LMTD DERIVATION PDF

The water leaves the heat exchanger at 60 oC. The saturation temperature of steam at 2 bar gauge is oC. The temperature on the heat exchangers surface on the steam side is constant and determined by the steam pressure. Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro. Only emails and answers are saved in our archive.

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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 limits, 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 will occur during operation but significant oversizing results in higher capital and unnecessary installation costs for thermal capacity.

The thermal capacity of a 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 transfer 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.

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 must first be calculated from the four fluid temperatures entering and leaving the heat exchanger. 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.

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 location 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.

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 greater the value of NTU, the larger the heat transfer surface area A required to meet the process conditions.

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.

Engineering Analogies Analogies are often made 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 of a control valve. 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 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 a true mean temperature difference across the heat exchanger.

Summary 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 often results in misunderstanding, miscommunication, and mistakes when integrating the work of the various groups.

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.

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