Prediction of Pressure Drop in Chilled Water Piping System Using Theoretical and CFD Analysis

Transcription

1 Shirish P. Patil et.al / International Journal o Engineering and Technology (IJET) Prediction o Pressure Drop in Chilled Water Piping System Using Theoretical and CFD Analysis Shirish P. Patil #1, Abhijeet B. Auti #, Dr. Arundhati S. Warke #3 Symbiosis International University, Pune, Maharashtra, India #1 patilshirish1008@gmail.com # autiabhijeet@sitpune.edu.in #3 arundhatiw@sitpune.edu.in Abstract In the present study, three dimensional models o chilled water piping system is created using design modeler o Ansys-13. Ansys-13 luent is used to analyses low through chilled water pipe or pressure drop prediction. Karman-Prandtl equation is used or deining velocity proile o turbulent low with the help o user deined unction. Result obtained rom CFD analysis is compared with results o 3K, K, ISHARE and Carrier equivalent length methods. Statistical analysis o perormance based relative error has been carried out and based on that optimum analytical method or pressure drop prediction in chilled water piping is suggested. Keyword- chilled water piping, K, 3K, Pressure drop, ISHARE, CFD analysis I. INTRODUCTION In designing air conditioning and rerigeration plant, prediction o correct pressure drop is the most important actor. Appropriate and economical selection o pump requires accurate prediction o pressure drop in chilled water piping circuit and hence calculation o pressure drop is the initial step o the pump selection process. In air conditioning plant, cold water is circulated through chiller (evaporator coil) where it reject the heat and then passed through air handling unit (AHU) where it absorbs heat. Chiller and air handling unit are connected through chilled water pipe. Length o chilled water pipe may depend up on the locations o both the units. This piping system contains dierent types o ittings like control valves, bends, expander, reducer, chiller coil and AHU coil. Fig. 1. Schematic diagram o chilled water piping system setup by Voltas Ltd. Pune Schematic diagram o chilled water piping system setup by Voltas Ltd. Pune is shown in Fig. 1. Chilled water piping system contains meter diameter (D) M.S. pipe through which water o density (ρ) = kg/m 3 ISSN : Vol 5 No 4 Aug-Sep

2 Shirish P. Patil et.al / International Journal o Engineering and Technology (IJET) and dynamic viscosity (µ) = Ns/m lows at rate o (Q) o m/sec. Whereas Fitting details are provided in table I. Pressure drop prediction in chilled water piping system is calculated in two ways: analytical methods and CFD analysis. Fitting 90 Elbow Butterly Valve TABLE I Fitting Details Motorised Butterly Valve Check valve Y strainer 3 Way Balancing Valve Abbreviation Elbow BFV MBFV NRV Y- St. 3WBV Quantity (Nos.) II. PRESSURE DROP CALCULATION BY ANALYTICAL METHODS There are dierent methods or calculation o pressure drop due to pipe ittings. 3K method by Darby [3][4], Equivalent length method by ISHRAE [5] and Carrier system design manual [], K method by William B. Hooper [8] are analytical methods used or pressure drop prediction in chilled water piping system. In these methods pressure drop due to each itting is calculated separately and then combined or pressure drop measurement in the entire piping system. Q Water velocity ( V ) = = = m/sec. A Π 4 ρ V D Reynolds Number (Re) = = = [1] μ Colebrook- white equation [7] is solved or riction actor () using newton raphson method with the help o C- program. Equations by Zarko and Brkic [9] are used or calculating initial value o riction actor. The calculated value o riction actor () is TABLE II Equipment Pressure Drop Details [6] Equipment Model No. Coil Pressure Drop Head Loss Due to Coli 48 TR Water cooled Chiller WCDS048DMNX 0.5 bar.5556 meters 5000 CFM AHU IAW 66 6 psi meters A. Equivalent Length Method This method assumes that the pressure drop across the length o a pipe is same as it occurs in the itting. Thus these ittings can be replaced by simple pipe o particular lengths which has a same pressure drop. Pressure drop calculations by equivalent length method are carried out using equation given below. Where h is head loss due to riction. h V Le = * * g D (1) TABLE IIII Equivalent Length o Fittings by ISHARE [5] and Carrier System Design Manual [] Fitting BFV MBFV NRV Y-St. 3WBV Elbow Equivalent length (L e ) in Meter by ISHARE standard Equivalent length (L e ) in Meter by Carrier System Design Manual ) Head loss due to riction by Carrier System Design Manual [] (0.98 8) (1.8 ) (1.5 3) ( h ) = = meters Pressure Drop ( Δ P) = h ρ g = ( ) Pressure Drop ( Δ P) = Pascal ) Head loss due to riction by ISHARE Standard [5] ISSN : Vol 5 No 4 Aug-Sep

3 Shirish P. Patil et.al / International Journal o Engineering and Technology (IJET) (1 8) (1.8 ) (1.5 3) ( h ) = = meters As ISHARE has not provided any details about the Y-strainer, pressure drop due to Y-strainer is calculated using equivalent length method as per details provided in Carrier System Design Manual []. Pressure Drop ( Δ P) = h ρ g = ( ) Pressure Drop ( Δ P) = Pascal B. William B. Hooper K Method This method requires two constant which correlates Reynolds number and size o itting with pressure drop. Pressure drop calculations by k method are carried out using equations given below. Where Re is Reynolds number, D is diameter in inches and K is loss coeicient [8]. K K1 1 = + K 1+ Re D () h V = k (3) g TABLE IV K 1 and K Values by William B. Hooper [8] K K Loss coeicient (K ) or BFV = ( K ) = = Values o loss co-eicient or all other pipe ittings is calculated similarly by using equation (3) and table IV and represented in table V. As William Hooper [8] has not provided any details about the pressure drop due to Y-strainer, thereore pressure drop due to Y-strainer is calculated using equivalent length method as per details provided in Carrier system design manual []. Pressure drop due to straight pipe is calculated using equation (1). TABLE V Loss Co-eicient or Pipe Fittings by K Method Loss co-eicient (K ) Head loss due to ittings ( h ) = [( ) ( ) ] =.3754 meters Head loss in straight pipe and Y-strainer (1.8 ) ( h ) = = meters Pressure Drop ( Δ P) = h g = [.3754 ] ρ Pressure Drop ( Δ P) = Pascal C. Darby 3K Method Pressure drop calculations by 3k method are carried out using equations given below [3] [4]. K K1 Kd = + Ki 1 Re + D (4) ISSN : Vol 5 No 4 Aug-Sep

4 Shirish P. Patil et.al / International Journal o Engineering and Technology (IJET) TABLE VI K 1, K and K D Values by Darby [3] [4] K K K d Loss coeicient or BFV = ( K ) = = Values o loss co-eicient or all other pipe ittings is calculated similarly by using equation (4) and table VI and represented in table VII. As Ron Darby [3][4] has not provided any details about the pressure due to Y- strainer, thereore pressure drop due to Y-strainer is calculated using equivalent length method as per details provided in carrier system design manual []. TABLE VII Loss Co-eicient (K F ) or Pipe Fittings by 3K Method Loss co-eicient (K ) h = = meters Head loss due to ittings ( ) [(5 8) ( ) ] Pressure Drop ( Δ P) = h g = [ ] ρ Pressure Drop ( Δ P) = Pascal Method Darby 3K Method TABLE VIII Results o Analytical Methods William B. Hooper K Method ISHARE Standard Carrier System Design Manual Standard Pressure Drop Pa Pa Pa Pa III. PRESSURE DROP PREDICTION BY CFD ANALYSIS For this study, three dimensional model o chilled water piping system is created using design modeler o Ansys-13. This geometry is then imported in to a meshing module. In which sweep meshing method is used along with ace inlation to capture correct wall boundary condition. Details o mesh created are shown in table IX. Whereas ig. shows the mesh generated or chilled water pipe. TABLE IX Meshing Details Type Method Element Size Cell Mesh Sweep Meshing Minimum 3.5 mm Maximum 8 mm Max. Face Size 4.5 mm Number o Elements Number o Nodes Aspect Ratio Max. Skewness Orthogonal Quality Max Mini ISSN : Vol 5 No 4 Aug-Sep

5 Shirish P. Patil et.al / International Journal o Engineering and Technology (IJET) Fig.. Meshing For Chilled Water Pipe Velocity inlet is used as an inlet boundary condition. Whereas usual no slip and adiabatic condition is assumed on wall surace. The pressure outlet is used as outlet boundary condition as shown in table IX. K- epsilon-realizable turbulence model is used with standard wall unction or near wall treatment. For solution SIMPLE pressure-velocity coupler with least squares cell based discretization scheme is used. Tolerance o 10-4 is used or convergence criteria. TABLE X Boundary Conditions Boundary Type Input Inlet Velocity Inlet m/sec. Chiller coil pressure drop Interace (Fan) 5000 Pascal AHU coil pressure drop Interace (Fan) Pascal Pipe surace Wall No-slip, stationary and adiabatic wall Outlet Pressure outlet - Fig. 3. Pressure contour o chilled water piping system ISSN : Vol 5 No 4 Aug-Sep

6 Shirish P. Patil et.al / International Journal o Engineering and Technology (IJET) Ater Appling all the boundary conditions as shown in table IX, it is observed that the CFD analysis shows the pressure drop o Pascals. TABLE XI Result o CFD Analysis Inlet Pressure Outlet Pressure Pressure Drop (ΔP) Pascal Pascal Pascal IV. RESULT AND CONCLUSION Fig. 4 shows the comparison o pressure drop in chilled water piping system calculated by analytical methods and CFD analysis. Relative errors o 3K, K, and equivalent length method o ISHARE and carrier system design manual is calculated with respect to CFD solution and plotted in ig. 4. Whereas ig. 5 shows the variation in pressure drop calculated by analytical methods with respect to CFD analysis. It is observed that variations in pressure drop are in the acceptable range o 1.85% to 7.75% o relative error and hence CFD analysis can be used or pressure drop prediction in chilled water piping system. Fig. 4. Pressure Drop and Relative Error o dierent methods o pressure drop prediction Fig. 5. Variations in pressure drop calculated by analytical methods with respect to CFD analysis Pressure drop calculated by ISHARE and Carrier shows similar results but higher pressure drop value with relative error o 6.19% and 6.11% respectively. Whereas 3K method under predicts pressure drop value as compared to CFD results with relative error o 7.7%. However K method shows optimum results o pressure drop prediction with 1.86% relative error. ACKNOWLEDGMENT The authors would like to thanks Mr H. J. Beloshe Sr. Design Engineer, and Mr Bhaskaran Nagarajan Manager, Planning Dept. rom Voltas Ltd. Pune or their grateul support and help in the present study. REFERENCES [1] A textbook o Fluid Mechanics by Er. R. K. Rajput, Part-1, by S. Chand publication. [] Carrier Corporation Carrier System Design Manual ninth printing, 197. [3] Darby R. Correlate Pressure Drops through Fittings Chemical Engineering, pp , [4] Darby Ron Chemical Engineering Fluid Mechanics nd edition, ISBN , published by Marcel Dekker- New York, 001. [5] Indian Society o Heating Rerigerating and Air-conditioning Engineers (ISHRAE) Bangalore chapter HVAC Handbook Part-I Air conditioning, 007. [6] Voltas Ltd. Product Catalog. [7] Srbislav Genić, Ivan Arandjelović, Petar Kolendić, Marko Jarić, Nikola Budimir A review o Explicit Approximation o Colebrook s Equation FME transactions, vol. 39 No., pp , 011. [8] William B. Hooper and Monsanto Co. The Two-K Method Predicts Head Losses in Pipe Fittings Chemical Engineering, pp , [9] Zarko Cojbasic and Dejan Brkic Very accurate explicit approximation or calculation o the Colebrook riction actor, International Journal o Mechanical Sciences, 01. ISSN : Vol 5 No 4 Aug-Sep