Cooling towers are used for the rejection of heat from water cooled chillers. Cooling towers draw heat rejection water (referred to as condenser water) from the chiller and this water is sprayed as droplets through a stream of outside air drawn by a fan, thereby evaporating a small proportion of the condenser water and cooling the remainder in the process. The ambient air drawn through the cooling tower increases in humidity and temperature, and is discharged to the outside atmosphere.
A chiller is more effi cient (performs with a higher COP) when it uses a cooling tower for heat rejection, rather than an air cooled condenser because the condensing temperature of the chiller can be lower due to the effects of evaporation of water. Water cooled chillers also last longer than air cooled chillers. A cooling tower can reduce the condenser water temperature down to typically 4–5ºC above the prevailing ambient wet bulb temperature. The difference in temperature between the water leaving the cooling tower and the ambient wet bulb temperature is referred to as the cooling tower approach. The difference between water entering temperature and water leaving temperature is known as the cooling tower range.
Systems that use cooling towers suffer the penalty of costs for water consumption and water treatment to minimise risks from Legionella. Due to the costs associated with water treatment, the use of cooling towers is not cost effective for small chillers, typically below 600–800kW capacity, unless they are part of a larger multi-chiller installation. The presence of a cooling tower in a building can signifi cantly lower the energy consumption for air conditioning but signifi cantly increase the water consumption. Where installed, cooling towers typically account for 30–40% of the total water consumption of a building.
lead cooling tower to maximum and staging the remainder. To obtain the best effi ciency, fans should be speed controlled through VSDs, rather than being switched on/off or two speed operation. As the demand for heat rejection increases from the chillers, initially each fan should be operated at the minimum permissible speed. Once all fans have been enabled, they should be modulated in unison
Control algorithms must be programmed to minimise fan power consumed, to deliver the desired condenser water temperature to the chiller. Often, the set point for control of cooling tower fans is below the prevailing ambient wet bulb temperature and this wastes energy because the fans are being driven to operate at 100%, attempting to deliver the impossible. This energy wastage can be minimised by controlling the cooling tower fans to deliver condensing water temperatures which track the prevailing ambient wet bulb temperature by 3–4ºC, depending on the capability of the tower. The control strategies for optimising chiller effi ciency and cooling tower effi ciency must be coordinated, to ensure that the benefi ts from one don’t negate the other
If a plate heat exchanger (PHE) is installed to protect equipment such as supplementary HVAC units connected to an open circuit cooling tower, ensure that the pressure drop through the PHE is not excessive and that the system is not fouled. If the pressure drop is excessive, typically >30kPa, then consider the addition of more plates to the PHE and re-balancing the water fl ow rate. Pumping energy can be saved by the installation of two port motorised valves at the supplementary air conditioning units. These valves should shut down water fl ow to the supplementary units when the refrigerant compressor is not in operation, thereby enabling the Tenants condenser water pump to be slowed down through VSD control. Ideally this measure should be implemented at design stage, or when supplementary HVAC units are replaced. If this measure is being retrofi tted, to avoid issues with nuisance tripping of compressors, it is important that systems are specifi ed to incorporate the necessary time delays and control interlocks that ensure adequate water fl ow through the units when the compressor is in operation. It is important for this issue to be covered in ‘tenancy fi t-out guidelines’ to ensure that supplementary HVAC units installed by the Building Occupants incorporate such features that save energy consumption of base building systems
Where multiple towers are installed, a lack of a balance pipe to equalise water levels. Water leaks from pipes and faulty valves must be
eliminated
Optimise the water bleed rate in accordance with the quality of the mains water, the water treatment regime and the type of cooling tower. Where automatic TDS controls are installed, they must be calibrated and adjusted correctly. Where such controls do not exist, their installation will be benefi cial because excessive water bleed is wasteful and insuffi cient bleed will lead to corrosion and/or scaling of heat exchangers. Operating cooling towers with cycles of concentration typically below 3–4, indicates water wastage, operating typically above 6–7 produces little gains in water effi ciency, with increased risks of corrosion and other water quality problems
The type and condition of splash guards and drift eliminators must be checked to ensure that water wastage due to poor design and/or condition of the splash guards and drift eliminators is minimised. Replacement drift eliminators must comply with AS 4180 which limits the drift loss to 0.002% of the maximum design water circulation rate through the cooling tower. Aggressive action from water treatment chemicals and exposure to the sun will degrade these components over time The cooling tower fi ll must not be allowed to
gather excessive fouling and regular inspection in accordance with AS 3666 should be carried out. Apart from the microbial hazards, excessive fouling on the fi ll material could also affect the breakdown of water fl ow and make the cooling tower less effi cient by increasing the temperature approach of the tower
The airfl ow around cooling towers must not be restricted. Discharge air from cooling towers must not be allowed to re-cycle into the intake. If prevailing winds are affecting the air fl ow through a cooling tower, it may be necessary to construct a wind barrier
Where a cooling tower bypass valve is installed, there must be no confl ict of controls between the valve being open and the fans being operated. Ensure that there is at least a 2ºC differential between the valve closing and the cooling tower fans operating
The strategy for staging cooling towers and their fans must be optimised. Best effi ciencies will be gained when all available cooling towers are operated in parallel, rather than running the
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Further Information
1. AIRAH DA 17 – Cooling Towers: ISBN 978 0 949436 46 7
2. Sydney Water: Best Practice Guidelines for Cooling Towers in Commercial Buildings. www.sydneywater.com.au
3. Water Effi ciency Guide – Department of the Environment and Heritage: ISBN 06425 52878.