Fresh water is a scarce resource in South Africa. Whilst energy availability is a challenge now, many experts predict that water shortages will become a critical issue for us within the near future. Energy and water costs are rising steeply every year with the result that energy & water conservation should form an important part of every organization’s sustainability strategy. Cooling Towers & Heating, Ventilation and Air Conditioning (HVAC) systems require significant energy for pumps and motors and water for the heat transfer and cooling, using the evaporative process.
Typical open-circuit cooling tower:
1. Energy optimisation
The energy requirement can be optimized by ensuring that motors and pumps are sized appropriately and flow rates optimized for the cooling requirements of the building’s tenants. This requires sophisticated testing and analysis to ensure that the energy objectives are achieved without compromising the effectiveness of the cooling equipment. Further, flow rate meters may need to be tested and recalibrated. Investment in optimizing the energy efficiency of the pumps and motors in a cooling tower system that is 10 years or older can achieve payback of less than 2 years, making it a good investment.
2. Water optimisation
Cooling towers can use approximately 4 kl of water per annum per 1 kW of energy required for cooling purposes. This typically makes cooling towers one of the largest users of potable water in commercial, institutional and industrial buildings, often accounting for 15-30% of total water consumption of the building. Office buildings are at the top of the cooling water use list, followed by manufacturing, education, hospitals and lodging.
Cooling towers lose water in a number of ways:
- Drift loss
- Bleed-off or Blow-down
- Leaks and overflows
- Make-up water
Water lost through “drift” can account for a substantial portion of the total water loss in a cooling tower system. New drift loss eliminators can cut down water lost through drift. This new equipment is relatively inexpensive and can normally be installed easily and quickly. The pay-back period on installing new drift loss eliminators can vary between 2.5 and 4.0 years, depending upon the age of the old drift loss eliminators and the cost of the make-up water. Investment in new drift loss eliminators in older cooling towers (10+ years) can achieve payback of less than 4 years, and save a substantial amount of potable water.
Bleed-off normally accounts for the largest operational water “loss” in a cooling tower. Consequently, reducing the quantity of bleed-off water by treating and recycling this bleed-off water is the single largest water saving opportunity with cooling towers.
Bleed-off is a function of total dissolved solids (TDS) in the water circulated through the cooling tower. As the TDS increases, water is dumped from the system and replaced with fresh make-up water in order to lower the average TDS of the water in the system.
The chemical treatment regimen is the first step in reducing bleed-off. As water circulates through the cooling system and a portion is evaporated in the cooling tower, the concentration of solids – minerals, organics, contaminants and corrosives – increases until it degrades the performance of, and can actually damage, the cooling equipment.
In-process water treatment can enable the water in the cooling system to hold a higher concentration of solids without interfering with system efficiency or causing corrosion or biofouling. This capacity to suspend solids and other problematic molecules is expressed as the system’s “cycles of concentration” or “concentration ratio” or “conductivity ratio”. It is the ratio of TDS in the bleed-off water to the TDS levels in the make-up water. The cycles of concentration should be maintained as high as is practically possible without compromising the performance of the cooling equipment. For example, safely increasing cycles of concentration from 2 to 6 will reduce your bleed-off and resultant demand for make-up water by 40%. It is possible to raise the cycles of concentration to 9 or 10 – depending on the quality of make-up water in terms of TDS, pH, hardness and bio-levels.
Whilst it is possible to test water in the cooling tower and bleed-off manually, automated controls are simple and relatively inexpensive. Timer-based controls are the first level of automation, suitable for smaller applications. A conductivity controller provides more accurate feedback. Conductivity is a measure of water’s ability to conduct electricity due to the relative concentration of dissolved salts, and is a good indicator of the TDS level in the water i.e., the more salts, the better the conductivity and the higher the TDS.
Other automated controllers can gauge the need for and amount of bleed-off and make-up water by pH and other factors. Investment in these automated controllers can achieve 2-3 year payback, making it a good investment.
Make up water
Make up water can also be sourced from rainwater harvesting or capture investments with the result that this water does not have to be treated to potable standards. Rain water can also be very low in TDS with the result that it can be used to increase the cycles of concentration. An investment in a rainwater harvesting system can have payback of less than 4 years provided it is sized appropriately and that the rainwater can be efficiently “captured” on site.