SUMMARY ON

Design and Construction of a Concentric Tube Heat Exchanger

Folaranmi Joshua

Department of Mechanical Engineering, Federal University of Technology 

Minna, Niger State, Nigeria

E-mail: <folajo@yahoo.com>

SUMMARY submitted by VAISHALI GURJER, PGDIE 42, NITIE, Roll No. 100

Introduction

The concentric tube heat exchanger was designed in order to study the process of heat transfer between two fluids through a solid partition. It was designed for a counter-flow arrangement and the logarithmic mean temperature difference (LMTD) method of analysis was adopted. Water was used as fluid for the experiment. The temperatures of the hot and cold water supplied to the equipment were 87^o and 27^oC, respectively and the outlet temperature of the water after the experiment was 73^oC for hot and 37^oC for cold water. The results of the experiment were tabulated and a graph of the mean temperatures was drawn. The heat exchanger was 73.4% efficient and has an overall coefficient of heat transfer of 711W/m2 K and 48^oC Log Mean Temperature Difference. The research takes into account different types of heat exchangers. 

There are three main types of heat exchangers:

a. The Recuperative type

b. The Regenerative type  

c. The Evaporative type

This research paper is on recuperative type of heat exchanger, which can further be classified, based on the relative directions of the flow of the hot and  cold fluids, into three types:

a. Parallel flow, when both the fluids move in parallel in the same direction.

b. Counter flow, when the fluids move in parallel but in opposite directions.

c. Cross flow, when the directions of flow are mutually perpendicular. 

The objectives of the research work are:

(i) To design and construct a concentric- tube heat exchanger in which two tubes are concentrically arranged and either of the fluids(hot or cold) flows through the tube and the other through the annulus.

(ii)To carry out test on the concentric- tube heat exchanger and obtain values which will be compared to theoretically determined ones.

Theory of Design and Analysis

Design Considerations

In designing heat exchangers, a number of factors that need to be considered are: 

1. Resistance to heat transfer should be minimized

2. Contingencies should be anticipated via safety margins; for example, allowance for fouling during operation.

3. The equipment should be sturdy.

4. Cost and material requirements should be kept low.

5. Corrosion should be avoided.

6. Pumping cost should be kept low.

7. Space required should be kept low.

8. Required weight should be kept low. 

Design involves trade-off among factors not related to heat transfer. Meeting the objective of minimized thermal resistance implies thin wall separating fluids. Thin walls may not be compatible with sturdiness.

The Energy Balance

Since the flow in a tube is completely enclosed, an energy balance may be applied to determine how the mean temperature (T_m) x varies with position along the tube and how the total convective heat transfer Q_conv is related to the difference in temperatures at the tube inlet and outlet.

This gives us the following equation, which can be used to find T_m and Q_conv:

DQ_conv +  M(CvT_m +  pv) + [M(CvT_m +  pv) d(pv)/dx] = 0,

The Overall Heat Transfer Coefficient 

 A heat exchanger typically involves two flowing fluids separated by a solid wall. Heat is first transferred from the hot fluid to the wall by convection through the wall by conduction and from the wall to the cold fluid again by convection. Any radiation effects are usually included in the convection heat transfer coefficients.

Fouling Factor

 The performance of heat exchanger usually deteriorate with time as a result of scaling or deposits from over the interior surface. Scaling or deposits on the inside and outside of the tubes is really a gradual build-up of layers of dirt due to impurities in the fluid, chemical reaction between the fluid and the metal, rust etc. The deposits can severely affect the overall heat transfer  coefficient U. It is related to the overall heat transfer coefficient under clean conditions and under fouled conditions by the equation:

1/U_foul = R_f + 1/U_clean

Logarithmic Mean Temperature Difference 

 The method used in the analysis of the heat exchanger in this research work is the Logarithmic Mean Temperature Difference (LMTD), and it is defined as that temperature difference which, if constant, would give the same rate of heat transfer as actually occurs under variable conditions of temperature difference.

In order to derive expression for LMTD, the following assumptions were made:  The overall heat transfer coefficient U is constant, the flow conditions are steady, the specific heats and mass flow rates of both fluids are constant, the is no loss of heat to the surroundings, there is no change phase either of the fluid during the heat transfer, the change in potential and kinetic energies are negligible, axial conduction along the tubes of the heat exchanger is negligible (Saunders 1981).

In this design, counter-flow LMTD was adopted because it is always greater than that for a parallel flow unit, hence counter-flow heat exchanger can transfer more heat than parallel-flow one; in other words a counter-flow heat exchanger needs a smaller heating surface for the same rate of heat transfer.

Conclusion

This study as a whole offers an overview of an analytical method applicable to the design of concentric tube heat exchanger (counter-flow type). Logarithmic mean temperature difference (LMTD) method was used in the design analysis. The overall heat coefficient and the efficiency were computed. Results obtained show that the heat exchanger was effective.