A Water-Saving, Hybrid Approach to Process Plant Cooling
Used in conjunction with a traditional, full-size cooling tower, the hybrid cooling system offers a way to reduce environmental impact.
In cooler weather, you may sometimes notice puffs or cloud plumes rising from cooling towers. They have been around for decades — a silent reminder of the millions of gallons of water consumed each year as industry seeks cost- and energy-efficient cooling for myriad systems in process plants. Even with modern plant technology, keeping these processes cost efficient is a challenge. Steady population and economic growth rates place price pressures on industrial plants due to rapidly increasing water prices and growing concerns about water availability. What happens to plant output and profitability when the cost of water increases or water supply is curtailed?
Process plants in Texas dealt with this reality when a severe drought struck the state in 2011. Plants faced looming curtailments, water quality degradation and physical water shortages. Yet these challenges are not unique to Texas. Similar droughts are occurring across the nation and globe at a much higher frequency than previously recorded. Formerly safe locations are no longer guaranteed immunity from drought events.
In chemical manufacturing and refining, 67 to 92 percent of the water used is dedicated for process cooling or steam systems. Within those industrial facilities, cooling towers are often the thirstiest devices. For example, a 500-ton base-loaded chiller requires about seven million gallons of cooling tower water makeup per year. Other examples of systems with high cooling tower makeup water demand include:
- 6,000-ton process load: 65 million gallons per year.
- 500 MW thermoelectric power plant: 2.5 billion gallons per year.
- 10 MW data center: 40 million gallons per year.
Historically in many processes, most of the focus has been on energy consumption and costs. Today, however, water, wastewater and chemical treatment costs are becoming significant components of the overall price of operating a cooling system. It is no longer sufficient to just look at the energy use. It is crucial to look at the total utility expense. Part of looking at this total expense is examining how water is currently used for these processes — and if other options make better sense.
Evaporative Heat Rejection
Evaporative heat rejection (EHR) is the primary driver of water consumption in many cooling processes. EHR works similarly to fanning away sweat on a hot day. During this process, open cooling towers reduce the temperature of water heated in chillers, industrial processes, data centers and other high heat practices. A closed loop of cool water is used to extract heat from the hot process equipment. The now warm water is circulated to the top of the cooling tower. The warm water is sprayed over the fill within cooling tower (fill increases the water contact area). Heat is removed from the water through evaporation of some of the water. The cool water that remains in the basin is recirculated back through the closed loop to complete its process cooling journey.
A constant supply of water is needed to replace the water evaporated from the cooling tower. In many regions, however, continuing droughts and increasing competition for this vital resource limit water availability. Additionally, some water is continuously bled from the system to reduce the buildup of undissolved solids created as water is evaporated. This generates a large wastewater stream, often containing many additional water treatment chemicals.
Evaporative heat rejection offers several advantages. On the hottest days, the process produces much cooler water temperatures than dry cooling. This improves process efficiency and capacity. EHR also has a lower initial cost, requires less parasitic energy and requires less overall space than dry cooling.
The main disadvantages of evaporative heat rejection are that the process consumes massive amounts of water and produces wastewater. It also requires chemical water treatment to combat issues related to corrosion, scale and biological growth. During cool or freezing weather, EHR creates the potential for plume and increases the potential for icing issues. Finally, EHR contributes to a plant’s higher water footprint.
A technology that could deliver the efficiency and capacity advantages of EHR with less annual water consumption could provide advantages for some plants.
When weighing options, it is important for plant owners to understand the total economic impact of water-saving cooling alternatives vs. base cooling tower systems. To do so, several economic factors must be considered.
- Some economic factors favor water-saving technologies. They include the ability to:
- Avoid the costs of purchased water, chemical treatment and blowdown disposal.
- Maintain full production during periods of water constraint.
By contrast, some economic factors disfavor the implementation of water-saving technologies. These include:
- The additional capital cost for equipment, energy and lost production capacity (if applicable).
- The need for additional space to house the equipment.
Plant owners also must consider:
- How easily the technology can be retrofitted into the facility.
- The water-saving technology’s impact on overall system reliability and maintainability.
- How the technology will affect the company’s public image relating to water conservation.
A hybrid cooling technology has shown positive results in reducing plumes and saving 25 to 80 percent of water usage when compared to all-evaporative heat-rejection systems. The hybrid system maintains peak evaporative heat-rejection performance on even the hottest days. Used in conjunction with a traditional, full-size cooling tower, the hybrid cooling system offers a way to reduce environmental impact. At the same time, the hybrid system can lower total utility operating costs and increase the resiliency of critical cooling operations.
The system offers dry cooling using a thermosyphon process. Refrigerant circulates naturally without need for a pump or compressor. Internet-connected controls coordinate the operation of the wet and dry system components, and they adjust in all weather and thermal load conditions for optimum efficiency. The controls utilize wet cooling when it is hot and modulate toward dry cooling when it is not. The dry heat-rejection device was specifically designed to work in open-loop cooling towers. Its design provides ease of tube cleaning, assurance against low water pressure drop and automatic freeze protection (figures 1 and 2).
The system is based on using an additive approach to a full-size cooling tower system. When it is too hot outside, or when the process temperatures are not warm enough, it is inefficient to use dry cooling. In addition, intelligent controls respond to changing weather and system conditions as well as the site’s energy and water prices. It adjusts the fan speeds for the wet and dry units for optimum system water and operational cost savings.
A Call to Action
Process and manufacturing plants are rightly concerned about their vulnerability to water restrictions along with the accompanying operational, productivity, economic and environmental image risks. Water is a precious resource and global demand is only increasing. Hybrid cooling technology has the potential to make a difference by reducing industry’s impact on the environment and freeing up water for more critical uses.