CHAPTER 5: ENERGY EFFICIENT ABSORPTION COOLING SYSTEM
5.5 Cooling Tower
Cooling towers provide a means of removing low grade heat from cooling water. Its function is similar to the function of a motor vehicle’s radiator. Hot water from the heat exchangers is sent to the
50 cooling towers. The cooled water exits the cooling tower and is sent back to the heat exchangers and/other components so that these components can continuously be cooled.
In the absorption chiller of this HVAC system, the cooling towers play a vital role in the condenser and absorption heat exchangers. It can be stated that the efficiency of the chiller depends on the efficiency on the cooling towers. Cooling towers fall into two categories according to their configuration: natural draft cooling towers and mechanical draft cooling towers. Natural draft cooling towers employ large concrete chimneys to introduce air through the water and are used for flow rates exceeding 45000m3/h.
These types of cooling towers are used for utility power stations. Mechanical draft cooling towers are used more often than natural draft cooling towers. They utilize large fans to force air through circulated water. The water falls downward over surfaces which help increase the contact time between the water and air. This maximises heat transfer (loss from water to air) between the two.
The cooling tower theory is that heat is transferred from water droplets passing through the cooling towers to the surrounding air by the transfer of sensible and latent heat. This movement of heat can be modelled with a relation known as the Merkel Equation: [72]
h
c
T
T w a
b
h h
dT L
KaV ……… (Equation 5.2)
Where:
KaV Tower characteristics [ - ] L
K mass transfer coefficient (kg water/ms2) a contact area/tower volume (m-1)
V active cooling volume/plan area (m) L water rate (kg/ms2)
T h Hot water temperature (C) T c Cold water temperature (C) T b Bulk water temperature (C)
h w Enthalpy of air-water vapour mixture at bulk water temperature (J/kgdryair) h a Enthalpy of air-water vapour mixture at wet bulb temperature (J/kgdryair)
51 In cooling tower design, charts are typically used to evaluate
KaV , although they can be calculated. L These charts can be found in the Cooling Tower Institute Blue Book [73], to estimate
KaV for given L design conditions. Three key points are important to note in cooling tower design: a change in wet bulb temperature (due to atmospheric conditions) will not change the tower characteristics
KaV ; a L change in the cooling range will not change
KaV ; however a change in the L
m
GL ratio (discussed m
below) will change KaV . L
Thermodynamics dictates that the heat removed from the water should equal the heat absorbed by the surroundings/ambient air:
) h h ( G ) T T (
Lm h c m 2 1 ………..…….….. (Equation 5.3)
When rearranged gives
c h
1 2 m m
T T
h h G
L
………..…...…. (Equation 5.4)
Where:
m
GL Liquid to gas mass flow ratio (kg/kg); m
T h Hot water temperature (C) T c Cold water temperature (C)
h 2 Enthalpy of air water vapour mixture at exhaust wet bulb temperature (J/kgdryair) h 1 Enthalpy of air-water vapour mixture at inlet wet bulb temperature (J/kgdryair)
The engineer defines the cooling water flow rate, inlet and outlet water temperatures for the tower and then designs the tower to be able to meet these criteria on a “worst case scenario” which is during the hottest months. The required tower size will be a function of the cooling range, approach to wet bulb temperature, mass flow rate of water, wet bulb temperature, air velocity through tower or individual tower cell and the tower height.
52 The components of a typical cooling tower will be the frame and casing; fill material, cold water basin, drift eliminators, air inlet, louvers, nozzles and fans. Refer to Figure 5.9 for the cooling tower arrangement.
Figure 5.9: Cooling tower arrangement
Not indicated in the Figure 5.9 is the make-up water for the losses that will occur in the process of cooling in the cooling towers. Water losses include evaporation, drift (water entrained in discharge vapour), and blowdown (water released to discharge solids) [72].
The losses in the cooling tower are expressed as follows:
Drift losses = 0.1% and 0.2% of water supply……….………..… (Equation 5.5)
Evaporation Loss (in m3/hr) = 0.00085*water flow rate (in m3/hr) * (T1 T2)….…... (Equation 5.6)
Blow down Loss =
1 Cycles
Loss n Evaporatio
………..……… (Equation 5.7)
Where cycles = the ratio of solids in the circulating water to the solids in the makeup water.
53 Total Losses = Drift Losses + Evaporation Losses + Blow down Losses………...….…. (Equation 5.8)
The make-up water is provided by a fixed source that is readily available to cover up for the total water losses calculated above. Wood or plastic is commonly used as fill to facilitate heat transfer. This fill maximizes water and air contact. Fill material used is of the splash and film type.
In splash fill, water is designed to fall over successive rows of horizontal splash bars, causing further breaking into smaller droplets as well as wetting the fill surface. The plastic type fill facilitates better heat transfer than the wood splash fill. The film fill is made up of plastic surfaces that are spaced closely over which water spreads – this allows a thin film of water to be in contact with air. The film fill may be honeycombed, flat or corrugated. The fill type is more efficient than the splash type and provides the same heat transfer in a smaller volume.
The water basin at the sump of the cooling tower collects the cooled water which flows/falls from the tower fill sections. This basin usually connects to the cold water discharge. The use of drift eliminators in the air stream are employed to capture water droplets. Air inlets introduce air into the tower by means of louvers. This allows the equalization of air flow into the fill and this assists in retaining the water in the cooling tower. Nozzles provide a means for the hot water (water to be cooled) to be distributed over the fill. Fans are used to induce draft and aid the cooling down of the water.
Even though a percentage of the water is evaporated into the air, the impurities are re-circulated into the system. Concentration of the dissolved solids increases rapidly, and will reach unacceptable levels, if not controlled. This may lead to scaling, corrosion and sludge accumulations which reduces heat transfer efficiencies. Thus it becomes necessary that chemical treatment be implemented and bleeding off a small amount of the circulating water so that the system can be topped up with fresh water. The growth of algae and other microorganisms calls for the use of biocides.
Cooling towers will be designed to be as energy efficient as possible as the core objective of this project is to utilize the minimum amount of grid power.