Maintaining Cooling Tower Operation in Cold-Weather Environments
Although sustained freezing conditions complicate cooling tower operation, key measures can ensure smooth performance.
In applications ranging from industrial process facilities to data centers, many buildings require cooling — even in cold weather. Careful oversight is needed when operating cooling towers in subfreezing temperatures to avoid complications. By following some basic operating guidelines, cooling towers can be successfully operated year-round even in cold climates.
It is important to discuss cold-weather operation requirements with your cooling tower manufacturer. Crossflow and counterflow cooling tower designs have different cold-weather operation characteristics, but there are some practices that are common to both tower designs. They include:
- Operators must maintain adequate heat load.
- Operators should use variable-frequency drives to modulate fan speed.
- Operators should incorporate a design that prevents water from splashing out and forming ice.
Inlet louvers and areas away from the heat source are most susceptible to icing. Light icing on the inlet louver area during subfreezing weather is normal. Typically, it is not a concern. The goal is to prevent excessive icing that can prohibit air from flowing through the cooling tower.
Recommended Practices for Cooling Towers in Frigid Climates
Cooling tower operators should establish cold-weather operation procedures that are implemented when the predicted temperatures are near freezing for more than 24 hours.
Listed below are some basic cold-weather operation principles. They include:
- Do not operate cooling towers without a heat load.
- Maintain a water flow rate over the cooling tower heat exchange media (fill) at the design minimum or greater at all times.
- To prevent icing in cold climates if the heat load drops too low, bypass operating water flow directly to the cold-water basin.
- While fan cycling or two-speed motors can be employed, variable-frequency drives are preferred.
- Do not operate the cooling tower unattended in subfreezing weather.
A closer look at each of these tips will help prepare cooling tower users for cold-weather operation.
Do Not Operate Without a Heat Load. Without a heat load, water flowing through the tower will equalize at the air wet-bulb temperature or form ice, whichever occurs first. This will happen quickly with fans running and more slowly with fans off. Excessive ice buildup can impede airflow and damage the fill, rendering the cooling tower ineffective.
Maintain Water Flow Rate over the Cooling Tower Fill at the Design Minimum or Greater. For any flow rate desired by the operator, care must be taken to maintain at least the cooling tower manufacturer’s minimum water flow rate per individual cooling tower cell. The number of cells receiving water must be adjusted to maintain the minimum flow per cell.
The cooling tower manufacturer may be able to extend the minimum flow percentage to a lower value by using water-distribution design provisions. These provisions accommodate low flow by appropriately reducing the active area of fill while keeping proper pressure head over the nozzles. (Methods of reducing the active area of fill include hot-water basin dams or overflow cups on a crossflow cooling tower.) This leads to even water distribution over the fill and predictable performance.
While ice control depends on a tower’s configuration and its mechanical components, crossflow cooling towers such as the one pictured promote contact between warm water and ice to encourage melting.
Bypass Operating Water Flow Directly to the Cold Water Basins. To prevent icing in cold climates if the heat load drops too low, bypass operating water flow directly to the cold-water basin. Do not modulate the bypass water flow, or the fill can easily freeze in low flow areas.
Use VFDs When Operating Cooling Towers In Sustained Freezing Conditions. While fan cycling or two-speed motors can be employed, variable-frequency drives (VFDs) eliminate cell-to-cell temperature gradients and are preferred when operating cooling towers in sustained freezing conditions. Sequential fan cycling (all on, one off, two off, etc.) can lead to significantly lower temperatures in the cells where the fans are on. This increases the risk of cell freezing. To avoid this, each cell should be equipped with a VFD, and each should operate at the same setpoint temperature.
Do Not Operate the Cooling Tower Unattended in Subfreezing Weather. No matter how automated the cooling tower’s operation, inspectors must physically check the tower regularly in sustained freezing conditions. During temperate weather, conditions can be monitored with remote cameras that communicate with the control room. During the colder weather, however, it is recommended that the operator should conduct on-site inspections of the cooling tower more frequently.
Why Crossflow Cooling Towers Are Better for Cold Weather Operation
Of the two basic types of towers — crossflow and counterflow — a crossflow cooling tower is recommended in most scenarios for cold-weather operation. The water distribution system of a crossflow tower permits significantly lower flows. This design simultaneously avoids development of water channeling in the fill and prevents ice formation in subfreezing temperatures.
Using hot-water basin dams or nozzle cups, crossflow towers keep the heat load toward the side of the fill exposed to weather elements. This promotes contact between warm water and iced surfaces to improve melting during low flow conditions. Furthermore, crossflow towers with inwardly sloping air inlet faces initiate faster melting (compared to a counterflow tower) at the air inlet louver when fans are shut down. Ice control also depends on the type of fill, whether it has louvers attached, the water-distribution system and mechanical equipment arrangements.
For crossflow towers, variable-flow systems such as a nozzle-cup system reduce the tower’s minimum flow while keeping the tower interior moist and heated.
Fill sheets with integral louvers, typically found in crossflow towers, are better suited for cold-weather operation. The louvers are in close proximity to the heat load, which helps limit ice buildup and increases the effectiveness of ice defrosting when the fans are off. Integral louvers, with their tighter spacing compared to blade-style louvers, also provide maximum water containment to avoid ice buildup. Typically, they are thermoformed into one continuous sheet with the heat transfer surface. This provides good water control and helps prevent splashout.
Water-Side Economizers in Freezing Conditions
To reduce energy consumption, design engineers often utilize air-side or water-side economizers to take advantage of cool, low humidity, ambient air conditions for more efficient cooling. An integrated economizer allows gradual transitions in either direction — from economizer operation only, to full chiller operation — allowing for the best control in freezing conditions. Air-side economizers introduce low humidity air to the system rather than recool recirculated air. Water-side economizers complement a cooling tower’s evaporative cooling to maximize heat rejection and minimize chiller power consumption.
This process, called free cooling, makes use of lower outdoor-air temperatures to provide colder water to the system. Such designs can eliminate or significantly reduce the need to operate a chiller. Processes or facilities that use water-cooled systems offer lower energy consumption for most cooling duties.
When operating under a free cooling system, crossflow towers often are specified. The design of crossflow systems provide easier access to perform service and to clean the distribution system even with pumps running. Other advantages include improved water distribution and adaptability to variations in water flow. As a result, crossflow towers keep more cells in operation at reduced loads.
Some icing on cooling towers during subfreezing weather is normal and typically not a concern. The goal is to prevent the excessive icing shown on this tower. Such conditions can impede airflow and render a cooling tower ineffective.
During subfreezing conditions, the water flow rate and the cooling range — or the difference in temperature of the water entering the cooling tower versus leaving the cooling tower — must be carefully considered for economizer operation. Understanding the dynamics of heat load, range and flow rate are crucial for successful economizer operation. Assuming a constant heat load, increasing the flow rate will decrease the cooling range. This leads to a smaller temperature gradient across the fill and relatively lower chance of icing.
Reducing flow and increasing range for the cold-water temperatures desired from an economizer do not ensure freeze protection. Keep the water flow rate up and cycle VFD-controlled fans from the minimum speed to off when needed to keep the system at the highest possible average temperature in the fill when in the economizer mode. The higher the setpoint for the economizer, the lower the freezing risk. A 45°F (7°C) or higher setpoint — at the highest water flow rate that can be maintained — will result in less freezing potential in economizer mode.
In conclusion, cooling towers can be operated successfully in all climate conditions, including freezing environments. Attention to some basic principles during tower operation and to key system design characteristics during tower selection is necessary for success. Use of VFDs on systems of all sizes reduces the potential for freezing.
Owners and designers of projects in freezing climates can confidently take advantage of the significant energy-saving benefits of water-cooled chillers with cooling towers. Utilizing free cooling with water-side economizers offers advantages as well. Success is ensured as the operator carefully follows the guiding principles for cold-weather operation. PC