DESIGN AND EXPERIMENTAL ANALYSIS OF SHELL AND TUBE HEAT EXCHANGER (U-TUBE)

Transcription

1 DESIGN AND EXPERIMENTAL ANALYSIS OF SHELL AND TUBE HEAT EXCHANGER (U-TUBE) Divyesh B. Patel 1, Jayesh R. Parekh 2 Assistant professor, Mechanical Department, SNPIT&RC, Umrakh, Gujarat, India 1 Assistant professor, Mechanical Department, SNPIT&RC, Umrakh, Gujarat, India 2 Abstract: Heat exchangers are one of the most important heat transfer apparatus that find its use in industries like oil refining, chemical engineering etc. Shell and tube (U-tube) type of heat exchangers have been commonly and effectively used in industries over the year. In this paper, shell and tube (U-tube) heat exchanger is designed which includes thermal design, mechanical design and hydraulic design. Different types of methods are carried out for optimum design. According to design parameters experimental analysis is carried out, which reveal the clear idea about temperature difference and dimension of heat exchanger. General Design consideration and design procedure is also illustrated. Keywords: Heat exchanger, heat transfer coefficient, LMTD, Mass flow rate, NTU. INTRODUCTION Heat exchanger is a device which is used to transfer the heat from one working fluid to another or we can say transfer of internal thermal energy between two or more fluids available at different temperatures. It is used in wide variety of applications i.e., power production, food industry, electronics, environmental engineering, waste heat recovery, air conditioning, refrigeration etc. Shell & tube type heat exchanger are most versatile type of heat exchangers. In most heat exchangers, the fluids are separated by a heattransfer surface, and ideally they do not mix. The U tube is least expensive construction because only one tube sheet is needed. The tube side cannot be cleaned by mechanical means because of the sharp U bend. Numbers of shell side & tube side flow arrangement are used in shell & tube heat exchanger depending on heat duty, pressure drop, fouling, cost, corrosion control and cleaning problems. Baffles are used in shell & tube heat exchanger to promote better heat transfer coefficient on shell side and to support the tubes. In this present work design of optimum size U type SHTE was carried out. For optimum heat transfer rate different method were carried out [1][2]. Sadic Kakac et.al[1], Ramesh k. shah et.al[2], had given the equation for heat transfer rate for hot and cold fluid by LMTD method, Є-NTU method, P-NTU method and also for Ѱ- NTU method. Sadic Kakac et.al [1] also showed that approximate overall heat transfer co-efficient for preliminary analysis for water to water is between W/m 2 K. He suggested that Tube Pitch, P T, is usually chosen in such a way that PT/dӨ is between 1.25 to 1.5. He gave that tube layout constant CL=0.87 for 300 triangular layout and tube count calculation constant CTP=0.9 for two tube passes. The number of tubes that All rights reserved by ISSN :

2 can be placed within a shell depended on a tube lay out, tube outside diameter, pitch size, number of passes and shell diameter. He had conclude that optimum baffle spacing, Bs is between 0.4 and 0.6 of shell diameter and baffle cut Bc, is 25% to 35% of shell diameter. Sadic Kakac et.al[1], T. Kupprn et.al[3] had given following four type of tube layout, Triangular (30 ), Rotated Triangular (60 ), Square (90 ), Rotated Square (45 ). T. Kupprn et.al [3] had given the basic equation to find out, the number of baffles, N b = (L/B c )-1 Bundle to shell clearance, L bb = ( D)/1000 in m for U tube exchanger. Outer tube limit diameter, D otl = (Ds-L bb ), Centerline tube limit diameter, D ctl = (D otl -dө), Baffle diameter = D s -B c He also given Centriangle of baffle cut θ ds = 2 cos -1 [1-2 Bc/100] DESIGN MEHODOLOGY Following are the operating parameters while designing the shell & tube heat exchanger: 1. Inlet temperature of hot water, T h1 = 63 C 2. Outlet temperature of hot water, T h2 = 53 C 3. Inlet temperature of cold water, T c1 = 33 C 4. Mass flow rate of cold water, ṁ c = 0.6 kg/s 5. Mass flow rate of hot water, ṁ h = 0.58 kg/s A. Thermal Design There are basic three approaches for thermal and hydraulic design: 1) LMTD METHOD 2) є-ntu METHOD 1) LMTD Method Outlet Temperature Of Cold Water : [1][2] Heat Transfer Rate: Q=U *A* Tm = ṁ h *c ph * (T h1 T h2 ) = ṁ c *c pc * (T c2 T c1 ) Here, ṁ h and ṁ c are mass flow rate of hot fluid and mass flow rate of cold fluid respectively. From the above equation we can find the outlet temperature of cold water. Heat Transfer Rate Of Shell And Tube Side: [1][2] Actual heat load can be estimated by Q = ṁ h *c ph * (T h1 T h2 ) Log Mean Temperature Difference: [1][2] Tm= Heat Transfer Area: [1][2] Qact = U * A * Tm Overall Heat Transfer Co-efficient [U] for the water to water fluid is between 1300 to 2500 w/m 2 C 2) є-ntu Method Maximum possible heat transfer rate is determined by Q max = C min (T h1 -T c1 ) All rights reserved by ISSN :

3 Here C min =minimum heat capacity which is given by C C = ṁ c *C pc or C h = ṁ h *C ph whichever is minimum. Effectiveness: [2] Heat exchanger effectiveness is defined as the ratio of actual heat transfer rate to the maximum possible heat transfer rate є= Q act / Q max Heat Capacity Ratio: [2] It is a heat exchanger operating parameter since it depends on the mass flow rate and temperature of the fluid in the heat exchanger C* = C min / C max = (ṁ*cp) min /( ṁ*cp) max Effectiveness and number of transfer unit relationship: [2] Effectiveness is function of NTU, heat capacity ratio and flow arrangement. = Number Of Transfer Unit: [2] Number of transfer unit is estimated as the ratio of overall thermal conductance to the minimum heat capacity NTU = (U* A) / C min From this equation we find Area of heat exchanger which is coming higher than the area found by LMTD method so We adopt LMTD METHOD for Thermal Design. B. Number Of Tube: [1] Heat transfer area for the tube of the diameter d 0 and length L can be given by following equation from which numbers of tube can be estimated A = π*d 0 *N t *L C. Shell Diameter: [1] Shell diameter can be obtained as Ds = * *[A*(PR) 2 *d o ] (1/2) CL=0.87 for 30 and 60 CTP=0.9 for two tube pass PR is usually between 1.25 to 1.5 D. Pitch: [1] Pitch can be determined as P T =PR*d E. Baffle Spacing: [1] The practical range of single segmental baffle spacing is 40% - 50% of shell diameter B s =( 0.4 to 0.5) * Ds F. Number Of Baffle: [3] Numbers of baffles for the tube length L and baffle spacing B s can be given by N b = (L / B s ) 1 G. Baffle Cut Geometry: Baffle Cut: [3] The baffle cut vary from 20-35% of shell diameter with the most common being 20-25% as it affords the highest heat transfer for a given pressure drop. B c = (0.20 to 0.35)* Ds All rights reserved by ISSN :

4 Figure 1: baffle geometry Bundle To Shell Clearance: [3] Bundle to Shell Clearance is calculated based upon the following equation L bb = [12+ (0.005*Ds)]/1000 (m) Outer Tube Limit Diameter: [3] It is difference between the shell diameter and Bundle to Shell Clearance D otl =Ds- L bb Centerline Tube Limit Diameter: [3] It is difference between the Outer Tube Limit Diameter and tube diameter D ctl = D otl -d o Diametral clearance The diametral clearance between shell diameter and baffle diameter is given by L sb = 3.1+(0.004* Ds))/1000 (m) Baffle Diameter: [3] It is difference between the shell diameter and diametral clearance D b = Ds- L sb H. Shell Side Heat Transfer Coefficient: Equivalent Diameter: [1] Figure 2: triangular pitch layout All rights reserved by ISSN :

5 Triangular pitch layout is shown in the figure2, for which the equivalent diameter is given by following equation. 2 For triangular pitch D e = P T /4)- 2 /8) / ( * Here d o is tube outer diameter. Clearance between Tubes: [1] As shown in figure 2 clearances between tubes is estimated as below. C= P T - Bundle Cross Flow Area: [1] Bundle Cross Flow Area can determined by following equation A s = D s *C*B s / P T Shell Side Mass Velocity: [1] The variable that affects the mass velocity are shell diameter D s, clearances between the adjacent tubes and the cross flow area A s. shell mass velocity is found with, U s = s/ A Here s is mass flow rate of fluid in shell side. Shell Side Reynolds Number: [1] Reynolds number for the shell side flow can be estimated as, R es = U s * D e / ʋ s Here ʋ s is kinematic viscosity of fluid at given fluid temperature. Approximate Wall Temperature: [1] Approximate Wall Temperature is determined as, T w = (T h1 + T h2 +T c2 +T c1 )/4 Shell side heat transfer coefficient is given by To find the shell side heat transfer coefficient it requires to find the properties like viscosity, thermal conductivity of working fluid at approximate wall temperature and viscosity at shell side fluid temperature. h o =[k/d e *0.36]*[D e *U s /ʋ s ] 0.55 *[C p * μ s /k] 0.33 *[ μ s / μ w ] 0.14 I. Tube Side Heat Transfer Coefficient: [1] ρ = 985 kg/m3 μ t = at average tube side temperature, Tube side flow area is determined as A tp = [π/4*di 2 ]*[ Nt/2] Here di is tube inner diameter and Nt is numbers of passes. Average Velocity In Tube: Average fluid velocity in the tube is given by V m = ṁ t / (ρ* A tp ) Here ṁ t is mass flow rate of tube side fluid; ρ is density of fluid at tube side fluid temperature. Tube Side Reynolds Number: Tube Side Reynolds Number given by, R et = ρ*vm* di/ μ t Friction factor for the tube is, f t =(1.58ln R et -3.28) -2 All rights reserved by ISSN :

6 It is require to find the properties like Prandtl number and thermal conductivity of fluid at tube side fluid temperature to find tube side heat transfer coefficient, Nusselt number for the tube side fluid is; N ub = [( f t /2)*( R et -1000)*Pr]/[(1+12.7(f t /2) 0.5 *(pr 2/3-1)] Tube side heat transfer coefficient is estimated as; hi= N ub *k/di J. Overall Heat Transfer Coefficient: [1] Overall heat transfer coefficient is determined by following equation, U= CALCULATED RESULTS Based on the above methodology the calculated results are summarised below. TABLE 1: CALCULATED RESULTS DESIGN PARAMETER VALUE Cold outlet temperature (T c2 ) o C Heat duty (Q) kw LMTD ( T m ) o C Heat Transfer Area by LMTD (A 1 ) m 2 Maximum Heat Transfer Rate (Q max ) kw Effectiveness (є) Heat Capacity Ratio (C * ) Number Of Transfer Unit ( NTU) Heat Transfer Area by NTU (A 2 ) m 2 Numbers of tubes (N t ) 26 Shell diameter (D s ) m Pitch (P T ) m Baffle spacing (B s ) m Numbers of baffles (N b ) 10 Baffle Cut (B c ) m Bundle To Shell Clearance (L bb ) m Outer Tube Limit Diameter (D otl ) m Centerline Tube Limit Diameter(D ctl ) m Diametral clearance between shell diameter m and baffle diameter (L sb ) Baffle Diameter (D b ) m Equivalent Diameter (D e ) m Clearance Between Tubes (C) m Bundle Cross Flow Area (A s ) m 2 Shell Side Mass Velocity (U s ) kg/s.m 2 Shell Side Reynolds Number (R es ) Approximate Wall Temperature (T w ) C All rights reserved by ISSN :

7 Shell side heat transfer coefficient (h o ) W/m²k Average velocity in tube (v m ) m/s Tube Side Reynolds Number(R et ) Tube side Friction factor (f t ) Nusselt number (N ub ) Tube side heat transfer coefficient (hi) W/m²k Overall heat transfer coefficient (U) W/m²K Experimental Setup: I. EXPERIMENTAL RESULTS Figure 3: Experimental setup of shell and tube heat exchanger (u-tube) In Experimental analysis of STHE, first we fix the mass flow rate of hot water at kg/s and vary the mass flow rate of cold water then we get the diff. Temperature reading. Here the cold water and hot water flowing through shell and tube side respectively. TABLE 2: EXPERIMENTAL DATA Sr. N o. 1 Tub e Shel l 2 Tub e Shel l 3 Tub e Shel l Mass Flow Rate (kg/sec) Inlet Temp. ( C) All rights reserved by ISSN : T 1 ( C) T 2 ( C) T 3 ( C) Out let Te mp. ( C)

8 Figure 4: Shell side temperature variation Figure 4 shows the temperature variation in the shell side along the length at different mass flow rate for the shell side fluid. Keeping hot water mass flow rate constant at 0.401kg/sec and varying cold water mass flow rate at , and kg/sec, we get increase in cold water temperature of 6.8 C, 5.7 C and 4.9 C respectively. CONCLUSION Based on the design methodology we design and fabricate the shell and U tube heat exchanger and perform the experimental test. Keeping hot water mass flow rate constant at 0.401kg/sec and varying cold water mass flow rate at , and kg/sec, we get increase in cold water temperature of 6.8 C, 5.7 C and 4.9 C respectively. REFERENCES [01] Karnik, Kakac Sadik, Hongten Liu, Heat Exchangers Selection,Rating And Thermal Design, Second Edition. Page No. [33-71, ] [02] Shah R.K, Sekulic D.P, John Wiley And Sons, INC., Fundamental Of Heat Exchanger Design, ISBN [03] T.Kuppan, Heat Exchanger Design Hand Book All rights reserved by ISSN :