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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Pages … Engineered Software, Inc. As with any engineering problem, there are various ways to approach a solution when sizing htu selecting a heat exchanger or analyzing its thermal performance. From Wikipedia, the free encyclopedia. Piping systems are built to transport fluid to do work, transfer heat, and make a product.
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. 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.
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For example, the effectiveness of a parallel flow heat exchanger is calculated with: Hence in this special case the heat exchanger behavior is independent of the flow arrangement. Voltage drop, current, and electrical resistance are analogous to pressure drop, fluid flow, and hydraulic resistance, which are analogous to the temperature difference, heat mtehods rate, and thermal resistance.
Wiley, New York F. 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. For this configuration, the Maximum Effectiveness for a given HCRR curve is greater than that for a pure single pass parallel flow configuration.
The HCRR is limited to values between 0 and 1. 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. Similar effectiveness relationships can be derived for concentric tube heat exchangers and shell and tube heat exchangers. This page was last edited on 17 Octoberat 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.
Effectiveness is dimensionless quantity between 0 and 1. If the selected heat exchanger is undersized, the design heat transfer conditions snd not be achieved.
NTU method – Wikipedia
Resolved comments Export to PDF. 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. Equations for NTU vary by heat exchanger configuration, but the mathematical relationship methors some types of heat exchangers is not readily available or easily derived.
These relationships are differentiated from one another depending on the type of the flow counter-current, concurrent, or cross flow pmtd, the number of passes in shell and tube exchangers and whether a flow stream is mixed or unmixed.
Similarly, a direct comparison can be made between the thermal capacity of a heat exchanger and the flow capacity of a control valve. The other fluid would change temperature more quickly along the heat exchanger length. The relationship mwthods these three parameters depends on the type of heat exchanger and the internal flow pattern.
Lavine Fundamentals of Heat and Mass Transfer ,6th edition, pp — To understand the difference between these two methods, we need to understand the key terminology and the equations used in each solution method. 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. After calculating P and R, CF is then determined graphically using the location of the P value on the appropriate R curve.
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.
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 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. The thermal capacity of a heat exchanger is its ability to transfer heat between two fluids at different temperatures.
The HCR is calculated for both fluids as the product of the mass flow rate times the specific heat anr of the fluid.
For example, for a pure single pass counter current flow heat exchanger: The division often results in misunderstanding, miscommunication, and mistakes when integrating the work of the various groups.
Summary Piping systems are built to transport fluid to do work, transfer heat, and make a product. Improperly sized 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. 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.
The greater the value of NTU, the larger the heat transfer surface area A required to meet the process conditions. Resulting in less heat transfer and higher outlet fluid temperatures, which leads to off-quality production, exceeding environmental limits, or creating safety hazards that require mitigation.
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Created by Jeff Sineslast modified on Jun 29, 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. Fundamentals of Momentum, Heat and Mass Transfer. The method proceeds by calculating the heat capacity rates i. To define the effectiveness of a heat exchanger we need to find the maximum possible heat transfer that can be hypothetically achieved in a counter-flow heat exchanger of infinite length.
The equation to calculate the heat transfer rate is given by:.
The Temperature Effectiveness P is the ratio of the tube side temperature change to the maximum temperature difference across the heat exchanger. Anv action would require the purchase and installation of a properly sized heat exchanger, causing additional downtime for installation. The control valve is slightly over-sized to ensure sufficient capacity to deliver the required flow.
Both methods share common parameters and concepts and will arrive at the same solution methoods heat exchanger thermal capacity.
Engineering Analogies Analogies are often made between concepts in many engineering disciplines. Therefore the effectiveness is given by: