1.1 BACKGROUND OF THE STUDY
According to the modern or dynamical theory of heat:
Heat a form of energy. The molecules of a substance are in parallel motion. The mean Kinetic energy per molecules of the substance is proportional to its absolute temperature. In description of heat, a molecule may consist of one or two or many atom depending upon the nature of the gas. The force of attraction between the molecules of a perfect gas is negligible. The atom in a molecules vibrate with respect to one another, consequently a molecules has vibration energy. The whole molecules may rotate about one or more axes, so it can have “notational energy”. A molecule has “translational energy” due to its motion, thus kinetic energy of a molecule is “the sum of its translational, rotational and vibrational energies. Summarily heat energy given to a substance e is used in increasing its internal energy. Increase in internal energy cause increase in Kinetic energy or potential energy or increase in both the energies. Due to increase in Kinetic energy of a molecules, its translational, vibrational or rotational energy may increase. In a nut shell “heat transfer is the science which deal with the rate of bodies called the Source aims receiver KERN, [2006:1]
1.2 MECHANISM OF HEAT
Heat transfer is of three distinct way in which heat may pass from a source to a receiver, although most engineering application are combination of two or three method, which are conduction convection and radiation Conduction: Conduction heat Transfer is energy transport due to molecular motion and interaction. Conduction heat transfer through solids is due to molecular vibration. Fourier determined that Q/A, the heat transfer per unit area (W/m2 ) is proportional to the temperature gradient ∂t/∂x. The constant of proportionality is called the material thermal conductivity K.
Fourier equation according to Colostate [2014:4]
Q/A = -K ∂t
The thermal conductivity K depends on the material and also some what on the temperature of the materials.
Convection heat transform is energy transfer due to bulk fluid motion. Convection heart transfer through gases and liquids form a solid
boundary results from the fluid motion along the surface.
Newton determined that the heat transfer/area Q/A, is proportional to the fluid sold temperature difference T2. if the temperature difference normally occurs across a thin layer of fluid adjacent to the solid surface. This thin fluid, layer is called a boundary layer. The constant of proportion is called the heat “transfer coefficient, h. Newton‟s equation: According to Colostate [2014:4]
Q/A = h ( Ts – Tf) …(1-2)
The heat transfer coefficient depends on the type of fluid and the fluid velocity. The heat flux) depending on the area of interest, is the local or area averaged. The various types of convective heat transfer are usually categorized into the following
CONVECTIVE HEAT TRANSFER COEFFICIENT CONVECTION
Natural Convection. Fluid motion induced by density difference Forced Convection Fluid motion induced by pressure differences from a fan or pump Boiling Fluid motion induced by a change of phase from liquid to vapour Condensation Fluid motion induced by a change of phase from vapor to liquid
Source: Colostate [2014:4]
Radiation heat transfer is energy transport due to emission of electromagnetic wave or photons form a surface or volume. The radiation does not require a heat transfer medium, an can occur in a
vacuum. The heat transfer by radiation is proportional to the fourth power of the absolute material temperature. The proportionality constant S or the stefom – Boltzman constant equal to 5.67 x 158 ml/m2k4. The radiation heat transfer also depends on the material properties represented by ℮, the emissivity of the material.
Q/A = бT4
Source: Colostate [2014:4]
1.3 PROCESS HEAT TRANSFER.
Heat transfer been described as the study of the rate at which heat is exchanged between heat source and receiver usually treated independently. Process heat transfer deals with the rate of heat exchanger as the occur in the heat – transfer equipment of the engineering and chemical processes. This approach brings to better focus the important of the temperature difference between the source and receiver, which is after all, the driving force whereby the transfer of heat is accomplished. A typical problem of process heat transfer, is considered with the quantities of heat to be transferred. The rate of which they may be transferred be cause of the nature of the bodies, the driving potential the extent and arrangement of the surface separating the source and receiver, and the amount of mechanical energy which may be expanded to facilitate the transfer, Since heat transfer involves can exchanger in a system, the loss of heat by the one body will equal
the heat absorbed by another within the confine of the same system.
1.4 HEAT TRANSFER PROCESS EQUIPMENT
Heat exchanger been one of the commonly used equipment in heat transfer a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall to prevent mixing o they may be on direct conduct. They are widely used in space heating, refrigeration, air conditioning, petro-chemical plant, petroleum engineers, and natural gas processing and sewage treatment.
Heat exchange is device that exchanges the heat between fluids of different temperatures that are separated by a solid wall. The temperature gradient or he difference in temperature facilitate the transfer of heat. Transfer of heat happens by three principle means: Convection, conduction, radiation. Conduction occurs as the heat from the higher temperature fluid passes through the solid wall. The biggest contribution to heat transfer on a heat exchanger is made through convection. To maximize the heat transfer, the walls should be thin and made of a very conductive materials.
In a heat exchanger forced convection allows for the transfer of heat of one moving as heat is transferred through the pipe wall it is mixed into the stream and the flow of the stream removes the transferred heat. This maintain a temperature gradient between the two fluids. The double-pipe heat exchanger because on fluid flows inside a pipe and the other fluid flows between that people and another people. That surrounds the first “this is a concentric tube construction”. Flow in a double pipe heat exchanger can be co-current or counter current. There are two flow configurations co-current is when the flow of the two streams are in the same direction, counter current is when the flow of the stream is in opposite direction.
1.4.1 TYPES OF HEAT EXCHANGER
In order to meet the widely varying applications, several types of heat exchanger have developed which are classified on the basis of nature of heat ex change process, relative direction of flow motion, design on constructional features and physical state of fluids. Nature of heat exchange process On the basis of nature of heat exchange process, heat exchanger
are classified as follows.
- Director contact heat exchanger: These type of heat exchanger are used predominantly in air conditioning condensing plants, water cooling, industrial hot water heating etc and it involves heat transfer between hot cold stream of two phases in the absence of a separating walls. Most direct contact heat exchange fills under the gas-liquid category where heat is transfer between a gas and begin in the form of drop, film, or spray.
- Direct Contact heat Exchanger: In this type of heat exchanger, the heat transfer between two fluids could be carried out by transmission through wall which separates the two fluids. On the other hand indirect – Contact heat exchanger.
“The fluid stream remain separate and the heat transfer continuously through an impervious diving wall or into and out of a well in a transient, thus c, ideally there is not direct contact between thermally interacting fluids. This type of heat exchanger also referred to as a surpass heat, exchanger, it can e further classified into direct –transfer type, storage type and fluidized – bed exchanger.
1.4.2 RELATIVE DIRECTION OF FLUID MOTION
Due to the relative direction of two fluid streams. The heat exchangers are classified into three categories. Parallel flows or unidirectional flow In parallel flow heat exchangers, the two medium enter the exchanger at sane bed, and travel in parallel to one another to the other side. Counter- Current
Source: Rajpat (2010:566)
Counter flow heat Exchanger
In counter flow heat exchanger, the fluids (i.e. hot and cold) enter a exchanger form opposite ends. The counter flow design is most efficient, in that it can transfer the most heat from the heat transfer medium
Cross-flow Heat Exchanger
In cross-flow heat exchanger, the low fluids (hot and cold) crossone another in space, usually at right angles, below is a schematic dragram of common arrangements of cross-flow heat exchangers
Cold fluid (in)
(mixed hot steam)
1.4.3 DESIGN AND CONSTRUCTIONAL FEATURES:
On the basis of the above sub-topic, heat exchanger are classified as follows Concentric tube heat exchanger Concentric tube (or pipe) heat exchanges are used in a variety of industries for purposes such as material processing, food preparation and air conditioning. They create a temperature driving force by passing fluid steams of different temperatures parallel to each other, separated by a physical boundary in the form of pipe. This includes forced convection, transferring heat to/from the product.
Theory and application
The thermodynamic behavior of concentric tube heat exchanger can be described by both empirical and numerical analysis. The simplest of these involves the use of correlations to model heat transfer; however the accuracy of these predictions varies depending on the design. For “turbulent”, non-viscous fluids the Dittus –Boelter equation can be used to determine the heat transfer coefficient for both the inner and outer streams; given their diameters and velocities (or flow rate). For conditions where thermal properties vary significantly such as for large
Source: Rajput (2000:566) Cold fluid (oat) (b) temperature difference, the Seidec Tale Correlation is used. This model takes into consideration the differences between bulk and wall viscosities.
Concentric tube Heat Exchanger design: The primary advantage of a concentric configuration, as opposed to a plate or shell and tube heat exchanger, is the simplicity of their design.
As such, the insides of both surface are easy to clean and maintain, making it real for fluids that cause fouling. Additionally their robust building means that they can withstand high pressure operation. They also produce turbulent coefficient, and hence the rate of heat transfer.
There are significant disadvantages however, the two most noticeable being their high cost in proportion to heat transfer area; and the impractical length required for high heat duties. They also suffer from ”comparatively high heat losses via their large, outers shells. The simplest form is composed of straight section of tubing encased within the outer shell, however, alternative such as corrugated or curved tubing conserve space while maximizing heat transfer area per unit volume. They can be arranged in series or in parallel depending on heating requirements.
Typically constructed from stainless, steel, spacer are inserted to retain concentricity while the tubes are sealed with O-ring, packing, or welded depending on the operating pressure.While both Co- and – counter configurations are possible, the counter current method is more common. The preference is to pass the hot fluid through the inner tube to reduce heat losses, while the annulus is reserved for the high viscosity steam to limit the pressure drop beyond double stream heat exchangers, designs involving triple (or more) streams are common; alternating between hot and cool stream, thus heat/cooling the product from both sides.
Shell And Tube Heat Exchanger: Shell and tube heat exchanger are typically used for high pressure application (with pressure greater them 30 hour and temperature e greater than 260oC).
The exchanger consists of a series of tubes. One set up the tube contains the fluid that must be either heated or cooled. The second fluid runs over the tube that are being heated or cooled so that it can either produce the heat or absorb the heat required. A set of the tube is called tube bundle.
Regenerative heat Exchanger: In a regenerative heat exchanger, the same fluid is passed along both side of the exchanger,which can be either a plate heat exchanger or a shell and tube heat exchanger.
Because the fluid can get very hot; the existing fluid is used to warm the incoming fluid, maintaining a near constant temperature. A large amount of energy is saved in a regenerative heat exchanger because the process is cyclical, with almost all relative heat being transferred from the existing fluid to the incoming fluid. To maintain a constant temperature only a little extra energy is needed to rise and lower the overall fluid temperature.
Plate Heat Exchanger: Plate heat exchanger consist of thin plates joined together, with a small amount of space between each plate, typically maintained by a small rubber gasket. The surface area is large and the corners of each rectangular plate feature; an opening through which fluid can flow between plate extracting heat from the plate as it flows the fluid channel themselves alternate hot and cold fluids, meaning that heat exchanger can effectively cost as well as heat fluid. They are often used in refrigeration application. Because plate heat exchanger has such a large surface area, key are often more effective then shell and tube heat exchanger.
Adiabatic Wheel Heat Exchanger: In this type of heat exchanger an intermediate fluid is used to store heat, which is then transferred to the opposite side of the exchanger unit. An adiabatic wheel consist of a large wheel with thread that rotate through the fluids – both hot and cold to extract or transfer heat.
1.5 HEAT TRANSFER COEFFICIENT
The heat transfer coefficient or film coefficient, in thermodynamic and in mechanics is the proportionately coefficient between the heat flux and the thermodynamic driving force for the flow of heat (i.e. the temperature difference, T): Wikipedia [2014:1]
H = q”/ T …(1-4)
q” = heat flux, W/m2 i.e., thermal power per unit area
q = dQ/dA
h : heat transfer co-efficient, w/(m2
T : difference in temperature between the sited surface and
surrounding fluid area, K
The heat transfer coefficient has SI unit in walt per squared meter Kelvin: W/(m2K) heat transfer coefficient is the inverse of thermal insulance. This is used for building materials c(RE. Value) and for clothing insulation.
OVERALL HEAT TRANSFER COEFFICIENT
The overall heat transfer coefficient it is a measure of the overall ability of a series of conductive and convective barriers to transfer heat. It is commonly applied to the calculation of heat transfer in “heat exchanger,” but can be applied equally well to other problems.
For the case of a heat exchanger U can be used to determine the total heat transfer between the two streams in the heat exchanger by the following relationships. Wikipedia [2014;7]
q = UA TLM …(1-7)
q = heat transfer rate (N)
U = overall heat transfer coefficient (w/(Cm2K)
A = Heat transfer surface area (M2)
TLM = Log mean temperature difference (k)
The overall heat transfer coefficient takes into account the individual heat transfer coefficient of each stream and the resistance of the pipe material. It can be calculated as the reciprocal of the sum of a series of thermal resistances (but more complex relationship exist, for example when heat transfer takes place by different routes in parallel):
1 + R = m …(1-8)
R = Resistance(s) to heat flow in pipe wall (K/W) other parameters are as above.
The heat transfer coefficient is the heat transferred per unit area per Kelvin. This area is included In the transfer of heat takes place. The areas for each flow will be different as they represent the contact area for each fluid side.
The thermal resistance due to the pope wall it is calculated by the following relationships. Wikipedia [2014:8]
R = X
X = the wall thickness (m)
K = The thermal conductivity of the material (W/(m.K)
A = The total area of t he heat exchanger (m2)
This represent the heat transfer by convection i.e the pipe
1.6 THERMAL CONDUCTIVITY
K = Q/A . dx/dt …(1-10)
The value of K – 1, when Q – 1, A = 1 and dt/dx = 1
Now K = Q/1. dt/dt (unit of K: Wx 1
/m2 K m = w/m/K
Thus, the thermal conductivity is the amount of energy conducted through a body of unit are and at thickness in unit time when the difference in temperature between the face causing heat flow in unit temperate difference. It follows grow equation above, that material with high thermal conductivity are good conductor of heat, whereas material with low thermal conductivity are good thermal insulator.
Thermal conductivity depends essentially upon the following factors.
Density of the material
Pressure and temperature (operating conditions) Rajput
1.7 PROBLEM STATEMENT
In heat transfer problem are classified below as follows.
This problem use similarity law for the transfer of heat from one point to another through the model of heat transfer, the knowledge is transferred by means of consistency of known characteristic quantitative of heat transfer field to the heat transfer of actual interest.
Analytical heat transfer Problem Analytical method of applied thermodynamic are used to solve basic heat transfer problem coefficient and the heat transfer medium.
1.8 OBJECTIVES OF THE STUDY
– To study the performance and the characteristic of double pipe, water to water concentric tube heat exchanger in both parallel and counter flow.
– Determining the heat transfer coefficient at different value opening of the co- and counter current arrangement
– To determine the comparative relationship of enthalpy approach for co and counter flow in a concentric tube heat exchanger
– To make poly recommendations on enthalpy approach for Coand counter flow in a concentric tube heat exchanger based on timing.
1.9 SIGNIFICANCE OF STUDY
The significance of this study emphasis on the importance of heat transfer in our modern word with respect to human heat and both on complex industrial process and domestic activities so with the view of heat transfer wit h respect to heat exchanger which is an essential agent, its significance direct us to; where heat transfer is valued most .Finally, it actually enlighten on overall view of heat transfer, its aim and application with respect to heat exchanger as an equipment involved in the process. Application of heat exchanger Chemical processing plant” Dehydration / Dissolving – Crude oil interchange, Water / crude oil inter changer Refinery – Brine cooling, crude oil / Water interchange treated Crude oil/ untreated crude oil interchange Food and Beverage:
Soft Drink – pasteurization of syrup and feral Product, water, heating and sugar dissolving of final product Brewing – beer pasteurization, wort Boiling, wort cooling Finally in food and beverage appellation; stabilize and prolong shelf life. Heat transfer has to be rapid and effective, in order to avoid as much as possible any damage to nutritive and organoleptic qualities of
1.10 SCOPE OF STUDY
The study make a great emphasis or impact on the Enthalpy approach in heat transfer pipe (i.e. heat exchanger) flow using fluid mechanics. And it tends to explain the comparative enthalpy approach for Co- and Counter current flow in concentric tube heat exchanger using the result or data obtained.
1.11 LIMITATION OF STUDY
The determination of heat transfer coefficient at different valve opening of the co- and counter current arrangement, limit the performance of more than one (i.e. either co-current or counter current
flow) experiment a day, in order to enhance or support the p roper cooling of the heat exchange, likewise time limit the performing of more than six (6) experiment (i.e. between 90o
to 15) to reduce the degree of error in the result obtained generally.
1.12 CHALLENGES OF THE STUDY
In the course of the experiment various challenges was experienced which includes, the complexity of the complicated technique used in performing the analysis, shortage of water flow as a result of inadequate water supply system, finally in consistence power supply system to run the equipment which require high capacity of power or electricity to run the pumps (i.e water flow system) and heaters.
- Colostate (2014). Heat Transfer Mechanism, Accessed, from Internet on 21st July, 2014 from http/www.engr.colostate.edu. p.4
- Kern, (1997). Process Heat Transfer, thirteenth Edition, Tata McGrawHill Edition, New Delhi. McGraw Hill Company Ltd. P 1
- Rajput, R. K. (2000). Fluid Mechanics and Hydraulic Mechanics, S, Chad Company Limited, New Delhi. pp 565-566
- Wikipedia (2014), Heat transfer coefficient. Accessed from Internet on the 18 July, 2014 from http//en.m.wikipedia.org. p. 1
- Wikipedia (2014), Overall heat transfer coefficient. Accessed from Internet on the 18th July, 2014 from http//en.m.wikipedia.org. p. 7
- Wikipedia (2014), Thermal resistance of heat transfer coefficient. Accessed from Internet on the 20th July, 2014 from http//en.m.wikipedia.org. p. 8