heattransfer rate

A. INTRODUCTION

A simple double tube heat exchanger consists of an inner tube covered by the outer tube functioning as a jacket to the inner tube (Abdullah et al., 1990 and Holman, 1997) as illustrated in Fig. 1.

Figure 1. Simple double tube heat exchanger.

When a fluid with a higher temperature flows inside the inner tube, and another temperature with a lower temperature flows in the outer tube, the rate of heat transfer can be expressed as (Heldman and Singh, 1981).

q₁ = w₁ cp₁ (T_e-T_L)....................................................................................... (1)

q₂ = w₂ cp₂ (T_i-T_o)........................................................................................ (2)

Where

q₁ = heat removed from the higher temperature fluid (kW/hr)

q₂ = heat absorbed by the lower temperature fluid (kW/hr)

w₁ = mass flow rate of the higher temperature fluid (kg/hr)

w₂ = mass flow rate of the lower temperature fluid (kg/hr)

cp₁ = specific heat of the higher temperature fluid (kW/kgK)

cp₂ = specific heat of the lower temperature fluid (kW/kgK)

T_e = entrance temperature, K

T_L = exit temperature, K

T_i = inlet temperature, K

T_o = outlet temperature, K

The flow of fluid in the inner tube could be parallel to the flow of fluid in the outer tube, or it could be counter to each other. Figure 2 illustrates the temperature profile of the heat exchanger in the parallel flow.

Fig 2. Temperature profile for a parallel flow in a double tube heat exchanger

(Holman, 1997).

The mean temperature differences along the tubes is calculated as the logarithmic mean temperature differences ∆Tm (Welty, 1978 and Heldman and Singh, 1981)

............................................................................. (3)

The rate of heat transfer, q, can also be expressed as (Holman, 1997)

q = A U ∆Tm............................................................................................... (4)

Where

A = area of tube wall, m²

U = overall heat transfer coefficient, kW/m²K

Thus, U can be calculated by dividing eq (1) with eq. (4)

..................................................................................... (5)

The efficiency of a heat exchanger, E, can be defined as the absorbed by the outer tube over the heat removed from the inner tube.

......................................................................... (6)

B. OBJECTIVES

1. Assessment of the performances of an experimental apparatus heat exchanger.

2. Determination of the log mean temperature differences (∆Tm), the overall heat transfer coefficient (U), and the efficiency of the heat exchanger (E).

C. MATERIALS AND EQUIPMENT

The equipment is an experimental heat exchanger apparatus Model HEP-1200 as illustrated in Fig 3, with a detailed piping system showed in fig 4. The material is water in liquid and steam phases.

D. METHODS

1. Water Inlet Test

Close all valves and open V₁ and V₄. Fill up the water tanks (for both the hot water tank and the cold-water tank) until they are full.

2. Air Removal from Pipes

The air bubbles inside the piping system have to be removed.

2.1. Removal from hot water piping

Open valves V₄, V₅, V₆, V₇ and V₈ and then close V₆. Put the power and the hot water pump on. Open V₂₆ until the water is full, then close V₂₆ and open V₁₃. Close V₁₃ after the water is full and open V₂₃, and then close it when the water is full. Repeat the procedures 2-3 times. Open V₆ and adjust the water flow rate using the flow rate meter. Close V₄ when the air bubbles have been removed.

2.2. Removal from cold water piping

Open valve V₁ and close V₂. Put the power and the cold-water pump on. Close V₁₁ and open V₁₅, V₁₇, V₂₀ and V₂₁ until the water is full, and then close them down. Open valve V₂ and adjust the water flow rate to a given level.

3. Experiments

The hot water will flow in the inner tube while the cold water in the outer tube or the jacket. The experiments can be carried out in two modes, parallel and counter flows.

3.1. Parallel Mode

Put the power off, close the valves V₅, V₆, V₇ and V₈. Adjust the hot water flow direction using the flexible tubes A and B. A is connected to V₅ and V₇, and B to V₆ and V₈.

3.2. Temperature Regulation

Regulate the temperature at the hot water tank by putting the temperature control (TEMP CONTROL) on and off, then adjust TEMP CONTROL (SLIDE) to a necessary level.

4. Observation

4.1. Temperatures by using the digital thermometer buttons

T_i : inlet temperature hot water (K)

T_o : outlet temperature, hot water (K)

T_e : entrance temperature, cold water (K)

T_L : exit temperature, cold water (K)

4.2. Water flow rate at the flow rate meters

w₁ : mass flow rate of hot water (kg/hr)

w₂ : mass flow rate of cold water (kg/hr)

E. RESULT AND DISCUSSION

Group 2A

qD = mD x CpD x (TiD – ToD)

=

Discussion

Based on the calculating data we get the minus value of heat transfer coefficient in area tube wall. This means that is missing heat from fluid in the inner tube as symptom in temperature compensating.

This experiment objectives is determination of heat transfer rate in heat exchanger, the function of heat exchanger for cooling and enrage after did the experiment, we obtained the average heat removed, temperature differences and efficient of heat exchanger.

Heat exchanger is a system to exchange energy among two difference fluid. The energy exchange is caused by the difference of the temperature among the fluid which flows in the inner tube and fluid which flows in outer tube.

The differences of temperature caused imbalance and resulting the happening of moving the fluid in the inner tube and the fluid in the outer tube. The temperature in the inner tube will decrease and the contrary the temperature of fluid in the outer fluid will increase until raised the balance condition.

CONCLUSION

From this experimental, we can know the performances of apparatus heat exchanger which used in the experiment. The heat was transferred from high temperature to low temperature of the low temperature is absorbing heat from high temperature. Effectiveness the tube in the mass velocity is 98.3 %. We also can conclude from the temperature profile in experiment is not very different with temperature profile in literature.

REFERENCES

Abdullah, K, A. H. Tambunan dan M. Yamin. 1990. Energi dan Listrik Pertanian. Pedoman Praktikum. JICA-DGHE/IPB Project/ADAET: Jta-9a (132), Bogor, IPB.

Heldman, D. R. and P. R. Singh. 1981. Food Process Engineering. The AVI Pub. Co., Westport, CT, USA.

Holman, J. P. 1997. Heat Transfer. McGraw Hill Book Co., Singapore.

Welty, J. R. 1978. Engineering Heat Transfer. John Wiley & Sons, New York, NY, USA.

Heat Transfer Laboratory 2010

Thermodynamics and Heat Transfer

DETERMINATION OF HEAT TRANSFER RATE IN HEAT EXCHANGER

By:

Ahmad Eriska Dwi Hutama Putra

F14080122

DEPARTMENT OF AGRICULTURAL ENGINEERING

FACULTY OF AGRICULTURAL TECHNOLOGY

BOGOR AGRICULTURAL UNIVERSITY

2010