pinch technology application in a hospital
TRANSCRIPT
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Pinch technology application in a hospital
A. Herrera a, J. Islas b,*, A. Arriola c
a HGZ No.1, IMSS, Josee Mariia Chaavez 1202, Lindavista, 20000 Aguascalientes, Ags., Mexicob Centro de Investigacioon en Energiia, UNAM, Apdo. Postal 34, Temixco 62580, Morelos, Mexico
c Gerencia de Procesos Teermicos, IIE, Av. Reforma 113, Col. Palmira, Temixco 62490, Morelos, Mexico
Received 5 December 2001; accepted 3 September 2002
Abstract
This paper is related to a pinch technology application in a hospital. The two most important results ofthis application are: a thermal power savings potential of 38% and the identication of the optimum heat-
exchanger network design. In relation to pinch technology development, this paper pretends to be a re-
ported case about the relevance of applying this technology to a low enthalpy thermal system as the one
encountered in a hospital.
2002 Elsevier Science Ltd. All rights reserved.
Keywords: Pinch technology; Hospital; Energy reduction potential; Threshold case
1. Introduction
Hospitals are important energy users, especially thermal energy and this characteristic is thereason for the appearance of many studies about energy savings alternatives in hospitals such asDOE [1], Ostroy [2], Thumann [3], Thumann and Mehta [4], IEA [5], Santamouris et al. [6],Jakeelius [7], CADDET [8], Hyman [9], BUREAU [10], European Commission [11], Williams [12],Getino [13], Gonzaalez [14], Lockie [15], Saanchez [16] and CMPL [17]. Nevertheless, none ofthese studies has been done applying pinch technology as an ecient tool to evaluate energysavings in these low entalphy thermal systems.The case study is a hospital complex of Instituto Mexicano del Seguro Social (IMSS) located in
the city of Aguascalientes, Mexico, which is integrated by a General Hospital, a regional laundry
*Corresponding author. Address: Energy Research Centre, National University of Mexico, Apartado Morelos
62580, Mexico. Tel.: +52-5622-9719; fax: +52-5622-9791.
E-mail address: [email protected] (J. Islas).
1359-4311/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.PII: S1359-4311(02)00157-6
Applied Thermal Engineering 23 (2003) 127139www.elsevier.com/locate/apthermeng
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centre, a sport centre, the Social Welfare Centre and the Family Health Centre. In 1999 dieselrepresented 75% of its total energy consumption and 68% of its total energy cost which was 396131 USAD1999 [18]. For the IMSS-Aguascalientes (IMSS-Ags.) it was very important to cal-culate the potential of thermal energy savings and to optimize the use of thermal energy in thecomplex.
2. Pinch technology
To achieve these goals pinch technology was applied to the hospital complex. The concepts andmethodology of pinch technology are well explained in the works of Linho et al. [19], Eastop andCroft [20], Linnho [21] [22], Mubarak Ebrahim [23] and Arriola [24].The benets of pinch technology application in industrial applications are several and include
according to Best [25] the identication of the maximum thermal energy recovery, the optimumheat exchanger network design and the minimum thermal utilities required. This paper will showthat these benets can be extended to low enthalpy thermal systems such us those of the servicesector.
3. Methodology
Pinch point technology is based on process thermodynamic analysis. The methodology used inthis work has therefore as fundamental points mass and energy balances and the optimum use ofthe heat ows in the process and consists of the following steps:
1. Obtaining the process diagram of the thermal system of the hospital complex.2. Obtaining thermodynamic data of the thermal system (temperatures, pressures, enthalpies,
mass ows and specic heats).3. Identication of the hot and cold streams in the hospital complex.4. Identication of the DTMIN (minimum approach temperature) for the original thermal system.5. Construction of the hot and cold composite curves.6. Elaboration of the problem table.7. Determination of the pinch temperature and the minimum heating and cooling utilities re-
quired in the hospital complex.8. Evaluation of the thermal energy savings potential.9. Modication of the heat exchanger network according to pinch technology.
4. The thermal system and the hot and cold streams of the IMSS-Ags. complex
The application of the pinch technology for any industrial processes starts with the repre-sentation of the process streams as vectors in temperature vs enthalpy graphs. Fig. 1 shows the
128 A. Herrera et al. / Applied Thermal Engineering 23 (2003) 127139
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thermal process diagram of the IMSS-Ags. complex and its thermodynamic data [26]. Processstreams can be classied as follows: Cold streams are those where heating is required. Hotstreams are those where cooling is required. Heating utilities have external heat inputs. Theseheating utility streams are produced in auxiliary service equipment, generally a steam generator(combustion boiler, heat recovery boiler) or a direct red boiler (furnace). Cooling utilities areintegrated by all streams used for cooling with thermal source external to the process such as anopen cooling system (natural water) or a closed one (cooling towers). In the IMSS-Ags. complexcase the thermal demand is covered by steam produced by boilers using a high price diesel,nevertheless, there is no heat recovery network in this complex. To apply the pinch technologyto the complex, the heat wasted in the soapy water from laundry and the ow of condensedsteam not recovered in the condensation network were identied as hot streams. Fig. 2 shows thehot and cold streams proposed for the complexs thermal system and their thermodynamicdata.Each process stream or utility starts with an inlet temperature (Tinlet) and after exchanging
heat in one or several heat exchangers, nishes with an outlet temperature (Toutlet). The thermalload DH (enthalpy change, Dh, multiplied by mass ow, Fm) of the streams is the heat transferredto the stream while heating (case of cold streams) or the heat rejected during its cooling (case ofhot streams). In the thermal system of the IMSS-Ags. complex it is considered that the massow Fm (kg/s) and the specic heat Cp (J/kg C) are constants. In this case the cold and hot
CALDERA5.5 kg/cm2
RED DEDISTRIBUCIN
ESTERILIZACIN
LAVANDERA
AGUA SANITARIA
AGUA DEALBERCAS
DIESEL
TANQUE DECONDENSADOS
REPOSICINDE AGUA
T= 240 CH=740 kW
T=60 C
FW=24526 lt./d
H=740 kW
P=2kg /cm 2
=77.3 kW
=77.12 kW
= 151.4 kW
P=2kg/ cm2 T=121 C
BOILER5.5 kg/cm2
STEAMNETWORK
STERILIZATIN
LAUNDRY
SANITARY WATER
COOKING
SWIMMINGPOOL
HEATING
DIESEL
CONDENSEDSTEAM TANK
WATER CONDENSEDSTEAM NETWORK
H= 45.9 kW
AIR
P=2kg cm T=121 C
P=5kg/ cm2 T=150 C
=12.5 kW
P=5kg/cm2 T=150 C
P=2kg/cm2 T=121 C = 59.6 kW
P=5kg/cm2 T=150 C
P=2kg cm = 100.8 kW
=625.3 kW
H=7.1 kW
D
H D
H=D
HD
HD
HD
HD
HD
HDHD
Fig. 1. IMSS-Ags. complex thermal system diagram.
A. Herrera et al. / Applied Thermal Engineering 23 (2003) 127139 129
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streams are represented by straight lines in a temperature vs enthalpy graph and dened bytheir thermal load H . Its slope equals the thermal capacity, dened as CP FmCp DH=DT(W/C).
5. Determination of the pinch position for a threshold case and composite curves of the IMSS-Ags.complex
Once all services and streams are quantied in terms of temperature (T ) and thermal load (DH )they are grouped together in temperature intervals for their graphic representation in which iscalled the composite curves of the process. The composite curves are bases on the thermal loadvalues, FmCpDT , and the temperature intervals, DT , of each streams. This information is graphi-cally represented in coordinate axis in which the X axis is the thermal load (DH expressed in MW)and the Y axis is the temperature (T expressed in C). In this way, two composite curves are ob-tained, one for cold streams and another one for hot streams. These curves show the quantity ofavailable or missing heat in the process for dierent temperature levels, and a value for mini-mum hot and cold utility requirements for each minimum approach temperature, DTmin, as shownin Fig. 3.
Heating
H CP Tinlet. Toutlet.(kW) (kW/oC) (oC) (oC)
Soapy water 23.7 0.53
Condensed steam 96.32 2.41
Cold StreamsLaundry sanitary water 17.60 0.59
Laundry 77.27 2.58
Boiler feed water
Sanitary water
7.13 0.24
77.12 2.20
Sterilization 12.50 0.14
Swimming pool water 151.67 50.56
Cooking 59.63 0.85
100.82 14.40
Bedpanwashers 4.94 0.05
Hot Streams
L
BF
SW
S
SP
CO
H
B
85
60
60
121
100
25
121 21
28
25
30
55LS
2555
1 85 40
280 40
Toutlet. Tinlet.
30
25
30
18
Fig. 2. Hot and cold streams proposed for the IMSS-Ags. complex.
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Nevertheless, as the case studied shows, not all thermal systems have these properties. Thereare thermal systems that reach a point in which one of the thermal utilities reduces to zero whenthe DTmin value diminishes (when the composite curves are brought together moving them inthe horizontal axis). The DTmin value at which this happens is known as the thresholdDTmin. From the threshold value, if the curves are brought closer by moving on the horizon-tal axis doesnt change the value of the required utility. It can be seen that in the compos-ite curves of Fig. 3 that three zones exist: the heating zone in the extreme right of the curves,the exchange zone, where they overlap, and the cooling zone in the extreme left of the curves.In the threshold problems, the hot composite curve, for example, has such a thermal loadthat from a DTmin approach value it stays completely inside the cold composite curve, result-ing with only two zones, one of heating and one of overlap. The same thing can happenwhen the cold curve is inside the hot curve resulting only in one zone of cooling and one ofoverlap.Fig. 4 is a graph of the hospital complex thermal system showing the required thermal services
against DTMIN, where a threshold case is found with a temperature range of 022 C. To resolvethis case an analytic method was used to nd by iteration. For this DTMIN no external heat wasrequired for the system. In terms of heat load balance this means the sum H of the last column inTable 2 equals zero. The result was DTMIN 22 C.In this process Table 1 was very useful. It contains the thermodynamic data of the hot and
cold streams for the complex thermal system and the modied temperatures for the solutionDTMIN 22 C, that is to say, the latest DTMIN tested in the iterative process. These modi-ed temperature data of the hot and cold streams of the system enables to set the problemtable showed in Table 2. This table shows the thermodynamic data of the complex systemthermal balance realized with the present hot and cold streams classied according to thetemperature intervals of Fig. 5. The last columns of this table show the sums of the thermal
PinchHeatsources
Heatreceive
Heatingutilities Processpinch
Processpinch
Coolingutilities
HeatThe heat surplus is
transfered throughpinch
C
Kwatt
170
30
20
300 400 800 900
140
80
60
600
Tmin
Fig. 3. Composites Curves.
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loads RDH realized. It can be seen that the next to last column, the RDH in all tempera-ture intervals is not zero and the largest negative value found is )388.64 kW in the intervalfrom 43 to 40 C. Consequently, the value of 388.64 kW starts the RDH of the last column
0
50
100
150
200
250
300
350
400
2 7 12 17 22 27 32 37
T (oC)
H (kW)
Hot thermal services Cold thermal services
PINCH
Fig. 4. Required thermal utilities for dierent DTMIN (threshold case).
Table 1
IMSS-Ags. complex thermal system data and modied temperatures for DTMIN 22 CStreams Tinlet (C) Toutlet (C) T 0inlet (C) T 0outlet (C) DH (kW) CP (kW/C)SW Hot 85 40 23.70 0.53
C Hot 80 40 96.32 2.41
LS Cold 25 55 47 77 17.60 0.59
L Cold 55 85 77 107 77.27 2.58
BF Cold 30 60 52 82 7.13 0.24
SW Cold 25 60 47 82 77.12 2.20
S Cold 30 121 52 143 12.50 0.14
SP Cold 25 28 47 50 151.67 50.56
CO Cold 30 100 52 122 59.63 0.85
H Cold 18 25 40 47 100.82 14.40
B Cold 21 121 43 143 4.94 0.05
132 A. Herrera et al. / Applied Thermal Engineering 23 (2003) 127139
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Table 2
The problem table
STREAM Cp(kW/
C)
T(C)
DT(C)
Cp (kW/C) DH Byrange
of
Tem-
pera-
ture
RDHBy
range
of
Tem-
pera-
ture
RDHQ 388:64
Hot
streams
Cold streams (*)1)
AJ C ASL L AR AS E AA CO CA LC
155.4 0.00
155 0.4 625.28
143 0.00 0.00 388.64
SW 0.53 122 21 )0.14 )0.05 )3.92 )3.92 384.72COND 2.41 112 10 )0.14 )0.85 )0.05 )10.39 )14.31 374.33
0.00 107 5 )0.14 )0.85 )0.05 )5.19 )19.50 369.14LSW 0.59 85 22 )2.58 )0.14 )0.85 )0.05 )79.52 )99.02 289.62L 2.58 82 3 0.53 )2.58 )0.14 )0.85 )0.05 )9.26 )108.28 280.36BF 0.24 80 2 0.53 )2.58 )0.24 )2.2 )0.14 )0.85 )0.05 )11.06 )119.34 269.30SW 2.20 77 3 0.53 2.41 )2.58 )0.24 )2.2 )0.14 )0.85 )0.05 )9.36 )128.70 259.94S 0.14 75 2 0.53 2.41 )0.59 )0.24 )2.2 )0.14 )0.85 )0.05 )2.26 )130.96 257.68SP 50.56 52 23 0.53 2.41 )0.59 )0.24 )2.2 )0.14 )0.85 )0.05 )26.02 )156.98 231.66CO 0.85 50 2 0.53 2.41 )0.59 )2.2 )0.05 0.19 )156.79 231.85H 14.40 47 3 0.53 2.41 )0.59 )2.2 )50.56 )0.05 )151.38 )308.17 80.47B 0.05 43 4 0.53 2.41 )14.40 )0.05 )46.07 )354.23 34.40
40 3 0.53 2.41 )14.40 )34.40 )388.64 0.00
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that nally equals zero. In this way, the pinch position which corresponds to DTMIN 22 Cis found at 40 C on the hot composite curve and at 18 C on the cold composite curve.Finally, Fig. 6 shows in a T vs DH graph the pinch position and the hot and cold compositecurves.
30
40
5060
70
8090
100
110
120
130
140
150
CP=(kW/oC)
SW C LS L BF SW S SP BHCO0.29 2.41 0.59 2.58 0.24 2.2 0.14 50.56 0.85 14.4 0.05
Pinch position
Fig. 5. Process thermal streams by temperature interval.
0
10
20
30
40
50
60
70
80
90
100
110
120
130
0 40 80 120 160 200 240 280 320 360 400 440 480 520
H (kW)
T (oC)
Cold CC Hot CC
PINCH
Fig. 6. Pinch temperatures (DTMIN 22 C).
134 A. Herrera et al. / Applied Thermal Engineering 23 (2003) 127139
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6. The energy targets and the optimum heat exchanger network design for the IMSS-Ags. complex
The calculation of energy targets, that is to say, the minimum requirements of cold and hotthermal utilities for the process, has as a starting point the thermodynamic analysis of the processrepresented by the composite curves.In order to make feasible the heat exchange between process streams, the hot composite curve
(available heat prole in a temperatureenthalpy diagram) must be located in temperatures abovethan those of the cold composite curve (prole of missing heat in a temperatureenthalpy dia-gram). The overlap in between composite curves represents the heat recovery potential throughthe heat transfer in between thermal streams of the process.The horizontal distance (thermal charge axis) in between nal points of the hot and cold curves,
in the higher part of the curves, represents the minimum hot utility requirements for the process(heating targets). The horizontal distance (thermal charge axis) in between nal points of the hotand cold curves in the lower part of the curves, represents the minimum cold utility requirementsfor the process (cooling targets).The minimum vertical distance in between curves represents the minimum approach temper-
ature for thermal exchanges, DTmin. Once the DTmin is known, the relative position of the com-posite curves determines the energy targets. If a higher value of DTmin is required, compositecurves get separated in their horizontal position, the heat recovery potential is reduced andconsequently the heat required for the hot and cold utilities increases. This point of minimumtemperature dierence, for heat exchanges in the process, represents the critical point of the heatrecovery and is called pinch point.The pinch point divides the process (above and below the pinch point) in two dierent
thermodynamic regions, in each one an enthalpy balance exists between process streamsand corresponding thermal utilities. For temperatures above the pinch point the processonly requires external heating and below the pinch point the process requires only externalcooling.From this, the following three rules are derived to optimize energy saving in a process.
1. For temperatures above the pinch point, cooling utilities inside the process must not be used(because in this region only external heating is required).
2. For temperatures below the pinch point heating utilities in the process must not be used (be-cause in this region only external cooling is required).
3. Heat transfer between streams above and below the pinch point is not allowed (because heatingand cooling utilities increase in the same quantity transferred).
In the IMSS-Ags. thermal system complex case, as result of the problem table, the minimumheat load that is necessary to transfer from heating utilities to the IMSS-Ags. complex thermalsystem is 388.64 kW. Fig. 7 is the representation of the heat exchanger network and the requiredthermal utilities in the grid diagram. The two vertical lines that separate the thermal system in twoparts represent the pinch position: the one that has higher temperatures than 40 C and the otherone that has lower temperatures than 18 C. In the IMSS-Ags. complex case there are no thermalstreams with temperatures below 18 C. Consequently, this thermal system does not need coolingutilities, only heating utilities are required.
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7. The grand composite curve of the IMSS-Ags. complex and the energy and economic savingspotential
The representation of the process with the composite curves indicates, for a DTmin, the maxi-mum potential of heat that can be recuperated inside the process and the minimum quantity ofheat to be given up or eliminated by thermal utilities.
Streams CPH T PINCH T(kW) (kW/oC) (oC) 40oC 18oC ( oC)
SW 23.7 0.53 85 40
COND 96.32 2.41 80 40
LS 17.6 0.59 55 25
57.5 kWL 77.2 2.58 85 55
BF 7.13 0.24 60 30
SW 77.12 2.2 60 25
12.5 kWS 12.5 0.14 121 30
151.6 kWSP 151.6 50.56 28 25
59.63 kWCO 59.6 0.8 100 30
100.8 kWH 100.82 14.4 25 18
4.94 kWB 4.94 0.05 121 21
LS
BF
SW
S
SP
CO
L
B
H
SW
CON7.31 kW
69.31 kW
19.7 kW
17.6 kW
72oC 69oC
56.5oC7.81 kW
Fig. 7. Proposed heat exchangers network after pinch analysis.
136 A. Herrera et al. / Applied Thermal Engineering 23 (2003) 127139
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From the composite curves perspective it is considered that the heating utilities are hot enoughto provide heat at any temperature level required by the process, and that the cooling utilities arecold enough to extract heat at any temperature level required by the process.However for the thermal integration analysis of utilities to the process, the questions to be
answered are: For heating utilities: Which is the minimum heat quantity that must be providedand which is the minimum temperature level? For cooling utilities: Which is the minimum heatquantity to eliminate and which is the maximum temperature level? In order to answer thesequestions a concept known as grand composite curve is used. It enables to make a more preciseanalysis in order to integrate thermal utilities in the process. The grand composite curve is thedierence between the heat supply (available heat) and heat demand (required heat) in dierenttemperature intervals of the composite curves, consequently it is the net heating quantity (inexcess or missing) inside the process. These net needs should be covered by external utilities.Parts of the grand composite curve with positive slope represent temperature intervals in which
heat has to be provided to the process using utilities. Parts of the grand composite curve withnegative slope represent temperature intervals in which heat has to be eliminated in the processusing utilities. The temperature in the grand composite curve for which there is zero thermal needcorresponds to the location of the pinch point. The grand composite curve is useful to identify notonly the energy quantity that the process requires but also to identify temperature levels in whichenergy is required; this enables to t utilities thermal load (quantity) and its temperature level
0
20
40
60
80
100
120
140
160
180
-50 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700
H (kW)
T ( oC)
625.28kW
388.64kW
Fig. 8. Grand composite curve (net heat demand from IMSS-Ags. complex) and heat load actually supplied.
A. Herrera et al. / Applied Thermal Engineering 23 (2003) 127139 137
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(quality) to avoid its degradation with low eciency, using excessive temperature gradients be-tween utilities and process.In the IMSS-Ags. complex case with the data of the rst and last column of Table 2, the grand
composite curve is constructed and shown in Fig. 8. This curve conrms that the heating loadtheoretically required by the IMSS-Ags. complex is 388.64 kW. This means a yearly energy re-quirement of 12.26 TJ. In the same gure the straight line of 625.28 kW represents the thermalservices that at present are transferred to the complex. Comparing both values, the theoreticalenergy savings potential is 38% which means a yearly energy savings of 246 thousand diesel litersand a yearly economic savings close to 100,000 USAD1999.
8. Conclusions
According to the pinch technology the maximum thermal power required to satisfy 100% of theheating demand in the IMSS-Ags. complex is 388.64 kW. This result means that the yearlythermal power saving potential is 236.64 kW; it represents 38% of the thermal power used atpresent. To achieve this energy saving potential the pinch point technology suggests to place fourheat exchangers in the complex (see Fig. 7). Two in the laundry zone to cover part of the thermaldemand of this nal use. One more in the machinery room no. 1 which helps to heat boiler feedwater. And nally, the last one in the condensation tank area that helps to heat sanitary water.The analysis of the grand composite curve shows that 60% of the thermal power demand re-
quired for the IMSS-Ags. complex thermal system is low enthalpy energy. That means that animportant part of the thermal power used in the complex can be satised from the thermodynamicpoint of view through more compatible energy technologies such as solar collectors and heatpumps. The analysis of the grand composite curve strongly suggests that the use of these tech-nologies could be a source of major energy and economics reductions of the present high leveldiesel consumption in the complex.
Acknowledgement
We thank to PAPIIT-UNAM for the nancial support through project IN303400.
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A. Herrera et al. / Applied Thermal Engineering 23 (2003) 127139 139
Pinch technology application in a hospitalIntroductionPinch technologyMethodologyThe thermal system and the hot and cold streams of the IMSS-Ags. complexDetermination of the pinch position for a threshold case and composite curves of the IMSS-Ags. complexThe energy targets and the optimum heat exchanger network design for the IMSS-Ags. complexThe grand composite curve of the IMSS-Ags. complex and the energy and economic savings potentialConclusionsAcknowledgementsReferences