Pressure drop in condensers

The pressure drop on the condensing side is difficult to predict as two phases are present and the vapour mass velocity is changing throughout the condenser. 

A common practice is to calculate the pressure drop using the methods for single-phase flow and apply a factor to allow for the change in vapour velocity. For total condensation, Frank (1978) suggests taking the pressure drop as 40 per cent of the value based on the inlet vapour conditions Kern (1950) suggests a factor of 50 per cent. 

An alternative method, which can also be used to estimate the pressure drop in a partial condenser, is given by Gloyer (1970). The pressure drop is calculated using an average vapour flow-rate in the shell (or tubes) estimated as a function of the ratio of the vapour flow-rate in and out of the shell (or tubes), and the temperature profile. 

These methods can be used to make a crude estimate of the likely pressure drop. A reliable prediction can be obtained by treating the problem as one of two-phase flow. For tube-side condensation the general methods for two-phase flow in pipes can be used see Collier and Thome (1994) and Volume 1, Chapter 5. As the flow pattern will be changing throughout condensation, some form of step-wise procedure will need to be used. Two-phase flow on the shell-side is discussed by Grant (1973), who gives a method for predicting the pressure drop based on Tinker s shell-side flow model. 

A method for estimating the pressure drop on the shell-side of horizontal condensers is given in the Engineering Sciences Data Unit Design Guide, ESDU 84023 (1985). 

---

Shell Side Pressure Drop in Condensers Kem recommends Equation 10-228 as being conservative ... 

Shin and Kim [8] obtained pressure drop measurements for condensation flow of R134a in a microtube having a hydraulic diameter of 691 xm. Figure 8 shows a comparison of their pressure drop measurements with available correlation equations for macrochannels. Figure 8a shows that the pressure drop in condensation flow at low mass flux (G < 200 kg/m s) in a microchannel is lower than that predicted by Friedel s correlation for macrotubes [9]. At higher mass fluxes (G = 400kg/m s, for example), however, their pressure drop measurements for condensation flow in a microchannel can be predicted well by Friedel s correlation, as shown in Fig. 8b. 

Fig. 3. Solvent-processing equipment using partial condenser. Level a on the water overflow line to the receiver should be about 3 cm below level b on the solvent-return line. Dimension b—c must be great enough to overcome pressure drop in the vapor piping, condenser, solvent piping, and rotameter. In a 4 m (1000-gaI) ketde, dimension b—c would be at least 1.25 m. The volume of the piping described by the dimension c—d—e should contain twice the volume of dimension b—c, thus providing an adequate Hquid seal against normal ketde operating pressures.

Pressure drop during condensation inside horizontal tubes can be computed by using the correlations for two-phase flow given in Sec. 6 and neglec ting the pressure recoveiy due to deceleration of the flow. 

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. 

The pressure drop of condensing steam is therefore a function of steam flow rate, pressure and temperature difference. Since the steam pressure drop affects the saturation temperature of the steam, the mean temperature difference, in turn, becomes a function of steam pressure drop. This is particularly important when vacuum steam is being used, since small changes in steam pressure can give significant alterations in the temperature at which the steam condenses. 

Sometimes insufficient differential across the regenerated catalyst slide valve is not due to inadequate pressure buildup upstream of the valve, but rather due to an increase in pressure downstream of the slide valve. Possible causes of this increased backpressure are an excessive pressure drop in the Y or J-bend section, riser, reactor cyclones, reactor overhead vapor line, main fractionator, and/or the main fractionator overhead condensing/cooling system. 

Calculate the pressure drop in, and the power required to operate, a condenser consisting of 400 tubes 4.5 m long and 10 mm internal diameter. The coefficient of contraction at the entrance of the nibes is 0.6. and 0.04 mJ/s of water is to be pumped through the condenser. 

Kawahara A, Chung PM, Kawaji M (2002) Investigation of two-phase flow pattern, void fraction and pressure drop in a micro-channel. Int J Multiphase Plow 28 1411-1435 Kawaji M (1999) Fluid mechanics aspects of two-phase flow Flow in other geometries. In Kand-likar SG, Shoji M, Dhir VK (eds) Handbook of phase change boiling and condensation. Taylor and Francis, Washington, DC, pp 205-259... 

Pressure drop during condensation results essentially from the vapor flow. As condensation proceeds, the vapor flowrate decreases. The equations described previously for pressure drop in shell-and-tube heat exchangers are only applicable under constant flow conditions. Again the exchanger can be divided into zones. However, in preliminary design, a reasonable estimate of the pressure drop can usually be obtained by basing the calculation on the mean of the inlet and outlet vapor flowrates. 

The output of such high-speed dryers is limited by the increasing density of the water vapor flow. The grains of the product are floating in the vapor stream as in a fluidized bed, and the smallest particles are carried along with the vapor to the condenser. Even if only 1 % of the dried product is carried away, it sumps up to 10 kg per day if the throughput is 1000 kg per day. In 4 weeks, this totals to 280 kg or 1 m3 of coffee powder. To remove this out from the vapor stream very large filters have to be used in order to minimize the pressure drop in the filters. 

In the above expression, the first term represents the accumulation and convective transport of enthalpy, where is the heat capacity of phase k. The second term is energy due to reversible work. For condensed phases this term is negligible, and an order-of-magnitude analysis for ideal gases with the expected pressure drop in a fuel cell demonstrates that this term is negligible compared to the others therefore, it is ignored in all of the models. 

For actual Rankine cycles, many irreversibilities are present in various components. Fluid friction causes pressure drops in the boiler and condenser. These drops in the boiler and condenser are usually small. The major irreversibilities occur within the turbine and pump. To account for these irreversibility effects, turbine efficiency and pump efficiency must be used in computing the actual work produced or consumed. The T-s diagram of the actual Rankine cycle is shown in Fig. 2.9. The effect of irreversibilities on the thermal efficiency of a Rankine cycle is illustrated in the following example. 

These porous structures may hinder the transport of solutes away from the membrane downstream surface, causing a local increase of the solute partial pressure and hence a decrease of the driving force (19.1). Eventually, solute condensation may occur if the solutes local partial pressure surmounts its saturation vapour pressure. This problem becomes particularly relevant when dealing with high-boiling aroma compounds [14] and when pressure drop in the downstream circuit increases owing to poor module design. 

Water is condensed out of the stripped gas at 100°F. After compression to 50 psig, that gas is combined with a recycle stream. The mixture is diluted with an equal volume of steam and charged to a reactor where pyrolysis of the propane occurs at a temperature of 1300°F. For present purposes the reaction may be assumed to be simply C3H8— C2H4 + CH4 with a specific rate k = 0.28/sec. Conversion of propane is 60%. Pressure drop in the reactor is 20psi. 

Although the rate of heat transfer to or from fluids is improved by increase of linear velocity, such improvements are limited by the economic balance between value of equipment saving and cost of pumping. A practical rule is that pressure drop in vacuum condensers be limited to 0.5-1. Opsi (25-50 Ton) or less, depending on the required upstream process pressure. In liquid service, pressure drops of 5-10 psi are employed as a minimum, and up to 15% or so of the upstream pressure. 

However, droplet systems can enable much higher energy input (via gas phase pressure drop in cocurrent systems) and, as a result, dominate applications where a quick quench is needed. See Examples 21 and 22. Conversely, droplet systems can also be designed for very low pressure drop which is advantageous in applications such as vacuum condensers. 

The test results with the ultrasonic nozzle were obtained with an estimated steam to copper (S/Cu) ratio of 23 and the humidified Ar was injected co-currently with the CuCl2 solution. Several variables remain to be investigated, i.e. lower S/Cu ratios, counter-current instead of co-current operation, and subatmospheric pressures. LeChatelier s Principle predicts that reducing the pressure in the hydrolysis reactor should reduce the S/Cu ratio. The effect of a reduced pressure was quantified by the results of a sensitivity study using Aspen. Aspen predicts that a S/Cu ratio of 17 is needed for essentially complete conversion at 375°C and atmospheric pressure while a S/Cu ratio of 13 is required at 0.5 bar. The conceptual process design specifies that the hydrolysis reactor be run at 0.25 bar. The pressure drop in the reactor is achieved by adding a low temperature steam ejector after the condenser at the exit of the hydrolysis reactor in the conceptual design. 

---