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.
|Published (Last):||6 April 2017|
|PDF File Size:||19.80 Mb|
|ePub File Size:||17.96 Mb|
|Price:||Free* [*Free Regsitration Required]|
The greater the value of NTU, the mrthods the heat transfer surface area A required to meet the process conditions. 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.
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. 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.
The Temperature Difference Ratio R is the ratio of the temperature change across the shell side to the temperature difference across the tube side. A control valve is sized and selected to meet the hydraulic requirements of the piping system, metnods includes the design flow rate and pressure drop across the valve.
The HCRR is limited to values between 0 and 1. 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. Fundamentals of Momentum, Heat and Mass Transfer. 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 equation to calculate the heat transfer rate is given by: Corrective action would require the purchase and installation of a properly sized heat exchanger, causing additional downtime for installation.
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. 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: 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. 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. 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 method proceeds by calculating the heat capacity rates i. 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.
Retrieved from ” https: Engineered Software Knowledge Base.
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 equation to calculate the heat transfer rate is given by:. The heat exchanger will operate at this thermal capacity as long as it has sufficient heat transfer area at these operating conditions, including a factor for fouling. Mdthods methods share common parameters and concepts and will arrive at the same solution to heat exchanger thermal capacity.
Wiley, New York F. Engineering Analogies Analogies are often made between concepts in many engineering disciplines. Effectiveness is dimensionless quantity between 0 and 1. The HCR of a fluid is a measure of its ability to metbods or absorb heat. The division often results in misunderstanding, miscommunication, and mistakes when integrating the work of the various groups. The thermal capacity of a heat exchanger is its ability to transfer heat between two fluids at different temperatures.
After calculating P and R, CF is then determined graphically using the location of the P value on the appropriate R curve.
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. Pages … Engineered Software, Inc.
The Effectiveness-NTU method takes a different approach to solving heat exchange analysis by using three dimensionless parameters: Hence in this special case the nty exchanger behavior is independent of the flow arrangement. The method, at this point, is concerned only with the fluid undergoing the maximum temperature change. 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 methoda exchanger.
NTU method – Wikipedia
Each HCRR curve flattens to a maximum value of Effectiveness as was the case for the pure single pass parallel flow heat exchanger. Summary Piping systems are built to transport fluid to do work, transfer heat, and make a product.
For example, the effectiveness of a parallel flow heat exchanger is calculated with: If the selected heat lmttd is undersized, the design heat transfer conditions will not be achieved.
Lavine Fundamentals of Heat and Mass Transfer ,6th edition, pp — The HCR is calculated for both fluids as the product of the mass flow rate times the specific heat capacity of the fluid.