heat exchanger

Summary
The topic investigated in this report is heat transfer in a concentric tube heat exchanger. A heat exchanger is a device used to transfer heat from a hot fluid to a colder fluid. Heat exchangers are widely used in the petroleum industry for various reasons. Some of the most important reasons are theta heat exchangers increase the process efficiency, conserve energy, reduce maintenance and provide employee safety among many other reasons.
The main objectives of the lab study were to determine the heat transfer rate, the logarithmic mean temperature difference and overall heat transfer coefficient. Other objectives included determining the surface heat transfer coefficient inside and outside the tube and the effect of fluid velocity on these. Lastly the final objective was to compare the differences between concurrent and counter current flow in a heat exchanger.
The apparatus used to carry out this lab study was a concentric tube heat exchanger. The concentric tube heat exchanger consists of a series of pipes or tubes in which fluids enter. It consists of two streams one for hot water and the other for cold water. The heat exchanger also consists of a control valve which is used to regulate the volume and mass. It also consists of cooling water couplings which are used to switch the cooling water direction from counter current to concurrent and vice versa.
Introduction
A heat exchanger is used in many different applications in a wide range of fields. The importance of a heat exchanger is to transfer fluid from a hot fluid to a cold fluid. A heat exchanger is a device which transfers heat from on fluid to another it consists of a series of tubes and pipes where fluids enter.in a heat exchanger two fluids never come into direct contact therefore heat transfers from the walls of the hotter fluid to the colder fluid. Heat exchangers are very important in the oil and gas industry in transferring heat, heat exchangers increases the process efficiency, reduce maintenance, conserve energy and provide employee safety. Different types of heat exchangers are used in the petroleum industry the most common ones are double pipe, plate, shell and tube, aerial coolers, concentric tube heat exchangers.
Theory heat transfer
Heat can be transferred by three main methods
Conduction-energy is transferred between solids or stationer fluids by the movement of atoms or molecules. For conduction through a wall the rate of heat transfer is given by: (1)
Where is the heat transfer rate Is the thermal conductivity of the wall A is the area normal to the direction of heat flow, x is the thickness of the wall Are the temperatures of the hotter and colder faces of the wall
Convection- the movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat. There are two types of convection forced convection and natural convection. Forced convection is a mechanism, or type of transport in which fluid motion is generated by an external source. Whereas natural convection is Natural convection is a mechanism, or type of heat transport, in which the fluid motion is not generated by any external source but only by density differences in the fluid occurring due to temperature gradients.

According to its flow rate fluid may be “laminar” or “turbulent”. Laminar flow occurs when a fluid flows in parallel layers, with no disruption between the layers in laminar flow Reynolds number is low and most of the heat transferred through the fluid is by conduction.

At higher Reynolds number the laminar flow disrupts and changes to turbulent flow. In turbulent flow the movement within the fluid quickly distributes the heat transferred from the walls. However when the majority of the fluid is mainly turbulent flow turbulence within the boundary layer against the wall is greatly suppressed.
Subsequently heat transfer within the boundary layer is mainly as a result of conduction. The benefit of turbulent flow is that it offers an improved heat transfer within an heat exchanger however as it requires high velocities the pressure drop must be increased.as a result of high pressures and the expense of wasted pumping power turbulent flow is unfavorable when highly viscous fluids are heated and cooled.

In both laminar and turbulent flow the rate of heat transfer to or from a surface is given by (3)
Where = heat transfer rate A =heat transfer area = surface heat transfer coefficient = temperature of fluid, = temperature of surface
This assumes that both the surface and the bulk of the fluid are at different constant temperatures.
As before, the resistance of the surface is Hence (4)
Radiation- can be defined as the emission of energy as electromagnetic waves which do not require direct contact between the hot and cold bodies nor the use of an intermediate carrier. Radiation is important in higher temperatures but has a lower effect at lower temperatures.

Overall heat transfer coefficient
It epitomises the ability of an heat exchanger to transfer heat from a hot fluid through a boundary to a cold fluid.

Temperature Distribution in Simple Concentric Tube Heat Exchangers-the temperature difference between the two streams fluctuates with respect to the position between hot and cold within the heat exchanger. The heat transfer calculations are simpler if a mean value of the local temperature differences is found. This is known as the logarithmic mean temperature .the mean temperature difference between the two streams can be given by :

= = (15)
The Rate of heat transfer is then given by: (16)
Evaluation of Heat Transfer Coefficients:
The values obtained for temperatures and mass flow rates of both streams are useful required when calculating the following:

1. Overall rate of heat transfer from hot stream: = ( -) (17)
2. Overall rate of heat transfer to cold stream: = ( -) (18)
3. Overall Heat Transfer Coefficient: U = = (19) 4. Overall surface Heat Transfer Coefficient between inner surface of tube and hot stream: = = (20) 5. Overall surface Heat Transfer Coefficient between outer surface of tube and cold stream: = = (21)

Hot water mass flow: Is calculated using = (.) = (Ɩ ) × . × (22)
Water velocity in hot core: Is calculated using : =
Water velocity in cold annulus tube: is calculated using: = = (23)

Apparatus
The concentric tube heat exchanger consists of a series of pipes or tubes in which fluids enter. It consists of two streams one for hot water and the other for cold water. The heat exchanger also consists of a control valve which is used to regulate the volume and mass. It also consists of cooling water couplings which are used to switch the cooling water direction from counter current to concurrent and vice versa.
Experimental procedure
Part A
1) Set the cooling water direction for counter current flow by choosing suitable cold flow couplings.
2) Turn on the main switch and heater switch and adjust the water temperature control to around 70 degrees Celsius. Conform the hot water flow rate Vi(l min-1) to 6 l min-1.
3) Adjust the cold water flow rate m0(g.s-1) to 10-15g.s-1.let the conditions to settle for a small period of time and then measure t1-t0.tabulate the results in table1.
4) Lower the flow rate for hot water to 5 min-1.let the conditions settle for a small period of time and then measure the temperatures t1-t0.
5) Repeat the last step however set the flow rate to 4 l min-1, and then 2 l min-1.Record the observations in table 1.
Part B
1) Set the cooling water direction for counter current flow by choosing suitable cold flow couplings.

2) Open the hot water control valve to set the volume flow rate in 6 l min-1,turn on the main supply and heater and set the water temperature control to around 50 degrees Celsius.

3) Adjust the cold water flow rate m0(g.s-1) to 10-15g.s-1.let the conditions to settle for a small period of time and then measure t1-t0.tabulate the results in table2.

4) To reverse the direction of the cooling water to concurrent (parallel) flow. Reverse the position of the cooling water couplings.

5) Adjust the cold water flow rate m0(g.s-1) to 10-15g.s-1.let the conditions to settle for a small period of time and then measure t1-t0.tabulate the results in table2.

6) For the sixth and seventh run from table 2.plot a graph of temperature distribution of hot and cold streams against distance from hot inlet. Compare and contrast the results obtained for counter current and concurrent flow.
Sample calculations for run 3
1) Mean hot temperature () :

= = = 56.65

2) Mean cold temperature () :

= = = 33.75

3) Density of hot water () at the mean hot temperature () : = -0.000004582 + (- 0.000040007) + 1.004 = -0.000004582 + (- 0.000040007) + 1.004 = 0.987. = 987.033.

4) Hot water mass flow ( ) at the mean hot temperature () :

(.) = (Ɩ ) × . × = 4 × 987.033 × = 0.0658022.

5) Conversion of the cold water mass flow from “.” to. :

= 15 × = 0.014.

6) Specific heat of water (:

= 0.000014177 + (- 0.0012833866 + 4.207683181) = 0.000014177 + (- 0.0012833866 + 4.207683181) = 4.18047643 J

7) Density of Water at :

ρ (at ) = -0.000004582 + (- 0.000040007) + 1.004 = -0.000004582 + (- 0.000040007) + 1.004 = 0.9870289. = 987.830.

8) Thermal Conductivity of Water :

= 0.569910538 + 0.001758899 + (- 4.855140 × ) + (-1.74252 × ) = 0.569910538 + 0.001758899 + (- 4.855140 × ) + (-1.74252 × ) = 0.6508276919

9) Viscosity of Water () :

= (1743.206226 + (-52.022586) + (0.89350510) + (-0.00805683 ) + (0.000028665) = (1743.206226 + (-52.022586) + (0.89350510) + (-0.00805683 ) + (0.000028665) = 494.055891

10) Water Velocity in hot core tube of heat exchanger :

= = = 1.3605 m/s

11) Water Velocity in cold annulus tube of heat exchanger :

= = = 0.58064 m/s

12) Overall rate of heat transfer from hot stream () :

= ( -) = 0.0986 4.18 (60.3 – 53.1) = 1.980608732kJ/s = 1980.608732 W

13) Overall Surface Heat Transfer Coefficient between inner surface of tube and hot stream (:

= = = = 12320

14) Overall Surface Heat Transfer Coefficient between outer surface of tube and cold stream (: = = = = 4681.37

15) Prandtl Number for Hot Water ( ) at the mean hot temperature ():

= 12.941105293 + (- 0.4265872 ) + (0.007685745 ) + (-0.00007090148) + (2.55712531E-07) = 12.941105293 + (- 0.4265872 ) + (0.007685745 ) + (0.00007090148) + (2.55712531E-07) = 3.183754339

16) Reynolds Number for Hot Water () :

=

But for a circular tube, L = = =
= 21473.12949

17) Measured Nusselt Number for hot water ():

= = = =136.153

18) Calculated Nusselt Number for hot water ():

0.023 = 0.023 =106.7622788

19) Logarithmic Mean Temperature difference :

= = = = 20.65858414

20) Overall Heat Transfer Coefficient (U) :

U = ==3.392342994