The once-through cooling system is designed very simply. Water is pumped from a source through the heat exchange equipment and then discharged. A general schematic diagram of a once-through system is shown below in figure I-a
Figure I-a. Once through cooling water system.
Because of its simplicity, the once-through cooling system was the first cooling water system designed. The water temperature was generally low and rapid cooling could be achieved with a minimum amount of problems. Once-through cooling can be used for many different applications ranging from small oil coolers to large utility condensers. The major advantages of once-through cooling systems are their simplicity and flexibility. Great quantities of unwanted heat can be removed very effectively and the relatively low water temperature allows cooling with a minimum heat transfer surface.
The water caused problems in once-through systems are the same as other cooling water systems: corrosion, scale, fouling and microbiological growth. The extent of problems encountered is very much determined by the design of the system and, particularly, the quality of the water used. Since the water is not changed significantly in chemical or physical characteristics, the nature of the raw water largely determines the problems which will develop.
Water for a once-through system may be drawn from wells, lakes, streams, rivers or municipal water systems. Surface waters such as lakes and rivers will tend to have more suspended matter and contaminants with the type and quantity of material changing continuously with the seasons. Well waters often contain large amounts of scale forming ions and metal cations. These water characteristics, plus flow and temperature changes within the system, complicate what would initially appear to be a fairly simple situation.
In general, we can classify once-through systems into three major categories:
The small system may range from a few gallons per minute of water flow to several thousand gallons per minute. These systems may be used to cool oil coolers, air compressors, air conditioning units, etc. For these systems, a once-through cooling mode is effective and allows a great deal of flexibility.
The water source is commonly private wells or municipal water systems. Since these waters usually contain very limited amounts of suspended material and microbial growth, they have very little potential for fouling or formation of microbiological slime. Corrosion and scale can be major problems if not recognized and corrected with the appropriate action. The tendency of the water used to form scale or corrosion can be determined by the use of the Langelier's index or the Ryznar Stability index (both of which will be discussed later in this manual).
The large system may range from 10,000 gallons per minute (gpm) of water flow to well over 100,000 gpm. These systems can be used in various industries and are often found in large utility plants. Unlike the small system, the large system rarely employs wells or municipal water systems as the primary water source. Instead, due to the volume of water required for cooling, these systems usually use water from a river, ocean or lake. Since these waters usually contain significant quantities of suspended matter and bacteria, the primary treatment concern is the prevention of system fouling.
Once-through systems designed to provide potable water comprise a special category because the treatment chemicals selected must be suitable for use in drinking waters. Corrosion and scale present the primary problems because potable water supplies (municipal and/or well) usually contain less than 5 ppm suspended matter, 0.3 ppm iron and a low bacterial level. If microbiological growth is noted in the system, chlorine or chlorine dioxide are usually the only acceptable toxicants.
The same parameters as apply to other systems govern pH control. When pH adjustment is inadequate, additional scale and corrosion control can be achieved by feeding up to 10 ppm of polyphosphate, sometimes supplemented by up to 5 ppm of zinc. Sodium silicate is also frequently used to reduce corrosion. Potable water treatment may be applied to protect entire municipal systems or to control specific problems within a single potable water distribution system.
PRODUCT LINE FOR ONCE THROUGH SYSTEMS
Water System Corrosion and Scale Inhibitor
WSC OT-8270is a stabilized, liquid polyphosphate formulation utilized for the treatment of potable water distribution systems. The product may also be used in once through cooling water systems and under certain conditions, in recirculating cooling applications. It is designed to provide both corrosion and scale control at low treatment dosages.
Water System Corrosion and Scale Inhibitor
WSC OT-8271is a stabilized, liquid polyphosphate and a silicate formulation utilized for the treatment of potable water systems. It is designed to inhibit corrosion, stabilize soluble iron and manganese and control scale formation in distribution systems supplying low hardness waters.
The feed of a supplemental alkaline material for pH adjustment may be required to optimize corrosion inhibition of copper and steel metallurgy.
Water System Alkalinity Builder and Inhibitor
WSC OT-8273is a stabilized, liquid highly alkaline silicate primarily for the treatment of municipal potable water systems. It is designed to provide corrosion control and pH adjustment in the treated systems at moderate treatment dosages. The product may also be used in once through cooling waters and under certain conditions, in recirculating cooling water applications.
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