Flow co-current

Figure 24, Countercurrent-flow packed Figure 25. Co-current-flow scrubber, scrubber.

The temperature difference may not remain constant throughout the flow path. Plots of temperature vs. pipe length for a system of two concentric pipes in which the annular fluid is cooled and the pipe fluid heated are shown in Figures 2-2 and 2-3. When the two fluids travel in opposite directions, as in Figure 2-2, they are in countercurrent flow. When the fluids travel in the same direction, as in Figure 2-3, they are in co-current flow. 

Figure 2-3. Change in AT over distance, co-current flow of fluids.

It is possible to inject glycol in a gas line and have it absorb the water vapor in co-current flow. Such a process is not as efficient as countercurrent flow, since the best that can occur is that the gas reaches near equilibrium with the rich glycol as opposed to reaching near equilibrium with... 

Note that the logarithmic mean temperature difference should he used when the following conditions generally apply for conditions of true counter-current or co-current flow ... 

Eor co-current flow (see Eigure 10-29B), the temperature differences will be (Tj — p), and the opposite end of the unit will be (Tg — tg). This pattern is not used often, because it is not efficient and will not give as good a transfer and counter-currenC flow. Because the temperature cannot cross internally, this limits the cooling and heating of the respective fluids. Eor certain temperature controls related to the fluids, this flow pattern proves beneficial. 

In most multipass exchangers, a combination of counter-current and co-current flow exists as the fluid flows through alternate passes (see Figure 10-29). The mean temperature is less than the logarithmic mean calculated for counter-cur-rent flow and greater than that based on co-current flow. 

Note F = 1.0 for pure counter-current flow. As co-current flow increases in design arrangement (not flow rate), the F is reduced, and the exchanger efficiency Mis, to a usual practical lower limit of 0.75-0.80 ... 

From Figure 26.7 it can be seen that for equal duties and flows the temperature difference for countercurrent flow is lower at the steam inlet than at the outlet, with most of the steam condensation taking place in the lower half of the plate. The reverse holds tme for co-current flow. In this case, most of the steam condenses in the top half of the plate, the mean vapor velocity is lower and a reduction in pressure drop of between 10-40 per cent occurs. This difference in pressure drop becomes lower for duties where the final approach temperature between the steam and process fluid becomes larger. 

The great majority of towers available fall into one of two categories counterflow or crossflow (co-current flow is available but is seldom used) (see Figure 34.1). 

Figure 9.2. Mean temperature difference for co-current flow...

A heat exchanger is required to cool 20 kg/s of water from 360 K to 340 K by means of 25 kg/s water entering at 300 K. If the overall coefficient of heat transfer is constant at 2 kW/m2K, calculate the surface area required in < a) a countercurrent concentric tube exchanger, and (b) a co-current flow concentric tube exchanger. 

It may be noted that using Underwood s approximation (equation 9.10), the calculated values for the mean temperature driving forces are 41.9 K and 39.3 K for counter- and co-current flow respectively, which agree exactly with the logarithmic mean values. 

Considering co-current flow as shown in Figure 9.84c/, for an elemental area AA of a heat exchanger, the rate of transfer of heat AQ is given by ... 

Mass transfer processes involving two fluid streams are frequently carried out in a column countercurrent flow is usually employed although co-current flow may be advantageous in some circumstances. There are two principal ways in which the two streams may be brought into contact in a continuous process so as to permit mass transfer to take place between them, and these are termed stagewise processes and continuous differential contact processes. 

The temperature of oil leaving a co-current flow cooler is to be reduced from 370 to 350 K by lengthening the cooler. The oil and water flow rates and inlet temperatures, and the other dimensions of the cooler, will remain constant. The water enters at 285 K and the oil at 420 K. The water leaves the original cooler at 310 K. If the original length is 1 m. what must be the new length ... 

What is the coefficient when the exchanger is operating with co-current flow if its value is proportional to the 0,8 power of the water velocity ... 

After plant modifications, the heat exchanger is incorrectly connected so that the two streams are in co-current flow. What are the new outlet temperatures of hydrocarbon and water, if the overall heat transfer coefficient is unchanged ... 

Coriolis flowmeter 267 Costich, E. W. 280,283,288.311 Coulson, J. M, 517, 565 Counter- and co-current flow, mass transfer 621 Countercurr ent contacting of phases 600 --flow 387... 

Kariyasaki A, Fukano T, Ousaka A, Kagawa M (1991) Characteristics of time-varying void fraction in isothermal air-water co-current flow in horizontal capillary tube Trans JSME 57(544) 4036-4043... 

The gas liquid contact in a packed bed column is continuous, not stage-wise, as in a plate column. The liquid flows down the column over the packing surface and the gas or vapour, counter-currently, up the column. In some gas-absorption columns co-current flow is used. The performance of a packed column is very dependent on the maintenance of good liquid and gas distribution throughout the packed bed, and this is an important consideration in packed-column design. 

The equation is the same for co-current flow, but the terminal temperature differences will be (T — fi) and (T 2 — t2). Strictly, equation 12.4 will only apply when there is no change in the specific heats, the overall heat-transfer coefficient is constant, and there are no heat losses. In design, these conditions can be assumed to be satisfied providing the temperature change in each fluid stream is not large. 

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