Cep plate and_frame_hx

Cep plate and_frame_hx

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  32 www.cepmagazine.org September2002 CEP Heat Exchangers onsidering available heat exchanger technolo- gies at the outset of process design (at the process synthesis stage) is not general prac- tice. In fact, procedures established in some companies 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 can be sent to all equipment vendors, in the mistaken belief that they are then operating on a “level playing field.” This omission is both unfortunate and costly. It results in unnecessary cap- ital expenditure and in reduced energy efficiency. It also hinders the develop- ment of energy saving technology. Pinch analysis is the key tool used by engineers 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, their higher heat-transfer coefficients, com- pact size, cost effectiveness, and unique ability to handle fouling fluids make them a good choice for many services. A plate-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 or an alternating combination of the two. It is not uncommon for plate-and-frame heat exchangers to have overall heat- transfer coefficients that are three to four times those found in shell-and-tube heat exchangers. This three-part series outlines the lost opportunities and the 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 Exchangers DDeessiiggnniinngg COMPACT HEAT EXCHANGERS — PART 1: C Christopher Haslego, Alfa Laval Graham Polley, www.pinchtechnology.com s Figure 1. Cutaway drawing of a plate-and-frame heat exchanger.
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  CEP September 2002www.cepmagazine.org 33 exchangers, outlines a procedure for accurately estimating the required area, and shows how these units can be used to simplify processes. Part 2 (which will appear next month) covers integrating plate-and-frame exchangers (and other compact technologies) into pinch analysis for new plants, while Part 3 (which also will appear next month) deals with the application of plate-and-frame exchangers and pinch technology to retrofits. Specifying plate-and-frame heat exchangers Engineers often fail to realize the differences between heat transfer technologies when preparing a specification to be sent to vendors of different types of heat exchangers. Consider the following example. A process stream needs to be cooled with cooling water before being sent to storage. The stream requires C276, an expensive high-nickel alloy, to guard against corrosion; this metallurgy 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 the water flow being placed on the shellside, larger flowrates will enhance the heat-transfer coefficient. The basis for the heat exchanger quotation was specified as shown in the table. According to the engineer’s calculations, these basic parameters should result in a good shell-and-tube design that uses a minimum amount of C276 material. A typical plate-and-frame exchanger designed to meet the specification would have about 650 ft2 of area, compared to about 420 ft2 for a shell-and-tube exchanger. A plate-and- frame unit designed to the above specification is limited by the allowable pressure drop on the cooling water. If the cool- ing water flow is reduced to 655 gal/min and the outlet water temperature allowed to rise to 115°F, the plate-and-frame heat exchanger would contain about 185 ft2 of area. The unit is smaller and less expensive, and it uses less water. The load being transferred to the cooling tower is the same. With shell-and-tube heat exchangers, increasing water flow will minimize heat-transfer area. However, with com- pact technologies, the effect is exactly the opposite. The larger water flow actually drives up the cost of the unit. Rather than supplying a rigid specification to all heat exchanger manufacturers, the engineer should have ex- plained the goal for the process stream. This could have been in the form of the following statement: “The process stream is to be cooled with cooling water. Up to 2,000 gal/min of water is available at 80°F. The maximum return temperature is 115°F.” This simple statement could result in vastly different configurations compared with the de- signs that would result from the original specification. Design charts for plate-and-frame exchangers When it comes to compact heat-transfer technology, engineers often find themselves at the mercy of the equipment manufacturers. For example, limited litera- ture correlations are available to help in the preliminary design 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 be compromised for most materials of construction. 3. Wetted-material thermal conductivity is taken as 8.67 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/ft3, heat capac- ity (Cp) = 0.85 Btu/lb-°F. The following physical properties for water-based fluids were used for the basis: thermal con- ductivity = 0.33 Btu/h-ft-°F, density = 62.0 lb/ft3, heat ca- pacity = 0.85 Btu/lb-°F. 5. Accuracy should be within ±15% of the service value for 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 cP line 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. 1 1 1 4 U h x k hhot cold = + + ( ) ∆ NTU T T LMTD cold cold out cold in = − ( ), , 3 NTU T T LMTD hot hot in hot out = − ( ), , 2 LMTD T T T T T T T T hot in cold out hot out cold in hot in cold out hot out cold in = −( )− −( ) − −       ( ), , , , , , , , ln 1 Table. Basis for heat exchanger quotation. Tubeside Shellside Flowrate, gal/min 500 1,800 Temperature In, °F 280 80 Temperature Out, °F 150 92 Allowable Pressure Drop, psi 15 15
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  Using the charts Considerthe following example. 150,000 lb/h of water is being cooled from 200°F to 175°F by 75,000 lb/h of SAE 30 oil. The oil enters the ex- changer at 60°F and leaves at 168°F. The average viscosity of the water passing through the unit is 0.33 cP and the average viscosity of the oil in the unit is 215 cP. The maximum- allowable pressure drop through the plate heat exchanger is 15 psi on the hot 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 NTUhot and NTUcold. From Eqs. 2 and 3, NTUhot = (200 – 175)/64.9 = 0.38 and NTUcold = (168 – 60)/64.9 = 1.66. Step 3: Read hhot from the appro- priate chart. Use Figure 5, the chart for hydrocarbons when 0.25 < NTU < 2.0. Although there is not a vis- cosity line for 215 cP, the line repre- senting 100 cP can be used for vis- cosities up to about 400–500 cP. The heat exchanger will be pressure- drop-limited and the heat-transfer coefficient will not change apprecia- bly over this viscosity range for plate-and-frame exchangers. Read- ing from the chart, a pressure drop of 15 psi corresponds to hhot ≈ 50 Btu/h-ft2-°F. Step 4: Read hcold from the chart. Use Figure 2, which applies to water- based liquids when 0.25 < NTU < 2.0. Again, the exact viscosity line needed for pure water (0.33 cP) in this case is not available. However, the 1.0 cP line provides a very good estimate of the heat-transfer coefficient for pure water. Reading from the chart, a pres- sure drop of 15 psi corresponds to hcold ≈ 3,000 Btu/h-ft2-°F. Step 5: Calculate U. Assume a stainless 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 = 49 Btu/h-ft2-°F. Heat Exchangers 34 www.cepmagazine.org September 2002 CEP s Figure 2. Heat-transfer correlations for water-based fluids, 0.25 < NTU < 2.0. Pressure Drop, psi h=LocalHeat-TransferCoefficient, Btu/h-ft2-˚F h=LocalHeat-TransferCoefficient, W/m2-K Pressure Drop, kPa 5 10 15 20 25 30 3,500 3,000 2,500 2,000 1,500 1,000 500 0 18,000 15,000 12,000 9,000 6,000 3,000 100 1 cP 2.5 cP 5 cP 10 cP 100 cP 50 200150 Water-Based Fluids s Figure 3. Heat-transfer correlations for water-based fluids, 2.0 < NTU < 4.0. Pressure Drop, psi h=LocalHeat-TransferCoefficient, Btu/h-ft2-˚F 5 10 15 20 25 30 3,500 3,000 2,500 2,000 1,500 1,000 500 0 18,000 15,000 12,000 9,000 6,000 3,000 100 1 cP Correction: For liquids with average viscosities less than 2.0 cP, reduce the local heat-transfer coefficient by 15% from 3.5 < NTU < 4.0. 2.5 cP 5 cP 10 cP 100 cP 50 200150 h=LocalHeat-TransferCoefficient, W/m2-K Pressure Drop, kPa Water-Based Fluids s Figure 4. Heat-transfer correlations for water-based fluids, 4.0 < NTU < 5.0. Pressure Drop, psi h=LocalHeat-TransferCoefficient, Btu/h-ft2-˚F 5 10 15 20 25 30 3,500 3,000 2,500 2,000 1,500 1,000 500 0 18,000 15,000 12,000 9,000 6,000 3,000 100 1 cP 2.5 cP 5 cP 10 cP 100 cP 50 200150 h=LocalHeat-TransferCoefficient, W/m2-K Pressure Drop, kPa Water-Based Fluids
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  Now let’s consideranother exam- ple. 150,000 lb/h of water is being cooled from 200°F to 100°F by 150,000 lb/h of NaCl brine. The brine enters the exchanger at 50°F and leaves at 171°F. The average viscosity of the water passing through the unit is 0.46 cP and the average viscosity of the brine in the unit is 1.10 cP. The maximum-allow- able pressure drop through the plate heat exchanger is 10 psi on the hot (water) side and 20 psi on the cold (brine) side. The LMTD is calculated to be 38.5°F. NTUhot and NTUcold are 2.59 and 3.14, respectively. From the charts for 2.0 < NTU < 4.0 (water based), hhot ≈ 2,000 Btu/h-ft2-°F and hcold ≈ 2,500 Btu/h-ft2-°F. Al- though the material of choice may be titanium or palladium-stabilized titanium, the properties for stain- less steel are used for preliminary sizing. U is calculated to be 918 Btu/h-ft2-°F. Implications of size reduction Alternative technologies offer significant size advantages over shell-and-tube heat exchangers. Let’s now consider the implications of this. The individual exchangers are smaller, and the spacing between process equipment can be reduced. Thus, a smaller plot is needed for the process plant. If the plant is to be housed in a building, the build- ing can be smaller. The amount of structural steel used to support the plant can be reduced, and because of the weight saving, the load on that structure is also reduced. The weight advantage extends to the design of the foundations used to support the plant. Since the spac- ing between equipment is reduced, piping costs are lower. However, we stress again that the savings associated with size and weight reduction can only be achieved if these advantages are rec- ognized and exploited at the earliest stages of the plant design. CEP September 2002 www.cepmagazine.org 35 s Figure 5. Heat-transfer correlations for hydrocarbons, 0.25 < NTU < 2.0. Pressure Drop, psi h=LocalHeat-TransferCoefficient, Btu/h-ft2-˚F 5 10 15 20 25 30 1,000 800 600 400 200 0 5,400 4,600 3,800 3,000 2,400 1,600 800 100 1 cP 2.5 cP 5 cP 10 cP 100 cP 50 200150 h=LocalHeat-TransferCoefficient, W/m2-K Pressure Drop, kPa Hydrocarbon-Based Fluids s Figure 6. Heat-transfer correlations for hydrocarbons, 2.0 < NTU < 4.0. Pressure Drop, psi h=LocalHeat-TransferCoefficient, Btu/h-ft2-˚F 5 10 15 20 25 30 1,000 800 600 400 200 0 100 Correction: For liquids with average viscosities less than 2.0 cP, reduce the local heat-transfer coefficient by 15% from 3.5 < NTU < 4.0. 50 200150 5,400 4,600 3,800 3,000 2,400 1,600 800 1 cP 2.5 cP 5 cP 10 cP 100 cP h=LocalHeat-TransferCoefficient, W/m2-K Pressure Drop, kPa Hydrocarbon-Based Fluids s Figure 7. Heat-transfer correlations for hydrocarbons, 4.0 < NTU < 5.0. Pressure Drop, psi h=LocalHeat-TransferCoefficient, Btu/h-ft2-˚F 5 10 15 20 25 30 1,000 800 600 400 200 0 5,400 4,600 3,800 3,000 2,400 1,600 800 100 1 cP 2.5 cP 5 cP 10 cP 100 cP 50 200150 h=LocalHeat-TransferCoefficient, W/m2-KPressure Drop, kPa Hydrocarbon-Based Fluids
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  Reduced plant complexity Theuse of alternative heat ex- changer technologies can signifi- cantly reduce plant complexity by reducing the number of heat ex- changers through improved thermal contacting and multi-streaming. This adds to the savings associated with reduced size and weight and also has safety implications. The simpler the plant structure, the easi- er it is for the process operator to understand the plant. In addition, plant maintenance will be safer, easier and more straight-forward. Mechanical constraints play a significant role in the design of shell-and-tube heat exchangers. For instance, it is common to find that some users place restrictions on tube length. Such a restriction can have important implications for the design. In the case of exchangers requiring large surface areas, the restriction drives the design toward large tube counts. If such large tube counts lead to low tubeside veloci- ty, the designer is tempted to in- crease the number of tubeside pass- es in order to maintain a reasonable tubeside heat-transfer coefficient. Thermal expansion considerations can also lead the designer to opt for multiple tube passes, because the cost of a floating head is generally lower than the cost of installing an expan- sion bellows in the exchanger shell. The use of multiple tube passes has four detrimental effects. First, it leads to a reduction in the number of tubes that can be accommodated in a given size shell, thereby leading to increased shell diameter and cost. Second, for bundles having more than four tube passes, the pass-partition lanes introduced into the bundle give rise to an increase in the quantity of shellside fluid by- passing the tube bundle and a reduction in shellside heat-transfer coefficient. Third, it results in wasted tubeside pressure drop in the return headers. Finally, and most significantly, the use of multiple tube passes results in the thermal contacting of the streams not being pure counter-flow, which reduces the effective- mean-temperature driving force and possibly produces a temperature cross (i.e., where the outlet temperature of the cold stream is higher than the inlet temperature of the hot stream, as shown in Figure 8). If a tempera- ture cross occurs, the designer must split the duty be- tween multiple heat exchangers arranged in series. Heat Exchangers s Figure 8. The situation on the left does not involve a temperature cross, while the one on the right does. 200˚F 168˚F 175˚F One shell-and-tube exchanger One plate exchanger 60˚F No Temperature Cross 200˚F 171˚F Three shell-and-tube exchangers One plate exchanger 50˚F 100˚F Temperature Cross 36 www.cepmagazine.org September 2002 CEP s Figure 9. A single plate-and-frame exchanger handles three process streams … 160˚F Styrene, 290,285 lb/h 130˚F 248˚F ∆P = 14.6 psi Styrene, 290,285 lb/h 285˚F Styrene, 100˚F ∆Ptotal = 30 psi 112˚F ∆P = 9.5 psi Water, 248,655 lb/h 80˚F 160˚F s Figure 10. … whereas the equivalent shell-and-tube design requires six units. Styrene (Hot) Styrene (Cold) Cooling Water
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  Many of thealternative heat-exchanger technologies allow the application of pure countercurrent flow across all size and flow ranges. This results in better use of the available temperature driving force and the use of single heat exchangers. Multi-streaming The traditional shell-and-tube heat exchanger handles only one hot stream and one cold stream. Some heat ex- changer technologies (most notably plate-fin and print- ed-circuit exchangers) can handle many streams. It is not uncommon to find plate-fin exchangers transferring heat between ten individual processes. (The principles behind the design of multi-stream exchangers and the operability of such units are discussed in Ref. 1.) Such units can be considered to contain a whole heat ex- changer network within the body of a single exchanger. Distribution and recombination of process flows is un- dertaken inside the exchanger. The result is a major re- duction in piping cost. Engineers often overlook the opportunities of using a plate-and-frame exchanger as a multi-stream unit. As mentioned earlier, this is a common oversight when ex- changer selection is not made until after the flowsheet has been developed. A good example of multi-streaming is a plate heat ex- changer that serves as a process interchanger on one side and a trim cooler on the other. This arrangement is par- ticularly useful for product streams that are exiting a pro- cess and must be cooled for storage. Another popular function of multi-streaming is to lower material costs. Some streams, once they are cooled to a certain temperature, pose much less of a corrosion risk. One side of the exchanger can be made of a more- expensive corrosion-resistant alloy while the other side can utilize stainless steel or a lower alloy. Figure 9 shows a plate-and-frame unit applied to three process streams. A single exchanger with 1,335 ft2 of ef- fective surface area is used. Figure 10 is the equivalent shell-and-tube solution — to avoid temperature crosses, six individual exchangers are needed: the cooler having two shells in series (each with 1,440 ft2 of effective sur- face area) and the heat recovery unit having four shells in series (each with 2,116 ft2 of surface area). So, the plate- and-frame design involves the use of 1,335 ft2 of surface area in a single unit, whereas the equivalent shell-and- tube design has 11,344 ft2 of surface area distributed across six separate exchangers. Budget pricing correlations For plate-and-frame heat exchangers with a design pres- sure up to 150 psi and a design temperature up to 320°F, the following cost equations can be used to estimate the purchased cost. For areas (A) less than 200 ft2: C = 401 A 0.4887 for Type 316 stainless steel (5) C = 612 A 0.4631 for Grade 1 titanium (6) For areas larger than 200 ft2: C = 136 A 0.6907 for Type 316 stainless steel (7) C = 131 A 0.7514 for Grade 1 titanium (8) Typical installation factors for plate-and-frame heat ex- changers can range from 1.5 to 2.0, depending on the size of the unit. Up next Being able to estimate the area and prices of plate- and-frame heat exchangers is an important first step in including alternative heat-transfer technology in process synthesis. The remaining articles in this series (which will appear next month) focus on the integration of plate-and-frame (and other compact technology) into pinch analyses. CEP CEP September 2002 www.cepmagazine.org 37 Literature Cited 1. Picon Nunez, M., and G. T. Polley, “Recent Advances in Analysis of Heat Transfer for Fin Type Surfaces,” Chapters 9 and 10, Sunden, B., and P. J. Heggs, eds., WIT Press, Southampton, U.K. (2000). CHRISTOPHER HASLEGO is a design engineer with the Process Technology Div. of Alfa Laval (5400 International Trade Dr., Richmond, VA 23236; Phone: (804) 236-1318; Fax: (804) 236-1360; E-mail: [email protected]). His focus is heat transfer in the inorganic base chemicals industry. He received a BS in chemical engineering from West Virginia Univ. A member of AIChE, he also maintains a website dedicated to chemical engineering entitled “The Chemical Engineers’ Resource Page” at www.cheresources.com. GRAHAM T. POLLEY (Phone: +44-1229-585-330; Fax: +44-1229-585-708; E-mail: [email protected]) is retired from the Univ. of Manchester Institute of Science and Technology, Manchester, U.K., where he was the director of the Centre for Process Integration. To promote the application of process integration technology, he has developed the website www.pinchtechnology.com. His research activities have involved condensation heat transfer, boiling heat transfer, heat-recovery-system design and the development of process and equipment design methodologies. He is the current president of the U.K. Heat Transfer Society. In 1990, he was awarded the Moulton Medal by the Institution of Chemical Engineers for his work on oil refinery revamping, and in 1992, he and Dr. Nasr were awarded the Ackrill Trophy by the U.K. Heat Transfer Society for their work on heat-transfer enhancement. He holds BTech, MSc and PhD degrees in chemical engineering from Loughborough Univ. of Technology, and is a member of AIChE. Correlations used to formulate charts were developed by Alfa Laval (formerly Alfa Laval Thermal) through comprehensive research and development and decades of industrial experience.