1.32 2.624 590 471 533 400 430 400 280 3 5 505.6 7 H 1 86.3 2 22.4 505.6 4 412.8 341.1 6 341.1
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§ PINCH DESIGN METHOD RULE 2: DECOMPOSITION - THE HEN PROBLEM IS DIVIDED AT THE PINCH INTO SEPARATE DESIGN TASKS. - THE DESIGN IS STARTED AT THE PINCH AND DEVELOPED MOVING AWAY FROM THE PINCH.
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DATA FOR EXAMPLE II Temperature Heat Capacity
Supply Target Flowrates Heat load Process Stream TS TT CP Q no. Type F F BTU/h/F BTU/h 1 Cold 2 Hot 3 Cold 4 Hot Tmin = 10 F QHmin = 50 104 BTU/h QCmin = 60 104 BTU/h
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PINCH DECOMPOSITION DEFINES THE SEPARATE DESIGN TASKS
260 2 250 4 240 1 240 3 C = 60 Btu/h H = 50 Btu/h Umin = 4 Umin = 3 PINCH DECOMPOSITION DEFINES THE SEPARATE DESIGN TASKS
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BELOW THE PINCH CP Q 190 2 3 190 4 4 G 60 190 3 4 1 ABOVE THE PINCH CP Q 260 2 1 250 4 2 235 H 2 1 20 90 240 H 1 3
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Cp Q 260 1 3 2 250 2 4 C 4 60 235 H 2 3 4 1 240 H 1 3 THE COMPLETE MINIMUM UTILITY NETWORK
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PINCH MATCH Pinch A Pinch Match Pinch 2 1 Exchanger 2 is not
Exchanger 2 is not a pinch match Pinch 1 Exchanger 3 is not a pinch match
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FEASIBILITY CRITERIA AT THE PINCH
Rule 1: Check the number of process streams and branches at the pinch point Above the Pinch : PINCH PINCH 90 80 90 80 1 1 2 2 3 3 (80+T1) 4 4 (80+T2) Q1 5 5 Q2 Tmin = 10C Tmin = 10C
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FEASIBILITY CRITERIA AT THE PINCH
Rule 1: Check the number of process streams and branches at the pinch point Below the Pinch : 90 80 (90-T1) 90 80 1 1 (90-T2) 2 2 3 3 4 4 Q1 5 5 Q2 PINCH PINCH Tmin = 10C
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FEASIBILITY CRITERIA AT THE PINCH
Rule 2: Ensure the CP inequality for individual matches are satisfied at the pinch point. Above the Pinch : Below the Pinch : CPH1 CPC3 1 1 CPH2 CPC4 2 2 3 3 Q2 4 4 PINCH Q1 PINCH 1 T 2 T Tmin Tmin 3 4 Q Q Q2 Q1 CPC CPH CPC CPH
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Stream data at the pinch NH NC? Yes No CPH CPC Split a for every
pinch match Split a cold stream No Yes Split a stream ( usually hot) Place pinch matches Figure Design procedure above the pinch. (From B. Linnhoff et al., 1982.)
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Stream data at the pinch NH NC? Yes No CPH CPC Split a for every
pinch match Split a cold stream No Yes Split a stream ( usually hot) Place pinch matches Figure Design procedure below the pinch. (From B. Linnhoff et al., 1982.)
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CRITERION #3 THE CP DIFFERENCE
ABOVE THE PINCH, INDIVIDUAL CP DIFFERENCE = CPC - CPH OVERALL CP DIFFERENCE = BELOW THE PINCH, INDIVIDUAL CP DIFFERENCE = CPH - CPC THE SUM OF THE INDIVIDUAL CP DIFFERENCES OF ALL PINCH MATCHES MUST ALWAYS BE BOUNDED BY THE OVERALL CP DIFFERENCE.
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Overall CP Difference = 8 - 6 = 2
PINCH CP 4 2 5 3 Overall CP Difference = = 2 Total Exchanger CP Difference = = 2 O.K.
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Overall CP Difference = 9 - 6 = 3
PINCH CP 4 2 5 3 1 Overall CP Difference = = 3 Total Exchanger CP Difference = = 2 O.K.
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Overall CP Difference = 9 - 5 = 4
PINCH CP 3 2 8 1 Overall CP Difference = = 4 Total Exchanger CP Difference = = 6 Criterion violated !
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Cp Q 260 1 3 2 250 130 2 4 C 4 60 235 180 135 H 2 3 4 1 240 H 1 3 Heat Load Loops heat loads can be shifted around the loop from one unit to another
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4 H 2 3 H 2 4 1 H C 1 3 C Heat Load Loops heat loads can be shifted around the loop from one unit to another
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260 1 3 2 250 130 2 C 4 60 235 H 2 3 1 240 H 1 3 Heat Load Path heat loads can be shifted along the path
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4 H 2 3 H 2 1 H C 1 3 C Heat Load Path heat loads can be shifted along the path
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Cp Q 260 1 3 2 2 250 C 4 60+X 235 165 2 3 H 1 20+X 240 H 1 3 X=7.5
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Two ways to break the loop
If: L1>L4 L2>L3 then: X=L4 or X= -L3 1 1 2 2 3 4 (a) 3 L2 + X L4 - X 4 L3 + X L1 - X 1 2 3 2 1 4 3 4
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heater/cooler can be included in a loop
1 3 4 2 (b) H1 - X 3 H L3 + X 4 H L4 - X H2 + X 1 H 3 4 3 4 Figure Complex loops and paths
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Match 1 is not in the path 1 2 (c) 3 4 H 1 2 4 3 C 2 3 1 4 C H 4 2 3
C + X 3 L3 + X L4 - X 4 H L2 - X H + X H 1 2 4 2 3 4 3 C Figure Complex loops and paths
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