As we discussed early in the first chapter that heat can transfer through materials and the surrounding medium whenever temperature gradient exists until thermal equilibrium is reached. Heat transfer by: Radiation is often categorized as either ionizing radiation or non-ionizing radiation depending on the energy of the radiated particles. Conduction is the transfer of heat through materials by the direct contact of matter. Convection is the transfer of heat by the motion of the fluid (liquids and gases). Natural convection Forced Convection
Natural and forced Convection Natural convection occurs whenever heat flows between a solid and fluid, or between fluid layers. As a result of heat exchange, change in density of effective fluid layers taken place, which causes upward flow of heated fluid. If this motion is associated with heat transfer mechanism only, then it is called Natural Convection
Forced Convection If this motion is associated by mechanical means such as pumps or fans, the movement of the fluid is enforced. And in this case, we then speak of Forced convection.
Heat Exchanger is a device that provide the flow of thermal energy between 2 or more fluids at different temperatures.. Usually involves convection in each fluid and conduction through the wall of separating the two fluids. The overall heat transfer coefficient U contributes to all these factors. Usually work with the LMTD. LMTD = Logarithmic Mean Temperature Difference
Classification of Heat Exchangers They are used in a wide variety of applications. These include power production process, chemical, food and manufacturing industries, electronics, environmental engineering, waste heat recovery, air conditioning, reefer and space applications. Heat Exchangers may be classified according to the following criteria. Recuperators/ regenerators Transfer process: direct and indirect contact Geometry of construction; tubes, plates, and extended surfaces. Heat transfer mechanism: single phase and two phase Flow arrangement: Parallel, counter, cross flow current.
As mentioned in the previous slight, according to transfer process heat exchangers are classified as direct contact type and indirect contact type. In direct contact type, heat is transferred between cold and hot fluids through direct contact of the fluids (e.g. cooling towers, spray and tray condensers) In indirect heat exchanger, heat energy is transferred throw a heat transfer surface.
Heat Exchanger Heat Exchangers prevent car engine overheating and increase efficiency Heat exchangers are used in Industry for heat transfer Heat exchangers are used in AC and furnaces
Double pipe heat exchanger Concurrent\Parallel Countercurrent
Advantages of Double Pipe Heat Exchanges: 1. Simplest type of heat exchangers 2. Can be easily assembled 3. Relatively low cost 4. Small sizes Disadvantages of Double Pipe Heat Exchanger: 1. Leakages are very common 2. Requires a lot of time in dismantling and cleaning 3. Small surface area of heat transfer/pipe 4. Space requirements are large Double pipe heat exchangers should be considered first in design. The heat transfer surface should not exceed 200 ft 2. If several double pipes are required, their weight increases and thus the shell and tube heat exchangers is better.
Shell and Tube Heat Exchangers A shell and tube heat exchanger is a class of heat exchanger designs. It is the most common type of heat exchanger in oil refineries and other large chemical processes. Shell and tube heat exchangers normally consist of a bundle of tubes fastened into holes, drilled in metal plates called tube sheets.
Shell and Tube Heat Exchangers The Tubular Exchanger Manufacturers Association (TEMA) provides a manual of standards for construction of shell and tube heat exchangers, which contains designations for various types of shell and tube heat exchanger configurations. The most common types are summarized below.
E-Type The E-type shell and tube heat exchanger, illustrated in Fig. 2, is the workhorse of the process industries, providing economical rugged construction and a wide range of capabilities. The E-type shell is usually the first choice of shell types because of lowest cost, but sometimes requires more than the allowable pressure drop, or produces a temperature, so other, more complicated types are used. Tubular Exchanger Manufacturers Association
F-Type The F-type shell can be effective in some cases if well designed, but has a number of potential disadvantages, such as : Thermal and fluid leakage around the longitudinal baffle. High pressure drop. Tubular Exchanger Manufacturers Association
J-Type When an F-type shell cannot be used because of high pressure drop, a J-type or divided flow exchanger, shown in Fig. 4, is considered. Tubular Exchanger Manufacturers Association
X-Type When a J-type shell would still produce too high a pressure drop, an X-type shell, shown in Fig. 5, may be used. This type is especially applicable for vacuum condensers, and can be equipped with integral finned tubes to counteract the effect of low shellside velocity on heat transfer. Tubular Exchanger Manufacturers Association
Baffle-Type Baffles are used to increase velocity of the fluid flowing outside the tubes (shellside fluid) and to support the tubes. Higher velocities have the advantage of increasing heat transfer and decreasing fouling (material deposit on the tubes), but have the disadvantage of more energy consumption. Baffle types commonly used are shown in Fig. 9, with pressure drop decreasing from Fig. 9a to Fig. 9c.
Shell and Tube Heat Exchangers Non-baffled Heat Exchangers W,C p,t 1 w,c p,t 1 w,c p,t 2 W,C p,t 2 IDs d i d o
Baffles are used to establish a cross-flow and to induce turbulent mixing of the shell-side fluid, both of which enhance convection. The number of tube and shell passes may be varied One Shell Pass and One Tube Pass One Shell Pass, Two Tube Passes Two Shell Passes, Four Tube Passes Heat Exchangers 24
Shell-side flow
TEMA Designations Tubular Exchanger Manufacturers Association
Plate heat exchangers The heat transfer surface consists of a number of thin corrugated plates pressed out of a high grade metal. The pressed pattern on each plate surface induces turbulence and minimizes stagnant areas and fouling. Unlike shell and tube heat exchangers, which can be custom-built to meet almost any capacity and operating conditions, the plates for plate and frame heat exchangers are mass-produced using expensive dies and presses.
Superior thermal performance is the hallmark of plate heat exchangers. Compared to shell-and-tube units, plate heat exchangers offer overall heat transfer coefficients 3 to 4 times higher. These values, typically 4000 to 7000 W/m 2 ºC (clean), result in very compact equipment. This high performance also allows the specification of very small approach temperature (as low as 2 to 3 ºC) which is sometimes useful in geothermal applications. Selection of a plate heat exchanger is a trade-off between U-value (which influences surface area and hence, capital cost) and pressure drop (which influences pump head and hence, operating cost).
Casketed plate heat exchangers (plate and frame heat exchangers) Brazed plate heat exchangers Welded plate heat exchangers
Finned - Both Fluids Unmixed Unfinned - One Fluid Mixed the Other Unmixed 35
Widely used to achieve large heat rates per unit volume, particularly when one or both fluids is a gas. Characterized by large heat transfer surface areas per unit volume (>700 m 2 /m 3 ), small flow passages, and laminar flow. 36
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Tubes Standard tube lengths are 8, 12, 16 and 20 ft. Tubes are drawn to definite wall thickness in terms of BWG (Birmingham Wire Gauge) and true outside diameter (OD), and they are available in all common metals.
Tube Pitch The spacing between the tubes (center to center) is referred to as the tube pitch (P T ). Triangular or square pitch arrangements are used. Unless the shell side tends to foul badly, triangular pitch is Used.
Heat Exchanger (HEX) Rating Checking the existing design for compatibility with the user requirements (outlet temperature, heat load etc.) given: flow rates, inlet temperatures, allowable pressure drop; thus HT area and passage dimensions. find: heat transfer rate, fluid outlet temperatures, actual pressure drop. HEX Sizing Thermal and pressure drop considerations, maintenance scheduling with fouling consideration. given: inlet and outlet temperatures, flow rates, pressure drop find: dimensions -type and size of HEX. 41
Assumptions for Basic Design Equations for Sizing steady-state, steady flow no heat generation in the HEX negligible ΔPE, ΔKE adiabatic processes no phase change (later) constant specific heats and other physical properties. 42
Because the temperature difference between the hot and cold fluid streams varies along the length of the heat exchanger, it is necessary to derive an average temperature difference from which heat transfer calculations can be performed. This average temperature difference is called the Logarithmic Mean Temperature Difference (LMTD) ΔT lm. T lm T o ln( T T o i / T i ) Where, ΔT o = T 1 T 4 ΔT i = T 2 T 3
Log Mean Temperature Difference (LMTD) is the heat flows between the hot and cold streams due to the temperature difference across the tube acting as a driving force. As seen in the Figure below, the difference will vary with axial position within the HX. Where, θ 1 = T 1 -t 2 θ 2 = T 2 -t 1 LMTD 1 2 1 ln 2
Parallel Flow Flow Counterflow : 45
LMTD Method Expression for convection heat transfer for flow of a fluid inside a tube: q conv mc p( Tm, o Tm, i ) q UA s T lm T U = Overall heat exchanger coefficient, q = heat transfer rate lm T o ln( T T o i / T i ) 46
In a two-fluid heat exchanger, consider the hot and cold fluids separately: ) ( ) (,,,,,, i c o c c p c c o h i h h p h h T T m c q T T c m q T lm UA q and Need to define U and T lm
q q h c m h c c m c p, h p, c ( T ( T h, i c, o T T h, o c, i ) ) Where: q h q c : the heat power emitted from hot fluid. : the heat power absorbed by cold fluid. ṁ h, ṁ c : mass flow rate of hot and cold fluid, respectively. h h,i, h h,o : inlet and outlet enthalpies of hot fluid, respectively h c,i, h c,o respectively. : inlet and outlet enthalpies of cold fluid, T h,i, T h,o : inlet and outlet temperatures of hot fluid, respectively. T c,i, T c,o : inlet and outlet temperatures of cold fluid, respectively. C ph, C pc : specific heats of hot and cold fluid, respectively 48
With the LMTD method, the task is to select a heat exchanger that will meet the prescribed heat transfer requirements. The procedure to be followed by the selection process is:
Example 1