Heat Transfer from Steam to Water - Lab Report Example

Table of Contents Summary 1 Background and Methods 2 Theory 5 Procedures 7 Results and Discussion 11 Data Analysis 13 Conclusion 16 References 16 Summary Heat is a form of energy and the level of energy contained in a certain object is shown by the level of temperature in the object. The main purpose of this experiment was to test on the heat transfer from steam to water. Stem is usually produced when the water is heated to its boiling point and the heat can also be transferred from steam to water. Unlike the heat transfer by the other processes, e.g. condensation, radiation and conduction, heat transfer by condensation for example in steam does not involve temperature change. The steam passes on its latent heat to the products when it condenses on the heat transfer surface. This product formed for the condensate still contains its own sensible heat and this heat is of the same temperature that is contained in the steam from which it was produced. This experiment uses two sets of equipment. The first set is described under “Heating Liquids in Tank Storage.” The second set is described under “Corning Heat Exchanger.” The main aim of this experiment is to test on the heat transfer from the steam to water and to produce analysis from the data collected in the lab. Usually, the latent heat that is contained within the steam is released instantly as the steam condenses into the liquid state. The amount of latent heat that is released ranges from 2-5 times greater than the amount of sensible heat available from water after cooling.The objective is to determine the overall heat transfer coefficient (Uo) for the external heat exchanger at two different water circulation rates. Background and Methods Equipment Specifications 1. The tank with the following specifications were used: 17.6" I.D., 31" deep, 30 gallon capacity (roughly). 2. Heat exchanger: 13 tubes, 12" long of 0.375" O.D. and 0.315" I.D. type L copper mounted in a 3" I.D. shell. There was water on the tube side and steam on the shell side. 3. Stirrer: Propeller on central shaft, powered by a 1/15 HP motor with adjustable speed (0-190 rpm). 4. Circulating pump: 1/3 HP centrifugal pump (32 gpm at 5' head, 12 gpm at 40' head). 5. Thermocouples: Copper/constantan thermocouples (TC's) connected to a computer via analog/digital converter. The thermocouples measure the temperature of the tank water (water entering the exchanger), water leaving the exchanger, steam in, and steam (or condensate) out. 6. Orifice meter (on water return line from heat exchanger to tank - used to measure the circulating water flow rate when the external exchanger is used.) The flowrate, Q, in lb/min, is given by: Q=27.5√ ∆P Where ΔP is the pressure drop across the orifice in pounds per square inch, and the pressure drop is six times the dP (differential pressure) cell voltage. 7. Computer with data acquisition program used to monitor temperature and flow rate data. The time versus temperature data are logged to a data file 1. Corning Heat Exchanger Apparatus The figure below shows the corning heat exchanger, shown from the water inlet end. The steam inlet is on the right side, and not visible in this photograph. Theory The rate of heat transfer that is often obtained during the condensation of the vapour is always very crucial since it is used in many industries in the steam heated vessels where the steam condenses and produces the heat. It is also applied in the distillation and evaporation where vapors produced must first be condensed. The latent heat of vaporization is produced at a constant temperature that is the boiling temperature of the liquid. The Equations were used to compute the over-heat transfer coefficient. The equations enabled the valves of both experimental as well as heat transfer coefficient to be calculated. ………………………………………….equation 1 Where: Ts = steam temperature, as an average of inlet and outlet steam. T1 = initial tank temperature T = temperature of the tank as a function of time W = rate of water circulation, (lbm/min) ρ = density of water V = Volume of water t = time (s) …………………………………………….equation 2 Where: Uo = overall heat transfer coefficient (based on Ao) Ao = heat transfer area cP = heat capacity of water When E-1 is plotted with respect to time we get the slope as following: ………………………………………….equation 3 Primarily, unit consistence time is converted into minutes in order to obtain a three significant figure. In oerde to obtain the trend analysis, the intercept is always plotted and set to zero. ………………………………………….equation 4 Where: Do = outside Diameter of the tube Di ­= inside Diameter of the tube hfi = heat transfer coefficient due to fouling on the inside tube surface­ hi­= water to metal heat transfer coefficient on the inside tube surface kw­= copper tube thermal conductivity ho= steam to metal heat transfer coefficient on the outside tube surface hfo= heat transfer coefficient due to fouling on the outside tube surface Equation 4 is applied when fouling is significant, but if not then the following equation can be used: 1/Uo=Do/Dihi+Doln(Do/Di)/2kw+1/ho………………………………………….equation 5 ho of 2000 Btu/hr-ft2-oF was used as given handout. Nu=0.023Re0.8Pr0.4…………………………………………………….equation 6 Nu=hiDi/k …………………………………………………………….….equation 7 Where Nu is the Nusselt and k is the thermal conductivity of water. Re=Diνρ/μ…………………………………………………….…………….equation 8 Re is the Reynolds number 𝜇 is the viscosity of water 𝜈 is the average liner velocity of water with unit’s length over time. Pr=Cpμ/k ………………………………………………………………….equation 9 Fundamentally, there are two types of condensation that are likely to occur in general; a) Film wise condensation b) Drop wise condensation Film wise condensation usually occurs when the water vapour strikes on the surface of the lower temperature vapour condenses and a film forms on the surface. If the mode of flow of the condensate is due to the gravity and the condition is of streamline flow exists throughout the film throughout the film thickness is by conduction. The thickness of the condensing film is has a direct influence to the quantity of heat transferred because the heat transfer is by conduction. Therefore, the thickness of the film depends on the rate at which the condensate is removed. On the other hand, the drop wise condensation occurs on the surfaces where the substances are contaminated and prevents the condensate from wetting the surface thus the steam condenses in drops and not as a film. Procedures 1. The computer was started up and the file heating Liquids was opened in the tank of the desktop. 2. Then the tank drain valve was closed. 3. The tank was filled with the cold water from the tap; the fill valve is a quarter turn ball valve above the tank and behind the stirrer and pump controls as shown in the figure below. There is a cross beam on top of the tank that supports the stirrer motor and the stirrer and pump controls. A little below this cross beam, there is a black mark on the stirrer shaft sleeve. The tank was filled to this level, the flow rate of the water was therefore slowed as the tank approached full in order to avoid slopping water out of the tank. 4. The stirrer was turned on and controlled according to the specifications, that is number 9 should be on top of the dial, the higher speed stirrer will result to be noisy. 5. Data recording was done by clicking the arrow icon near the top of the screen. 6. The pump was turned on and the water control valve was adjusted in order to get a water flow rate of about 40lb/min. The indicated flow rate bounces rapidly because the water is turbulent. The voltage corresponding to a 40lb/min flow rate should be near the center of the bounce. 7. The data recording was stopped by clicking the icon near top of the screen that looks like a stop sign. The program was required to be stopped at this point in order to start a new data file when the heating of the tank starts. 8. The steam inlet valve was opened which is the quarter-turn ball valve behind and right to the tank. 9. The logging of the data started immediately after opening the steam inlet valve by clicking the arrow icon. 10. The steam pressure was adjusted to about 5 psi, this was done by the steam regulator valve which is located just to the right of the steam inlet valve. A regulator is like a valve, but it is twisted in the opposite direction of most valves. To increase steam pressure it was opened and the handle was twisted clockwise. To decrease steam pressure, it was closed and the handle was twisted counter clockwise. 11. The water control valve was re-adjusted to maintain a 40lb/min flow rate.The flow rate was monitored regularly since it was drifting. 12. The data was continued logging until the tank reached 130oF. 13. The data logging was stopped by clicking the stop sign icon. 14. The steam was shut off. 15. The pump and the stirrer were shut off. 16 The tank drain valve was then opened. Equation 4 is applied when fouling is significant, but if not then the following equation can be used: 1/Uo=Do/Dihi+Doln(Do/Di)/2kw+1/ho………………………………………….equation 2 ho of 2000 Btu/hr-ft2-oF was used as given handout. Nu=0.023Re0.8Pr0.4…………………………………………………….equation 6 Nu=hiDi/k …………………………………………………………….….equation 7 Where Nu is the Nusselt and k is the thermal conductivity of water. Re=Diνρ/μ…………………………………………………….…………….equation 2 Re is the Reynolds number 𝜇 is the viscosity of water 𝜈 is the average liner velocity of water with unit’s length over time. Pr=Cpμ/k ………………………………………………………………….equation 2 Results and Discussion Table 1.Calculation of over-all heat transfer coefficient for 40 lb. /min Tank 40 Values   slope K ln K Uo Uo(Btu/ft^2-hr-F) Week 1 0.0262 1.196123 0.179085 5.601531 336.0918354 week 2 0.0317 1.247482 0.221127 6.916554 414.9932467 week 3 0.0297 1.228304 0.205634 6.431943 385.916572 Table 2. Calculation of over-all heat transfer coefficient for 70 lb. /min Tank 70 Values   slope K ln K Uo Uo(Btu/ft^2-hr-F) Week 1 0.0333 1.135183 0.126794 6.940394 416.4236267 week 2 0.0356 1.145882 0.136175 7.453878 447.2326724 week 3 0.0344 1.140275 0.13127 7.185372 431.1223342 A figure 5.The graph below illustrates the analysis of the 40lb/min vs 70lb/min flow rate of water for the first week. Figure 6.The graphs below illustrates the analysis of the 40lb/min vs 70lb/min flow rate of water for the second week. Figure 7.The graphs below illustrates the analysis of the 40lb/min vs 70lb/min flow rate of water of the third week. Equation 1 was used to calculate temperatures of the heating liquids in tanks. A graph was drawn as a function of time. The gradient was then computed from the graphs by use of the trend line options from excel. Data Analysis Table 3.Shows computations for the average standard deviation of the whole heat transfer coefficients both for the 40 lb. /min and 70 lb. /min flow rate of water Average 379.0006 (Tank 40) 431.5929 (Tank 70) Standard Deviation 32.58048 (Tank 40) 12.58214 (Tank 70) Table 4. Indicates the percentage errors between the predicted and the experimental values for the over-all coefficient of heat transfer for the 40lb. /min flow rate of water.   UoExp Uo Pre week 1 5.601531 5.99378 week 2 6.916554 5.99378 week 3 6.431943 5.985644 Ave 6.316676 5.991068 std 0.665046 0.004697 % error 5.15% Table 5. Shows the percentage error between the predicted and experimental value for the over-all coefficient of heat transfer for the 70lb. /min flow rate of water.   UoExp Uo Pre week 1 6.940394 8.495554 week 2 7.453878 8.495554 week 3 7.185372 8.485107 Ave 7.193215 8.492072 std 0.256832 0.006032 % error -18.06% From the data obtained above, the water density, water viscosity, heat capacity of water, thermal conductivity of water as well as Prandtl number of water all with respect to temperature were used to calculate the constants during the course of the experiment. From the calculations detailed above, a relationship was derived between the experimental and predicted data of the over-all heat transfer coefficient. Through this, errors were also computed. The errors for 40lb. /min flow rate was found to be 5.15% whereas for the 70lb. /min flow rate of water was found to be -18.06%. Table6: computations for the over-all coefficient of heat transfer. Figure 8: A Graph of over-all Heat Transfer coefficient against water flow rate Conclusion Primarily, the experiment indicates that the error increases with increase in flow rate. An error of 5.15% and -18.06% were recorded for 40 lb. /min and 70 lb. /min of water respectively. It is also clear that errors obtained were as a result of systematic and random errors subjected to the experiment. From the data, the over-all coefficient of heat transfer for the 40 lb./min flow rate was found to be 6.32 (Btu/ft2-hr-F) while for the other 70 lb./min (Btu/ft2-hr-F) was found to be 7.19. From the analysis, it can be deduced that the flow rate increases with increase in the over-all coefficients of heat transfer. Similarly, the same results were obtained when the corning heat exchanger was used. In conclusion, the graphs drawn of natural log against time is a straight line and starts from the origin. References Anna M. and Alyssa H. Effects of Flow Rate on Heat Transfer, Cameron Pearce, 2012. ASHRAE Guide and Data Books, American Society of Heating, Refrigerating and Air Conditioning Engineers, New York, 2006. Incropera F. and Dewitt D., Fundamentals of heat and mass transfer 5th edition, McGraw-Hill, 2005. Education for Chemical Engineers Design of a laboratory experiment on heat transfer in an agitated vessel. 2011. Fan, Maohong. Heat Transfer from Steam to Water; Laramie, WY, 2012 Read More