Along with providing storage for liquid, a chief function of most vessels used in industrial refrigeration is to separate liquid from vapor to assure that the vapor reaching the compressor is free of significant amounts of liquid. The vessel should be sized for whichever of the two functions, storage or separation, controls. The process of separation that occurs in vertical vessels is different than that in horizontal vessels, so design of the two orientations will be addressed separately.
While it is usual to refer to liquid separation as though it is a process of complete removal of liquid, such is not the case. In the vessel there is a spectrum of droplet sizes, and the separation techniques are successsful in removing only the largest droplets. The small droplets are carried out, but some are evaporated in the suction line and others vanish immediately on entering the suction of the compressor. The droplet size is a defining characteristic in the separation principles that will be explained, which immediately raises the question of what is the largest droplet size that should be permitted to escape. This question cannot be answered by analytical means alone and must be decided on the basis of field experience. Thus, if values used for the separation criteria in the design of the vessel result in liquid carryover problems, the criteria must be tightened.
Gravity is the fundamental force used for separating liquid from vapor and retaining the liquid in the vessel. When a drop of liquid falls freely in a motionless vapor, its maximum velocity occurs when the force of gravity just equals the drag force, so no net force is available for acceleration. The upward velocity in the vertical separator of Fig. 10.3 must be low enough that all but the small diameter drops settle. It is reasonable to expect that separation will not be perfect, but if the only drops carried out are the small ones, the total mass of liquid carried out will be small and the small drops can be vaporized more easily than the large ones.