Transcript
Ryerson University
Department of Chemical Engineering
CHE 415 Unit Operations II
Lab Report
Experiment #6
Heat Transfer: Steam to water
Experiment Performed on 21 September 2009
Report Submitted to
Dr.
By Group # 3 Section # 3 1. (Leader)
2. (Data Reporter)
3. (Safety Officer)
Submitted on: 28 September 2009
Marking Scheme
Formatting Used all 6 writing points in each section of the report / 10
General Appearance; Grammar and Spelling / 5
Complete and Informative Tables and Graphs / 15
Contents Calculation of Accuracy and of Precision of Results / 20
Comparison with Literature Data (expected results) / 10
Discussion on Influence of Procedural Design on Results / 10
Logic of Argumentation / 20
Sample Calculations / 10
_____ Total: / 100
Table of Contents
Introduction 3
Theoretical Background 4
Experimental Procedure 8
Co-Current Flow 8
Counter-Current Flow 8
Experimental Apparatus 9
Results 10
Discussion 11
Error Analysis 14
Conclusions 15
Appendix 16
Raw Data 16
Equations and Calculations 17
Sample Calculation – Co-Current Flow 18
Justification for altering experiment objectives 20
References 22
List of Figures and Tables
Figure 1. Co-current flow in a concentric tube heat exchanger 4
Figure 2. Counter-current flow in a concentric tube heat exchanger 4
Figure 3. Temperature distributions for parallel-flow and counter-flow heat exchangers 5
Figure 4. Temperature distributions for parallel-flow and counter-flow heat exchangers with steam as a hot fluid 6
Figure 5. Flow Diagram for Steam-Water Heat Exchanger 9
Table 1. Mass flow rate and Velocity for water 10
Table 2. Overall Heat Transfer Coefficient 10
Table 3: Pertinent Data 16
Table 4: Counter-Current Time Temperature Data 16
Table 5: Co-Current Time Temperature Data 16
Introduction
Heat exchangers are one of the mostly used chemical engineering equipment. In general, heat exchanger deals with transferring heat from one medium to another, which might be separated by a solid barrier or be in direct contact with each other. It is often desired to characterize a heat exchanger based on its properties such as the overall and local heat transfer coefficients, heat transfer efficiency and nuances of different modes of operation.
Current experiment objectives were to familiarize ourselves with a shell-and-tube heat exchanger and to perform the following tasks:
Determine the process instrumentation diagram from the pipes on the lab wall, including the characteristics and limitations of all valves and measuring / controlling equipment
Determine the overall heat transfer coefficient between the steam and the water, and compare it with the literature value (1000-6000W/m2*K)
Verify that the heat transfer during counter-current operation is the same as during co-current operation
Heat Exchanger’s mode of operation was set-up as co-current and then as counter-current, using the four controlling valves (see Figure 5 for an illustration). Water valve was opened fully and steam valve was opened to attain 10 psi pressure in the shell. Inlet and outlet temperatures for steam and water were subsequently recorded every 5 minutes for a total of 20 minutes.
Equations from the theory part were used to calculate the overall heat transfer coefficient U to be 2.7024W/m2*K, which fit well within the range of 1000-6000W/m2*K specified by literature. Heat transfer to the water was found to be 47.228 KJ m2/s, which was the same for both co-current and counter current modes of operation, proving our hypothesis.
Theoretical Background
The process of heat exchange between two fluids at different temperatures, which are separated by a solid wall, occurs in many engineering applications. The device that implements such transfer is a heat exchanger, which can be utilized for heating spaces, power production, waste heat recovery and chemical processing. There is a wide variety of different types of heat exchangers, such as: shell-and-tube, cross-flow, fin-tube, plate-fin and others.
This experiment is going to examine the simplest type of a heat exchanger: a concentric tube heat exchanger, in which the colder fluid (tap water) flows through the smaller inner tube, while the hotter fluid (steam) flows through a larger tube around it. In the parallel flow arrangement (see Fig 1) both cold and hot fluids at the same side, flow in the same direction and then exit at the same side. In the counter-current flow arrangement the two fluids enter at the opposite ends, flow in the opposite directions and then exit at the opposite ends.
Figure 1. Co-current flow in a concentric tube heat exchanger
Figure 2. Counter-current flow in a concentric tube heat exchanger
Figure 3 shows the temperature distribution profiles for parallel-flow and counter-flow modes of operation. In parallel flow arrangement the initial ?T is large, and moves to zero asymptotically towards the pipe outlet. The outlet temperature of the cold fluid never exceeds the inlet temperature of the hot one. In contrast the counter-flow mode of operation provides for heat transfer between the hotter portions of two liquids at one end of the heat exchanger and colder portions at the other end. For this reason ?T is smaller than for a parallel-flow heat exchanger and now the outlet temperature of the cold stream may exceed the outlet temperature of the hot stream.
Figure 3. Temperature distributions for parallel-flow and counter-flow heat exchangers
The above situation, however, does not entirely apply to our experimental set-up. Heat transfer, in our case, is provided by the steam condensing on the outside surface of the inner tube. Since the pressure remains constant, so does the condensation temperature, therefore the temperature of the hot fluid can be assumed to remain constant throughout the length of the pipe. The resulting temperature distributions are depicted in Figure 4 below. Therefore, we expect to observe identical amount of heat transfer for both co-current and counter-current modes of operation.
Figure 4. Temperature distributions for parallel-flow and counter-flow heat exchangers with steam as a hot fluid
Experimental overall heat transfer coefficient will be determined using the equation:
U=qA(?T1-?T2ln?(?T1?T2)) (1)
where q is the heat transferred to the water by steam, A is the area of heat transfer and ?T will be inlet and outlet temperatures, which will vary depending on the heat-exchanger operation mode (co- or counter-current) as follows:
For co-current flow: ?T1 = Thot,in-Tcold,in; ?T2 = Thot,out-Tcold,out
For counter-current flow: ?T1 = Thot,in-Tcold,out; ?T2 = Thot,out-Tcold,in
Heat transferred to the water (q) will be obtained by:
q=mccp,cTc,o-Tc,i (2)
where mc is the mass flow-rate of water, cp is the specific heat capacity of water, Tc,o is the outlet temperature and Tc,i is the inlet temperature of water.
Once we have experimental overall heat transfer coefficient, we can compare it with the literature value which gives an estimated range for an overall heat transfer coefficient for a steam condenser to be U=1000-6000 W/m2*K.
Experimental Procedure
Co-Current Flow
Before the experiment was begun the safety check was performed; all the vales were checked to be closed and all the flows to the system were shut off. The condition of the experimental equipment was verified and no visible leaks or damages were indentified. After the safety check was completed the vent valves were opened to release any non-condensables that were trapped within the apparatus from previous experiments. To obtain co-current flow valve 2 and valve 3 were opened while valves 4 and 1 remained closed. Once the valves were opened it was safe to turn on the steam and water. The valves were opened very slowly and carefully using a thermal glove for safety. Once all the flows and valves were opened the temperatures and pressures were monitored at set points in the experimental apparatus. Once the fluctuations in temperature had minimized and the system reached steady state the timer started and the initial reading on the water rotometer was recorded. The temperature and pressure readings were then recorded every 5 minutes for a total of 20 minutes. The final reading on the rotometer was also noted when the last set of data was gathered. The above experiment was then repeated for reduced water flowrate.
Counter-Current Flow
To carry out the experiment under counter-current conditions the flow of the steam had to be changed. While apparatus was running valves 4 and 1 were slowly opened as valves 2 and 3 remained opened. Once all the valves were opened it was safe to slowly close valves 2 and 3 and obtain counter current flow. The experimental procedure was then duplicated of the co-current flow. For counter-current flow, just like for co-current, the experiment was performed for 2 different water flowrates.
Experimental Apparatus
Figure 5. Flow Diagram for Steam-Water Heat Exchanger
Results
The overall heat transfer coefficient was calculated for co-current flow at fully open valve and half open valve position for the water flow (Table 1). It can be seen that there is no significant deviation and that and the water flow does not affect the heat transfer coefficient. The overall heat transfer coefficient was not calculated for counter current operation because the temperature reading for outlet steam did not work properly (see error analysis). The steam heat transfer coefficient was omitted (see Appendix) this resulted in a 34.76 % error between the experimental and theoretical value which can be explained by the removal of the coefficient.
Case
Flow Rate kg/s
Velocity m/s
Counter-Current
0.694
1.73
Co-Current
0.6
1.496
Co-Current Half valve position
0.505
1.258
Table 1. Mass flow rate and Velocity for water
Case
Uth ()
Uexp ()
Counter-Current
N/A
N/A
Co-Current
4.142
2.7024
Co-Current Half valve position
3.722
2.665
Table 2. Overall Heat Transfer Coefficient
Discussion
The main purpose of this lab was to evaluate the characteristics of the heat exchanger. The steam used in this experiment was supplied by a pipeline. Once the steam valve was open it was possible to read the pressure and the temperature of the entering steam. Using those values and the steam tables all the needed remaining steam properties were calculated. Since the steam was supplied from a main source it was impossible to control it properties such as temperature. When evaluating the drop along the steam pipe it was found that there was no noticeable pressure. This is mainly cause by the improper sizing of the pressure gauges that are not sensitive enough to small changes in pressure.
Throughout the calculations, to obtain certain properties of the fluid such as Reynolds number or density, the temperature was approximated by calculating an average from the inlet and outlet values. This method was used only when necessary.
When analyzing the obtained data it was determined that the inlet and outlet temperatures of steam vary only by about 2oC for co-current flow, for counter current they vary a lot more but more detail about that will be provided in the error analysis section of the report. This means that enough steam was provided to ensure the condensation over the complete length of the pipe. This is because if there was not enough steam and all of the steam condensed early on in the heat exchanger the temperature difference would be a lot greater and the outlet steam temperature would have been below 100oC. Since the heat exchanger is not transparent it is hard to determine whether the steam inside the heat exchanger condenses drop wise or film wise. From theoretical observations the steam should be condensing film wise because the furnace of the pipe, which is made out of copper, is not water repellent.
The instrumental apparatus included a pressure safety valve as well as electro pneumatic injection valve. The safety release valve was located on the inlet steam side of the apparatus. The steam valve is designed to open, in our case into the atmosphere, when a certain maximum pressure is reached to prevent rupture of the equipment. The electro pneumatic steam injection valve is also located on the inlet of the stream side of the apparatus. The valve is designed to control the steam flow to maintain constant properties on the inlet of the steam stream. It also prevents early steam condensation, from the pressure change across the valve, by adjusting the valve accordingly. The adjuster can set the % valve opening and monitor it real time and make adjustments accordingly. This is accomplished by an electrical motor on the valve operating the valve mechanism. The adjuster is located on side console and can be programmed and set to specific tolerances depending on the needed function.
The fluid allocation is very important in heat exchanger design. Allocating the most fouling fluid to the tube side will be more favorable as mechanical cleaning of the inside of the tubes will be much easier. As the allowable velocity in the tubes is usually higher than at the shell side and a high fluid velocity causes attrition of the deposits, it is possible to reduce fouling by design. The layout of the laboratory heat exchanger does not follow the Tubular Exchanger Manufacturers Association (TEMA). This is because the exchanger itself did not have a tag specifying it was designed and operating under TEMA standards.
The experimental overall heat transfer coefficient for counter-current operation was not calculated, since the exiting steam temperature was on average 89.9 C. Based on the theory of heat transfer by a condensing vapor there should be no change in the exiting steam temperature compared to the inlet temperature. Through discussion with the supervising professor it was concluded there would be no value to using these experimental results. Possible reasons for this could be due to the fact that it was a thermocouple and the reading is calibrated to a standard.
Error Analysis
The inlet and outlet steam temperatures varied by approximately 2 C. Based on the theory there should have been no drop in temperature. It was noticed that while steam was turned on in the beginning of the experiment that the inlet temperature dropped when the water was turned on. This shows that the water affected the temperature reading for the inlet and outlet steam. It is suggested that the thermocouple be moved so that it represents a more accurate measurement of the steam temperatures.
The outlet temperature reading for the steam in a counter current operation gave a reading on average of 89 C. After conversation with the advising professor it was a agreed this was incorrect and that the values had no objective value. A possible reason for this error is that the thermocouple used is not calibrated properly and that it should be looked at.
Conclusions
Current experiment familiarized us with the principles involved in evaluating and characterizing a concentric tube heat exchanger. Further the objectives of determining an overall heat transfer coefficient and evaluating the effects of co-current and counter-current modes of operation of heat transfer. The overall heat transfer coefficient was found to be 2.7024W/m2*K, which fit well within the range of 1000-6000W/m2*K specified by literature. Further, heat transfer to the water was found to be 47.228 KJ m2/s, which was the same for both co-current and counter current modes of operation, proving our hypothesis that the mode did not have an effect on heat transfer. This resulted from the hot fluid (steam) being at approximately the same temperature over the whole length of the heat exchanger.
It has also been established that theoretical heat transfer coefficient could not be accurately obtained prior to the experiment due to the fact that local convection heat transfer coefficient for the steam couldn’t be determined, because the unavailability of flow-rate. Absence of the steam transfer coefficient resulted in a deviation of 34.76 % of the value obtained experimentally.
Appendix
Raw Data
Variable
Measurement
Shell Outer Radius
2.706 cm
Tube Outer Radius
1.26 cm
Tube Inner Radius
1.13 cm
Steam Input Pressure
8.0 psi
Steam Outlet Pressure
8.5 psi
Tube Length
2.81 m
Table 3: Pertinent Data
Table 4: Counter-Current Time Temperature Data
Time (min)
Water
Steam
In
Out
In
Out
0
11.1
30.0
109.8
86.9
5
11.1
30.0
109.8
86.7
10
11.3
30.2
109.7
86.8
15
11.2
30.1
109.9
86.9
20
11.1
30.1
109.8
86.7
Average
11.16
30.08
109.8
86.8
Water Valve Approx ½ open
5
11.2
33.7
113.5
89.9
Table 5: Co-Current Time Temperature Data
Time (min)
Water
Steam
In
Out
In
Out
0
11.2
30.1
109.6
107.3
5
11.3
30.0
109.6
107.1
10
11.3
30.1
109.7
107.3
15
11.2
30.2
109.8
107.4
20
11.2
30.0
109.7
107.2
Average
11.24
30.08
109.68
107.26
Water Valve Approx ½ open
0
11.3
33.7
112.6
110.7
5
11.2
33.5
112.4
110.4
10
11.3
33.5
111.9
110.3
Average
11.27
33.57
112.3
110.47
Case
Total Flow Initial (USG)
Total Flow End (USG)
Counter-Current
450710
450930
Co-Current
450310
450500
Co-Current Half valve position
450550
450630
Case
Flow Rate kg/s
Velocity m/s
Counter-Current
0.694
1.73
Co-Current
0.6
1.496
Co-Current Half valve position
0.505
1.258
Case
Uth ()
Uexp ()
Counter-Current
N/A
N/A
Co-Current
4.142
2.7024
Co-Current Half valve position
3.722
2.665
Equations and Calculations
Flow Rate:
Flow=Flow Total Start-Flow Total EndTotal Time (1)
Theoretical overall heat transfer coefficient
ro = outer process tube radius (m)
r i = inner process tube radius (m)
hi = water heat transfer coefficient (W/m2K)
k = is the thermal conductivity of the process tube (W/mK)
Di = the inside diameter of pipe (m)
k = the thermal conductivity of water (W/mK)
V = the velocity of water (m/s)
= the viscosity of water (Ns/m2)
s = surface viscosity of water (Ns/m2)
cp = the specific heat of water (KJ/Kg•K)
= the density of water (kg/m3)
** Conditions 0.7 < Pr < 16700; Re > 10000; L/D > 10 **
Experimental Overall Heat Transfer coefficient
q = m Cp ?Tlm
q = rate of heat transfer, steam to water (KJ m2/s)
cp = the specific heat of water (KJ/Kg•K)
m = mass flow rate (kg/s)
A = the surface area of process pipe (m2)
?T lm= the log mean function of the temperature differences between hot and cold fluid based on input and out put
U=qA?T lm
U = overall heat transfer coefficient ()
A = total surface area (m2)
Sample Calculation – Co-Current Flow
Experimental:
Flow=Flow Total Start-Flow Total EndTotal Time
Flow=450500 USG-450310 USG20 min×0.003785 m31 USG×1000 kg gal1 m3×1 min60 s
m = 0.6 kgs
V = Q/Across -sectional
=0.0006m3/s ?×(0.0113 m)2
= 1.496 m/s
Average water temperature to determine average water properties:
Tav =30.08+11.242
=20.66 oC
?=1000 Kg/m3
Cp = 4.183 KJ/Kg•K
kwater =
kpipe = 1
q = mCp ?T
= (0.6)(4.183)(30.08 – 11.24)
= 47.228 KJ m2/s
A = ?DL
=?(0.0226m)(2.81m)
= 0.20 m2
U= q/(A?T lm )
= (47.228/(0.2)(87.382))
= 2.7024
Theoretical Overall Heat Transfer Coefficient
=
= 6.095
= 6.1355
= 0.24143
U = 4.142
=
= 34.76 %
Justification for altering experiment objectives
The previous objective of the experiment stated in the pre laboratory submission was to calculate the overall heat transfer coefficient from experimental results and to calculate a theoretical value for co-current and counter current operation. Since the steam entering and exiting was not superheated, but saturated there is condensate film resistance through steam condensing on the pipe. The derivation of the Nusselt equation gives the condensate heat transfer coefficient for a single saturated vapor on a smooth surfaced horizontal tube see equation (*). All variables are known expect for (the tube loading). W is the mass flow rate of the steam. The measured parameters of the steam are inlet and outlet temperature and pressure. There is no way to determine the flow rate of the steam, and in consequence the heat transfer coefficient for the condensing steam cannot be calculated. The theoretical value for overall heat transfer was calculated for co-current operation see Appendix II at 4.142 ho had been omitted from the calculation and as a result there is 34.76% error between the experimental and theoretical values of U. Since 1/U = 0.24143 and 1/ho would be a positive value it would reduce the value of U theoretical.
(*)
= W/4L
W = steam mass flow rate (kg/s)
L = Pipe length (m)
k = thermal conductivity of condensate (W/mK)
g = acceleration do to gravity (m/s2)
PL = density of condensate (kg/m3)
= the viscosity of steam (Pois)
= weighting factor
References
Incropera, DeWitt, Bergman, Lavine, 2007. Introduction to heat transfer. Hoboken: John Wiley & Sons Inc.
STEAM - Arithmetic and Logarithmic Mean Temperature Difference. 2006. http://www.steam-toolbox.com/arithmetic-logarithmic-mean-temperature-d_436.html (accessed September 25, 2009).
R.E. Peck, W. R. (December 1951). Heat Transfer Coefficients for Vapors Condensing on Horizontal Tubes. Engineering and Process Development , 2926 - 2932.
Turcotte, D. G. (2009, September 21). General Questions Regarding Heat Transfer Steam to Water. (P. K. Charles Gilmour, Interviewer)
