Devices used to achieve the division of flows from a single source to many outlets are usually termed headers and manifolds. The former term is usually confined to heat exchangers and cases where the outlets are close together. Manifolds are more often involved in the distribution between pieces of equipment.
For single phase flow, the proportion of the flow emerging from each outlet depends on the resistance in the downstream pipework. Figure 1 shows that when the resistance in the branches is low, there can be significant maldistribution which is noticeably decreased when the resistances are increased. Where it is important that the flow from each outlet is approximately uniform, additional resistances in the form of orifices (or ferrules) are provided. In cases such as fired boilers these ferrules are individually sized.
As well as the pressure drops in the downstream pipes, there are additional pressure drops associated with the junction, one for each outlet, which should be taken into account. In the modelling of these multibranch systems, there have been two approaches. The first [Acrivos et al. (1959), Baruja and Jones (1976)] used the concept of porosity to represent a regular array of closely spaced branches. Such an approach might be suited to cases with a single flow direction but not for more complex configurations. Alternatively, the system is treated as a series of individual junctions. Care must be taken to account for any interaction between branches. McNown (1954) examined the effect on interbranch distance on the pressure changes along the main pipe. For the branch pressure drop, Zenz (1962) suggests that the value for an individual junction be increases by 25%. However, in a systematic study, Scruton et al. (1988) showed that the branch pressure drop is well represented by the correlation of Gardel (1957) for interbranch spacing of 1.5 to 15 times the branch diameter.
Separation of the phases occurs in the inlet or return headers of heat exchangers operating with the Gas/Liquid Flows. The liquid collects at the bottom. Hence tubes on the lower rows will be overloaded with liquid.
There is very little information available for the division of gas/ liquid flows at manifolds. What few data exist, e.g., Collier (1976) and Coney (1980), show that the manifold acts as a series of junctions. However, because of the almost inevitable maldistribution of the phases this can mean that one phase accumulates at the last branch as in the case shown in Figure 2. Similar results have been reported with junctions in the horizontal plane. If the branches were connected to a series of condensers, the one connected to the final branch could seriously underperform. If the branches are placed very close together, there could be interaction between them but it is not known at which interbranch spacing this becomes important as this parameter has not been researched.
Figure 1. Effect of downstream resistance on maldistribution of single-phase flow between six branches.
Figure 2. Division of gas and liquid at a four branch manifold with a vertical main pipe and horizontal branches – mass flux = 580 kg/m2s; inlet quality = 16%.
Acrivos, A., Babcock, B. D., and Pigford, R. L. (1959) Flow distribution in manifolds, Chem. Eng. Sci., 10, pp 112-124. DOI: 10.1016/0009-2509(59)80030-0
Baruja, R. A. and Jones, E. H. (1976) Flow distribution in manifolds, Trans ASME, J. Fluids Eng., 98, pp 654-666.
Collier, J. G. (1976) Single-phase and two-phase behaviour in primary circuit components, N.A.T.O Advanced Study Institute on Two-phase Flow and Heat Transfer, Istanbul, Turkey.
Coney, M. W. E. (1980) Two-phase flow distribution in a manifold system, European Two-Phase Flow Group Meeting, Strathclyde.
Gardel, A. (1957) "Les pertes de charge dans les ecoulementes au travers de branchements en te", Bulletin Technique de la Suisse Romande, 9, pp 122-130 and 10, pp 143-148.
McNown, J. S. (1954) Mechanics of manifold flow, ASCE Trans., 119, pp 1103-1142.
Scruton, B., Longworth, D., and Mays, C. J. (1988) Pressure losses for dividing single-phase flows in headers with multiple branches, 2nd National Heat Transfer Conference, Birmingham.
Zenz, F. A. (1962) Minimise manifold pressure drop, Hydrocarbon Processing and Petroleum Refiner, 41 (12), pp 125-130.