CHPR 3530 Executive Summary Heat transfer is a crucial part of the Chemical engineering industry. A heat exchanger is used to do this process. There are many types of heat exchangers available; each has its own use depending on its application. For the given assignment, the shell and tube heat exchanger was compared with two other ones and helping us conclude that shell and tube was the most economical and efficient option. The parts of a shell and tube heat exchanger can be divided into 3 major components:
The front head Rear-end Type of shell
Heat Exchanger Design
After careful evaluation of the different designs available according to TEMA standards, in this assignment a BEU design was chosen. The justification is given in the appropriate sections of the report. The thermal calculations are based on tedious iterations which were done on Microsoft Excel. The iterations required a “guess” for the overall heat transfer coefficient. The iterations were done till all the variables, such as Reynolds’s number, number of tubes, pressure drop, error margin etc.
2051259
The final results of the iterations are shown in the report.
Daksh Parashar
2011
Introduction
Heat Exchanger Design A heat exchanger is a device that is used to transfer between two or more fluids at different temperatures and in thermal contact. Within a heat exchanger there are usually no external heat and work interactions. In most heat exchangers, heat transfer between the fluids takes place through a separating wall or into and out of a wall in a transient manner. A shell and tube heat exchanger is the most widely used heat exchanger in the industry. Its importance comes from its unique versatile design and the ability to handle a broad range of temperatures, as well as pressure differentials. The major components of a shell and tube heat exchanger include: tubes, baffles, shell, and frontend head, rear-end head and tube sheets.
Figure 1: Basic schematic of a shell and tube heat exchanger (Boxall 2010)
Due to the unique design of a shell and tube exchanger, they are relatively easy to clean. Shell and tube heat exchangers are easily constructed and can be altered to specific needs as they are well documented eg, TEMA standards/ booklets. Shell and tube heat exchangers are relatively cost-effective due to their relatively large ratio of heat transfer to volume and weight. Can be constructed from a wide range of materials.
Plate heat exchanger: Plate type heat exchangers are usually built of thin plates in series held apart by gaskets. The plates are either smooth or some of corrugation, and they are either flat or wound in exchanger. Corrugations are there to induce the onset of the turbulence flow in order to provide an increased heat transfer coefficient. The corrugations are so effective such that for a liquid-liquid exchanger we are able to obtain turbulent flow at a Reynolds number between 100-400. The hot and cold fluid flows into alternate spaces between the plates; heat is then transferred through the plate medium. Plate heat exchangers are classified either as gasket, brazed, welded (one or both fluid passages), depending on the leak tightness required. (T Kuppan 2000)
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Heat Exchanger Design
Figure 2: Basic schematic of a plate heat exchanger (aimradiantheatinf 2011). Table 1: Advantages and Disadvantages of a Plate heat exchanger Advantages
Disadvantages
Simple and Compact Design
Initial setting costs are high.
Increased Heat transfer
Capacity can be easily increased, just by introduction of additional plates
Finding Leakages is a complex task, since pressure tests are not easily done. Sensitive to operating temperatures due to the bonding material between the plates Pressure drops are generally high in comparison with other types of heat exchangers
Deteriorated/Leaking plates are easily removed in pairs, without the need for complicated methods.
Caution must be taken while assembling and dismantling the plates
Maintenance is simple/cost effective
Joints are prone to deterioration under operating conditions Corrosion is a likely nuisance
Easily clean and sanitized
Turbulent flow profiles are easily attainable.
The spiral heat exchanger The spiral heat exchangers consist of one or more spirally wound coils fitted in a shell. In a spiral tube the heat transfer rate is higher in a comparison to a straight tube. Since spiral tube has a higher surface area to volume ratio, they tend to be very compact. As of the spiral (helical) design, there tends to be an onset of high turbulence. Turbulent flow reduces the amount of scale build up and fouling and aids in the self-cleaning process. Thermal expansion is insignificant. Table 2: Advantages and disadvantages of a Spiral heat exchanger Advantages
Disadvantages
High efficiency: tightly packed tubes increase the heat transfer area.
Limited working temperatures and pressures.
Low installation costs: Takes up less space due to its lightweight and unique vertical design.
Not easy to repair: Once leakage occurs, generally have to replace the whole heat exchanger.
Flexible Designs: Can be used for wide range of pressures, temperatures and flows. Low Pressure drop
Suitable for non-Newtonian fluids.
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Heat Exchanger Design
The spiral heat exchanger would be an excellent choice for the task of exchanging heat between ethyleneglycol and water, but its application limits the use of Non-Newtonian fluids. Since, both of our working fluids are classified as Newtonian, a shell and tube heat exchanger is a better choice.
DISCUSSION The design of a heat exchanger requires a number of variables to be determined. Some of these major variables include relative directions of the fluid flow, front end head, rear end head, number of shells and tube passes, baffles; number, position and the orientation, construction materials, tube layout, designation of the shell and tube side fluid.
Relative Direction of the Flows According to Newton’s Law of cooling, heat transfer rate is related to the instantaneous temperature difference between hot and cold media. In a heat exchanger, the temperature difference varies with position and time. Hence to calculate the total heat transferred or the duty of the heat exchanger we use Log Mean temperature difference. There are three different types of flows:
Parallel or co-current flow: flow in the same direction. Counter current flow: in the opposite direction Cross-flow: perpendicular to each other.
Figure 3: Temperature profile in a Parallel and counter flow profile.
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Heat Exchanger Design
Now unless there is strong reason to use parallel flow, counter-flow is preferred. As observed from Figure3 above, in a parallel flow there is a greater (LMTD) log-mean temperature difference, therefore it requires more duty. . This can be shown with a simple comparison of parallel and counter-current flow in a 1-1 shell and tube heat exchanger. Using thermochemical data from the table in 3 and using the design requirements as stated in the assignment.
Table 3 Simple calculation for duty in Counter and parallel flow profile.
Table 4
Counter-Current
Parallel Flow
∆T1 (K)
65
∆T1 (K)
87
∆T2 (K)
27
∆T2 (K)
5
LMTD (K)
43.25
LMTD (K)
28.71
Required UA (J/m^2. K)
32370
Required UA (J/m^2. K)
48770
For this design that uses a 2 tube passes and 1 shell pass a counter-current flow can be characterized as having the first tube pass parallel to the shell-side flow and the second counter-current to the flow Since this design will in-corporate both parallel and counter flow, we require a correction factor for the LMTD. The introduction of the correction factor to the LMTD will decrease it relative to the LMTD’s of the 1-1 exchanger as shown above. Baffles
Baffles are provided in the shell of the heat exchanger in order to direct the fluid flow, support the tube structure, and prevent vibration and sagging. . Longitudinal baffles control the overall flow direction of the shell fluid. Longitudinal baffles are generally only implemented in shell and tube heat exchangers when there is more than 1 shell-side pass
Figure 4: A basic schematic of baffle construction.
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Heat Exchanger Design Transverse baffles may be classified as plate baffles. They also support the tubes during assembly and operation and to direct the fluid in the tube bundle approximately right andles to the tubes to achieve higher heat transfer coefficients. However, In keeping with these design guidelines; there will be no longitudinal baffle, the spacing of the transverse baffles will be 2/5 times the internal shell diameter and the baffle cut will be 25%. Though these features will not be calculated for, instead a perfect cross-flow is assumed. Number of shell and tube passes The number and length of the tubes are key design fundamentals. The velocity of fluid inside the tubes is proportional to both number and length number of tubes. Inside the tubes we need increased velocities in order to set up turbulent flow fields. Hence, one of the key reasons multipass exchangers is preferred over single pass, ensuring high velocity and tube side coefficient. Increasing the tube side passes adds to the complexity, costs and friction. Some sections may also experience parallel flow which limits the temperature approach. In practice, commonly employed heat exchangers are the 1-1, 1-2 and 2-4exchangers so these were the only orientations considered. In order to avoid complex problem solving, for this design I propose of 1-2 shell and tube heat exchanger. Reasons for the choice are such that, there are not significant cost difference between 1-2 and 1-1 heat exchanger. The advantages of a 1-2 design include, compactness, higher heat transfer rate etc.
Choice of shell type According to TEMA there are 7 different types shell’s available. Longitudinal baffles are absent in this type hence it facilitates only one shell. Further research has shown it is widely used in the industry; also it is the cheapest option. The entrance and exit of the E shell are located at the end of the shell. The K-type is used for a 2-shell pass; hence it won’t be elaborated here. The remaining shell types induce counter flow for improved heat transfer, at the same time they have higher initial and maintenance costs.
Choice of Rear-end type TEMA specifications give 8 different options for the rear end of the shell and tube heat exchanger. The decision making process majorly broken into the following categories:
Associated Costs Ability to handle thermal stresses Complexity of replacement Ease of cleaning process
Simplest and cheapest is the U-tube bundle (U) closely followed by the fixed tube designs; L, M or N. On top of the price range is the floating head design. Therefore, U, L, M or N are desirable. Thermal stresses are generated within the shell due to temperature differentials expansions of the working fluids. Fixed tube sheets are only employed when there is a temperature difference of less than 56°C. Since our design process exceeds 56°C we are forced to ignore the L, M and N type rear-ends. Hence, we are down to Utube bundle and depending on the cost constrains; a floating head. The U-tube head (U) is a easily manufacturable design, requiring a bundle of U-tubes with only one tube-sheet and no expansion joint or rear head; hence allowing for easy removal of the bundle.
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Heat Exchanger Design The relative ease of cleaning is an important deciding factor. The floating head design helps with easy cleaning, removal and maintenance. U-tube bundle hinders cleaning in the shell side of the heat exchanger. Since the fouling factors are relatively low, if significant velocities are maintained fouling may be ignored. Worst comes, the product of fouling will be Calcium Carbonate deposit, which in turn is easily cleaned via an acidic wash. Based on the decision making process, it can be concluded that U-tube bundle is sensible choice. Front head design The thermal stresses are of major importance at the rear-end of the heat exchanger. Hence, the front design is solely based on the ease of cleaning. There are two categories for the front end design; channels and bonnets. For this design we will be using the (B) as it is easier to clean and lower initial set up costs.
Materials The following is the table used for decision making process in the selection of the materials.
Tube side (water) In the tube side, the water ranges 23-45 . This means Aluminium and and austentic Cr-Ni steel are a good choice. For mildly Aluminium is aconstruction. valid choice,(Lancaster since its relatively inert oxide provides good Figure 5: Options of thecorrosive available fluids, materials required for 1998) protection against corrosion. Carbon steel is also a valid choice. Shell side (ethylene glycol) The design range of ethylene glycol is 50-110 . Hence carbon steel is a safer option. Even though, it is relatively expensive, in long term it will turn out to be more economical due to its inert properties. In carbon steel, the carbon content may be altered in order to increase the thermal conductivity.
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Heat Exchanger Design Layout of tubes The pitch is defined as the distance between the centres of the adjacent tubes; the available options are either triangular or square pitch. Triangular pitch provides an improved heat transfer area, hence it is preferred unless the shell side fluid tends to foul badly. Tringular pitch is able to provide better heat transfer area due to its compact size. If the center to center distance between the tubes is too small, tubes in triangular pitch cannot be cleaned by running a brush between rows, whereas tubes in square pitch are readily cleaned. Also square pitch gives a lower shell side pressure drop in than a triangular pitch. TEMA standards specify a minimum pitch of 1.25 times the outside diameter of the tubes for a triangular pitch and a minimum cleaning lane of .25 inches for a square pitch. Overall decision is to choose triangular pitch, since ethylene glycol doesn’t foul easily, so by choosing triangular pitch we are maximizing heat transfer area. Designation of tube side fluid Several factors influence the fluid allocation in a heat exchanger. The tube side is preferable for fluid has a higher pressure, higher temperature or is more corrosive. This is because the tube side is less likely to leak expensive/hazardous fluid and also is easier to clean. Calculations for pressure drop and laminar heat transfer are able to be predicted to higher precision. Turbulent flow is easily attainable on the shell side; hence the fluid with lower density is preferred in the shell side. From these requirements, water should be placed in the tube side, leaving ethylene glycol in the shell side. The water can be considered comparably more reactive, especially when we are told it is of ‘poor quality’; this implies impurities. These impurities would also make the water a candidate for fouling. In addition, ethylene glycol has a much higher viscosity than water. (R.Shah & D.Sekulic 2003) Thermal Calculations: Data: Water
Ethylene Glycol
Tc (˚C)
23
Th (˚C)
36.62
110
4.21
2.06
6
4
Cp(J/g.K) P(barg) m(kg/hr) MP (˚C) BP (˚C) µ (N.s/m²)
unknown
50
35000 0
-13
100
197
1.00E-03
1.61E-02
*45 degrees Celsius is the maximum allowable outlet temperature, since we it is an iterative the value may be lower than 45. a) To find the cooling water mass flow rate, we put an. The equations are following: ( ) ( ) ̇ ̇
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Heat Exchanger Design
̇
( ( ̇
) )
b) The un-corrected LMTD of the heat exchanger is;
(
)
Calculating LMTD correction factor: ( (
Where
) (
)
(
)
(
(
) )
(
(
) )
)
and
Taking Tha, Thb, Tcb and Tca as 383K, 323K, 318K and 296K respectively, as specified in the design brief the equation yields a correction factor (Fg) of 0.86. Thus;
c) Sizing and estimation of the heat exchanger was achieved through the iterative process.
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Heat Exchanger Design
Layout and Tube size Outside diameter of each tube (do)
0.75
inch
0.01905
m
Wall thickness of each tube (dt)
0.078740157
Inside diameter of the tube (di)
0.592519685
inch
0.002
m
inch
0.01505
m
Tube Length (L)
20
ft
6.096
m
Tube pitch (pt)
0.9375
inch
0.0238125
m
Surface area of each tube (m^2)
0.364829385
Total number of tubes
240.8219343
Total number of tubes (Rounded up)
241
Total number of tubes in each pass
120.4109672
Total number of tubes in each pass (Rounded up)
120.5
Tube cross sectional area (m^2)
0.000177895
Total cross-sectional area in each tube pass (m^2)
0.021436305
Volumetric flow rate in each pass (m^3/s)
0.03068107
Velocity of tube fluid(m/s)
1.431266723
d) Inside (tube-side) heat transfer coefficient, h1 Reynolds Number (Re)
26167.80251
Prandtl Number (Pr)
5.626186947
L/di
405.0498339
jh
0.0039
jH
102.0544298
Nusselt Number (Nu)
181.5111243
Inside fluid film co-efficient (hi) (W/(m^2*°C))
7344.868749
e) Outside heat transfer coefficient, h0
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Heat Exchanger Design
lB:Ds
0.45
Baffle spacing (lB) (m)
0.2025
Number of baffles
30.1037037
Number of baffles
32
Area for cross flow (As) (m^2)
0.018225
Shell side mass velocity (Gs) (kg/(m^2*s))
533.455266
Shell linear velocity (us) (m/s)
0.503339804
Shell-side equivalent diameter (de) (m)
0.013526453
Reynolds number (Re)
1689.676436
Prandtl Number (Pr)
13.28041803
Baffle cut-off (%)
25
jh
0.015
Shell-side heat transfer co-efficient (hs) (W/(m^2*°C))
1144.812904
f) Fouling and tube wall resistance Fouling Factors (Step 11) Tube fouling factor (1/hid)
0.0008
(hard water)
Shell fouling factor (1/hod)
0.0002
ethylene glycol)
g) Overall heat transfer coefficient. Overall heat transfer co-efficient (Step 12)
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1/U0
0.002396226
U0 (W/(m^2*°C))
417.3229835
Difference between estimate (W/(m^2*°C))
7.322983511
Heat Exchanger Design Difference between estimates (%)
1.786093539
Hydraulic and pressure for tube side pressure drop
Re (Tube)
26167.80251
Tube side friction factor (jf)
0.004
∆Pt (N/m^2)
31543.88019
∆Pt (Bar)
0.315438802
Re (Shell)
1689.676436
Baffle cut-off (%)
25
Shell side friction factor (jf)
0.0475
∆Pt (N/m^2)
48011.39965
∆Pt (Bar)
0.480113997
Summary of Final Design Final design of TEMA components B,E,U, they are based on cost effectiveness and application effectiveness.
Heat Exchanger Tube side stream Shell side stream Head Design Layout Pitch Flow type (Tube) Flow type (Shell) Materials
1-2 Heat Exchanger Water Ethylene glycol U-Tube Triangular 1.25 do Turbulent Transitional Plain carbon steel
Schematic Design ( TEMA STYLE) 11 | P a g e
Heat Exchanger Design
References 1. Mccabe, W. L., Smith, J. C. & Harriott, P. (2005) Unit Operations of Chemical Engineering, 7th ed. New York, McGraw-Hill. Page 441-455 2. Coulson J.M. and Richardson J.F. (Eds) (1999), Chemical Engineering Vol. 6: Chemical Engineering Design (3rd Edition) (Sinnott R.K), Butterworth-Heinemann, Oxford Pg. 640-697 3. Perry R.H. and Green D.W. (Eds) (1997) Perry’s Chemical Engineers’ Handbook (7th Edition), McGraw-Hill, New York 4. TEMA (1988) Standards of the Tubular Exchanger Manufacturers’ Association (7th Edition) 5. Boxall. J. 2010. CHPR 3530 Lecture notes.
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