Heat Exchanger Installation Method Statement
Overall Heat Transfer CoefficientA heat exchanger typically involves two flowing fluids separated by a solid wall. Many of the heat transfer processes encountered in industry involve composite systems and even involve a combination of both. Heat is first transferred from the hot fluid to the wall by convection, through the wall by conduction, and from the wall to the cold fluid again by convection.With these composite systems, it is often convenient to work with an, known as a U-factor. The U-factor is defined by an expression analogous to Newton’s law of cooling. The overall heat transfer coefficient, U, is related to the and depends on the geometry of the problem. For example, in a involves convection from the bulk of the reactor coolant to the steam generator inner tube surface, conduction through the tube wall, and convection (boiling) from the outer tube surface to the secondary side fluid.In cases of combined heat transfer for a heat exchanger, there are two values for h. There is the convective heat transfer coefficient (h) for the fluid film inside the tubes and a convective heat transfer coefficient for the fluid film outside the tubes.
The (k) and thickness (Δx) of the tube wall must also be accounted for. Online monitoring of commercial heat exchangers is done by tracking the overall heat transfer coefficient, because the overall heat transfer coefficient tends to decline over time due to fouling.
Fouling is the accumulation of unwanted material on solid surfaces to the detriment of function. The fouling materials can consist of either living organisms or a non-living substance (minerals or organic compounds). The layer of deposits represents additional resistance to heat transfer and causes the rate of heat transfer in a heat exchanger to decrease. As a result, fouling in heat exchangers reduces thermal efficiency, decreases heat flux, increases temperature on the hot side, decreases temperature on the cold side, induces under-deposit corrosion, increases use of cooling water.
The net effect of these accumulations on heat transfer is represented by a fouling factor, Rf, which is a measure of the overall thermal resistance introduced by fouling. Logarithmic Mean Temperature Difference – LMTDIn order to solve certain heat exchanger problems, engineers often use a log mean temperature difference (LMTD), which is used to determine the temperature driving force for heat transfer in heat exchangers. LMTD is introduced due to the fact, the temperature change that takes place across the heat exchanger from the entrance to the exit is not linear.The heat transfer through the wall of heat exchanger at a given location is given by the following equation:Here the value of overall heat transfer coefficient can be assumed as a constant. On the other hand the temperature difference continuously varies with location (especially in counter-flow arrangement). In order to determine the total heat flow, either the heat flow should be summed up using elemental areas and the temperature difference at the location or more conveniently engineers can average the value of temperature difference. The heat exchanger equation can be solved much easier if we could define a “Mean Temperature Difference” (MTD). It can be seen from the figure that the temperature difference varies along the flow and the arithmetic average may not be the real average, therefore engineers use the logarithmic mean temperature difference.
The “ Logarithmic Mean Temperature Difference“ (LMTD) is a logarithmic average of the temperature difference between the hot and cold feeds at each end of the heat exchanger. The larger the LMTD, the more heat is transferred. LMTD – Condensers and Boilers Temperature gradients in typical PWR steam generator.and are also examples of components found in nuclear facilities where the concept of LMTD is needed to address certain problems. When the subcooled water enters the steam generator, it must be heated up to its boiling point and then it must be evaporated.
Because evaporation is taking place at constant temperature, it cannot be used a single LMTD. In this case the heat exchanger has to be treated as a combination of two or three (when superheat occurs) heat exchangers. NTU Effectiveness MethodThe log mean temperature difference (LMTD) method discussed in previous section is easy to use in heat exchanger analysis when the inlet and the outlet temperatures of the hot and cold fluids are known or can be determined from an energy balance.
Therefore, the LMTD method is very suitable for determining the size and performance of a heat exchanger.When direct knowledge of the LMTD is not available and the NTU method ( Number of Transfer Units method) can be used. This method is based on a dimensionless parameter called the heat transfer effectiveness, defined as:As can be seen, the effectiveness is the ratio between the actual heat transfer rate and the maximum possible heat transfer rate. To define the effectiveness of a heat exchanger, we must first determine the maximum possible heat transfer rate, q max, for the heat exchanger.Further reading. Example: Calculation of Heat ExchangerConsider a parallel-flow heat exchanger, which is used to cool oil from 70°C to 40°C using water available at 30°C. The outlet temperature of the water is 36°C.
The rate of flow of oil is 1 kg/s. The specific heat of the oil is 2.2 kJ/kg K.
The overall heat transfer coefficient U = 200 W/m 2 K.Calculate the logarithmic mean temperature difference. Determine the area of this heat exchanger required for this performance. LMTDThe logarithmic mean temperature difference can be calculated simply using its definition:. Area of Heat ExchangerTo calculated the area of this heat exchanger, we have to calculate the heat flow rate using mass flow rate of oil and LMTD.The required area of this heat exchanger can be then directly calculated using general heat transfer equation. Heat Transfer:. Fundamentals of Heat and Mass Transfer, 7th Edition. Bergman, Adrienne S.
Lavine, Frank P. John Wiley & Sons, Incorporated, 2011.
ISBN: 253. Heat and Mass Transfer.
Heat Exchanger Installation Method Statement Pdf
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Flat Plate Heat Exchanger Installation
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