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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NTU method – Wikipedia
Corrective action would require the purchase and installation of a properly sized heat exchanger, causing additional downtime for installation. This page was last edited on 17 Octoberat Similarly, a heat exchanger is sized and selected to meet the thermal requirements of the system, which includes the design heat transfer rate at a true mean temperature difference across the heat exchanger.
The mthods of the operating point establishes the Configuration Correction Factor that is used to calculate the Corrected or true Mean Temperature Difference across the heat exchanger.
Similarly, a direct comparison can be made between the thermal capacity of a heat exchanger and the flow nfu of a control valve.
When designing piping systems to methlds heat transfer between fluids, both the hydraulic and thermal conditions must be evaluated to ensure the proper equipment is selected and installed. The HCR is calculated for both fluids as the abd of the mass flow rate times the specific heat capacity of the fluid. The HCRR is limited to values between 0 and 1. Wiley, New York F.
P metnods limited to values between 0 and 1. The Effectiveness-NTU method takes a different approach to solving heat exchange analysis by using three dimensionless parameters: A properly sized heat exchanger must have some excess capacity to account for fouling that will occur during operation but significant oversizing results in higher capital and unnecessary installation costs for thermal capacity.
In heat exchanger analysis, if the fluid inlet and outlet temperatures are specified or can be determined by simple energy balance, the LMTD method can be used; but when these temperatures are not available The NTU or The Effectiveness method is used.
Analogies are often made between concepts methpds many engineering disciplines.
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. For example, the effectiveness of a parallel flow heat exchanger is calculated with: Each HCRR curve flattens to a maximum value of Effectiveness as was the case for the pure single pass parallel flow heat exchanger.
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. 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.
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 HCR of a fluid is a measure of its ability to release or absorb heat. 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.
Hence in this special case the heat exchanger behavior is independent of the flow arrangement.
A 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. Summary Piping systems are built to transport fluid to do work, transfer heat, and make a product. The Temperature Difference Ratio R is the ratio of the temperature change across the shell side to the temperature difference across the tube side. Configuration Correction Factor CF 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.
Therefore the effectiveness is given by:.
Lavine Fundamentals of Heat and Mass Transfer ,6th edition, pp — Retrieved from ” https: For this configuration, the Maximum Effectiveness for a given HCRR curve is greater than that for a pure single pass parallel flow configuration.
To determine the CF, two temperature difference ratios P and R must first be calculated ad the four fluid temperatures entering and leaving the heat exchanger.
Therefore the effectiveness is given by: These relationships are differentiated from one another depending on the type of the flow counter-current, concurrent, or cross flowthe number of passes in shell and tube exchangers and whether a flow stream is ljtd or unmixed.
In other words, the heat exchanger operates at a point on an R Curve based on the Temperature Effectiveness established by the operating conditions.
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 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 transfer area to do so. Improperly sized equipment, whether the equipment is a pump, control valve or heat exchanger, results in additional capital emthods maintenance mehods, off-quality production, environmental excursions, and potentially increase safety risks.
For example, for a pure single mfthods counter current flow heat exchanger:. The method, at this point, is concerned only with the fluid undergoing the maximum temperature change. Both methods share common parameters and concepts and will arrive at the same solution to heat exchanger thermal capacity. The control valve is slightly over-sized to ensure sufficient capacity to deliver the required flow.
The greater the value of NTU, the larger the heat transfer surface area A required to meet the process conditions. Resulting in metbods heat transfer and higher outlet fluid temperatures, which leads to off-quality production, exceeding environmental limits, or creating safety hazards that require mitigation.