Experiment - Find Out the Efficiency of Double Pipe Heat Exchanger

Experiment Objective: Find out the efficiency of double pipe heat exchanger in co current flow arrangement. Theory: Heat exchanger is a device that exchanges the heat between two fluids of different temperatures that are separated by a solid wall. The temperature gradient , or the differences in temperature facilitate this transfer of heat. Transfer of heat happens by three principle means: • Radiation • Conduction • Convection The double-pipe heat exchanger is one of the simplest types of heat exchangers. It is called a double-pipe exchanger because one fluid flows inside a pipe and the other fluid flows between that pipe and another pipe that surrounds the first. This is a concentric tube construction. Flow in a double-pipe heat exchanger can be co-current or counter-current. There are two flow configurations: • Co-current It is when the flow of the two streams is in the same direction. • Counter current It is when the flow of the streams is in opposite directions. Effect of changing the conditions on heat transfer rate: In this double-pipe heat exchanger a hot process fluid flowing through the inner pipe transfers its heat to cooling water flowing in the outer pipe. The system is in steady state until conditions change, such as flow rate or inlet temperature. These changes in conditions cause the temperature distribution to change with time until a new steady state is reached. The new steady state will be observed once the inlet and outlet temperatures for the process and coolant fluid become stable. In reality, the temperatures will never be completely stable, but with large enough changes in inlet temperatures or flow rates a relative steady state can be experimentally observed. Working Principle: The basic principle of heat exchanger is the transfer of heat between two fluids. Two fluids are brought in close contact with each other but are prevented from mixing by a physical barrier. The temperature of the two fluids will soon come to an equilibrium temperature. The energy from each fluid is exchanged and no extra heat is added or removed. Since the heat in the process is not constant and the heat amount of the fluids is also not constant thus the Heat exchanger must be designed in a way that it is suited for all the cases of heat exchange and the performance is best suited for all conditions. Also the design should be such that the heat exchange is at a particular rate required by the process. Heat exchangers are originally designed to be over sized so that in cases of fouling, the surface of heat exchanger is still large enough to carry out operations. Once cleaned the heat exchanger would be again oversized. For the use of any heat exchanger the proper study of various technical and economical parameters is required such as life of heat exchanger, cost per unit area , Overall Heat Transfer Coefficient , Low heating value of fuel, Effectiveness, Efficiency , Heat Capacities, Annual variation of temperatures of fluid under observation. Various types of heat exchanger follow this general principle. Whether it is a Double Pipe heat exchanger or Shell and Tube or Plate Heat exchanger or others of various kinds the underlying principle is same although specifics differ greatly. Some factors that influence the heat exchanger performance are: • width of the material of the tubes • Temperature variation between two fluids • Thermal Conductivity of the material of fabrication • Physical features of the exchanger and Surface Area of the tubes • Type of flow i.e. Counter current or co current or mixed flow • Properties of the liquid i.e. viscosity, Heat capacity. Explanation: The determination of the overall heat-transfer coefficient is necessary in order to determine the heat transferred from the inner pipe to the outer pipe. This coefficient takes into account all of the conductive and convective resistances (k and h, respectively) between fluids separated by the inner pipe, and also takes into account thermal resistances caused by fouling (rust, scaling, i.e.) on both sides of the inner pipe. For a double-pipe heat exchanger the overall heat transfer coefficient, U, can be expressed as In a heat exchanger the log-mean temperature difference is the appropriate average temperature difference to use in heat transfer calculations. The equation for the log-mean temperature difference. Where T1=Hot Stream Inlet Temperature. T2=Hot Stream Outlet Temperature. t1 =Cold Stream Inlet Temperature. t2=Cold Stream Outlet Temperature. Construction: They simply consist of two concentric pipes: one fluid flows in inner pipe and other fluid flows in the annular space between the inner pipe and outer pipe. Double pipe Heat exchangers can be made with various materials: • Carbon steel • Alloy steels • Copper alloys • Exotic materials (tantalum). • Iron • Aluminum alloys Industrial applications: In many industrial processes there is waste of energy or a heat stream that is being exhausted, heat exchangers can be used to recover this heat and put it to use by heating a different stream in the process. This practice saves a lot of money in industry, as the heat supplied to other streams from the heat exchangers would otherwise come from an external source that is more expensive and more harmful to the environment. Heat exchangers are used in many industries, including: • Waste water treatment • Wine and beer making • Refrigeration • Petroleum refining Limitations: ❖ It is not as cost effective as most shell and tube exchangers ❖ It requires special gaskets ❖ Limited volumetric capacity ❖ Fouling Procedure: 1. Startup • Fill the steam generator with clean water. • Switch on steam generating unit until it creates your desired pressure in unit. • Open cold water inlet • Measure the flow rate of inlet water 2. Operation • Open the valve which control flow of steam. • Inlet temperature of steam can be found from steam table corresponding to steam pressure • Note down the inlet temperature of cold water. • Also note down the outlet temperature of cold water. • Measure the condensate flow rate. 3. Shutdown • Shut off the hot stream first • Close the cold water inlet. Observation and Calculations: Results: Discussion: Safety and Precautions: 1. Use PPE i.e. Lab Coat & Safety goggles to prevent any harm. 2. As we are using steam leather gloves are recommended for operation. 3. Do not let Steam to be accumulated in the steam chamber high pressure me cause it to burst. 4. Do not rip the insulation of the exchanger. 5. Clean Water must be used to prevent corrosion. 6. Learn where the safety and first-aid equipment is located. This includes fire extinguishers, fire blankets, and eye-wash stations. 7. Report any accident immediately. 8. In case of fire evacuate the lab immediately. Application: Double pipe heat exchanger found its greatest applications in the industries where the total heat transfer area is small, 100 to 200 ft2 or less, and is usually use in the refineries, fertilizers, food, petrochemical industries etc.