Variable-speed Drives On Cooling Tower Fans Reduces Energy And Water Costs

Phil Black - PII Editor 11/11/2016

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Any manufacturing or processing facility that requires heat to convert raw materials into a product is likely to require some form of cooling. In industrialised nations, cooling towers are plentiful – used on large manufacturing projects, light industry, HVAC and refrigeration applications.

In South Africa, a small economy by global standards, it is estimated that more than 50 000 package cooling towers (including closed circuit fluid coolers) are in operation. Each of these is equipped with an electrically-driven fan of some description. These 50 000 package cooling towers potentially represent an installed electrical power base, for fan motive power only, exceeding 350 MW.

Globally, energy prices are increasing and South Africa is no exception. More than this, worldwide, water scarcity has become a major issue. This is particularly true in developing countries. Uneven distribution of water around the globe has resulted in it being a non-market to a market goods, which has impacted on water policy and water pricing.

So it would make sense that one of the most profitable ways to increase energy efficiency and reduce energy and water costs is by making use of Variable-speed Drives (VSD) on cooling tower fans.

Says Roger Rusch, MD of IWC (Industrial Water Cooling), “Variable Speed Drives adjust fan speed as conditions change, regulating airflow to compensate for fluctuations in ambient air and load conditions, while maintaining the exact flow required.”

Most cooling towers are manufactured to meet predicted peak-heat loads in the most unfavourable wet-bulb atmospheric conditions.

Says Rusch, “In examining weather data for stations located in Johannesburg and Durban, January, February and December are the peak wet-bulb periods for each of the two stations – where the cooling towers fan shaft-power are at maximum for the given operating points, thus for the most part, a cooling tower’s lifespan is spent operating at much lower than the designed duty point and wasting energy unnecessarily.”

The following graphs show the potential saving on a typical package cooling tower unit having a design fan shaft power of 7.5 kW.
In Johannesburg, significant savings on power is available from March through to November – a saving of approximately 51%.

The average shaft power for the year for this Johannesburg site is calculated as 3.7 kW. On the basis of the weather data used, this represents a potential electrical saving of approximately 51?% or 31 832 kWh.

In Durban, savings on power is available from April through to November – a potential electrical saving of approximately 53.5 %.

The average shaft power for the year for this Durban site is calculated as 3.49 kW. On the basis of the weather data used, this represents a potential electrical saving of approximately 53.5 % or 33 371 kWh.

Says Rusch, “Considerable energy and water savings are possible with the use of Variable Speed Drives (VSD) on mechanical draught cooling towers. These coupled with the use of the latest-generation, high-efficiency electrical motors pose substantial long-term savings in operational costs annually. Variable Speed Drives offer not only cost savings but provide a long-term environmentally sustainable solution to reducing global energy and water consumption.”

The cost of electricity saved per year for each location, based on a global average price in US$ cents/kWh given in Figure 1 & 2 above is as follows:

Johannesburg = 31 832 kWh x $ 0.0981
= $3122.72 per year saved
Durban = 33 371 kWh x $ 0.0981
= $3273.69 per year saved

At time of writing, the average cost of a Variable Speed Drive for a 7.5 kW electrical motor is approximately US$ 1 500.00. At both locations, the payback on capital expenditure would be about 6 months. This is based entirely on the electrical savings presented, and doesn’t consider the cost of water nor the savings from reduced water treatment costs that are attributable to the reduction in water consumed by the cooling tower resulting from lower evaporation and blow-down rates.

On face value, water savings aren’t of the same magnitude as the electrical savings but this should not be ignored. This is especially relevant in larger cooling tower applications where cooling water recirculating flow rates can be in excess of 100 times the flow rate contemplated herein. This is particularly relevant in South Africa where both water restriction and the cost of power are on the rise.

Says Rusch, “When using a VSD, slowing down a pump or fan from 100 to 80% can reduce a motor’s energy usage by up to 50%, and thus considerable energy and water savings can be achieved across all industrial cooling tower applications. Furthermore every kilowatt-hour (kWh) of electricity saved means one less kilogram of carbon dioxide generated by a power station”.

Concludes Rusch, “When IWC supplies the VSD, we guarantee the power saving.”