文档介绍：HEAT WORK
SYNTHESIS 1: TARGETING
OBJECTIVES:
1) Determine the minimum energy inputs required
for a process
2) Determine the minimum number of heat exchangers
required to transfer this energy
3) Develop a methodology for determining where these
heat exchangers should be placed in the process
2/21/99 HENS 1: Targeting 1
SINGLE HEAT EXCHANGER - REVIEW
W
COLD
T
COLDin
W
HOT
T T
HOTin HOTout
T
COLDout
Stream W(kg/h) Cp (kJ/kg-K) Tin(K) Tout(K) Q(gJ/h)
Hot 20,000 180 -
Cold 15,000 60 150 +
2/21/99 HENS 1: Targeting 2
Heat Exchanger Q-T Diagram
200
180
160
140
120
100
80
60
40
20
Temperature (K) 0
0 1 2 3 4 5 6
Q (gJ/h)
Thot Tcold
2/21/99 HENS 1: Targeting 3
IMPORTANT POINTS
1) The First Law says that the heat transferred from the hot
stream must equal that transferred to the cold stream.
Therefore, of the six process variables (two flow rates
and four temperatures), only five can be specified
independently.
2) The Second Law says that heat can only be transferred
from a hotter fluid to a colder one. Therefore, the
temperature of the cold fluid must be less than that of the
hot fluid at all points along the length of the exchanger.
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IMPORTANT POINTS (Cont’d.)
3) As a practical matter, for heat to be transferred at an
acceptable rate, the temperature of the cold fluid must be
less than that of the hot fluid at each point in the
exchanger by a reasonable amount. We refer to the
minimum acceptable temperature difference as the
MINIMUM APPROACH TEMPERATURE or ∆Tmin.
This is also known in the literature as the PINCH
TEMPERATURE. The methodology for synthesizing
heat works is sometimes called
PINCH TECHNOLOGY
2/21/99 HENS 1: Targeting 5
THE HEAT WORK
PROBLEM
A chemical plant generally has more than one hot
steam to be cooled and more than one cold stream
to be heated. To better understand this problem
let us