Cooling Tower Water Treatment Principles
Make-Up Water – Water added to the system to replace water lost through evaporation, bleed of and windage.
Recirculation – The rate at which water circulates in the cooling tower (in m3/hour litres/sec).
Bleed Off – Water deliberately bled from the system to prevent the cooling system water from becoming too concentrated.
Maximum acceptable concentration factor – An expression of how concentrated the cooling water can become if problems with scale formation and corrosion are to be avoided.
Windage – Water lost from the cooling tower through mechanical rather that evaporative losses.
Temporary calcium hardness – The lower of either the calcium hardness or the Alkalinity (M).
Typical Evaporation Rules
One tonne of refrigeration requires an evaporation rate of approximately 11.4L/hr.
One KW of cooling requires an evaporation rate of approximately 1.5L/hr.
1.8% of the recirculation rate evaporates for each 10oC temperature difference across a cooling tower (1% for each 10oF)
Cooling System Concentration Factors
Your cooling tower will at some stage reach a natural limit as it cannot cycle up in concentration indefinitely like a boiler. However, it may be possible that this natural limit will result in a control limit being exceeded, be on the lookout for:
- Systems with water chloride concentration exceeding 200pppm if stainless steel is present.
- System water contains temporary hardness exceeding the limit specified by your treatment.
- The system concentration factor exceeds the recommended maximum. As a general rule concentration factors should not exceed 5 but where the product is primarily inhibiting corrosion then this should be reduced to 3.5-4.0 to ensure sufficient inhibitor is added to maintain corrosion inhibiting film.