For decades, many plastics processors have relied on open-loop cooling towers to meet the bulk of their process cooling requirements. These structures typically cycle warm, process-return water through baffles to expose it to ambient air. As a portion of the warm water evaporates into the air, the rest is cooled as it falls back into the cooling tower reservoir, ready for recirculation.
For all of its simplicity and effectiveness, however, tower-based process cooling systems have their costs. Now, an increasing number of processors are recognizing the potential of adiabatic cooling towers, a closed-circuit cooling system design that dramatically reduces water usage, energy, and maintenance costs and evaporation, while they are cooled by the ambient air.
Open-loop cooling towers have their drawbacks
For decades, many plastics processors have relied on open-loop cooling towers to meet the bulk of their process cooling requirements. These structures typically cycle warm, process-return water through baffles to expose it to ambient air. As a portion of the warm water evaporates into the air, the rest is cooled as it falls back into the cooling tower reservoir, ready for recirculation. For all of its simplicity and effectiveness, however, tower-based process cooling systems have their costs: Because cooling capacity is based on evaporation, water consumption is continuous and fresh water must be continually replenished. And, because these systems are open to ambient air, cooling water quality must be continually monitored and maintained. Multiple forms of chemical treatment are necessary, not only to prevent the growth of harmful bacteria but also to prevent the accumulation of harmful particles or deposits in cooling lines, molds, or machinery linked to the cooling system. These concerns, together with increased water-use regulations, the drive to conserve energy and natural resources, as well as rising water, sewer, and electrical costs, make tower-based cooling less attractive than it used to be.
Closed loop, energy efficient, environmentally friendly
Now, an increasing number of processors are recognizing the potential of adiabatic cooling towers, a closed-circuit cooling system design that dramatically reduces water usage, energy, and maintenance costs and evaporation, while they are cooled by the ambient air. The only water consumed in an adiabatic cooling system is that which is applied onto evaporative cooling pads mounted at the front of the cooling tower, ahead of the cooling coils through which the process coolant flows. As ambient air passes through these wetted panels, the water within them evaporates, increasing the cooling capacity of the tower by reducing the temperature of the air flowing through the cooling coils.
Understanding the adiabatic process
Adiabatic cooling is a simple, natural process that has long been used as a low-cost way to cool volumes of air. Scientifically, here is how it works: When warm air is exposed to moisture – such as when warm air is drawn through the wetted pads in an adiabatic tower – the air is cooled as the water evaporates. In that process, water absorbs heat from the air and uses that energy, in the form of latent heat, to change its state from liquid to vapor (evaporation). The water or water vapor itself does not undergo any temperature change during this process. Interestingly, the total amount of heat in the volume of air, or air plus humidity, does not change due to adiabatic cooling. However, the measured temperature(dry-bulb, or thermometer temperature) of air flowing through an adiabatic cooling tower is reduced because of the latent heat that is bound up with the water vapor. Adiabatic towers are cost-effective because, in many cases, they are capable of cooling very effectively without the need to utilize the adiabatic process at all. When air temperatures are low, the flow of ambient air alone – drawn upward through the adiabatic panels and the cooling coils – is often sufficient to meet process cooling requirements. “Dry” or water-free operation is often possible during the cool months that occur in many climates, minimizing or eliminating the need and costs of supplemental mechanical chilling. Compared to conventional cooling towers, an advanced adiabatic cooling tower can reduce water usage by as much as 80%.
Adiabatic control systems optimize water and energy efficiency
The efficiency and performance of the today’s adiabatic cooling towers, including the latest EarthSmart™ towers from Conair, are optimized with the help of advanced control systems, managed by application-specific PLCs. All feature variable-speed, electrically commutated fans and circulating motors that ensure precise regulation, making it possible to coordinate motor speeds, match specific cooling load requirements, and minimize energy consumption. EarthSmart adiabatic cooling towers offer three programmable operating modes that give processors the flexibility to manage their water and power consumption costs as needed to meet changing costs and plant operating requirements: In dry (normal) mode, the control system regulates fan speed, air flow, and process coolant flow to achieve the desired process cooling set-point. No adiabatic cooling water is used. In water-saving mode, the control minimizes water consumption in favor of increased airflow to achieve the desired process-cooling set-point. In this mode, the tower makes maximum use of fan speed and ambient air flow to provide process cooling capacity. The control system allows for use of water on the adiabatic cooling pads only if airflow at maximum fan speed is insufficient to achieve or maintain the process cooling set-point. In energy-saving mode, the control system reduces electrical consumption by activating use of adiabatic cooling as soon as power consumption reaches a user-defined set-point. Adiabatic cooling is then increased until process cooling needs are satisfied or adiabatic capacity is up to 100% before power consumption is increased. The closed-loop design of adiabatic cooling towers not only minimizes the consumption of water due to evaporative cooling requirements, but also dramatically reduces the possibility of coolant contamination, coolant fouling, and the related costs of water monitoring and chemical treatment. In place of water as a process coolant, processors may adopt a mix containing 35% propylene glycol mix, which will also help prevent freezing during cold weather. Reduced water consumption also dramatically reduces sewage costs.
Adiabatic coolers serve a range of cooling capacity requirements
Conair offers eleven models of EarthSmart™ adiabatic cooling tower (ESTW Series), to handle nominal process coolant flow rates from as little as 45 gal/min (170 l/min) up to 510 gal/min (1930 l/ min), with cooling capacities ranging from 17 tons to 195 tons.