The woldwide source of the right solutions for power plants
|
The woldwide source of the right solutions for power plants
|
|
|
General Description
Since the dissolved solids in the water are not concentrated by evaporation, deposition by scaling is usually not a problem. Fouling is also minimized because the system is not open to outside contaminants. The main exception to this is microbiological fouling which can cause serious problems in closed systems. Be aware that these assumptions are based on ideal design conditions where the only water losses are minor ones due to pump seal leaks, expansion tank overflows and surface evaporation from system vents. With those systems that have severe water losses and, consequently, high makeup rates, the potential for scale can become significant. Since there is no bleedoff in a closed system, there is an opportunity for rust or suspended solids to drop out in low flow areas and deposit on heat transfer surfaces. Consequently, for those systems with high makeup rates, it is usually advisable to either use high quality makeup water (either softened or demineralized) or employ a corrosion inhibitor that is blended with deposit control agents. Later on, we will discuss methods by which to control the primary water problem associated with closed loop systems: corrosion.
. Typical closed cooling system In engineering terms, the "closed system" is actually two interrelated systems:
The engine cooling system of an automobile provides a classic example of a closed recirculating system. The engine gives off its generated heat to the water recirculated through it, and the water is cooled as it circulates through the radiator. In order to further clarify the concept of closed loop cooling, above figure shows a diagram of a simple closed recirculating system. Basically, heat is transferred to the closed loop by typical heat exchange equipment and is removed from the closed system by an exchange of heat from the closed loop to a secondary cooling water cycle. The secondary loop could use either evaporative or once-through water cooling, or air cooling. You should note, that, included in the diagram is an expansion tank (surge tank), which not only allows for temperature induced water volume changes, but also functions as an addition point for makeup water. Basically, there are two types of expansion tanks: open and closed. Open tanks are vented to the atmosphere and are usually located at the highest point in the system. Closed tanks, which operate under pressure, are not capable of venting oxygen and other dissolved gases that enter through the makeup water and leaking valves. Therefore, air vents must be installed at high points throughout the system. While these vents are designed to remove dissolved gases from the system, they can serve as an oxygen contamination point if changes in system pressure cause air to be drawn in through them. Leaking air vents can significantly accelerate corrosion in closed systems, especially those that operate at higher temperatures because oxygen is more aggressive at higher temperatures.
Curve A, plots data for a completely closed system without any means of removing dissolved oxygen. Curve B, plots data for a vented system. For reasons that we will discuss in more detail later, the rate of corrosion tends to increase with temperature. On curve A, there is a steady increase in corrosion as the temperature increases. Curve B, on the other hand, shows a steady increase in corrosion up to approximately 170eF. At this temperature, the loss of dissolved oxygen through venting exceeds the amount made available through diffusion and, consequently, a decrease in the corrosion rate occurs.
|
|
The worldwide source of the right solutions for power plants
|
Unlike the
once-through and the open recirculating systems, the makeup water usage in a
closed system is usually minimal. Specifically, a system is considered closed if
it does not employ open evaporation for cooling and has an average water loss of
less than 0.5% of the circulation rate.
