Free cooling systems can generate significant savings for the owners of such systems. However, the amount of potential energy savings available depends almost totally on the overall system design and on the selection of equipment for use in the system. In general, the designer must balance higher equipment cost with greater opportunity for energy savings. Fortunately, these savings — and their associated costs —are reasonably quantifiable so that designers can make intelligent choices guided by reliable information. This article will describe Capacity Control Methods in free cooling design schemes.
Induced draft and forced draft cooling towers require separate guidelines for capacity control during free cooling applications, which are detailed in the following. The sequence of control for a cooling tower operating during low ambient conditions is much the same as a cooling tower operating under summer conditions, provided the ambient temperature is above freezing. When the weather becomes very cold, additional precautions must be taken to avoid the potential for damaging ice formation.
It is very important to maintain close control of the cooling tower during winter operation. EVAPCO recommends that a MINIMUM leaving water temperature of 45°F must be maintained during free cooling operation. However, laboratory testing and field experience has shown that 42°F should serve as the ABSOLUTE MINIMUM leaving water temperature. Obviously, the higher the leaving water temperature from the tower, the lower the potential for ice formation. This assumes that the proper water flow over the cooling tower is maintained and the fan operating procedures mentioned in the bulletin are followed.
The following provide a summary of the capacity control sequences for both induced and forced draft units. A sequence of operation for towers utilizing single speed, two-speed, and variable speed motor controls are shown. Also included are the capacity control considerations for multiple cell cooling towers. In the Appendix of this bulletin, the recommended capacity control sequences are shown in further detail for cooling towers operating during free cooling conditions.
Capacity Control (Induced Draft)
Capacity control of induced draft cooling towers operating during free cooling applications can be achieved using a variety of methods. The most common methods of capacity control are:
- Cycling Single Speed Fan Motors
- Using Two Speed Fan Motors
- Using Variable Frequency Drives (VFD’s)
The simplest method of capacity control of a cooling tower during free cooling operation is cycling the fan motor on and off in response to the leaving water temperature of the tower. However, this method of control results in larger temperature differentials and longer periods of time with the fans off. During extremely low ambient conditions, the moist air may condense and freeze on the fan drive system. Therefore, fans must be cycled during extremely low ambient conditions to avoid long periods of idle fan operation whether the water is flowing over the fill, or in bypass. Please note that excessive fan cycling may cause fan motor damage: the number of fan motor starts and stops should be limited to six per hour. If the building loads are small, the tower will see extended periods with the fans off, causing a greater potential for icing to occur on the intake louvers of the unit. As a result, cycling single-speed fan motors is the least recommended method of capacity control.
Another method of capacity control is to use two-speed fan motors, which include an additional step of capacity control. This step reduces the water temperature differential, and therefore the amount of time that the fans are off. In addition, two speed motors provide a significant savings in energy costs, since the tower has the potential to operate on low speed for the majority of the free cooling season.
The most accurate method of capacity control for an induced draft tower is to use variable frequency drives. This allows much closer control of the leaving water temperature by allowing the fan(s) to run at the appropriate speed to closely match the building load. However, the application of a VFD with an induced draft cooling tower could contribute to the formation of the ice during sub-freezing conditions. As the building load decreases, the variable frequency control system may operate for long periods of time at fan speeds below 50 percent. Operating at low leaving water temperatures and low air velocity through the unit can cause ice to form at various locations in the unit.
Therefore, it is recommended that the MINIMUM speed of the variable frequency drive be set at 50 percent of full speed to minimize the potential for ice to form in the unit during low ambient cooling tower operation. See the Ice Management section for additional information.
Capacity Control (Forced Draft)
Capacity control of forced draft cooling towers operating during free cooling applications can be achieved by several different methods. Similar to the induced draft units, the most common methods of capacity control are cycling single speed fan motors, using two speed fan motors or pony motors, or utilizing variable frequency drives to control the cooling tower fan(s). Although capacity control methods for forced draft units are similar to those used for induced draft units, there are several major differences detailed below.
The simplest method of capacity control for a forced draft units is to cycle the fan(s) on or off. However, this method of control results in larger temperature differentials and periods of time with the fans off. When the fans are cycled off, the water falling through the unit can induce air flow through the fan section.
During extremely low ambient conditions, this moist air may condense and freeze on the cold components of the fan drive system. If conditions change and cooling is needed, the excessive amounts of ice that may have formed on the drive system can destroy fans and fan shafts that are suddenly required to operate. Therefore, fans MUST be cycled during low ambient operation to avoid long periods of idle fan operation. Please note that excessive cycling can damage the fan motors. Limit the number of starts to six per hour.
Two speed or pony motors offer a better method of control than single speed motors. The two speed motor control will allow an additional step of capacity control, reduce water temperature differentials and the amount of time that fans are off, and provide savings in energy costs. This method of capacity control has proven effective for applications where load variations are excessive and winter conditions are moderate.
Variable frequency drives provide the most flexible method of capacity control for forced draft cooling towers. The variable frequency drive control system allows the fans to run at a nearly infinite range of speeds to match the unit capacity to the system load. During periods of reduced load and low ambient temperatures, the fans can be maintained at a minimum speed (25% of full speed) which ensures a positive pressure inside the unit. This positive pressure in the unit prevents moist air from migrating towards the cold fan drive components reducing the potential for condensation to form and then freeze on them.
The variable speed drive method of control should be implemented for applications that experience fluctuating loads and severe winter temperatures.
Capacity Control (Multiple Cell Units)
Multiple-cell induced and forced draft cooling towers require different control sequences than single cell cooling towers. It is important that the fans and water flow over each cell be controlled properly on multiple cell cooling towers.
Proper fan control is essential to avoid the potential for building ice in one of the cells of a multiple-cell cooling tower. All fans in operating cells MUST be controlled simultaneously to avoid freezing conditions in any one cell. The correct method for fan control of a multiple-cell cooling tower during free cooling operation is shown in figure below.
Instead of cycling one fan on and one fan off; both fans should be operated together at low speed in order to reach a 45 degree leaving water temperature. This method of operation has the same 65 degree water entering both cells; however with both fans operating at low speed, this allows 45 degree water to enter the basin from each cell. The final leaving water temperature of the cooling tower is 45 degrees, and the potential for freezing in any one cell is eliminated.
The INCORRECT method of multiple cell fan control is shown below in figure below. In this example, 65 degree system water is pumped through each cell’s water distribution system. However, the fan in Cell 1 is off and the fan in Cell 2 is on. Since the fan is off in Cell 1 water entering the basin is 55 degrees, and the fan of Cell 2 is on water entering the basin is 35 degrees, the resulting net water temperature between the basins is 45 degrees. This meets the required minimum temperature of 45 degrees; however the Cell 2 temperature is too low which may result in localized freezing.
Along with providing the proper fan control, it is also strongly recommended that the leaving water temperature be monitored in all cells. The basin temperature sensors can be placed in either the suction piping or the cold water basin. This can help determine possible icing conditions in each cell. However, if two cells operating together have too much capacity causing excessive fan cycling, then directing all of the load to one cell and shutting down the other cell completely should be considered.
Typically, the winter cooling loads are much less than what is seen during the summer cooling season. Therefore, the water flowrate through the tower may be reduced, which can create the potential for ice buildup on or inside the unit.
It is recommended that the water flowrate be directed to as few cells as possible, since the cooling tower performs better with the system flowrate as near the design flow per cell as possible. This ensures that the water distribution system maintains a proper spray pattern over the fill and avoids a low flow condition that can lead to ice formation inside the cooling tower.
In some cases, design winter flow rates are reduced beyond the lower limits of the spray nozzle performance range. In this case, a special water distribution system can be incorporated to accommodate the winter design flowrate. This design may utilize an additional water distribution system with spray nozzles that are capable of performing under very low flow conditions. Typically, the additional water distribution system is limited to one cell of the induced draft cooling tower.