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. Its the equipment hidden in the back room or outside the building. Its 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?
Were in a competitive global market, trying to reduce costs and drive sustainable and green processes. In the meantime, were trying to stay ahead of regulations and increased utility costs.
Thats why its time to look more closely at process cooling. Over the years, weve 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 worlds 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 manufacturers 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.
Enter an eco-friendly, closed-loop, dry-cooling system. This intelligent process cooling system provides economic and environmental advantages that are particularly important to industry today. These closed-loop systems are designed to dramatically reduce water use and keep water clean while minimizing costly chemicals.
· Closed-loop design ensures that the process water is never exposed to outside elements and never disposed of into ground water.
· Water returning from a process is pumped into a heat exchanger and cooled with ambient air, providing clean water at the right temperature year-round.
· Intelligent control systems maintain the desired water temperatures, even during extreme hot and cold weather conditions.
· To maintain water temperature in hot weather (85°F or higher), outside air passes through an adiabatic chamber before reaching the air-to-water heat exchange coils. A fine mist of water (from a separate source) is pulsed into the air stream, evaporates immediately, and cools the air before it impinges on the cooling coils. Only cool, dry air makes contact with the heat exchange coils hence the term dry cooling. (See Chart 1 below.)
· To ensure consistent cooling, the control system continuously adjusts the amount of water sprayed. During colder months, the system offers a fully automatic, self-draining function that protects it from freezing and also provides for free-cooling, if applicable, when ambient temperatures permit. Non-draining units also are available for non-freezing climates or for use with a glycol solution in freezing climates.
· Advanced microprocessor control systems feature easy-to-use, remote interface for monitoring temperatures, pressures, and alarms, while also controlling fan speeds, adiabatic functions, and pumping stations. Systems also will take into account real-time ambient temperature and automatically adjust system operation accordingly, taking into account the set point.
Using the closed-loop, dry-cooling system described here, water use is typically reduced to 20,000 to 40,000 gallons per year per 100 tons of capacity, compared to millions of gallons for an open tower with heat exchanger of similar capacity as shown in this article. In addition, these systems save up to 95 percent of the energy typically associated with conventional tower/central chiller systems. In the quest for greater efficiency, sustainability and competitiveness, such potential results should not be overlooked by any processor.
Lou Zavala is national sales manager for Frigel, a leader in intelligent process cooling since the 1960s. Frigel has expanded its reach to bring the Ecodry technology to North America in recent years. A provider of process cooling and temperature control equipment, Frigel also helps companies reach their performance and sustainability goals through process improvements. For more information or for an energy/water usage audit and analysis, contact Frigel at (847) 540-0160 or visit www.frigel.com/na.