CHE 415 - Steam to Air Formal Report 2006

Ryerson University Department of Chemical Engineering CHE 415 Unit Operations II Prof: Dr. T.A.: Experimental Determination of the Heat Transfer Coefficient of Air Team Members: Table of Contents 33 44 66 99 1010 1111 1313 1414 1414 1414 Error! Bookmark not defined.Error! Bookmark not defined. 1717 17 Introduction In this experiment, Theoretical Background Heat transfer is the passage of thermal energy. The transfer of heat is normally from a high temperature object to a lower temperature object. According to the first law of thermodynamics, heat transfer changes the internal energy of the systems. The heat transfer by convection is investigated in this experiment. Convection is comprised of two mechanisms: diffusion and bulk motion of fluid. Convection can be classified as the nature of flow. One form of convection is forced convection, where the flow is produced by an external source/energy; for example, a fan. The air surrounding a heat source in forced convection receives heat, becomes less dense and rises. According to ideal gas law, when the air heats, the molecules spread out and the air becomes less dense than the surrounding. The hot air will subsequently rise due to buoyant forces. And the cold air moves to replace. When the heat and the process are continued, the convection current is formed, and the driving forces for force convection by pumps or fans 1. For convection, the energy transferred is called sensible or internal thermal energy of the fluid. In essence, convection is the energy transfer between a surface and surrounding fluid over the surface. The heat transfer coefficient is used to calculate the convection transfer between moving air in thermodynamics. The heat transfer coefficient (h) is often calculated from a dimensionless number, the equation is used to find the heat lost from a hot tube to the surrounding area: (1) Where, Q is the power input or heat lost U is the overall heat transfer coefficient A is the outside solid-fluid contact surface area ?Tm is the difference in temperature between the solid surface and surrounding fluid area In order to determine or predict the performance of an heat exchanger(used in this experiment) the relationship between the total heat transfer rate to inlet and outlet fluid temperatures, overall heat transfer coefficient, and the total surface area for heat transfer is required. For this experiment, the following variation of Newton’s Law of Cooling is used to determine the rate of heat transfer 2: (2) The coefficients i and o denote inner and outer in reference to the fluids used. This equation can be used for both hot and cold fluids. Since ?T varies along the length of the heat exchanger, this explains why ?Tm is used(the mean temperature difference). Although, when dealing with overall heat transfer coefficient, the log mean temperature difference (LMTD) is more appropriate in finding the average temperature difference along an heat exchanger. Log mean temperature difference (LMTD) equation is expressed by 3: (3) Where Ts is the temperature of the heat transfer surface, and Tm is the temperature of the fluid flowing above the outer heat transfer surface. ?T1 = the small temperature difference between the two fluid streams at either the entrance or the exit to the heat exchanger. Also, in order to determine the heat transfer coefficients of the fluids used, the following dimensionless expressions can be used 4: Nuslet Number: Reynolds Number: Where Pr is the Prandtl number, m is the mass flowrate, D is the diameter, and ? is viscosity. These values are found in heat transfer tables. Experimental Method and Setup To begin all local steam valves connected to the seven steam coils were closed. Those valves are located at the left hand side of the dryer. The main switch is then turned on at the back of the dryer followed by the blower being turned on. An arbitrary low value for steam pressure was chosen and kept constant throughout the experiment. To begin, the blower voltage was set on a low value for the first run. Two steam tubes were opened, and the system was allowed to reach steady state. The temperature at the inlet and outlet while at steady state was taken. This was followed by subsequently increasing the heat transfer area by opening more steam tubes. These steps were repeated for different blower voltages in each individual run. After the experiment was completed, the system was shutdown with all valves and switches turned off. The following figure represents the process flow of the experimental apparatus: Figure 1: Process Flow Diagram of Heat Exchanger Setup The following is a schematic of the experimental apparatus: Experimental Results Table 1: Determination of Average Heat Transfer Coefficient for Run#1 # of Tubes Total Heat Transfer Rate (kJ/s) Mean Temperature (K) Overall Heat Transfer Coefficient (W/m2?K) 2 3.39 11.39 168.84 3 3.55 12.00 118.98 4 5.04 17.00 136.76 5 5.89 19.89 139.01 6 6.26 21.11 130.21 Table 2: Determination of Average Heat Transfer Coefficient for Run #2 # of Tubes Total Heat Transfer Rate (kJ/s) Mean Temperature (K) Overall Heat Transfer Coefficient (W/m2?K) 2 4.81 13.74 251.32 3 4.73 13.50 166.59 4 5.01 14.18 135.87 5 6.04 17.22 139.86 6 6.92 19.72 139.97 Table 3: Determination of Average Heat Transfer Coefficient for Run#3 # of Tubes Total Heat Transfer Rate (kJ/s) Mean Temperature (K) Overall Heat Transfer Coefficient (W/m2?K) 2 4.86 11.11 254.13 3 5.68 13.00 201.31 4 6.91 15.80 193.66 5 7.78 17.78 181.95 6 7.78 17.78 151.63 Discussion The objectives of this experiment were to determine the average and local heat transfer coefficient of air. The convection coefficient is a measure of how effective a fluid is at carrying heat to and away from the surface. It is dependent on factors such as the fluid density, velocity, and viscosity. Generally, fluids with higher velocity and/or higher density have greater h. This experiment was an example of forced convection whereby the flow was caused by a pump. For equation (1), the convection coefficient is dependent on three variables: the change in temperature (?T), the total heat transfer rate (q), and the surface area of heat transfer. To begin, the average heat transfer coefficient of air decreased as the number of tubes in operation increased. The larger the number of tubes in operation, the more surface area for heat transfer. Therefore, according to the equation, the heat transfer coefficient is directly proportional to the rate of heat transfer, but inversely proportional to both surface area and the temperature difference. As the results show that as the air mass flowrate increases the heat transfer coefficient decreases overall, which suggests that as the mass flowrate increases, the less the heat is transferred from the steam in the tubes to the surrounding air traveling past the bank of tubes. These results correspond to the decreasing heat transfer coefficient for air on average. The average heat transfer coefficient of air ranges from 118.98 to 254.13 W/m2?K. Figure 1 shows that by linear regression analysis how much U decrease as the heat transfer area increases. The third run has the highest R2 value which tells that these conditions gave the best average heat transfer coefficients of air that ranged from 151.63 to 254.13 W/m2?K, and that this experimental setup is a good and reliable method in determining average convection heat transfer coefficients. By using Run#1(2 tubes) data from Tables 1 and 4, the average heat transfer coefficient of air was determined to 167.68W/m2?K. The overall heat transfer coefficient was found to be 168.84W/m2?K. This experiment showed that in this case, average heat transfer coefficient can be considered to be similar to U. This is despite the fact that the heat exchanger used has thermal resistances that needed to be taken into account. If there were no thermal resistances, U could be considered equal to h in the heat transfer rate equation. The individual heat transfer coefficients were found for hi and ho: 625.2 and 126.85W/m2K respectively. Typically, heat transfer coefficients for air range between 10 - 100 W/m2K; therefore, there is some error in our determination. Error Analysis In this experiment, there are some sources of error which may have affected in achieving our objectives set out. To begin, some of valves used were leaking with steam being released. This possibly can cause some heat loss to the atmosphere. Also, the valves themselves may not totally shut off even if the valve itself was turned off causing excess steam to flow affecting results. Another potential source of error might be the in the original construction of the steam tubes inside the chamber of the tray dryer. No knowledge is known of how the steam tubes are aligned inside: are they aligned in a staggered line or completely over one another in a vertical straight line? It is reasonable to suspect that the alignment is a vertical straight line, by looking at how the tubes are aligned from the outside in. Therefore, by aligned the tubes vertically over one another, this could potentially cause a certain level where the maximum heat transfer rate is achieved no matter have much mass air flow is increased. The other source of error could be in the maintaining a constant steam pressure. It was observed that for Run 1 and the begin of Run 2, the steam pressure had to be constantly monitored and adjusted (manually) to a predetermined steam pressure setting, because it would not stay constant as the number of tubes operating were increased in a run. Percent Error for hi: Percent Error for ho: Conclusion and Recommendations The objective of this experiment was to determine the average and local heat transfer coefficient of air. Appendix A Raw Data O.D. of heating tubes: 0.625 inches Wall thickness of heating tubes: 0.040 inches Heating area per bank: 9.87 ft2 Table 4: Experimental Results for Run#1 Number of Tubes Tair,in (K) Tair,out (K) Cp(KJ/kg?K) Heat Transfer Area (m2) 2 299.15 322.04 1.0074 1.833 3 299.15 323.15 1.0074 2.7495 4 299.15 333.15 1.0076 3.666 5 301.15 340.93 1.0078 4.5825 6 303.15 345.37 1.0079 5.499 Table 5: Experimental Results for Run#2 Number of Tubes Tair,in (K) Tair,out (K) Cp(KJ/kg?K) Heat Transfer Area (m2) 2 300.15 327.59 1.0076 1.833 3 301.15 328.15 1.0076 2.7495 4 302.15 330.73 1.0077 3.666 5 303.15 337.59 1.0078 4.5825 6 303.15 342.59 1.0079 5.499 Table 6: Experimental Results for Run#3 Number of Tubes Tair,in (K) Tair,out (K) Cp(KJ/kg?K) Heat Transfer Area (m2) 2 303.15 325.37 1.0076 1.833 3 302.15 328.15 1.0076 2.7495 4 302.65 334.26 1.0077 3.666 5 303.15 338.71 1.0078 4.5825 6 303.15 338.71 1.0078 5.499 Sample calculations: Note: The following calculations used data from Table 4(Run #1 with 2 tubes) Run #1 with 2 tubes, m = 0.33 lb/s = 0.47 kg/s Tm = (Tout +Tin)/2 = (322.04 + 299.15)/2 = 310.65 K Heat transfer from steam = 0.47 kg/s x 1.0074 Kj/kg.s x (322.04 – 299.15) K = 3.389x103 J/s ?Tlm = 10.64 K Therefore, 3.389x103 J/h = U x 0.262 m2 x 76.63 K U = 168.84 W/m2?K Assume the heat exchanger is unfinned and no significant fouling factors. Therefore, Therefore, the average heat transfer coefficient is: References Incropera, Frank P. Fundamentals of Heat and Mass Transfer, 5th Edition, John Wiley&Sons, New York, 2002, p. 6-8, 484, 647-650, and 655-657.