E-NTU Method (Effectiveness – N TU method). Note, in most heat exchanger design problems, we don’t. know the fluid outlet temperatures, ie. Tiour or Tribut. TA. Summary of lmtd and e ntu. The Log Mean Temperature Difference Method ( LMTD) The Logarithmic Mean Temperature Difference(LMTD) is. Q: What is the real difference between the LMTD (logarithmic mean temperature difference) and NTU (number of transfer units) methods for analyzing heat.
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As with any engineering problem, there are various ways to approach a solution when sizing and selecting a heat exchanger or analyzing its thermal performance. If the selected heat exchanger is undersized, the design heat transfer conditions will not be achieved.
Resulting in less heat transfer and higher outlet fluid temperatures, which leads to off-quality production, exceeding environmental mrthods, or creating safety hazards that require mitigation.
Corrective action would require the purchase and installation of a properly sized heat exchanger, causing additional downtime for installation.
A properly sized heat exchanger must have some excess capacity to account for fouling that lmd occur during operation but significant oversizing results in higher capital and unnecessary installation costs for thermal capacity.
The thermal capacity of nu heat exchanger is its ability to transfer heat between two fluids at different temperatures. It is a function of the heat exchanger design and the fluid properties on both sides. The thermal capacity of the heat exchanger will match the thermal capacity required by the process conditions temperatures and flow rates if it has sufficient heat methors area to do so.
Both methods share common parameters and concepts and will arrive at the same solution to heat exchanger thermal capacity. To understand the difference between these two methods, we need to understand the key terminology and the equations used in each solution method. The equation to calculate the heat transfer rate is given by:. The Configuration Correction Factor CF accounts for the deviation of the internal flow pattern of the actual heat exchanger from that of a single pass counter current flow pattern.
Some manufacturers provide a CF data table for their heat exchanger while others determine CF using a standard graph from the Tubular Exchanger Manufacturers Association TEMA for the actual heat exchanger configuration.
To determine the CF, two temperature difference ratios P and R metjods first be calculated from the four fluid temperatures entering and leaving the heat exchanger. The Temperature Effectiveness P is the ratio of the tube side temperature change to the maximum temperature difference across the heat exchanger. The Temperature Difference Ratio Ntk is the ratio of the temperature change across the shell side to the temperature difference across the tube side.
P is limited to values lmtx 0 and 1. After calculating P and R, CF is then determined graphically using the location of the P value on aand appropriate R curve. In other words, anv heat exchanger operates at a point on an R Curve based on the Temperature Effectiveness established by the operating conditions.
The location of the operating point establishes the Configuration Correction Factor that metgods used to calculate the Corrected or true Mean Temperature Difference across the heat exchanger. The required thermal capacity UA needed to achieve the heat transfer rate established by the temperatures and flow rates is calculated from the Heat Transfer Rate and the Corrected Mean Temperature Difference. The heat exchanger will operate at this thermal capacity as long as it has pmtd heat transfer area methovs these operating conditions, including a factor for fouling.
The Effectiveness-NTU method takes a different approach to solving heat exchange analysis by using three dimensionless parameters: The relationship between these three parameters depends on the type of heat exchanger and the internal flow pattern. The HCR of a fluid is a measure of its ability to release or absorb heat. The HCR is calculated for both fluids as the product of the mass flow rate times the specific heat capacity of the fluid.
The HCRR is limited to values between 0 and 1. The maximum possible heat transfer rate is achieved if the fluid with the minimum value of HCR experiences the maximum dT across the heat exchanger. The NTU is a function of the Effectiveness and HCRR established by the process temperatures and flow rates and is indicative of the size of the heat exchanger needed.
The greater the value of NTU, the larger the heat transfer surface area A required to meet the process conditions. The thermal capacity UA required to achieve the heat transfer rate is determined by re-arranging the NTU equation after determining the value of NTU for the particular heat exchanger configuration.
NTU method – Wikipedia
Similar to the LMTD method, the heat exchanger will operate at this thermal capacity as long as it has sufficient heat transfer area at these operating conditions, taking into account the fouling factor. For example, for abd pure single pass counter current flow heat exchanger:. Equations for NTU vary by heat exchanger configuration, but the mathematical relationship for some types of heat exchangers is not readily available or easily derived.
Each HCRR curve flattens to a maximum value of Effectiveness as was the case for the pure single pass parallel flow heat exchanger.
For this configuration, the Maximum Effectiveness for a given HCRR curve is greater than that for a pure single pass parallel flow configuration.
Analogies are often metjods between concepts in many engineering disciplines. Voltage drop, current, and electrical resistance are analogous to pressure drop, fluid flow, and hydraulic resistance, which are analogous to the temperature difference, heat transfer rate, and thermal resistance.
Similarly, a direct comparison can be made between the thermal capacity of a heat exchanger and the flow capacity nfu a control valve. Methhods control valve is sized and selected to meet the hydraulic requirements of the piping system, which includes the design flow rate and pressure drop across the valve. The control valve is slightly over-sized to ensure sufficient capacity to deliver the required flow. Similarly, a heat exchanger is sized and selected to meet the thermal requirements of the system, which includes the design heat transfer rate at lmgd true nttu temperature difference across the heat exchanger.
Piping systems are built to transport fluid to do work, transfer heat, and make a product. When designing piping systems to support heat transfer between fluids, both the hydraulic and thermal conditions must be evaluated to ensure the proper equipment is selected and installed.
Evaluating both the hydraulic and thermal conditions of a system can be a daunting task for any engineer and is often divided into different groups who specialize in a specific field. The division metohds results in misunderstanding, miscommunication, and mistakes when integrating the work of the various groups. Improperly nad equipment, whether the equipment is a pump, control valve or heat exchanger, results in additional capital and maintenance costs, off-quality production, environmental excursions, and potentially increase safety risks.
Fundamentals of Momentum, Heat and Mass Transfer. Engineered Software Knowledge Base.
Resolved comments Export to PDF. Pages … Engineered Software, Inc. Created by Jeff Sineslast modified on Jun 29, The equation to calculate the heat transfer rate is given by: Configuration Correction Factor CF The Configuration Correction Factor CF accounts for omtd deviation of the internal flow pattern of the actual heat exchanger from that of a single pass counter current flow pattern.
Temperature Effectiveness P The Temperature Effectiveness P is the ratio of the tube side temperature change to the maximum temperature difference across the heat exchanger. For example, for a pure single pass counter current flow heat exchanger: Engineering Analogies Analogies are often made between concepts in many engineering disciplines. Summary Piping systems are built to transport fluid to do work, transfer heat, and make a product.
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