Determining the Heat Exchanger - Essay Example

Heat exchanger al affiliation The main aim of this experiment is to enable the familiarize with the shell and tube heat exchanger and to determine the overall efficiency of the current rates of different fluids. The experiment aims to determine the effect of changing current on the shell and tube heat exchanger efficiency and overall heat transfer. Moreover, this experiment aims to find out the temperature efficiency for co-current and counter current, the concurrent and the counter current operation overall heat transfer coefficient, the number of transfer units and the efficiency of the heat exchanger. In this experiment, the shell and tube exchanger that was used comprised of a number of tubes that were parallel in a cylindrical shell which had no significance until 15%. The experiment was conducted in environmental ambient temperature and distilled was added to make it successful since it does not contain impurities and has less probability to cause damage to the equipment. There was no significant change because the percentage error obtained for heat transfer coefficients for both concurrent and counter current is below 15%. In comparison to the counter current, the overall heat exchanger for concurrent flow is under control since there was a significant change for counter current has a significant change. Based on the estimated overall heat transfer, the expected exit temperature has the percentage error for both co-current and counter current was below 15% and thus no significant change. During the experiment, the heat losses for both co-current and counter current were above the expected percentage value, therefore creating a mean of high heat losses. The percentage error of the overall heat transfer coefficient for the co-current and counter current were not as expected indicating a significant change in the co-current. This shows that the efficiency of the heat exchanger effects on parallel flow more than counter flow. Key words: Shell and tube exchanger, co-current, and counter current Introduction Heat transfer is a widely used method of cooling warm fluids to lower temperatures while using a barrier to separate the two flows and is majorly applied in petroleum and chemical engineering. The shell and the tube heat exchanger are the equipment commonly used in this experiment because of the users’ ability to manage the pressures and temperature. In the shell and tube exchanger, the cylindrical shell is lined with some tubes internally. Its operation underlies the principle of two fluids flowing, one inside the tube and the other externally. The flows can assume either parallel or cross counter paths. In the co-current connection, the hot and the cold fluid flow in the same direction from both inside and outside the seven tubes. The co-current shell and tube heat exchange operation equation define the behavior of each fluid, hot and cold. During the co-current operation, the value of T3 is considered the cold fluid and T4 is considered as the cold fluid inlet. The counter current heat exchanger connection, the warm and the hot fluids flow in opposite directions. The hot fluid flows through the seven parallel tubes inside the heat exchanger while the cold fluids flows through the outside of the parallel tubes. The equation of counter current shell and tube heat exchanger operation shows that the equations that there is a likelihood of finding equations that define the behavior of each fluid to be the same but this is not the case practically since there could be heat loss or gain from the environment. The heat exchanger operation, the fluid that flows inside the tubes defines the inlet and the outlet temperature. The temperature difference between the inlet and outlet is represented effectively using the Log Mean Temperature Equation (LMTD), since the change is not linear. Due to the measurement of the temperatures taken at specific points on the heat exchanger, the LMTD equation for the co-current operation is the same as the counter-current operation. The area of heat transfer activity is calculated using the mean diameter of the inner tubes. Considering n as the number of tubes, which is 7, and l as the heat transmission of each tube, 0.144 m, the heat transmission length is written as L = n * l. Heat transmission area is defined as therefore overall heat transfer coefficient ( ). Equipment setup The pump, heater, heat exchanger and all the piped are mounted on a service bench. The type of heat exchanger used was Shell and tube heat exchanger which is commonly used in the industry. Tap water was used for cold water and pure water was used for hot water to prevent deposition of impurities after being heat up in the piping system. However, tap water temperature fluctuated a lot as observed in the experiment. The temperature needs to be controlled by a heater control. The hot water flow is being recycled and it goes through a closed loop. In addition, temperature sensors were vital in the experiment. Thermocouples made of different two alloys with a different expansion rates to heat and as a result, different voltage was used as to measure temperature. The water enters and exits the heat exchanger through the two headers. The manufacturer colored tubes distinguishes the current, that is, if it is co-current or counter current. In assigning the temperature sensors, the same method is applied to assign the temperature sensors. Only one pass was used in this exchanger despite that more heat transfer could be made by using more passes. Experiment Procedure The service bench is comprised of the heat exchanger, pump, heater and all the pipes all installed on a PVS base. The base’s function is to eliminate any fluid spills or unwanted leakage. The hot fluid enters the heat exchanger from the top header and exits from the other end in a normal heat exchanging operation. The hot fluid enters the heat exchanger through the top header to ensure smooth passage through the baffles and exchange accordingly. Headers and baffles of the outer case are made up of clear acrylic due to clarity and educational purposes. All the pipes used for connections of inlets and outlets from the heat exchanger are flexible rubber that make it easier to setup. The system was tried in both counter current and co-current to compare the results The heat exchanger that was used had seven stainless steel tubes with an outer diameter of 6.35 mm and 5.65 mm inner diameter. To get a desired flow rate (1L/Min), the valve is adjusted first but the indicator switch to the cold tube is first set. The same procedure is applied to water but the desired flow rate is 3L/Min. The system should be given a few minutes to reach a steady state. Subsequent to every change made in the flow rate, there should be a small wait to reach a steady state and the temperature sensors (T1, T2, T3 and T4). The flow rates for both hot and cold current read and recorded. Safety In every lab environment, safety is a high priority. Safety precautions should be considered to prevent any accidents or injuries. No foods or drinks should be allowed in the laboratory. In the laboratory, it is advisable to wear personal protective equipment such as safety glasses to avoid injury to the eyes and gloves should be used to avoid skin damage in case of contact with hazardous chemicals. In case on an incident occurrence, the location of safety items such as fire extinguisher, eye wash station, and first aid box should be known. The knowledge of safety procedures is equally important. Injuries or accidents should be reported to the instructor as soon as possible. First aid should be administered in case of any chemical splash in the eyes by taking then to the eye-wash station. It advisable to refrain from using tools that are not relevant to the experiment. Generally, basic laboratory rules and regulations should be adhered to. Original data The inlet and outlet temperature, and the volumetric both hot and cold water make up the original data. The temperatures are in °C while the flow rates are in L/min and have to be converted to cubic meters per second for calculations. Calculations For co-current flow, the heat transfer coefficients for cold are 12.983, 12.803, 12.809, 12.856 and 12.761 which will result in 1.7% error. For the hot, the heat transfer coefficients are 14.95, 15.361, 15.762, 16.275 and 16.979 which will resulting in a 12% error. This is below 15% showing no significant change. The o overall heat transfer coefficient, U are 1.196, 1.202, 1.318, 1.355 and 1.341 kW/Km^2. The percentage error was 11.7% and was under control. There was also no significant change since the percentage error for the estimated overall heat transfer was below 15%. The expected exit temperatures based on the estimated overall heat transfer were 44.480, 44.0812, 43.782, 44.369 and 44.948. The heat losses during the experiment for co-current are -0.0334, -0.033, -0.0307, -0.0573 and -0.108 will result in 71.6% error. This indicates a lot of heat losses during the experiment. U (calculated from NTU equation) for co-current are 1.192, 1.1982, 1.314, 1.451 and 1.523 will result in 21.75% error. The percentage error shows a significant change. Most of the errors are for either experimental error or human error or because of the quality of the equipment. For the counter current flow calculated values, the heat transfer coefficients cold are 14.94, 15.36, 15.762, 16.275 and 16.979 which will result in 12% error, and hot are 17.0007, 16.293, 15.862, 15.23 and 15.049 which will result in 11.5% error. The percentage error is under 15%, so there is no significant change. The percentage error resulted in 23% which is significant with the overall heat transfer coefficient U are 1.692, 1.508, 1.485, 1.423 and 1.302 kW/Km^2. In the overall heat transfer coefficient 44.567, 44.491, 44.064, 43.5006 and 43.041. The expected temperatures yielded 3.4% error which is below 15%, therefore non-significant. The heat losses during the experiment for counter-current are 0.1334, 0.1736, 0.11516, 0.1659 and 0.0721 will result in 58.5% error, showing significant heat losses during experiments. U (calculated from NTU equation) for counter-current are 1.4778, 1.235, 1.522, 1.621 and 1.552 will result in 23.8% error. The percentage error shows a significant change. Similar to co-current flow, errors resulted either from experimental and human error or the quality of the equipment. Conclusions and Recommendations The main aim of the experiment, Shell and tube heat exchanger, was to familiarise with the system and to understand the concept of temperature difference due to change in flow rates for co-current and counter current as well as to observe the effect of changing the flow rate on heat exchanger efficiency and overall heat transfer coefficient. Since the equipment is educational. There was no significant change until 15%. The heat transfer coefficient for co-current cold are 12.983, 12.804, 12.809, 12.856 and 12.761 will result in 1.7% error, and hot are 14.95, 15.362, 15.762, 16.275 and 16.979 will result in 12% error. On the other hand, the hot heat transfer coefficients 14.950, 15.362, 15.762, 16.275 and 16.979 will result in 12% error. For counter current flow, the heta transfer coefficient for hot are 17.0007, 16.293, 15.862, 15.237 and 15.0492 will result in 11.5% error. There is no significant change since the percentage error for both are below 15%. The overall heat transfer coefficient, U, for co-current are 1.196, 1.203, 1.319, 1.355 and 1.342 kW/Km^2 will result in 11.7% error. Similarly for counter-current flow are 1.693, 1.509, 1.485, 1.423 and 1.303 kW/Km^2 will result in 23% error. The percentage error for co-current flow is under control with an error below 15% whereas, for counter-current has a significant change since it is above 15%. Based on the estimated overall heat transfer coefficient for co-current, the expected exit temperatures are 44.480, 44.081, 43.782, 44.370 and 44.948 will result in 2.6% error, and for counter-current are 44.568, 44.494, 44.064, 43.5006 and 43.0411 will result in 3.4% error. There is no significant change because both errors were below 15%, There were a lot of heat losses during the experiment since the heat losses during the experiment for the counter current were are 0.1335, 0.1736, 0.115, 0.1659 and 0.0721 will result in 58.5% error while concurrent system were are -0.03338212, -0.03304028, -0.03068, -0.0573 and -0.1081 resulting to a 71.6% error. The calculation of the over heat transfer coefficient of heat exchanger U based on the definition, NTU = UA/ Cmin, for co-current are 1.1919, 1.1982, 1.314, 1.4509 and 1.523 will result in 21.75% error whereas for counter-current are 1.478, 1.235, 1.522, 1.621 and 1.552 will result in 23.8% error. Both co-current and counter current flows shows significant change due to the percentage error. There is also a substantial change in co-current since the percentage error of the overall heat transfer coefficient for co-current changes from 11.7% to 21.75% while for counter current flow changes from 23% to 23.8%. This means that the efficiency of the heat exchanger effects on parallel flow more than counter flow. It can be noted that from both flows, there is an increase in the temperature difference between hot and cold fluid and increasing the efficiency by increasing the flow rate. It is more advisable to use counter current flow because this has a greater efficiency despite its larger percentage error that may be caused by either human or experimental error. The flow rate can also be increased to elevate the rate of heat transfer. This creates turbulence therefore greater rate of heat exchange occurs in the gelatinous sub layer. Heat transfer in the system can be made better by use of fluid with less viscosity leading to less percentage error. Furthermore, percentage errors can be minimized by changing the acrylic material with other material would also result in more heat transfer thereby increasing the thermal conductivity of the solid. Read More