Pressure drop heat exchanger example

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Although reduced heat transfer efficiency is of prime importance there may also be pressure drop problems. The presence of the foulant will restrict flow that results in increased pressure drop. In severe examples of fouling the exchanger may become inoperable because of the back pressure. Indeed the pressure drop problems may have a more pronounced effect than the loss of thermal efficiency. 

The shape of the coohng and warming curves in coiled-tube heat exchangers is affected by the pressure drop in both the tube and shell-sides of the heat exchanger. This is particularly important for two-phase flows of multicomponent systems. For example, an increase in pressure drop on the shellside causes boiling to occur at a higher temperature, while an increase in pressure drop on the tubeside will cause condensation to occur at a lower temperature. The net result is both a decrease in the effective temperature difference between the two streams and a requirement for additional heat transfer area to compensate for these losses. 

Example A ventilation system (Fig. 9.64) handling 20 mVs of air needs to heat the supply air from 10 °C to 20 C. Doubling the number of heat exchangers from one to two increases the heat-recovery efficiency from 50% to 75% and introduces an extra pressure drop of 300 Pa. As we can see from Table 9.19, this is probably a cost-efficient measure. 

The plate heat exchanger, for example, can be used in laminar flow duties, for the evaporation of fluids with relatively high viscosities, for cooling various gases, and for condensing applications where pressure-drop parameters are not excessively restrictive. 

Using Bell s method, calculate the shell-side heat transfer coefficient and pressure drop for the exchanger designed in Example 12.1. 

This method uses simple, unsophisticated, methods to estimate the two-phase pressure drop through the exchanger and piping, and the convective boiling heat transfer coefficient. The calculation procedure is set out below and illustrated in Example 12.11... 

The tube-in-tube or multitube-in-tube heat exchangers are useful in small Linde lique-fiers or in the final Joule-Thomson stage of any liquefier. The performance of Linde-type exchangers is easy to calculate, and their realization is simple. In the examples shown in Fig. 5.12 (a)-(c), the tubes are concentric and the outer wall contributes appreciably to the pressure drop in the outer stream without contributing to the heat transfer. Usually, the smaller inner tube is used for the high-pressure stream and the low-pressure stream flows through the outer annular space. The tubes in Fig. 5.12 (d) and (e) are solder bonded while that in (f) is flattened and twisted before insertion into an outer tube. 

In some situations it is very important to be able to increase the flow rate above the design conditions (for example, the cooling water to an exothermic reactor may have to be doubled or tripled to handle dynamic upsets). In other cases this is not as important (for example, the feed flow rate to a unit). Therefore it is logical to base the design of the control valve and the pump on having a process that can attain both the maximum and the minimum flow conditions. The design flow conditions are only used to get the pressure drop over the heat exchanger (or fixed resistance part of the process). 

The basic reason for using different control-valve trims is to keep the stability of the control loop fairly constant over a wide range of flows. Linear-trim valves are used, for example, when the pressure drop over the control valve is fairly constant and a linear relationship exists between the controlled variable and the flow rate of the manipulated variable. Consider the flow of steam from a constant-pressure supply header. The steam flows into the shell side of a heat exchanger. A process liquid stream flows through the tube side and is heated by the steam. There is a linear relationship between the process outlet temperature and steam flow (with constant process flow rate and inlet temperature) since every pound of steam provides a certain amount of heat. 

The chemical process gives the enthalpy of reaction, the flow rate, the reaction time, and the required reaction temperature. The first step in the sizing procedure is to calculate the required number of channels for the heat exchanger. Then the pass arrangement is selected in order to achieve the highest possible Reynolds number within an acceptable pressure drop. For example, if the total number of channels is fixed by the residence time channels in series will induce high velocities and high pressure drop channels in parallel will induce low velocities and low pressure drop. The second step is to estimate the heat transfer coefficient and to check that the heat flux can effectively be controlled by the secondary fluid (the lower heat transfer coefficient should be on the reaction side). 

To give an example of an internally numbered-up device and to illustrate the concept, a new I MM development is shown in Figure 4.92. A novel micro-flow heat exchanger has 6685 parallel micro channels of 250 pm depth, 2 mm width and 240 mm length, giving a gas throughput in the range of about 1 m3 min 1 at a pressure drop of about 100 mbar. 

Examples of the use of models for the design of large-scale systems include the measurement of pressure drop and heat transfer in model heat exchangers, the mixing and rate of reaction in a bench-top batch reactor and the prediction of pressure drops in pipelines. 

Calculate the pressure drop for the water flowing through the air-cooled heat exchanger designed in Example 7.37 if the number of tube-side passes is 10. The density of the water is 60 lb/ft3 (961.1 kg/ m3), and the viscosity is 0.74 lb/(ft)(h) (0.31 cP). Assume that the velocity in the nozzles is 10 ft/s (3.05 m/s) and that the viscosity change with temperature is negligible. 

We are rarely able to extract much work from chemical reactors. WTe mostly take out energy from the reactor in the form of heat. Also, when we let a hot stream heat up a cold stream in a heat exchanger we lose some of the work potential of the hot stream. The same is true for mixing and material flow across a pressure drop we take advantage of the spontaneity of the process and make no attempt to recover work from it. In contrast, we often want to perform operations that are the reverse to the spontaneous direction. This always requires work. For example, separation is the opposite of mixing. The work demand of separating an ideal mixture of n components into pure products at constant temperature T is... 

Today contractors and licensors use sophisticated computerized mathematical models which take into account the many variables involved in the physical, chemical, geometrical and mechanical properties of the system. ICI, for example, was one of the first to develop a very versatile and effective model of the primary reformer. The program REFORM [361], [430], [439] can simulate all major types of reformers (see below) top-fired, side-fired, terraced-wall, concentric round configurations, the exchanger reformers (GHR, for example), and so on. The program is based on reaction kinetics, correlations with experimental heat transfer data, pressure drop functions, advanced furnace calculation methods, and a kinetic model of carbon formation [419],... 

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