|Process Cooling: Optimize Plant Processes and Drive Down Costs|
|by Lou Zavala, Frigel|
|Energy Savings Summer 2010|
For years, process cooling has been an important part of the plastic processing
plant. Properly controlled cooling is critical to the overall productivity of
the plant, the life of its equipment, and the quality of the products it makes.
Yet, in my experience, the plant cooling system is often an afterthought. It’s the equipment hidden in the back room or outside the building. It’s only recognized when there is a high-temperature alarm or when parts are being rejected because of a plugged mold. Do we really know the cost of operation in water and energy consumption?
We’re in a competitive global market, trying to reduce costs and drive sustainable and green processes. In the meantime, we’re trying to stay ahead of regulations and increased utility costs.
That’s why it’s time to look more closely at process cooling. Over the years, we’ve found ways to overcome some of the challenges of an open tower and chiller system.
Regulatory Pressures and Utility Costs
Industry uses 22 percent of the world’s clean water and the increasing water and energy pressures, combined with processors’ need to operate more cost effectively, poses a great challenge.
Increasingly, the federal government and local municipalities are pressuring manufacturers in all industrial sectors to reduce energy and water consumption and minimize process water discharge into sewer systems. Many local municipalities are providing rebates or incentives to invest in equipment that will reduce consumption.
Processors have many good reasons to make these investments. According to Dr. Robin Kent, founder of Tangram Technology, a U.K.-based plastics industry consultancy:
· Approximately 92 percent of a manufacturer’s energy consumption is attributed to processing machinery and associated services.
· 16 percent of that energy goes to process cooling (chillers and pumps).
· Improperly maintained cooling towers lose heat transfer efficiency and scale, causing chillers to consume 2.5 to 3.5 percent more energy for each degree rise in condenser temperature.
· Energy savings of 25 percent are easily achievable with virtually no technical risk.
According to the U.S. Department of Energy, energy efficiency projects are the most attractive investments in industry, with internal rate of returns above 20 percent and investment risk rivaling the safest opportunities available anywhere. Given this combination, why are energy projects so difficult to sell? Part of the problem rests with corporate structure - most projects are championed from the facilities side and have to be sold to management bottom-up. Companies need to get all stakeholders involved.
Conventional Cooling Towers
Traditional cooling towers are, by nature, dependent on continuous water use by evaporation. There also is the requirement for continuous disposal of the process water (known as bleed-off) to control hardness levels. Such systems require a high level of maintenance and consume large amounts of water, energy, and chemicals.
Evaporation accounts for the largest loss of water from a cooling tower system. To achieve one ton of cooling, a tower must evaporate around 0.03 gallons of process water each minute. This evaporation rate is independent of the system flow for typical operating temperatures. The following example of evaporative loss is based on 8,760 operating hours per year (24 hours per day, 365 days per year) with tower operating at full capacity:
100 Ton Tower 3 GPM 1,576,800 GPY
Typical open loop cooling towers have been called “air scrubbers,” because airborne dust and other contaminants inevitably end up in the process water loop. In addition, they also suffer from algae, bacterial/legionella, and microbiological build-up, as well as scale accumulation. These must be corrected with scale and corrosion inhibitors, microbiocides, and heavy filtration. This results in the need for constant testing of the water and injection of replacement chemicals.