Heat transfer studies in a spiral plate heat exchanger for water: palm oil two phase system

Abstract

Experimental studies were conducted in a spiral plate heat exchanger with hot water as the service fluid and the two-phase system of water – palm oil in different mass fractions and flow rates as the cold process fluid. The two phase heat transfer coefficients were correlated with Reynolds numbers (Re) in the form h = a Re m, adopting an approach available in literature for two phase fluid flow. The heat transfer coefficients were also related to the mass fraction of palm oil for identical Reynolds numbers. The two-phase multiplier (ratio of the heat transfer coefficient of the two phase fluid and that of the single phase fluid) was correlated with the Lockhart Martinelli parameter in a polynomial form. This enables prediction of the two-phase coefficients using single-phase data. The predicted coefficients showed a spread of ± 10 % in the laminar range.

Heat transfer coefficient; Two - phase flow; Lockhart Martinelli parameter

FLUID DYNAMICS; HEAT AND MASS TRANSFER; AND OTHER TOPIC

Heat transfer studies in a spiral plate heat exchanger for water – palm oil two phase system

S. Ramachandran^I,*; P. Kalaichelvi^II; S. Sundaram^III

^IDepartment of Chemical Engineering and Materials Science, Amrita School of Engineering, Phone: +(91) 422 2656422, Amrita Vishwa Vidyapeetham, Ettimadai, Coimbatore, Tamil Nadu, 641 105, India. E-mail: [e-mail]

^IIDepartment of Chemical Engineering, National Institute of Technology, Tiruchirapalli, Tamil Nadu, 620 015, India

^IIIDepartment of Electronics and Instrumentation Engineering, Sastra University, Thanjavur, Tamil Nadu, 613 402, India

ABSTRACT

Experimental studies were conducted in a spiral plate heat exchanger with hot water as the service fluid and the two-phase system of water – palm oil in different mass fractions and flow rates as the cold process fluid. The two phase heat transfer coefficients were correlated with Reynolds numbers (Re) in the form h = a Re^m, adopting an approach available in literature for two phase fluid flow. The heat transfer coefficients were also related to the mass fraction of palm oil for identical Reynolds numbers. The two-phase multiplier (ratio of the heat transfer coefficient of the two phase fluid and that of the single phase fluid) was correlated with the Lockhart Martinelli parameter in a polynomial form. This enables prediction of the two-phase coefficients using single-phase data. The predicted coefficients showed a spread of ± 10 % in the laminar range.

Keywords: Heat transfer coefficient; Two - phase flow; Lockhart Martinelli parameter.

INTRODUCTION

Conventional shell and tube heat exchangers have certain operational limitations. These are successfully addressed in compact exchangers such as plate / spiral type equipment. The advantages of these equipments include higher heat transfer rates, less fouling, operational flexibility, ease of maintenance and lower space requirement. They are also better suited to handle slurries, viscous liquids and can be operated where the approach temperatures are low.

In chemical industries, two phase flow is a process necessity. A better understanding of the rates of momentum and heat transfer in multi phase flow conditions is a must for the optimal design of the heat exchanger. (Ho et al., 1995). To simplify the complexities in design, transfer coefficient correlations are being developed using pure phase thermo-physical properties and system parameters like flow geometries and flow velocities. (Jensen, 1988; Gut et al., 2004) Considerable research is being pursued in two phase flow areas particularly in the area of fluid dynamics. The first detailed study in two phase flow was carried out by Lockhart and Martinelli in the year 1949. A number of such studies are cited in the references section (Naphlon and Wongwises., 2002; Manglik and Bergles, 1995; Downing and Gunol Kojasoy, 2002; Chen et al., 2004 Rani Hemamalini, et al., 2005).

However the field which has received relatively less attention is the study of heat transfer involving two phases (especially two immiscible liquids) in a compact heat exchanger. In the present work, experiments were done in a spiral plate heat exchanger with hot water as the service fluid and two-phase mixtures of water and palm oil in different ratios and flow rates as the cold process fluid. Experimental runs with single-phase fluids (pure water and pure palm oil) on the process side were also carried out. The heat transfer coefficients on the cold side were correlated with Reynolds numbers. The heat transfer coefficients were then related to the quality for identical Reynolds numbers. The two-phase multiplier (φ_L) based on heat transfer coefficients of pure fluid and two-phase mixture correlated well with the Lockhart – Martinelli Parameter (L – M Parameter -χ_tt² ). This enables prediction of the two-phase, service side coefficients (for the range of Reynolds numbers studied) using single-phase data. The predicted coefficients showed a variance of ± 10 % over the experimental values for the Laminar flow range.

MATERIALS AND METHODS

The heat exchanger dimensions are given in Table 1. The experimental setup is illustrated in Figure 1.

The service fluid used was water, heated in a stainless steel vessel by steam purging. A temperature controller was used to maintain the inlet temperature to the heat exchanger. The process fluid was stored in a separate stainless steel tank. Weighed quantities of food grade palm oil and demineralized water were charged into this tank to obtain the experimental range of mass fractions of palm oil (0% to 100%). Agitation in the tank was maintained by air bubbling. Two fractional horsepower centrifugal pumps were used for the circulation of the two streams of fluids. The two phase side rotameter was calibrated for each experimental mass fraction before the experimental run. Online, calibrated Resistance Temperature Detectors (RTDs) with digital indicators were used for the temperature measurements of the inlet and outlet streams of the service and process fluids.

The service fluid side inlet temperature and flow rate were kept steady. The two phase side flow rate was varied and for each selected flow rate observations of all four temperatures and two flow rates were recorded after steady state was reached. Experimental runs with pure liquids in the process side (water, palm oil) were also carried out. Fouling possibilities were eliminated by cleaning both process side and service side with hot water before each run. This was accomplished by pumping hot, mild detergent solution on both the process and service side followed by rinse pumping with pure hot water.

CALCULATION METHODOLOGY

a) The following basic relations were used for calculating the overall heat transfer coefficients and individual heat transfer coefficients on the single phase and two phase sides.

This correlation between Nusselt Number (Nu) and Graetz Number (Gz) is adopted from equation 12.25 in the book of McCabe et al. (2001)

b) The Quality Parameter X is defined as

c) The Lockhart Martinelli (L – M) Parameter (χ_tt²) is defined as

d) The factor m is obtained from the correlation

e) The two phase multiplier Ø_L is defined as

RESULTS AND DISCUSSION

Single Phase Results

The experimental results of single phase studies are presented in the form of a plot between Reynolds Number and h_1φ in Figure 2. Re and h_1φ were correlated by regression analysis in the form given in equation 9 and the values of a and m are given in Table 2.

Two Phase Results

Two phase studies were carried out with different mass fractions of palm oil in water (20%, 40%, 60%, and 80%). The experimental values of the inlet and outlet temperatures of the hot and cold fluids and the corresponding Re values are provided in Table 3.

Figure 2 also presents the two phase experimental heat transfer coefficients, h_2φ as a function of Re. For the two phase system, Re is based on the weighted average thermo-physical properties of the fluids at the respective mean bulk temperatures. It is seen that the two phase data falls in between the values for the single phase. These data are fitted by regression to the correlation given in equation 9 and the values of a and m are given in Table 2. The calculated values of h_2φ based on these constants agreed with the experimental data with an error of ± 15 % as shown in the trend lines in Figure 2.

The experimental data shown in figure 2 is used to calculate the values of the two phase multiplier (φ_L) and the L – M parameter (χ_tt²).

Figures 3, 4 and 5 present the relations φ_L Vs X, χ_tt² Vs X and χ_tt² Vs φ_L respectively.

The variation of φ_L with χ_tt², shown in Figure 5 is represented by equation 11.

The Correlation coefficients (R²) for the trend equations in Figures 3, 4 and 5 are given in Table 4.

The experimental heat transfer coefficients (h_2φ) and their corresponding calculated values based on equation 11 for different quality values (X) and Reynolds Numbers and the corresponding % error are given in Table 5. It is seen from this Table that the error ranges between ± 10 % for the laminar range. The results were re-ascertained by conducting validation runs.

Equation 11 can also be rewritten as

where is the modified two phase multiplier for water – palm oil system. This modified two phase multiplier is expressed as

Equation 12 is of the form

suggested by Chisholm and Laird (1958) .The value of C is - 18.4 for water - palm oil two phase system.

CONCLUSION

Two phase flow studies were conducted in a spiral plate heat exchanger using water – palm oil system. Heat transfer coefficients were related to the quality of the two phase systems. The correlations between quality (X), φ_L and L – M parameter show a good agreement with experimental data. This correlation can be used for the prediction of two phase heat transfer coefficients and are useful in the design of heat exchangers for two phase duties in the Re and temperature ranges investigated. The validation experimental runs have demonstrated the reliability range of this correlation. Further work at higher Re and for different two phase systems is in progress in this laboratory.

NOMENCLATURE

a

Experimental correlation constant

(-)

b

Channel height

m

Cp_h

Specific heat of hot fluid

J/kg K

d _e

Equivalent diameter of the flow channel

m

h_h

Heat transfer coefficient on hot fluid side

W/m² K

h_1Ø

Heat transfer coefficient of pure palm oil

W/m² K

h_2Ø

Heat transfer coefficient of palm oil – water mixture

W/m² K

k _h

Thermal Conductivity of hot fluid

W/m K

k _ss

Thermal conductivity of the wall

W/ m K

L

Length of the Flow Channel

m

M_h

Mass flow rate of hot fluid

kg/s

m

Experimental correlation constant

(-)

Q

Heat transferred

W

Q_f

Volumetric Flow rate of palm oil

kg/s

Q_w

Volumetric Flow rate of water

kg/s

T_h1

Inlet Temperature of water

K

T_h2

Outlet Temperature of water

K

T_c1

Inlet Temperature of palm oil

K

T_c2

Outlet Temperature of palm oil

K

t

Wall thickness of the spiral plate

m

U

Overall heat transfer coefficient

W/m² K

w

Channel width

m

X

Quality

(-)

Greek Letters

Ø _L

Two Phase Multiplier

(-)

ρ_f

Density of palm oil

kg/m³

ρ_w

Density of water

kg/ m³

µ_f

Viscosity of palm oil

kg/ms

χ_tt²

Lockhart Martinelli parameter

(-)

(ΔT) _h

Temperature drop of hot fluid

K

(ΔT) _lm

Logarithmic Mean Temperature

(-)

Difference between hot and cold fluid

K

(Received: August 18, 2006 ; Accepted: March 04, 2008)

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*

To whom correspondence should be addressed

Publication Dates

• Publication in this collection
  02 Sept 2008
• Date of issue
  Sept 2008

History

• Accepted
  04 Mar 2008
• Received
  18 Aug 2006