Transcript
Objectives of Experiment:
The objective of this experiment is to determine the overall heat transfer coefficients of a double pipe heat exchanger for saturated water and saturated steam in co-current and counter-current flow arrangements experimentally and comparing it to the theoretical values. The experimental overall heat coefficient shall be calculated using the first law of thermodynamics, where the overall heat transfer coefficient within the system shall be same as the heat gained by the water. The theoretical overall heat transfer coefficient shall be determined by the overall thermal resistance equation.
Design Basis:
Heat exchangers are commonly used in industries to transfer heat from one fluid (heating medium) to another (cooling medium). Fluids such as water, air, steam and fuel oils can be used for heat exchange as heating medium and cooling water or coolants/refrigerants as cooling medium. There are many types of heat exchangers used in the industry, such as shell and tube heat exchangers (with one pass or multi-passes), concentric tube heat exchangers, stacked and non-stacked, compacted or not compacted and finned cross-flow heat exchangers. For this experiment a concentric tube heat exchanger will be used which is made up of two tubes, one with a smaller diameter inside the other. This experiment will use the hot saturated steam fluid as the heating medium (in the outer pipe), and the cold water (in the inner pipe). The typical classification of a concentric tube heat exchangers is based on the flow arrangement and type of construction [1]. The flow arrangement is the direction in which the two fluids move, such as co-current, counter-current, and cross flow. In this experiment both co-current and counter-current flow between steam and water will be examined as shown in figure 1 and 2.
Cold Water
Hot Steam Pipe Section
Hot Steam Pipe Section
Heat Transferring from Hot Steam
Figure 1: Concentric double-pipe heat exchanger for counter-current flow
Hot Steam Pipe Section
Hot Steam Pipe Section
Heat Transferring from Hot Steam
Cold Water
Figure 2: Concentric double-pipe heat exchanger for co-current flow
During the experiment the direction of flow of steam shall be manipulated to achieve co-current by opening valve #2&3 while closing Valve #1&4 and counter-current flow by opening valve #1&4 while closing Valve #2 &3. Also inlet and outlet temperatures of each fluid will be measured along with the flow rate of the liquid and pressure drop across the heat exchanger.
Figure 4: Temperature profile for a co-current concentric tube heat exchanger [1]
Figure 4: Temperature profile for a counter-current concentric tube heat exchanger [1]
Introduction and Theoretical Background
Heat transfer is the thermal energy in transit due to a temperature difference or temperature gradient [1]. There are three modes of heat transfer: Conduction is the transfer of thermal energy from more energetic particles to less energetic particles through their interactions and occurs when there is no bulk motion [1]. Convection is the transfer of energy through random motion (diffusion) and through bulk motion of a fluid [1]. Lastly, Radiation is the energy emitted from a substance at a finite temperature [1]. In general, thermal energy (heat) is transferred from a high temperature to a low temperature medium. This phenomenon, known as the first law of thermodynamics, is greatly utilized in the industry as cold substances can be heated or hot substances can be cooled. This is accomplished through heat exchangers. Heat exchangers are devices that separate two fluids with different temperatures by a solid wall [1].
To calculate the overall heat transfer coefficient of the system, two relationships are used. Equation (1) relates the amount of heat, Q, gained by the liquid water from the steam [1]. Equation (2) relates the amount of heat transferred based on Newton’s law of cooling [1]. Equation (2) assumes heat loss to the surrounding, kinetic and potential energy changes, tube wall thermal resistance, and fouling factors are negligible.
(1)
(2)
Where: Q = heat transferred/gained
m = mass flow rate of liquid
U = overall heat transfer coefficient
A = area of heat transfer
?TLM = log mean temperature difference
Cp, L = specific heat capacity of liquid
?TL = temperature difference between the inlet and outlet
The log mean temperature difference, LMTD, is different for co-current and counter-current. Thus, an energy balance on differential elements coupled with a few assumptions (such as negligible axial conduction, negligible potential and kinetic energy, overall heat transfer coefficient and fluid specific heat are constant), gives the LMTD for the co-current and counter-current [1].
(3)
For co-current: ?T1 = TS,IN – TW,IN and ?T2 = TS,OUT – TW,OUT
For counter-current: ?T1 = TS,IN – TW,OUT and ?T2 = TS,OUT – TW,IN
The subscript “w” and “s” represents water and steam respectively.
Combining equation (1) and (2) allows the overall heat transfer coefficient to be determined:
(4)
To determine the theoretical overall heat transfer coefficient, the convection coefficient is utilized. For an unfinned tubular heat exchanger, the overall heat exchanger is shown in equation (5).
(5)
Where: ri = inner diameter of the pipe
ro = outer diameter of the pipe
hi = heat transfer coefficient for the inner tube
ho = heat transfer coefficient for the outer tube
k = thermal conductivity of the pipe
The inner heat transfer coefficients hi can be determined using Nusselt’s number, which can be computed by the Dittus-Boelter equation for heating shown in equation (6). This is based on Chiltron-Colburn’s analogy, a classical expression for local Nusselt number for fully developed turbulent flow in a smooth circular tube [1].
(6)
Where Re – Reynolds Number;
Where: Pr = Prandtl Numbers,
?L = viscosity of the liquid
CpL = heat capacity of the liquid
kL = thermal conductivity of the liquid
m = mass flow rate of liquid
D = characteristic length
Also, the general definition of Nusselt’s number is expressed in equation (7) [1].
(7)
where hi = the inner heat transfer coefficient
D = characteristic length
k = thermal conductivity
Combining (6) and (7) yields the inner heat transfer coefficient for the inner pipe.
The outer heat transfer coefficient for the outer pipe can be calculated with drop wise condensation. The drop wise condensation heat transfer coefficient is much larger than the film condensation, about an order more [1]. For heat exchangers, the drop wise condensation dominates more than the thermal resistances [1]. Therefore, the heat transfer drop wise coefficient can be used for the outer pipe as the saturated steam losses heat causing some vapors to form droplets. For copper surfaces, the drop wise condensation heat transfer is correlated in equation (8), where the sub cooling effect is neglected and no “crowding” occurs [1].
for 22ºC < Tsat < 100ºC
for Tsat > 100ºC (8)
Substituting the values of the two heat transfer coefficient (hi and ho) into equation (5) yields the theoretical overall heat transfer coefficient.
Duty Distribution:
Varun Tomar (Leader) will be responsible to make sure the correct valves are open to achieve co-current and counter-current flow. Also supervise implementation of technical procedure and coordinate clean up of equipment and area after the completion of experiment.
Ziyad Shaath (Data Reporter) will be responsible for the recording all relevant data: Temperature, pressure, flow
Marwa al-Haji (Inspector) will be responsible for the safety aspect of the experiment; and making sure all members are wearing proper laboratory attire (i.e. hardhat, lab coat, safety glasses)
References:
[1]. DeWitt, D.P., Incropera, F.P., Fundamentals of Heat and Mass Transfer, 5th Ed., 2002, John Wiley & Sons
[2] Turcotte, G. CHE415 - Unit Operations II Lab Manual – Exp 3 – Steam to Water, Ryerson University, Fall 2008
