Pressure drop exchangers

When recycling material to the reactor for whatever reason, the pressure drop through the reactor, phase separator (if there is one), and the heat exchangers upstream and downstream of the reactor must be overcome. This means increasing the pressure of any material to be recycled. 

Polley, G. T., Panjeh Shahi, M. H., and Jegede, F. O., Pressure Drop Considerations in the Retrofit of Heat Exchanger Networks, Trans. IChemE, part A, 68 211, 1990. 

Hydrate formation is possible only at temperatures less than 35°C when the pressure is less than 100 bar. Hydrates are a nuisance they are capable of plugging (partially or totally) equipment in transport systems such as pipelines, filters, and valves they can accumulate in heat exchangers and reduce heat transfer as well as increase pressure drop. Finally, if deposited in rotating machinery, they can lead to rotor imbalance generating vibration and causing failure of the machine. 

The measurement of a crude oil s viscosity at different temperatures is particularly important for the calculation of pressure drop in pipelines and refinery piping systems, as well as for the specification of pumps and exchangers. 

Friction Coefficient. In the design of a heat exchanger, the pumping requirement is an important consideration. For a fully developed laminar flow, the pressure drop inside a tube is inversely proportional to the fourth power of the inside tube diameter. For a turbulent flow, the pressure drop is inversely proportional to D where n Hes between 4.8 and 5. In general, the internal tube diameter, plays the most important role in the deterrnination of the pumping requirement. It can be calculated using the Darcy friction coefficient,, defined as... 

A )m sc represents miscellaneous pressure drops associated with valves, fittings, etc, in the flow circuit, and these devices are not usually present inside a heat exchanger. 

Frictiona.1 Pressure Drop. The frictional pressure drop inside a heat exchanger results when fluid particles move at different velocities because of the presence of stmctural walls such as tubes, shell, channels, etc. It is calculated from a weU-known expression of... 

Eurther research on convective transport under low Reynolds number, quasicontinuum conditions is needed before the optimal design of such a micro heat exchanger is possible. The cooling heat exchanger is usually thermally linked to a relatively massive substrate. The effects of this linkage need to be explored and accurate methods of predicting the heat-transfer and pressure-drop performance need to be developed. 

Flow Maldistribution. One of the principal reasons for heat exchangers failing to achieve the expected thermal performance is that the fluid flow does not foUow the idealized anticipated paths from elementary considerations. This is referred as a flow maldistribution problem. As much as 50% of the fluid can behave differently from what is expected based on a simplistic model (18), resulting in a significant reduction in heat-transfer performance, especially at high or a significant increase in pressure drop. Flow maldistribution is the main culprit for reduced performance of many heat exchangers. 

Examples of pressure drop variation for new resin as a function of flow rate and water temperature are shown in Eigure 5 for a standard styrenic strong acid cation exchanger. The lower pressure drop at the higher temperature is a reflection of water viscosity. 

Eig. 5. Pressure drop as affected by resin type, flow rate, and temperature, where A, B, and C, correspond respectively to acryUc strong base anion exchanger (Amberlite IRA-458), styrenic strong base anion exchanger (Amberlite IRA-402), and styrenic strong acid cation exchanger (Amberlite IR-120), all at 4°C. D represents styrenic strong acid cation resin (Amberlite IR-120) at 50°C (14). To convert kg/(cm -m) to lb/(in. -ft), multiply by 4.33 to convert... 

Heat-exchange pressure drop Optimum pressure drop... 

Fig. 1. Effect of energy use on total cost where total cost is the sum of capital and energy costs for the lifetime of the plant, discounted to present value. Point D corresponds to the design point if the designer uses an energy price that is low by a factor of four in projected energy price. Effects on costs of (a) pressure drop in piping, (b) pressure drop in exchangers, (c) heat loss through insulation, (d) reflux use, and (e) energy recovery through waste-heat boiler...

Heat exchangers use energy two ways as frictional pressure drop, and as the loss in ability to do work when heat is degraded. 

Optimum Pressure Drop. For most heat exchangers there is an optimum pressure drop. This results from the balance of capital costs against the pumping (or compression) costs. A common prejudice is that the power costs are trivial compared to the capital costs. The total cost curve is fairly flat within 50% of the optimum (see Fig. lb), but the incremental costs of power are roughly one third of those for capital on an aimualized basis. This simple relationship can be extremely useful in quick design checks. 

This term is a measure of the unit s length. Sometimes it is referred to as the number of transfer units. This simply says that the optimum pressure drop increases as the heat exchanger gets longer, ie, has more transfer units. The forms of F, and F both foUow from the fact that in turbulent flow the... 

---