Professor: Eduardo Cabrera Thermal Engineering Laboratory Heat Exchangers Damian Luna - 33509 Yetziel Sandoval – 78820 Alberto Gonzales – 80546 Fernando Fresse – 56319 Jaen Soto - 51080 Professor: Eduardo Cabrera Thermal Engineering Laboratory

Table of Contents Objective Introduction Theoretical Background Theory Flow Arrangements Energy Balance Temperature Efficiency Log Mean Temperature Difference (LMTD) Global Heat Transfer Coefficient Equipment and Experimental Set Up Experimental Procedure References

Objective Study the operating principle and performance of Heat Exchangers Specially the characterization of tube and shell heat exchanger and plate heat exchangers Understand the principles of heat transfer between two fluid streams separated by a solid wall Also how geometry affects its performance.

Introduction Cooling or Heating processes is something important to any industry to produces a product Typical types of Heat Exchangers are: Cooling Tower Boiler Car Radiator This project will target the Heat Exchangers commonly used for Oil or Chemical Industry that are: Shell and Tube Plate In order to evaluate the performance of this heat exchanger is important to understand the concepts of indirect heating, flow configuration, energy balance and overall heat transfer coefficient.

Theoretical Background Heat exchangers are devices used in the industry to transfer efficiently heat from one hot fluid to another cold fluid. The most common type is that in which fluids are not mixed such as the shell and tube heat exchanger and the plate heat exchanger. Shell and Tube In this one fluid goes through the tubes and the other flows outside the tubes but inside the shell, heat is transferred from one fluid to the other through the tube walls, either from tube side to shell side or vice versa. The fluids can be either liquids or gases on either the shell or the tube side, and in order to transfer heat efficiently, a large heat transfer area should be used, so there are many tubes. Plate Heat Exchanger In this one the fluids flow through grooves carved on plates. Many plates are attached together to form the channels by which the hot and cold fluid circulate. This heat exchanger is characterized by having a very large surface area to heat transfer. This plate type arrangement can be more efficient than the shell and tube. Advances in gasket technology have made the plate type increasingly practical.

Flow Arrangements Parallel Flow or Co-Counter Flow Two Fluids enter the exchanger at the same end and travel in parallel to one anorther to the other end. Counter Flow The fluid enters the exchanger from opposite ends.

Flow Arrangements Cross-Flow Fluids travel roughly perpendicular to one another through the exchanger. Shell and Tube or Plate heat exchanger can work as co-current and countercurrent.

Energy Balance Passing through the exchanger hot fluid releases energy as heat and cold fluid absorbs it. This heat rate given by the fluids is described by Considering adiabatic external surface if you will perform an energy balance you will see that: If the system is not adiabatic it has losses and you will produce the following relation: The efficiency of the system that transfer heat from hot to cold fluid can be defined as: In shell and tube you can have a case of more than 100% efficiency when average cold temperature is lower than ambient temperature

Temperature Efficiency A useful measure of the heat exchanger performance is the temperature efficiency of each fluid stream. This is the temperature change in each fluid stream compared with the maximum temperature difference between to fluids streams.

Log Mean Temperature Difference To design or to predict the performance of a heat exchanger, it is essential to relate the total heat transfer rate to quantities such as the inlet and outlet fluid temperatures, the overall heat transfer coefficient, and the total surface area for heat transfer. The expression that quantifies this heat rate can be written, where dQ is the heat rate thru the differential heat transfer area dA between the hot fluid at Th and the cold fluid at Tc having a global heat transfer coefficient U. dQ=U⋅dA⋅(T −T) hc

Log Mean Temperature Difference The temperature difference is not constant along the length of the heat exchanger and to quantify the effective heat transfer rate, an effective average temperature difference must be considered. This average temperature difference is called the Log Mean Temperature Difference. LMTD, which is defined as expressed by: Is the same equation for co-current and countercurrent flow operation because the temperature measure points are fixed on the exchanger. The total heat transfer is given by:

Global Heat Transfer Coefficient The overall heat transfer coefficient U is: The and were already defined and the A is the total heat transfer and in shell and tube is: Where L =1.0m is the heat transmission length ( 7 tubes of 0.144m) and dm=1/2(d0+di) is the arithmetic mean diameter, d0= 6.35mm and di=5.15mm The steps to perform this characterization is similar to the last; the only differences is that you must keep a constant rotational speed by using the power controller each time you set the flow. The rotational speed will be 50 Hz and an outlet valve V5 setting corresponding to one of the data points of T. 1.7 pump 2 is off.

Equipment and Experimental Set Up Console of HT30X Service Unit: Is an instrument which provides streams of hot water and cold water at variable flowrate to the heat exchanger under evaluation. Shell and Tube Heat Exchanger HT33 This unit is designed to demonstrate liquid to liquid heat transfer. It comprises an outer shell and 7 internal tubes with 2 transverse baffles inside the shell. Four temperature sensors are supplied in tapping at fluid inlets and fluid outlets. he heat exchanger is constructed from stainless steel tube and clear acrylic. It is mounted on a PVC plate which is designed to install on the HT30X unit without need of tools . The steps to perform this characterization is similar to the last; the only differences is that you must keep a constant rotational speed by using the power controller each time you set the flow. The rotational speed will be 50 Hz and an outlet valve V5 setting corresponding to one of the data points of T. 1.7 pump 2 is off.

Equipment and Experimental Set Up Plate Heat Exchanger HT32 This unit has a single heating section configured for multi-pass operation with passes in series. It comprises 7 individual 316-stainless-steel plates, which are clamped together using two-stainless steel threaded bars and nuts. Plates are pressed with a chevron pattern to promote turbulence and provide multiple support points. Even though there are 7 plates in the HT32 unit, there are only 5 plates that effectively work to transfer heat. The total heat transfer area is 0.04 m2 Concentric Tube Heat Exchanger HT31 The tubular heat exchanger is the simplest form of heat exchanger and consists of two concentric (coaxial) tubes carrying the hot and cold fluids. On the heat exchanger the inner tube is used for the hot fluid and the outer annulus for cold fluid. This minimizes heat loss from the exchanger without the need for additional insulation. The inner tubes are constructed from stainless steel and the outer annulus from clear acrylic, providing visualization of the heat exchanger construction and minimizing thermal losses. The heat transfer area is 0.02 m2. The steps to perform this characterization is similar to the last; the only differences is that you must keep a constant rotational speed by using the power controller each time you set the flow. The rotational speed will be 50 Hz and an outlet valve V5 setting corresponding to one of the data points of T. 1.7 pump 2 is off.

Experimental Procedure This procedure must be performance using all heat exchanger available. In this procedure it used the Shell and tube heat exchanger as example. Before proceeding with the experiment, make sure that the Shell and tube heat exchanger HT33 unit has been properly located on the HT30X. Ensure also that the thermocouples are properly connected to the sockets and the water supply connections provide countercurrent operation.

Experimental Procedure Schematic for countercurrent operation of the HT33 shell and tube heat exchanger unit

Experimental Procedure The procedure to perform this task is as follow: Turn On the main switch Set the temperature controller to set point of 500 C, then switch On the hot water circulator. Set the flow indicator switch to Fcold and adjust the cold water control valve Vcold to give approximately 1 liter/min. Then set the flow indicator switch to Fhot and adjust the hot water control valve Vhot (see Figure 3.12) to give approximately 1 liters/min. When the heat exchanger stabilizes(monitor temperatures using the switch meter), record the data for T1 , T2 , T3 , T4 , Fhot , and Fcold in table 3.2 Repeat the above for different setting of hot and cold flowrates as flowrates as follows, and for temperature setting and current flow direction setup

Experimental Procedure Volumentric Flows and Set Point Temperature Volumetric Flows Set Point Temperature (°C) Fhot lt/min Fcold lt/min countercurrent Flow  Co-Current Flow  1 40 50 60 2 --- 2.5 1.5

Experimental Procedure Repeat the whole process for the concentric tube heat exchanger (HT31), and the plate heat exchanger (HT32). For the concentric tube heat exchanger (HT31) and the plate heat exchanger unit (HT32) the pressure drop across the heat exchanger will prevent reaching the high values of flow rate make this task with the combination of flow rates possible. For co-current flow reversing the hot connections from the service unit for shell and tube heat exchanger (HT33) otherwise reversing the cold connections for concentric tube heat exchanger (HT31) and the plate heat exchanger (HT32).

References Thermal Engineering Laboratory Manual Heat and Mass Transfer Fundamentals & Applications, 5th Edition Fundamentals of Heat and Mass Transfer, 7th Edition

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