Cooling towers are rated in terms of approach and range.
The approach is the difference in temperature between the cooled-water temperature and the entering-air wet bulb temperature.
Approach = LWT – WBT
The range is the temperature difference between the water inlet and exit states.
Range = EWT – LWT
Where
• EWT = Entering hot water temperature (°F)
• LWT = Leaving cold water temperature (°F)
• WBT = Ambient wet bulb temperature (Design WB, °F)
Water Circulation through Cooling Tower
Q = H / (Range X 500)
Where
• H = Cooling tower heat rejection in Btu/hr
• Q = Water flow rate in GPM
Cooling Tower Water Balance
The amount of water that enters as make-up must be equal to the total water that exits the system
or
MR = water lost through evaporation (ER) + bleed (BR) + drift (DR)}
Where
• MR = Makeup water requirement in GPM
• DR = Typical drift rate in GPM
• ER = Evaporation rate in GPM
• BR = Bleed rate in GPM
The evaporation rate of cooling tower is
ER = Q X Range / 1,000
The drift loss is roughly 0.2 to 0.5%
DR = 0.002% X Q
When we ignore the insignificant drift losses
Then, MR = ER + BR …… (eq.1)
Recognizing that in order to keep off from making scale, all of the solids that enter as make- up must exit as bleed, it follows that:
MR = COC x BR …… (eq.2)
And that:
MR = ER [(COC)/ (COC -1)] ……. (eq.3)
Combine (eq.2) and (eq.3) to get:
BR = ER / (COC -1)
Where
• COC = Cycles of concentration.
Ideally the COC is maximized to 5 to 7 by addition of water treatment chemicals.
Cooling Tower Efficiency
Since the cooling towers are based on the principles of evaporative cooling, the maximum cooling tower efficiency depends on the wet bulb temperature (WBT) of the air. The cooling tower efficiency can be expressed as:
Efficiency = (EWT - LWT) x 100 / (EWT - WBT)
Where
• Efficiency = cooling tower efficiency - common range between 70 - 75%
• EWT = inlet temperature of water to the tower (0F)
• LWT = outlet temperature of water from the tower (0F)
• WBT = wet bulb temperature of air (0F)
The temperature difference between inlet and outlet water (EWT - LWT) is normally in the range 10 – 150 C. The water consumption - the make up water - of a cooling tower is about 0.2-0.3 liter perminute per ton of refrigeration.
Cooling towers use the principle of evaporative cooling in order to cool water. They can achieve water temperatures below the dry bulb temperature (DBT) of the air used to cool it. They are in general smaller and cheaper for the same cooling load than other cooling systems.
Cooling Tower Tons
A cooling tower ton is defined as:
1 cooling tower ton = 15,000 Btu/hr (3782 kCal /hr)
This is roughly 25% more than chiller ton because the heat of compress ion of the refrigeration compressor is added to the condenser/cooling tower.
Efficiency = (EWT - LWT) x 100 / (EWT - WBT)
Where
• Efficiency = cooling tower efficiency - common range between 70 - 75%
• EWT = inlet temperature of water to the tower (0F)
• LWT = outlet temperature of water from the tower (0F)
• WBT = wet bulb temperature of air (0F)
The temperature difference between inlet and outlet water (EWT - LWT) is normally in the range 10 – 150 C. The water consumption - the make up water - of a cooling tower is about 0.2-0.3 liter perminute per ton of refrigeration.
Cooling towers use the principle of evaporative cooling in order to cool water. They can achieve water temperatures below the dry bulb temperature (DBT) of the air used to cool it. They are in general smaller and cheaper for the same cooling load than other cooling systems.
Cooling Tower Tons
A cooling tower ton is defined as:
1 cooling tower ton = 15,000 Btu/hr (3782 kCal /hr)
This is roughly 25% more than chiller ton because the heat of compress ion of the refrigeration compressor is added to the condenser/cooling tower.
