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...