1.0

The objective of this experiment is to compare the heat transfer capabilities of a coil heat exchanger ( HE1 ), a korobon graphite plate heat exchanger ( HE2 ) and a shell and tube type heat exchanger ( HE3 ) by evaluating the overall heat transfer coefficient when condensing steam.

Heat exchanger is defined as a device or apparatus in which heat is transmitted from one fluid to another. Generally, heat exchanger is used in space heating and air-conditioning. In this experiment, three different types of heat exchanger viz., (i) coil in shell HE1, (ii) plate HE2 and (iii) shell and tube HE3, are operated in turn and studied separately. Each of the heat exchanger possess the same internal heat transfer area of 0.5m² but they are constructed by different materials (like glass and graphite). As a result, they will give a different performance.

All the heat exchangers are operated independently and evaluated by using low pressure steam on one side of the heat exchanger and cold water on the other side. Additionally, the cold water and the steam are operated in the countercurrent flow condition.

The rate of heat transfer in a heat exchanger and the overall heat transfer coefficient U are defined by the relation below

where Q = rate of heat transfer

U = Overall heat transfer coefficient

A = Area ever which heat transfer takes place

D T_lm = Log mean temperature difference

For the shell and tube heat exchanger

where F = correction factor ; F = 1.0 for boiling or condensation

From the equation above, it shows that the heat transfer rate Q is proportional to the value of the overall heat transfer coefficient U. However, U is itself dependent on the flow condition of the fluid within the heat exchanger ( either laminar or turbulent ), the material of construction and the thickness of the heat exchanger wall and most important the design of the heat exchanger.

1. The pre start-up checks are carried out and completed by either opening or closing the certain valves, as listed.
2. Heat exchanger HE1 is chosen to be operated by opening V.6 and closing V.7 and V.8.
3. The cold water valves are set for countercurrent operation and is started up with a flow rate of 5 liters/min.
4. The steam is turned on by opening V.9 slowly and the steam pressure is maintained in the range 0.4-1.0 bar gauge.
5. The HE1 system is allowed to run and readings for every two minutes are taken until the equilibrium is reached.
6. The temperature, pressure, flow rate readings are taken and the condensate rate is measured when equilibrium is reached
7. Instructions (4) to (5) are repeated for an increased flow rate to 7.0 and 10.0 liters/min.
8. Heat exchanger HE1 is shut down by first closing the V.9 and then the cold water circuit.
9. The whole experiment is repeated for the other two heat exchangers, HE2 and HE3.

Note :

FI1 = Cold water flow rate, liters/min PI1 = Cold water outlet pressure, bar

TI1 = Cold water outlet temperature ^oC PI2 = Cold water inlet pressure, bar

TI2 = Cold water inlet temperature ^oC PI3 = Steam inlet pressure, bar

TI3 = Steam inlet temperature ^oC PI4 = Condensate pressure, bar

TI4 = Condensate temperature ^oC PI5 = Steam supply pressure, bar

¯ = Contercurrent flow

Calculation is carried out by using certain equation and formula to calculate the U_loss, U_i and other parameters. The calculation is based on the data and readings taken at 8 minutes, which the equilibrium is reached already. In addition, the condensate rate is measured at this time. The calculated results are tabulated in the table below:

# All the calculation and symbols used are shown in Appendix I and II

In making the comparison of effectiveness and capabilities among the three heat exchangers, HE1, HE2 and HE3, there are three major factors or aspects have to be considered. These are, (I) the overall heat transfer coefficient U_i, (II) the effect of different cold water flow rate and (III) the heat loss from heat exchanger.

First, for several identical operations at 5.0, 7.0 and 10.0 liters/min cold water flow rate, it was found HE2 has the biggest value of U_i, followed by HE1 and HE3. From equation Q_C = U_iA_inD T_lm, we can see that Q_C is proportional to U_i. That means HE2 will transfer the greatest amount of heat input to the cold water. Since the efficiency of heat exchanger,e can be defined as fraction of heat input transferred to the cold water, HE1 is the best heat exchanger followed by HE1 and HE3 although all have the same internal area 0.5 m².

From energy balance equation, the rate of heat transferred from the steam (Q_L) must equal to the rate of heat absorbed by the cold water (Q_C). However, from the result calculated, it was found that almost half the amount of heat input from steam has been lost to the surroundings. This condition may be due to the imperfect insulation, piping and connection problems. Heat is lost to the surroundings mainly by radiation and convection. In this case, the heat transfer coefficient for heat loss to the surroundings U_loss for three heat exchangers is calculated. It was found that HE2 has the smallest value. The smaller value of U_loss, the less heat input will loss to the surroundings and this is generally true when the fraction of heat input transferred to the cold water is big. However, from the result calculated, there are some doubts. Theoretically, HE3, which has the smallest value of Ui should have the biggest value of U_loss. But from the experimental results, U_loss for the HE3 is smaller than U_loss for HE1 whereas HE1 has the bigger value of U_i than HE3. This situation may be caused by the experimental error and the assumption we made. In this experiment, we assumed that the steam input to the heat exchanger is saturated steam and the output is saturated water to simplify our calculations. During the experiment, it was observed that the condensate was drained out together with the steam. Besides that the values of TI4 for HE3 are somehow ambiguous

Now, let’s consider the effect of different cold water flow rate to the heat exchanger capabilities. From our results, it can be seen that the heat transfer coefficient U_i will increase with any increment of the cold water flow rate. Increasing the flow rate means a unit amount of cold water is being rapidly washed out of the heat exchanger after absorbing only little amount of heat. It will cause the greater gradient temperature between cold water and steam and this leads to higher heat transfer. In addition, by increasing the cold water flow rate will also change the flow condition from laminar to turbulent. Once the flow becomes turbulent, the overall heat transfer coefficient U_i will increase too. Thus, we can say that heat transfer can be improved for any heat exchanger by increasing the flow rate.

We can conclude that the effectiveness of the heat exchanger HE2 is the highest, followed by HE1 and HE3. The explanation for the difference can be seen to be a function of the physical design, layout and materials of construction of the heat exchanger. For HE2, the material used is korobon graphite. From general knowledge, graphite has higher thermal conductivity (less resistance) than glass (material of construction for HE1 and HE3). Since the outer fluid is steam that condenses on the tube/plate wall, it will transfer heat to the graphite with almost no resistance. Due to its high thermal efficiency, plate heat exchanger (HE2) can result in the need for much less water to cool than required by HE1 and HE3. HE1 is more efficient than HE3 because the flow in coil and shell heat exchanger is more turbulent than the in shell and tube heat exchanger.

8.0

1. In real condition, there is no heat exchanger can operate with 100% efficiency. Not all the heat input will be transferred to the cold fluid.
2. For identical flow rate and heat transfer area, HE2 is the most effective heat exchanger because it has the largest value of U_i, followed by HE1 and HE3.
3. By increasing the cold water flow rate, it will increase the heat transfer between the steam and water.
4. The different capabilities of HE1, HE2 and HE3 are due to the difference in materials of construction, physical design and layout.

1. Axial conduction along the tubes/plate is negligible.
2. Potential and kinetic energy changes can be negligible.
3. The overall heat transfer coefficient is constant.
4. Bulk temperature is considered in evaluating the properties of the cold water.
5. In heat exchanger, the hot fluid input is saturated steam and the output is saturated water

10.0

1. Calculate the D T_lm

= [ (100.0-54.0) – (56.0-28.0)] / [ ln(46.0/28.0)]

= 36.3 ^oC

2. Calculate the heat transfer to cold water Q_C.

= (5 liters/min)´ (1min/60saat)´ (1m³/1000liters)´ (4174J/kg^oC)´ (991.6kg/m³)´ (54.0-28.0)^oC

= 8967.7 W.

where m_cw = qr q = cold water flow rate in liters/min D T = TI1- TI2

C_P = 4174 J/kg for water in temperature range of 20 to 60 ^oC

2. Calculate the heat input Q_R Q_R = m_cond( H_steam – H_water )

= (0.364kg/min)´ (1min/60saat)´ [ (2676-234.4)´ 1000J/kg]

= 14812.4 W

where H_steam and H_water are taken straight from the steam table with temperature as the main reference.

 

3. Calculate the heat loss to the surroundings Q_L Q_L = Q_R - Q_C

= (14812.4 – 8976.7) W

= 5844.7 W

5. Calculate the overall heat transfer coefficient for cold water to steam Ui

From, Q_C = U_iA_inD T_lm Thus, Ui = (8967.7W)/[ (0.5m²)(36.3^oC)]

= 494.1 W/m^2oC

6. Calculate the overall heat transfer coefficient from HE1 to the surroundings U_loss

From, Q_L = U_lossA_ex(T_steam-T_air) Thus, U_loss = (5844.7W)/[ (0.217m²)(100.0-28.0)^oC]

= 374.1W/m^2oC

11.0

A_in = internal surface area of heat exchanger ( m² )

A_ex = external surface area of heat exchanger ( m² )

C_P = specific heat of cold water ( kJ/kg^oC )

H_steam = entalphy of saturated steam ( kJ/kg )

H_water = entalphy of saturated water ( kJ/kg )

m_cw = cold water flow rate ( kg/minutes )

m_cond = condensate flow rate ( kg/minutes )

Q_C = heat transferred to cold water ( kW )

Q_L = heat loss from external surface of the heat exchanger ( kW )

Q_R = heat input to heat exchanger ( kW )

T_air = ambient air temperature ( ^oC )

T_b = bulk temperature of cold water ( ^oC )

U_loss = overall heat transfer coefficient from heat exchanger to surroundings ( W/m^2oC )

U_i = overall heat transfer coefficient for cold water to steam ( W/m^2oC)

D T_lm = log mean temperature driving force for heat transfer ( ^oC )

• = density of cold water ( kg/m3 )

1. Heat Transfer, 7^th Edition, J.P. HOLMAN
2. Introduction to Chemical Engineering Thermodynamics (Steam Table pg.574-579),4^th Edition, J.M.SMITH/H.C.VAN NESS
3. Heat Transfer Equipment, NICHOLAS P.CHEREMISINOFF