Cooling Water Treatment ( Part 3 )

(b)         Heat flux

                Heat flux is defined as the heat removed from the process fluid by the cooling water per unit area per unit time, and can be calculated by the following equation:

 

The higher the heat flux, i. e. the heat load, the greater the possibility of scale or corrosion problems in heat exchangers. 

(c)          Over-all heat transfer coefficient              

               Over-all heat transfer coefficient is the index of thermal efficiency, referred to as U-value, and can be determined by the following equation:

The U-value is decreased by fouling with scale, corrosion products, and slime during the operation of heat exchangers.  The decrease in U-value  is more pronounced with heat exchangers of high U-values under the same level of fouling.

(d)         Fouling factor

                Fouling factor indicates the degree of fouling by scale and other deposits in the heat exchangers.   The factor is calculated by the following equation:



 



The fouling factor is also determined by the thermal conductivity and the thickness of the

fouling material using the following equation:

Heat exchangers are commonly designed with fouling factors (designed r₂) in the range of 0.0002 to 0.0006 m2 · hr· ^oC /kcal, depending on the expected fouling tendency of the cooling water. Therefore,  r₂ in service must be maintained below the designed r₂.

                The permissible scale thickness is estimated from the designed r₂ and l₂. The l₂ can be estimated to a certain degree from the cooling water quality and the chemical treatment program. Figure 3.8 shows an example of the relations between values of r₂ and L₂.
 

 

 

 To be continued in Part 4...

Cooling Water Treatment ( Part 2 )

Usually, round counter flow type cooling towers are used in small scale systems where less than about 300 m3/hr of water is circulated.  Square cross flow type cooling towers are used in larger size systems (Fig. 3.5 and 3.6)

 

(2)         Heat exchangers

(a)          Structure and features

                Heat exchangers can be used as coolers, heaters, condensers, or evaporators, depending on the requirement.

                By structure, they are classified as tubular heat exchangers, double tube heat exchangers, coil heat exchangers, irrigation coolers, plate heat exchangers, and air fin coolers.  The tubular heat exchanger is the most widely used.  It includes horizontal and vertical types.  The horizontal type is generally used although the vertical type has the advantage of a smaller installation area.

                Typical features of horizontal tubular heat exchangers are shown in Fig. 3.7 and Table 3.2.  Cooling water flows through either tube or shell side.  Flow and characteristics of cooling water in tubular heat exchangers are listed in Table 3.3.

 

 

 

 To be continued in Part 3...

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



Cooling Water Treatment ( Part 1 )

        Cooling water is used directly or indirectly to cool chemical products, steel products, etc.

Generally, cooling water is used indirectly to cool process fluids (liquids and gases) through heat exchangers. This system is called an indirect cooling system.  Solid products are often cooled directly by spraying water through a nozzle.  This system is termed a direct cooling system and is used in plants handling solid products, such as iron works and food factories. 

Cooling water systems are classified as shown in Table 3.1

The demand for cooling water has a great influence on the entire industrial water supply.  To reduce the water requirements for cooling systems, the changes have been made, such as changing from once through to recirculating cooling systems and the reduction of blowdown water quantities by higher cycle operation of open recirculating cooling systems.

Such effective utilization of the water causes concentration of dissolved solids, resulting in frequent problems from corrosion, scale and slime.  Various cooling water treatment chemicals are used to prevent these problems. The development of these chemicals and the related technology makes possible more effective utilization of cooling water.   Recent developments in treatment have made it possible to prevent the loss of thermal efficiency in heat exchangers even under lower water flow rates and higher outlet water temperatures.  Application of advanced cooling water treatment technology provides "energy saving operation'' of cooling systems by reducing the speed of cooling tower fans and decreasing the number of water circulating pumps.

Cooling water treatment chemicals can cause environmental pollution because they are discharged with the cooling water.  Therefore, nonpolluting chemicals have been developed.

The cooling water treatment chemicals and technology described here are mainly concerned with open recirculating cooling water systems. 

3.1         Outline of Cooling Water Systems

3.1.1      Types and Features of Cooling Water Systems

(1)         Open recirculating cooling water system

                Figure 3.1 shows a typical water flow in an open recirculating cooling system. In the open system, water which has been heated by flowing through heat exchangers is cooled in a tower by partial evaporation and the release of the latent heat of evaporation.  In this way, the water is recirculated and reused.  This system is widely used to cool products, refrigerants, etc., in oil refineries, petrochemical and other plants.  Fresh water is usually used in this system.

(2)         Closed recirculating cooling water system

                In a closed recirculating cooling water system, the water which is heated by passage through heat exchangers or other equipment is cooled in a secondary cooler using seawater, air, or the cooling water from the open recirculating cooling water system. It is then recirculated and reused .

                Usually water losses are low in this type of system and concentration of dissolved solids by evaporation is not a problem.  The closed system includes cooling systems for engines, bearings, air conditioners, etc.  Figure 3.2 shows an example of the water flow in a typical closed system.

(3)         Once through cooling water system

                In a once through cooling system, the water after being heated is discharged directly into a receiving stream without recirculation.  As this system needs a large amount of water, seawater and underground water have been used.  However, the once through cooling systems are being used less frequently since the utilization of underground water is being regulated due to ground subsidence.  Figure 3.3 illustrates the water flow in this system.

             This system may be used for cooling secondary coolers in closed cooling water systems.  Thermal power plants frequently install once through systems using seawater because they require large amounts of cooling water for the steam condensers.

3.1.2      Equipment and Materials in Cooling Water Systems

(1)         Cooling tower

                Cooling towers are used to bring warmed water into contact with cool air (precisely, air of wet bulb temperature below that of the water being cooled) to evaporate some portion of the water and to release the latent heat of evaporation in order to lower the water temperature.

                Cooling towers can be classified into two major types. One is the natural draft type where air is supplied by natural convection and the other is the mechanical draft type where air is supplied by fans (Fig. 3.4).

              

             The mechanical draft type includes forced draft and induced draft types.  Also, they are classified as counter flow or cross flow according to the flow direction of water and air. In the counter flow type, air moves vertically upward through the packing, counter to the downward fall of water.  In cross flow towers, air flows horizontally across the downward fall of water.  Each type has its advantages and disadvantages, and the most suitable type is selected depending on such conditions as installment site and tower capacity.

To be continued in Part 2...