Variable 1: Marketing digital
3.2 Contrastación de Hipótesis
(1) Cooling towers
Cooling tower is used to cool hot water for the recycling. In the cooling tower, hot water contacts with cold air and some portion of the water evapo-rates. As the result, the hot water is cooled by releasing the latent heat of evaporation.
Cooling towers are 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 fan.
The mechanical draft type includes forced draft
Make-up water
Circulating pump Reservoir
Air, seawater, etc.
Fig. 3.4 Types of cooling towers
Tower
2. Forced draft type 1. Natural draft type
3. Induced draft counter flow type Water outlet
4. Induced draft cross flow type Water outlet
and induced draft types. Also, they are classified into counter flow and cross flow types according to the flow directions of water and air. In a counter flow tower, air moves upward through the tower packing, counter to the downward flow of water. In a cross flow tower, air flows horizontally across the downward flow of water. Figure 3.4 illustrates these types of cooling towers. As each type has its advan-tages and disadvanadvan-tages, the most suitable type is selected depending on conditions such as tower capacity, the installation area, etc.
In Japan, round counter flow type cooling towers (Figure 3.5) are used for small scale systems with the circulation rate of less than about 300 m3/h.
Square cross flow type cooling towers (Figure 3.6) are used for larger size systems.
(2) Heat exchangers (a) Structures and features
Heat exchangers are used as coolers, heaters, condensers or evaporators depending on the requirement.
By structure, they are classified as tubular heat exchangers, double tube heat exchangers, spiral tube heat exchangers, irrigation coolers, plate heat exchangers, and air fin coolers. The tubular heat exchanger is most widely used. It includes horizon-tal and vertical types. The horizonhorizon-tal type is gener-ally used although the vertical type has the advan-tage of a smaller installation area.
Typical structures and features of horizontal tu-bular heat exchangers are shown in Figure 3.7 and
Fig. 3.5 A typical small size cooling tower (counter flow type)
Noiseless motor
Sprinkler head
Cooling tower pit Louver
Casing Ascending pipe
Noiseless fan
Sprinkler pipe
Tower packing
Silencer mat
Table 3.2. Cooling water flows through either tube inside or shell side. The flows of cooling water in tubular heat exchangers and their characteristics are listed in Table 3.3.
(b) Heat flux
Heat flux is defined as the heat removed from the process fluid by the cooling water per unit area and per unit time. It is calculated by the following equation:
where
Q = heat flux (kcal/m2·h)
∆T = temperature difference of the cooling water between the inlet and the outlet of heat exchanger (°C)
R = flow rate of cooling water (m3/h)
C = specific heat of water at constant pressure (kcal/kg·°C)
A = heat transfer area (m2)
The higher heat flux, the greater possibility of scale or corrosion problems in heat exchangers.
(c) Over-all heat transfer coefficient
Over-all heat transfer coefficient (U-value) is an index of thermal efficiency, and it is determined by the following equation:
where
U = over-all heat transfer coefficient (kcal/
m2·h·°C)
α1= heat transfer coefficient of laminar film at the process side (kcal/m2·h·°C)
α2= heat transfer coefficient of laminar film at the cooling water side (kcal/m2·h·°C)
λ = thermal conductivity of tube material (kcal/
m·h·°C)
l = tube wall thickness (m)
γ1 = fouling factor at the process side (m2·h·°C/
kcal)
γ2 = fouling factor at the cooling water side (m2· h·°C/kcal)
The U-value is decreased by fouling with scale, corrosion products and slime during the operation of heat exchanger. The reduction rate in U-value becomes bigger for the heat exchanger of higher U-value under the same level of fouling.
Q = ... (3.1)
U = 1 ... (3.2) + γ1+ l + + γ2
λ1 1 α2 1
α1
∆Τ X C X R X 103 A
Leg
Fan
Distribution deck
Louver
Eliminator
Cooling tower basin G. L
Fig. 3.6 A typical large size cooling tower (cross flow type)
Fig. 3.7 Typical structures of tubular heat exchangers
U-tube type Floating head type Fixed tubesheet type
Figure 3.8 shows an example of relationship between the thickness of fouling and the reduction of U-value.
(d) Fouling factor
Fouling factor indicates the degree of fouling by scale and other deposits in the heat exchanger.
The factor is calculated by the following equation:
where
γ = over-all fouling factor (m2·h·°C/kcal) γ1 = fouling factor at the process side (m2·h·°C/
kcal)
γ2 = fouling factor at the cooling water side (m2·h·°C/kcal)
Uf= over-all heat transfer coefficient with fouling (kcal/m2·h·°C)
Uo= initial over-all heat transfer coefficient (without fouling) (kcal/m2·h·°C)
The fouling factor is also determined by the thermal conductivity and the thickness of fouling material using the following equation:
Motor Reduction gears
Tower packing
G. L
Floating tubesheet Floating
head
1 Uf
1 Uo
γ = γ1 + γ2 = - ... (3.3)
γ1= L1 , γ2 = ... (3.4) λ1
L2
λ2
Table 3.2 Types and features of tubular heat exchangers
Features (1) Easy to manufacture; less expensive
(2) Difficult to clean the shell side because of the impossibility of removing the tube bundle (1) Applied to cases of large difference in temperature between the tube and shell side fluids or in
thermal expansion coefficient between tube and shell materials (2) Possible to clean the shell side by removing the tube bundle Type
Fixed tube-sheet
Floating head U-tube
Table 3.3 Flows of cooling water in tubular heat exchangers and their characteristics Water flow
Tube side
Shell side
Characteristics
Easy to get good effects of cooling water treatment because the flow rate is usually more than 0.5 m/s
Likely to cause fouling problems due to sludge accumulation because of the low and complicated water flow
Note: In general, corrosive fluids, fluids of high fouling potential or fluids under high pressure flow through the tube side. However, recently heat exchanger using cooling water have been frequently designed with the tube side water flow because of the easy maintenance.
Tube side flow
Shell side flow
where
L1= fouling thickness at the process side (m) L2= fouling thickness at the cooling water side
(m)
λ1= thermal conductivity of fouling at the process side (kcal/m·h·°C)
λ2= thermal conductivity of fouling at the cool-ing water side (kcal/m·h·°C)
Heat exchangers are commonly designed with the water side fouling factors (designed γ2) in the range of 0.0002 to 0.0006 m2·h·°C/kcal, depending on the expected fouling tendency of the cooling water. Therefore, the γ2 in service must be main-tained below the designed γ2.
The permissible scale thickness is estimated from the designed γ2 and λ2. The λ2 is reasonably estimated from the cooling water quality and the chemical treatment program.
(3) Cooling water circulation pump
Turbine pumps are usually used for circulating water in cooling water systems.
(4) Metals in general use
Equipments in contact with the cooling water are heat exchangers, pipings, pumps, cooling towers, cooling tower basins and the sensors of measuring instruments. Except for the cooling tower, most of the other equipments are made of metals. Table 3.4 shows the characteristics of metals widely used in pipings and heat exchangers.
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 600
650 700
0.09 Initial U value: 700 kcal/m2·h·°C Thermal conductivity of fouling: 0.5 kcal/m·h·°C
Over-all heat transfer coefficient (kcal/m2·h·°C)
Fouling thickness (mm)
Fig. 3.8 Relationship between the fouling thickness of an heat exchanger and the reduction of the overall heat transfer coefficient
0