Heat Exchanger: Conical shell and Nozzle Reinforcement Effectiveness: Case Study

Heat Exchanger: Conical shell and Nozzle Reinforcement Effectiveness: Case Study

Heat Exchanger: Conical shell and Nozzle Reinforcement Effectiveness: Case Study

1. Prof. L. S Utpat

   Professor, Mechanical Engineering Dept., MMCOE, Pune -52 Pune University, Maharashtra, India

2. Prof. Chavan Dattatraya K

   Professor, Mechanical Engineering Dept., MMCOE, Pune-52 Pune University, Maharashtra, India

   PhD scholar, JJT University, Rajasthan

3. Talke R V, Kalbhor M V Graduate Students MMCOE, Pune

1. INT RODUCTION

   Industrial heat e xchangers are much less comple x although the technological civ ilization ma kes the m no less vital. Manufactured heat exchangers are found in facet of our life. Automobiles are equipped with heat exchangers called as radiators and all electronic and electrical equip ment must be provided with heat e xchanger for cooling.

2. ESSENTIAL COMPONENT OF A HEAT EXCHA NGER In shell and tube heat exchanger, the tubes are mechanically attached to tube sheets, which are contained inside a shell with ports for in let and outlet fluid and gas. They a re designed to prevent the liquid flowing inside the tubes fro m mixing with the fluid outside the tubes. Tube sheet can be fixed to the shell or allowed to e xpand and contract with therma l stress. In the later design an expansion blows is used or one tube sheet is

   allo wed to float inside the shell. Heat e xchanger studied here has fixed tube sheet without bellows and without floating head. The brief description of each component goes as follows:

   1. Shell:

      It is the main body of heat e xchanger enclosing tubes, baffles (only for supporting), imp inge ment plate, with inlet and outlet connections for working flu id. The shell is constructed either fro m pipe up to 24 inch or rolled and welded plate materia l.

      The main shell is welded to tube sheet and channel shell is welded to the integral type flange.

   2. Headers/Channel dished ends:

      It forms the tube side of the shell and contains pass partition plates in mu lt iple pass heat exchangers with in let and outlet connections for working flu id, and a drain nozzle . The dished end is formed fro m steel plates, considering the appropriate forming allowances. A straight flange portion is kept to match channel shell while we lding.

   3. Tubes

      The number and length of the tubes decide heat transfer area of e xchanger. Dra wn and seamless stainless tubes are we lded to the tube sheet so as to provide for a double pass heat exchanger. The baffles are used for supporting the tubes as per construction code requirement, Radiographic tests, eddy current tests, pneumatic tests are conducted on tubes prior to its fit up.

   4. Tube Pitch

      Tube shall be spaced within the minimu m centre to centre distance of 1.25 OD of tube.

      Tube patterns:

      1. Square pattern:

         In re movable bundle units when mechanical clean ing of tubes is specified. Tube lanes should be continuous.

      2. Triangular pattern:

      Triangular or rotated triangular pattern where mechanica l c leaning is not specified.

   5. Tube sheets (TS)

      Tube sheets are used to support the tubes of heat e xchanger at e xtre me ends.

      TS are made fro m round plate piece of metal. Holes are drilled for the tube ends at precise locations and pattern relative to one another. Tube sheets are manufactured fro m the same range of materials as tubes. Tubes are attached to the tube sheets by pneumatic or hydraulic pressure by roller e xpansion if needed. Tube holes can drilled and reamed. Th is greatly increases tube joint strength.

      The tube sheet is in contact with both fluid s. Thus it has corrosion resistance allowances and metallurgica l and electrochemical properties appropriate for the flu ids and velocities. The tube hole pattern or pitch varies the distance fro m one tube to another as well as the angle of the tubes relative to each other and direction of flow. This allo ws the flu id velocit ies and pressure drop to be manipulated to provide the ma ximu m a mount of turbulence and tube surface contact for effective heat transfer.

   6. Tube bundle

As the name imp lies it is a bundle of tubes held together by baffles and tie rod spacers.

The tube bundle is assembled and it is inserted in to the shell. At the end, tubes are welded to tube sheet and finally tube sheet to main shell and channel.

3 PROBLEM STATM ENT:

To design a heat exchanger to meet the therma l and mechanica l require ments. The scope here is limited to study the stress variation at conical shell and nozzle re inforce ment pad. Only mechanica l design is considered here. Design

should be based on ASME standard. Given data is as follows:

3.1 MECHANICA L DESIGN CA LCULATIONS:

Internal design Pressure of shell = 20 kg/c m2 Internal design Pressure of Tube = 15.5 kg/c m2 Design Te mperature of She ll = 90C

Design Te mperature of Tube = 160C

Static head of the fluid is added to the internal design pressure and design is made fo r a total pressure of

20.151 kgf/c m2

Fig1. Arrangement of the Heat Exchanger

4.DESIGN OF CONICA L SHELL

Material of Channel = SA240 304L Inside Dia mete r of Channel = 1040 mm

Allowab le Stress at 900 (So) = 1174.13 kgf/c m2

Allowab le Stress at Ambient (Sa) = 1174.13 kgf/c m2 Efficiency of Longitudinal Sea m = 0.85

Corrosion Allowance (C) = 1mm

Minimu m Required Thic kness of Conical Shell due to Internal Pressure[t]:

………………..UG 27 (c )(1)

———– taking angle as 30o

= 12.7 + 1= 13.7 mm 14 mm

Maximu m Allowab le Working Pressure (M.A.W.P.):

= 22.9 Kgf/c m2

Actual Stress at given Pressure and Thickness [Sact]:

= 1030.5 Kgf/c m2

Sact < So, and M.A.W.P > Internal Pressure (P) Therefore at t = 14 mm Design is Safe .

Actual thickness used = 16mm

1. DESIGN OF NOZZLE AND OPENING IN UNFIRED PRESSURE VESSEL/ HEAT EXCHA NGER.

   Openings are provided in the heat e xchanger for functional require ment. They are required for

   1. Inlet & outlet opening connections

   2. Dra in pipe connections

   3. Pressure gauge connections

   4. Safety device connections

Nozzles are formed or we lded around these openings. Openings or hole causes discontinuity in the vessel wall which creates stress concentration in the vicinity of openings.

Higher stresses at the openings can be reduced by providing reinforce ment in the vic inity of opening. This can be achieved either by one or more co mb inations of follo wing methods:

1. Providing addit ional thic kness to the shell wall itself near the nozzle.

2. Use of separate re inforce ment pad attached to the heat

   e xchanger wa ll covering an area surrounding the opening .

3. Providing additional thickness to the nozzle:

Most widely used method for designing reinforcement for nozzle is area for area method of compensation.

1. Area for area method of compensation

   In this method the area o f the materia l re moved is compensated by providing additional area.

   1. In the portion of shell as e xcess thickness.

   2. In the portion of the nozzle outside the vessel as excess thickness.

   3. In the portion of the nozzle inside the ves sel as excess thickness.

   4. In re inforcing pad (co mpensation ring)

      Fig. 2 shows the reinfore ment boundary limits . The

      Fig2. Area for a rea method of co mpens ation

      Pi= internal pressure =1.96 N/ mm2 Required thic kness of shell = 13 mm

      1. Inner dia meter of the nozzle in corroded condition

         dnc=dn+2c

         = 50+2

         =52 mm

      2. Required thickness nozzle

         p .d

         area of the opening to be compensated is dnc*trs which is the

         trn

         i n

         ———UG37(a )

         minimu m area of the shell that is required to sustain the pressure. Require ment of the re inforc ing pad is first

         2..S.

         all pi

         established as below.

2. Calculation for reinforcement pad:

Let;

dn= dia meter of no zzle = 50 mm.

all= allowab le tensile stress for shell and nozzle material is =115.4 N/ mm2

ts= thickness of shell

=16 mm

tn =Thickness of nozzle = 8.7 mm

C= corrosion allowance for shell and nozzle

= 1 mm

trn= (1.96×50)/(2×1×115.7-1.96)

= 0.337 mm I

1. He ight of nozzle H1= dnc(tn-c)

   Or

   (Actual length of nozzle outside the heat exchanger)

   = 52(8.7-1)

   = 20 mm or 225 mm Selecting s maller value, Taking H1=20 mm

2. He ight of nozzle projecting in side H2= 0 mm

3. Estimat ion of co mpensation:

   The addition area required is estimated as follows

   1. Area for opening in corroded condition is Ar=dnc×trs = 52 × 14

      =728 mm2 I

   2. Area available for co mpensation(Aa)

      1. The area o f e xcess in shell thic kness in the portion of shell ;

         A1= dnc(ts-trs-C) = 52(16-13-1))

         = 104 mm2

      2. The area o f e xcess in shell thic kness in the portion of nozzle project ing outside of shell ;

         A2= 2H1 (tn-trn-C) = 2×20 (8.73-0.367-1)

         = 294.5 mm2

      3. The area o f e xcess in shell thic kness in the portion of nozzle p rojecting inside of shell ;

A3= 0 mm2

So total area available for co mpensation Aa=A1+A2+A3 = 104+294.5+0

= 398.5 mm2 II

Required area for re inforc ing pad; A= Ar – Aa

A= 728-398.5 A=329.5 mm2

Ca lculating the dimension for pad

tp = 9.0 = 10 mm

So taking thic kness of pad = 10 mm

1. FE A NALYSIS PROCEDURE

   A programmed FE pre-processor is used in order to standardize the analysis approach and significantly speed up the input process. This can also be used to further refine individual geometry. The post-processor provides a plot of stresses in the nozzle and shell utilizing the highest peak stress intensity indication to locate the line to plot. The post processor may also inc lude the acceptance criteria plotted for reference in position to the indicated stresses. This will a llow an immediate visual determination whether the loading and geometry is acceptable. As this procedure can be used for many geo metrys and loading conditions, it can a lso be used to analyze many so-called standard geometrys in order to arrive at acceptable nozzle loading criteria for a range of conditions.

   Methodology and F.E. Idealization:

   1. System of Units:

      The following system of units is fo llo wed for consistency throughout this analysis and results evaluation

      Table 1: system units used

      S .No	Parameter	Units	Conversion factor used in Analysis.
      1.	Length	M illimetres	1.0
      2	Force	Newton	1.0
      3	M ass	Kg	1.0
      4	M oment	N-mm	1.0
      5	Pressure, M odulus of elasticity, stress	N/mm2	1.0

   2. Ansys Elements Used

      The comp lete assembly is modeled using ANSYS Ele ment Types as follows:

      S .No	Element	ANS YS Element	Parts Modeled
      1.	3-D Elastics shell	8 node shell 93	Shell plate, cone plate, nozzle and pad etc.
      2	Rigid Constrain	MPC-184	Rigid Element

      Table 1: Ele ment types used

      A= (d

      po-d

      pi) tp

      329.5 = (104 – 67.4) tp

   3. Material Properties

      Material: Austenitic Stain less Steel. Isotropic Youngs Modulus = 189860 N / mm 2.

      Poissons Ratio = 0.3 Density: 8000 Kg / m3.

      Figure 3: F.E.A. Mesh Model

   4. Loading (Design Condition):

The Model has been analyzed for co mb inations of one or mo re of the fo llo wing loads

1. Internal Design pressure = 1.96 N / mm 2

2. External Design Pressure = 0.1013 N / mm 2

3. Design Te mperature = 90C

4. Hydrostatic Pressure = 2.54 N / mm 2

5. Hydrostatic Temperature = A mbient

   Fig.4She ll at loading condition

   The figure 4 and 5 show the conical shell portion and the nozzle subjected to various loading conditions.

   Conical shell as we ll as the nozzle is subjected to internal pressure of 1.96 N/ mm2 and the conical shell is connected to cylindrica l channel on both ends. The shell side te mperature is 90C.

   Fig.5 nozzle at loading condition

6. RESULTS INTERPRETATION AND CODE CHECKING:

Figure 6 shows the stress pattern in the conical shell without reinforce ment pad for the nozzle. The stresses for the above mentioned load co mbinations are summarized and compared with code allowab le limits for a ll critica l parts in the Table no.3.The stress plots are also enclosed and referred.

Fig.6 Stresses in shell without nozzle pad

Due to asymmetry in the conica l shell, the stresses are not uniform and the s tress variation is shown in figures 7 and 8. These stresses are difficu lt to ca lculate by hand calculations and therefore the stress variation plot becomes useful informat ion.

Fig.7 Stresses in shell without nozzle pad The various locations in the conical shell at section

AA are shown in fig. 8 belo w and the corresponding stresses in the shell are su mmarised in the table 3. As e xpected, the

stresses at ma ximu m d iscontinuity at point 1is ma ximu m.

Fig.8 Node points at A-A section Table 3: Stress at various node points.

The stress pattern can be used to locate a nozzle opening or any other auxilia ry opening at lowly stressed zone wherever choice e xists.

Stress Pattern at Nozzle opening:

As per hand calculations shown earlier, re inforce ment pads were found essential and 10 mm was found as the required

thickness. The stress pattern was studied using FEA after applying the reinforce ment pads of diffe rent thicknesses.

Fig.9 Stresses in nozzle pad and nozzle (at 10 mm)

Four node locations we re identified as shown in figure 10 and the stress values were estimated by FEA at all these locations . The exercise was carried out for thicknesses varying fro m 4 mm to 16 mm keeping the outer dia meter same.

Fig.10 No zzle She ll Junction

The observations are tabulated in table 4. It may be noted that the ma ximu m stress at node 1 location re flects the stress peak in the nozzle rather than the stresses in the pad due to stress concentratin.

Table 4: Stress Analysis Results.

It can be seen that the stresses at node locations of 2, 3, and 4 are significantly low for even sma lle r values of pad

thicknesses such as 4, 6, or 8 mm. Moreover the effect is highly concentrated at nozzle point.

160

140

Stress Intensity

120

100

80

60

40

20

0

4 6 8 10 12 14 16

Thickness

Node Point 1

Node Point 3

Node Point 2

Node Point 4

160

140

120

Stress Intensity

100

80

60

40

20

1. REFRENCES

   Book s :

   1. Heat Exchanger design by Mr.Young and Mr.Browne ll, 1998 ed itions for integral type of the flange design (Pg.No 242-260)

   2. Heat transfer by Donald Kern.

   3. Heat transfer by Mr.Ra jput,2002 edition for the study of heat transfer area

   4. Preassure vessel deign manual by Mr. Dennies

      R.Moss,1987 ed ition

   5. Heat e xchanger design by Mr.Ganeshan and Mahajan ,1996 ed ition

   6. The fundamentals of heat e xchanger by Ra mesh.K.Shah and Dusan.John 2003 edit ion

   7. ASME SEC. VIII Div.1 Ede 2010

Websites:

1. http://www.te ma.org

2. http://www.heate xchangeronline.co m

3. http://www.en.wikipedia.org/wiki/heat_exchanger#s hell_&_tube_heatexchanger

0

1 2 3 4

Node Point

4mm

12mm

6mm

14mm

8mm

16mm

10mm

7. CONCLUSION:

The stress pattern has been studied at conical shell and the nozzle reinforce ment locations.

The shell stress variation can be used to advantage by properly locating the openings in the shell at low stress areas.

Stress peak due to nozzle opening is observed main ly in the close vicinity of the nozzle.