experimental validation of thermal performance of compact heat...

IPASJ International Journal of Mechanical Engineering (IIJME) Web Site: http://www.ipasj.org/IIJME/IIJME.htm

A Publisher for Research Motivation........ Email: [email protected] Volume 3, Issue 7, July 2015 ISSN 2321-6441

Volume 3, Issue 7, July 2015 Page 46

ABSTRACT Today’s competitive industrial market puts aggressive demands on compressor for flow rate, pressure ratio and efficiency. One of the key factor that plays significant role in compressor performance is effectiveness of inter-stage heat exchanger. Highly effective heat exchanger with strict control on space and cost is today’s need. Quick, accurate as well as optimum performance prediction of heat exchanger is critical for overall design of compressor. Various analytical methods can quickly predict thermal performance but on conservative side. No design cycle is complete without experimental validation of result. This paper describes analytical calculation methodology of plate fin type compact heat exchanger based on Kern’s method.Results of analytical calculations are validated by experimental measurements carried out on heat exchanger of air compressor system. Keywords:Compact heat exchanger, NTU, effectiveness, Kern method

1. INTRODUCTION Basic objective of heat exchanger designer is to have desired thermal performance i.e. desired outlet conditions of one fluid with given inlet conditions of other fluid with minimum pressure drop of working fluid. Other constrains that influence the heat exchanger design are space available for installation & cost. Designer tries to achieve heat exchange objective with trade-off among these factors. In compressor applications, heat exchanger is used as intercooler to remove heat of compression between stages & also as after cooler to cool the gas before its final application. Inter stage cooling in compressors saves the work required for compression in subsequent stage by lowering temperature of hot gas compressed by pervious stage. Power saving due to intercooling stage cooling is partly offset by pressure drop of hot gas in heat exchanger. For particular flow rate of compressor, Pressure drop is inversely proportional to volume provided for hot gas cooling i.e. higher the volume lower the pressure drop. Hence it is necessary to strike a balance between space requirement & outlet temperature of hot gas for given set of operating conditions.[2],[5] A plate-fin heat exchanger is a type of heat exchanger that uses tubes and plate type fins to transfer heat between fluids. It is often categorized as a compact heat exchanger to emphasize its relatively high heat transfer surface area to volume ratio. The plate-fin heat exchanger is widely used in many industries. [5]

2. ANALYTICAL CALCULATIONS OF THERMAL PERFORMANCE OF HEAT EXCHANGER There are three rating methods to calculate thermal performance of plate fin type heat exchanger:

2.1 Kern method This method is a simplified approach suitable for shell side flow without baffles.[1]

2.2 Taborek method The Taborek version of the Delware method is most accurate, reliable and complete method available in open

literature.[4]

2.3 Bell Delaware method This method is most complex but accurate way of rating aheat exchanger withbaffles.[4] In this paper Kern method (1950) is used.Thismethod is based on experimental work on commercial exchangers with standard tolerances and will give a reasonably satisfactory prediction of the heat-transfer coefficient for standard designs. The prediction of pressure drop is less satisfactory, as pressure drop is more affected by leakage and bypassing than heat transfer. The shell-side heat transfer and friction factors are correlated in a similar manner to those for tube-side flow by using a hypothetical shell velocity and shell diameter. The method is simple and more explanative. [1],[3],[6],[7]

Experimental Validation of Thermal performance of Compact Heat Exchanger

Omkarsing Bhosale1*, Dipak Patil2 , Dipak Pawar3

1*, 2G.H Raisoni College of Engineering & Management, Pune, India

3Kirloskar Pneumatic Company Limited, Pune, India




Figure 1 Typical Plate fin & tube compact heat exchanger

Figure 2 Arrangement of heat exchanger

Figure 1 shows typical plate fin & tube type heat exchanger used in air compressor application. Hot air flows on fin side and cooling water flows through the tubes. Figure 2 shows line diagram of air and water flow heat exchanger. Following sections describe the analytical calculation methodology using Kern method:

2.1.1 Nomenclatures Inlet air temperature at compressor ( ) Inlet air pressure at compressor ( ) Air relative humidity (RH%) Air flow rate ( ) Inlet air temperature of heat exchanger ( Outlet air temperature from heat exchanger (T2) Inlet air pressure of heat exchanger ( ) Specific heat of dry air ( ) Specific heat of vapour ( ) Cooling water inlet temperature ( ) Specific heat of water ( ) No. of Tubes (Nt)

2.1.2 Total Heat load (Q) (1)

Sensible heat load (Qs)

(2) Latent heat load (QL)

(3) Mass flow rate of dry air (ṁda)

(5)

Air Pressure ( )




(6) Relative humidity (RH%)

(7)

Saturated pressure of water vapour (Pswv)

(8)

2.1.3 Geometry of tube bundle Geometrical and material details of heat exchanger are provided below:

Table 1: Geometrical and material details of heat exchanger

Tube details Fin details Layout Material copper Material Aluminium 4 pass

No. of tubes 156 Fin density

22 fins/inch

Tube diameter 3/8'' Thickness 0.15mm

Thickness 0.9mm

Figure 3 Tube Layout

Qair = Qwater From Qwater, mass flow rate of cooling water is calculated as below: Mass flow rate cooling water )

Cooling water flow rate ( )

Flow area of single pass of heat exchanger is decided from geometry of tube bundle and by using volume flow rate and area water velocity is calculated as below: Cooling water flow velocity ( )



Volume 3, Issue 7, July 2015 9

2.1.4 Tube side heat transfer coefficient From water velocity calculated by equation 11 and using following chart from Kern tube side heat transfer coefficient is determined: By using Kern chart Fig.25,

Figure 4 Kern chart: velocity v/s heat transfer coefficient

2.1.5 Shell side heat transfer coefficient

Air heat transfer coefficient tabulated by using air velocity ( ) [1],[3],[5]

Dry air heat transfer coefficient = Wet air heat transfer coefficient =

2.1.6 Overall heat transfer coefficient (U) Overall heat transfer coefficient is calculated as below [8]

where Uo= the overall coefficient based on the outside area of the tube, W/m2°C, ho= outside fluid film coefficient, W/m2 °C, hi= inside fluid film coefficient, W/m2 °C, hod= outside dirt coefficient (fouling factor), W/m2 °C, hid= inside dirt coefficient, W/m2 °C, kw= thermal conductivity of the tube wall material, W/m °C, di= tube inside diameter, m, do= tube outside diameter, m.



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2.1.7 Log mean temperature difference (Δ )

Figure 5 LMTD Correction factor (F)

2.1.8 Required heat transfer area (

Area ratio

2.1.9 Pressure drop air side (PD)

2.1.10 Pressure drop water side (

Where Np = number of tube passes

= tube side velocity

L = length of one tube = friction factor




3. EXPERIMENTAL RESULTS Test measurements points (a, 1, 2, 3, 4)

Figure 6 Experimental measurement set up

Table 2: Comparison of analytical and experimental results

Thermal Performance test of Heat Exchanger

Sr. No.

Heat Exchanger Air in Temperature

Heat Exchanger Water inlet temperature

Air out temperature from Heat Exchanger

Analytical Experimental Analytical Experimental 1 150.0 32.0 32.3 41.8 45.4 2 150.0 33.0 33.0 42.5 46.4 3 150.0 34.0 33.8 43.4 46.6 4 150.0 35.0 35.2 44.3 47.1 5 150.0 36.0 36.2 45.0 47.9 6 150.0 37.0 37.4 45.8 48.4 7 150.0 38.0 38.0 46.7 48.8 8 150.0 39.0 39.1 47.5 50.5 9 150.0 40.0 40.2 48.3 50.8

Figure 7 Effect of cooling water Inlet Temperature v/s Air outlet Temperature




An Accurate prediction of thermal performance of heat exchanger for various operating conditions of compressor is done in this paper.

4. CONCLUSION Analytical calculation method presented in this paper can be used for quick estimation of thermal performance of compact plate fin type heat exchanger with reasonably good accuracy.Analytical method can be used quick study of effect of various parameters on performance of heat exchanger.Analytical method provides good basis for optimization of heat exchanger.Results of experimentation and analytical calculations match within 10% of accuracy.Hence if we overdesign by 10% extra margin on area calculated by analytical method, heat exchanger thermal performance in field can be ensured as per design without need for extensive testing.Accurate experimental data for following factors will ensure better agreement in analytical & experimental results. Fin efficiency is assumed 100%. Experimental data is required for correct values of fin efficiency.Radiation and convection heat exchange from and to heat exchanger outer surface are not accounted for.Seal leakage losses internal to heat exchanger are not accounted as exact data for this is not available. References [1] Kern D. Q., Process Heat Transfer, Tata McGraw Hill, New Delhi, 1997. [2] Manjunath M. B., Jagadish S. B. “Analysis comparing performance of a conventional shell and tube heat

exchanger using Kern, Bell, and Bell Delaware method”, IJRET, 2014, vol. 3, pp. 486-496. [3] Yusuf Ali Kara, Ozbilen Guraras, “A computer program for designing of shell-and-tube heat exchangers”,

Applied Thermal Engineering, 2004, vol. 24, pp. 1797–1805. [4] Anand K., Pravin V. K., “Experimental Investigation of Shell and Tube Heat Exchanger Using Bell Delaware

Method”, IJRASET, 2014, Vol. 2 Issue I, pp. 73-85. [5] Uttam H. Patel, Kedar N. Bhojak, “An Overview of Shell & Tube Type Heat Exchanger Design by Kern’s

Method”, IJIERE, 2015, vol. 2, pp 61-65. [6] Veena P., Anand K., “Experimental Investigation of Shell and Tube Heat Exchanger Using Kern Method”,

IJPRET, 2014, vol. 2, pp. 64-82. [7] Kuppan T., Heat Exchanger Design Handbook, Marcel Dekker Inc., New York, 2000. [8] Sinnott R. K., Chemical Engineering Design, 5th ed., Vol. 6, Pergamon Press, New York, 2009, pp. 511-618.

Photograph of Experimental Set Up

Heat Exchanger Water Flow meter




AUTHOR Omkarsing Bhosale graduate in B.E Mechanical engineering and pursuing Post Graduation (M. E.) in Heat Power from G.H. Raisoni College of Engineering & Management, Pune, India. Dipak Patil Associate Professor, Department of mechanical, G.H. Raisoni College of Engineering & Management, Pune, India. Dipak Pawar Kirloskar Pneumatic Company Limited, Pune, India.

experimental validation of thermal performance of compact heat...

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