Gravity separator fundamentals and design

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Presented at IIAR 2001 Ammonia Refrigeration Convention & Exhibition Long Beach, CA March 18-21, 2001


Executive Summary The objective of this paper is to review the literature on theprinciples governing gravitydriven separation of liquid-vapor mixtures, review design methods for separators, and develop a model that predicts separator performance given operating requirements (i.e. size or velocity, and design droplet size) subject to design constraints. The model presented can serve as a basis to establish a fundamentals-based new design method for gravity separators. It is notthe purpose of this paper to develop or recommend design guidelines; rather it is a literature search and analysis to put the existing design guidelines on the same basis for ammonia. The paper summarizes landmark literature in the history of gravity separation and reviews the assumptions made in both the literature and the techniques developed in the paper. Equations of motion that define thedroplet trajectories in both vertical and horizontal vessels are presented and implemented in a computer model. Results of in-depth analysis aimed at characterizing liquid-vapor separation in both vertical and horizontal vessels are presented. ASHRAE recommendations for vessel sizing are quantified using the techniques developed in this paper. Other author’s recommendations for vertical vessel sizingare also analyzed and compared to the ASHRAE recommendations. A design example is presented for both vertical and horizontal vessels. Background Separators are essential components in industrial refrigeration systems. Separators (also known as suction traps, knock-out drums, low pressure receivers, accumulators, recirculators) are pressure vessels that may serve multiple functions includingseparation of liquid from a liquid-vapor stream (to protect compressors from liquid carry-over), maintain adequate supply of liquid for mechanical pumps, and to provide a buffer for accumulation of liquid during transient system operation. With the application of refrigerant separators in ammonia refrigeration systems, the catastrophic or accelerated failure of compressors due to liquid-carry over hasbeen greatly reduced. Virtually all of the liquid-vapor refrigerant separators used in the ammonia refrigeration market today rely on gravity forces to “knock out” or separate liquid from vapor (so called gravity separators). For additional background on vessels, refer to ASHRAE (1998).

Literature Review Much of the work reviewed for this project has roots originating from Souders and Brown’s(1934) work on fractionating columns in the petroleum industry. Fractionating columns are vertical vessels fitted with plates or trays that physically divide the vessel into stages. Each tray is perforated with small holes through which the vapor and entrained liquid droplets pass. The jets of vapor entrain liquid that has fallen by gravity onto the plate surface; the authors describe this as “thethrowing of liquid particles by the dynamic action of vapor jets.” This situation is quite different from both the vertical and horizontal configuration of accumulators common in ammonia refrigeration today. In fact, Souders and Brown state in their paper:
“Although this discussion deals exclusively with plate fractionating columns, it is well to indicate that much greater entrainment may beexpected in other types of equipment which do not contain plates or other types of entrainment separating devices. The actual entrainment in a flash chamber of a cracking plant (chamber free of any entrainment separating device) is … more than twice the entrainment observed in a fractionating tower. The vapor-liquid mixture in this case entered the large chamber through a single pipe at high...
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