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On condenser bypass applications, we are usually dealing with a considerable spray water flow. On a mass basis, it can exceed 40 percent of the total steam mass flow through the system. In order to achieve good mixing and distribution, as well as rapid vaporization, the cooler section has to be designed properly. To this end, most manufacturers utilize multi-point injection systems. By multi-point, we are describing the distribution of the total water flow through multiple injection ports or nozzles. Research has shown that the smaller the nozzle, the smaller the spray particle it can produce. With ever-smaller particle sizes, the total surface area for interaction and heat transfer with the flowing steam improves, thus increasing the vaporization rate. It should also be noted that as nozzles become smaller, their susceptibility to clogging or debris interference increases. Even moderately sized nozzles, when contaminated with pipeline debris, will lose efficiency in forming the desired spray pattern, particle size, and distribution. To eliminate this problem, it is always recommended that strainers be used in the spray water system and located as close as possible to the nozzles themselves. The size of the mesh should be carefully considered both from a pressure drop and nozzle passage viewpoint. If the mesh is selected too large, debris that could be damaging to the nozzles may pass. If the mesh is too small, the added restriction to flow may consume too much pressure drop for the system. This will restrict the necessary quantity of water from reaching the system and prevent the spray water from attaining the correct pressure required for atomization by the spray nozzles.
At the present time, there are two types of nozzles used for cooler sections, fixed and variable geometry. Generally speaking, the variable geometry styles will provide greater rangeability and more consistency in the spray pattern created. This translates directly to improved turndown and more efficiency in the process.
The available water pressure is also a factor in terms of performance. Besides reducing the size of the nozzle, increasing the pressure differential also produces smaller spray particle sizes. In a similar manner, the water temperature also improves nozzle performance. With increased water temperature, the surface tension of the water is reduced. This allows greater efficiency in the formation of droplets with a smaller diameter. Additionally, these smaller and hotter droplets interact better with the flowing steam. Their latent heat of vaporization is reduced, as is their resistance to shear by momentum transfer. This results in further droplet size reduction and more rapid vaporization even though the fluid particles are hotter, an idea that is contrary to discussions on cooling processes.
Unfortunately, the source of water selected by most users for this process is from the low-pressure condensate system. Water temperatures are normally in the 100° F-150° F range and pressures in the 150 psi to 250 psi range. Besides being relatively cool and with low pressure potential, this water source also affects another component in the system, the backpressure device.
The backpressure device, also referred to as a sparger, is utilized to create an elevated pressure downstream of the valve. Without this device, the vacuum conditions of the condenser would exist in the discharge piping. At these extremely low pressures, the velocity of the steam exiting the valve would reach sonic or choked conditions almost immediately unless the outlet pipe size and resultant cross-sectional flow area, were sized to accommodate the large specific volume of the free expanding steam. As this is normally not economically feasible, nor logistically possible, the backpressure device is installed to provide a fixed resistance to the flow entering the condenser. This translates to an intermediate pressure that is variable based on the valve inlet conditions and the mass flow through the system. Ideally, we would prefer that this variable intermediate pressure be as large as possible to keep the outlet pipe size to a minimum. In most condenser bypass applications, the desired outlet flow velocity from the valve is in the 200 FPS-300 FPS range. As mentioned previously, the available water pressure has a direct affect on the sizing of the backpressure device. Within the design and layout of the spray water circuit, we have multiple orifices or restrictions that require pressure differentials to operate, i.e., strainer, spray water valve, nozzles. If the backpressure is selected too high, we may not have sufficient pressure energy remaining to get the spray water to the discharge pipe. Thus, a compromise must be made relative to the distribution of water pressure as compared to the desired backpressure for the maximum flowing steam conditions. Normally, backpressures of 50 psi to 100 psi are utilized, but under certain conditions this could rise to as much as 200 psi.