Documents

hxdesign.pdf

Description
Description:
Categories
Published
of 6
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
Share
Transcript
  32 www.cepmagazine.org September 2002 CEP Heat Exchangers onsidering available heat exchanger technolo-gies at the outset of process design (at theprocess synthesis stage) is not general prac-tice. In fact, procedures established in somecompanies preclude it. For instance, some purchasing de-partments’“nightmare” is having to deal with a single sup-plier — instead they want a general specification that canbe sent to all equipment vendors, in the mistaken belief that they are then operating on a “level playing field.”This omission is both unfortunateand costly. It results in unnecessary cap-ital expenditure and in reduced energyefficiency. It also hinders the develop-ment of energy saving technology.Pinch analysis is the key tool used byengineers to develop flowsheets of ener-gy-intensive processes, where heat ex-changer selection is particularly impor-tant. Yet, this tool is hindering the adop-tion of a more-progressive approach be-cause of the way it is restricted to tradi-tional heat exchangers.Numerous articles have been pub-lished regarding the advantages of com-pact heat exchangers. Briefly, theirhigher heat-transfer coefficients, com-pact size, cost effectiveness, and uniqueability to handle fouling fluids makethem a good choice for many services.Aplate-and-frame heat exchanger(Figure 1) consists of pressed, corrugat-ed metal plates fitted between a thick, carbon-steel frame.Each plate flow channel is sealed with a gasket, a weld oran alternating combination of the two. It is not uncommonfor plate-and-frame heat exchangers to have overall heat-transfer coefficients that are three to four times those foundin shell-and-tube heat exchangers.This three-part series outlines the lost opportunities andthe importance of proper heat exchanger selection. This arti-cle discusses some general aspects of plate-and-frame heat Use these design charts for preliminary sizing. Plate-and-Frame Heat ExchangersDesigning  COMPACT HEAT EXCHANGERS — PART 1:   C Christopher Haslego, Alfa Laval Graham Polley, www.pinchtechnology.com  Figure 1. Cutaway drawing of a plate-and-frame heat exchanger.  CEP September 2002 www.cepmagazine.org 33 exchangers, outlines a procedure for accurately estimatingthe required area, and shows how these units can be used tosimplify processes. Part 2 (which will appear next month)covers integrating plate-and-frame exchangers (and othercompact technologies) into pinch analysis for new plants,while Part 3 (which also will appear next month) deals withthe application of plate-and-frame exchangers and pinchtechnology to retrofits. Specifying plate-and-frame heat exchangers Engineers often fail to realize the differences betweenheat transfer technologies when preparing a specification tobe sent to vendors of different types of heat exchangers.Consider the following example.Aprocess stream needs to be cooled with cooling waterbefore being sent to storage. The stream requires C276, anexpensive high-nickel alloy, to guard against corrosion; thismetallurgy makes the stream a candidate for the tubeside of a shell-and-tube heat exchanger. The cooling water is avail-able at 80°F and must be returned at a temperature no high-er than 115°F. The process engineer realizes that with thewater flow being placed on the shellside, larger flowrateswill enhance the heat-transfer coefficient. The basis for theheat exchanger quotation was specified as shown in thetable. According to the engineer’s calculations, these basicparameters should result in a good shell-and-tube designthat uses a minimum amount of C276 material. Atypical plate-and-frame exchanger designed to meet thespecification would have about 650 ft 2 of area, compared toabout 420 ft 2 for a shell-and-tube exchanger. Aplate-and-frame unit designed to the above specification is limited bythe allowable pressure drop on the cooling water. If the cool-ing water flow is reduced to 655 gal/min and the outlet watertemperature allowed to rise to 115°F, the plate-and-frameheat exchanger would contain about 185 ft 2 of area. The unitis smaller and less expensive, and it uses less water. Theload being transferred to the cooling tower is the same. With shell-and-tube heat exchangers, increasing waterflow will minimize heat-transfer area. However, with com-pact technologies, the effect is exactly the opposite. Thelarger water flow actually drives up the cost of the unit. Rather than supplying a rigid specification to all heatexchanger manufacturers, the engineer should have ex-plained the goal for the process stream. This could havebeen in the form of the following statement: “The processstream is to be cooled with cooling water. Up to 2,000gal/min of water is available at 80°F. The maximum returntemperature is 115°F.” This simple statement could resultin vastly different configurations compared with the de-signs that would result from the srcinal specification. Design charts for plate-and-frame exchangers When it comes to compact heat-transfer technology,engineers often find themselves at the mercy of theequipment manufacturers. For example, limited litera-ture correlations are available to help in the preliminarydesign of plate-and-frame heat exchangers. This article introduces a series of charts (Figures 2–7)that can be used for performing preliminary sizing of plate-and-frame exchangers. Examples will help clarify their use. The following important points should be noted regard-ing the charts and their use:1. The heat-transfer correlations apply to single-phase,liquid-liquid designs.2. These charts are valid for single-pass units with 0.50-mm-thick plates. The accuracy of the charts will not becompromised for most materials of construction.3. Wetted-material thermal conductivity is taken as8.67 Btu/h-ft-°F (which is the value for stainless steel).4. The following physical properties for hydrocarbon-based fluids were used for the basis: thermal conductivity( k  ) = 0.06 Btu/h-ft-°F, density ( ρ ) = 55.0 lb/ft 3 , heat capac-ity ( C   p ) = 0.85 Btu/lb-°F. The following physical propertiesfor water-based fluids were used for the basis: thermal con-ductivity = 0.33 Btu/h-ft-°F, density = 62.0 lb/ft 3 , heat ca-pacity = 0.85 Btu/lb-°F.5. Accuracy should be within ±15% of the service valuefor the overall heat-transfer coefficient, assuming a nomi-nal 10% excess heat-transfer area.6. For fluids with viscosities between 100 and 500 cP,use the 100 cPline on the graphs. For fluids in excess of 500 cP, consult equipment manufacturers.Equations 1–3 are used to calculate the log-mean tem-perature difference (  LMTD ) and number of transfer units(  NTU  ) for the hot and cold streams. After the local heat-transfer coefficients ( h ) are read from the charts, the over-all heat-transfer coefficient ( U  ) is calculated by Eq. 4.1114 U h xk h hot cold  = + +  ( ) ∆  NTU T T  LMTD cold cold out cold in = − ( ) ,, 3  NTU T T  LMTD hot hot in hot out  = − ( ) ,, 2  LMTDT T T T T T T T  hot in cold out hot out cold inhot in cold out hot out cold in = − ( )  − − ( ) −−      ( ) ,,,,,,,, ln1  Table. Basis for heat exchanger quotation. Tubeside ShellsideFlowrate, gal/min 5001,800 Temperature In, ° F 280 80 Temperature Out, ° F 150 92 Allowable Pressure Drop, psi  15 15  Using the charts Consider the following example.150,000 lb/h of water is being cooledfrom 200°F to 175°F by 75,000 lb/hof SAE 30 oil. The oil enters the ex-changer at 60°F and leaves at 168°F.The average viscosity of the waterpassing through the unit is 0.33 cPand the average viscosity of the oil inthe unit is 215 cP. The maximum-allowable pressure drop through theplate heat exchanger is 15 psi on thehot and cold sides. Step 1: Calculate the LMTD. From Eq. 1,  LMTD = [(200 – 168)– (175 – 60)]/ln[(200 – 168)/(175 –60)] = 64.9°F. Step 2: Calculate NTU  hot   and  NTU  cold  . From Eqs. 2 and 3,  NTU  hot  =(200 – 175)/64.9 = 0.38 and  NTU  cold  = (168 – 60)/64.9 = 1.66. Step 3: Read h hot   from the appro- priate chart. Use Figure 5, the chartfor hydrocarbons when 0.25 <  NTU  < 2.0. Although there is not a vis-cosity line for 215 cP, the line repre-senting 100 cPcan be used for vis-cosities up to about 400–500 cP. Theheat exchanger will be pressure-drop-limited and the heat-transfercoefficient will not change apprecia-bly over this viscosity range forplate-and-frame exchangers. Read-ing from the chart, a pressure dropof 15 psi corresponds to h hot   ≈ 50Btu/h-ft 2 -°F. Step 4: Read h cold   from the chart. Use Figure 2, which applies to water-based liquids when 0.25 <  NTU  < 2.0.Again, the exact viscosity line neededfor pure water (0.33 cP) in this case isnot available. However, the 1.0 cPline provides a very good estimate of the heat-transfer coefficient for purewater. Reading from the chart, a pres-sure drop of 15 psi corresponds to h cold   ≈ 3,000 Btu/h-ft 2 -°F. Step 5: Calculate U  . Assume astainless steel plate with a thick-ness of 0.50 mm is being used.Type 316 stainless steel has a ther-mal conductivity of 8.67 Btu/h-ft-°F. Then from Eq. 4, 1/  U  = (1/50 +0.000189 + 1/3,000) and U = 49Btu/h-ft 2 -°F. Heat Exchangers 34 www.cepmagazine.org September 2002 CEP    Figure 2. Heat-transfer correlations for water-based fluids, 0.25 < NTU < 2.0. Pressure Drop, psi    h   =   L  o  c  a   l   H  e  a   t  -   T  r  a  n  s   f  e  r   C  o  e   f   f   i  c   i  e  n   t ,   B   t  u   /   h  -   f   t    2   -    ˚    F    h   =   L  o  c  a   l   H  e  a   t  -   T  r  a  n  s   f  e  r   C  o  e   f   f   i  c   i  e  n   t ,    W   /  m    2   -   K Pressure Drop, kPa 510152025303,5003,0002,5002,0001,5001,000500018,00015,00012,0009,0006,0003,000100 1 cP2.5 cP5 cP10 cP100 cP 50200150 Water-Based Fluids  Figure 3. Heat-transfer correlations for water-based fluids, 2.0 < NTU < 4.0. Pressure Drop, psi    h   =   L  o  c  a   l   H  e  a   t  -   T  r  a  n  s   f  e  r   C  o  e   f   f   i  c   i  e  n   t ,   B   t  u   /   h  -   f   t    2   -    ˚    F 510152025303,5003,0002,5002,0001,5001,000500018,00015,00012,0009,0006,0003,000100 1 cP Correction: For liquids with average viscositiesless than 2.0 cP, reduce the local heat-transfercoefficient by 15% from 3.5 < NTU < 4.0. 2.5 cP5 cP10 cP100 cP 50200150    h   =   L  o  c  a   l   H  e  a   t  -   T  r  a  n  s   f  e  r   C  o  e   f   f   i  c   i  e  n   t ,    W   /  m    2   -   K Pressure Drop, kPa Water-Based Fluids  Figure 4. Heat-transfer correlations for water-based fluids, 4.0 < NTU < 5.0. Pressure Drop, psi    h   =   L  o  c  a   l   H  e  a   t  -   T  r  a  n  s   f  e  r   C  o  e   f   f   i  c   i  e  n   t ,   B   t  u   /   h  -   f   t    2   -    ˚    F 510152025303,5003,0002,5002,0001,5001,000500018,00015,00012,0009,0006,0003,000100 1 cP2.5 cP5 cP10 cP100 cP 50200150    h   =   L  o  c  a   l   H  e  a   t  -   T  r  a  n  s   f  e  r   C  o  e   f   f   i  c   i  e  n   t ,    W   /  m    2   -   K Pressure Drop, kPa Water-Based Fluids  Now let’s consider another exam-ple. 150,000 lb/h of water is beingcooled from 200°F to 100°F by150,000 lb/h of NaCl brine. Thebrine enters the exchanger at 50°Fand leaves at 171°F. The averageviscosity of the water passingthrough the unit is 0.46 cPand theaverage viscosity of the brine in theunit is 1.10 cP. The maximum-allow-able pressure drop through the plateheat exchanger is 10 psi on the hot(water) side and 20 psi on the cold(brine) side.The  LMTD is calculated to be38.5°F.  NTU  hot  and  NTU  cold  are 2.59and 3.14, respectively. From thecharts for 2.0 <  NTU  < 4.0 (waterbased), h hot   ≈ 2,000 Btu/h-ft 2 -°Fand h cold  ≈ 2,500 Btu/h-ft 2 -°F. Al-though the material of choice maybe titanium or palladium-stabilizedtitanium, the properties for stain-less steel are used for preliminarysizing. U  is calculated to be 918Btu/h-ft 2 -°F. Implications of size reduction Alternative technologies offersignificant size advantages overshell-and-tube heat exchangers.Let’s now consider the implicationsof this. The individual exchangers aresmaller, and the spacing betweenprocess equipment can be reduced.Thus, a smaller plot is needed forthe process plant. If the plant is tobe housed in a building, the build-ing can be smaller. The amount of structural steel used to support theplant can be reduced, and becauseof the weight saving, the load onthat structure is also reduced. Theweight advantage extends to thedesign of the foundations used tosupport the plant. Since the spac-ing between equipment is reduced,piping costs are lower. However, we stress again that thesavings associated with size andweight reduction can only beachieved if these advantages are rec-ognized and exploited at the earlieststages of the plant design. CEP September 2002 www.cepmagazine.org 35    Figure 5. Heat-transfer correlations for hydrocarbons, 0.25 < NTU < 2.0. Pressure Drop, psi    h   =   L  o  c  a   l   H  e  a   t  -   T  r  a  n  s   f  e  r   C  o  e   f   f   i  c   i  e  n   t ,   B   t  u   /   h  -   f   t    2   -    ˚    F 510152025301,00080060040020005,4004,6003,8003,0002,4001,600800100 1 cP2.5 cP5 cP10 cP 100 cP 50200150    h   =   L  o  c  a   l   H  e  a   t  -   T  r  a  n  s   f  e  r   C  o  e   f   f   i  c   i  e  n   t ,    W   /  m    2   -   K Pressure Drop, kPa Hydrocarbon-Based Fluids  Figure 6. Heat-transfer correlations for hydrocarbons, 2.0 < NTU < 4.0. Pressure Drop, psi    h   =   L  o  c  a   l   H  e  a   t  -   T  r  a  n  s   f  e  r   C  o  e   f   f   i  c   i  e  n   t ,   B   t  u   /   h  -   f   t    2   -    ˚    F 510152025301,0008006004002000100 Correction: For liquids with average viscositiesless than 2.0 cP, reduce the local heat-transfercoefficient by 15% from 3.5 < NTU < 4.0. 502001505,4004,6003,8003,0002,4001,600800 1 cP2.5 cP5 cP 10 cP 100 cP    h   =   L  o  c  a   l   H  e  a   t  -   T  r  a  n  s   f  e  r   C  o  e   f   f   i  c   i  e  n   t ,    W   /  m    2   -   K Pressure Drop, kPa Hydrocarbon-Based Fluids  Figure 7. Heat-transfer correlations for hydrocarbons, 4.0 < NTU < 5.0. Pressure Drop, psi    h   =   L  o  c  a   l   H  e  a   t  -   T  r  a  n  s   f  e  r   C  o  e   f   f   i  c   i  e  n   t ,   B   t  u   /   h  -   f   t    2   -    ˚    F 510152025301,00080060040020005,4004,6003,8003,0002,4001,600800100 1 cP2.5 cP5 cP10 cP100 cP 50200150    h   =   L  o  c  a   l   H  e  a   t  -   T  r  a  n  s   f  e  r   C  o  e   f   f   i  c   i  e  n   t ,    W   /  m    2   -   K Pressure Drop, kPa Hydrocarbon-Based Fluids

Mobile App

Apr 15, 2019

Desertation

Apr 15, 2019
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks