Cooling towers are used to dissipate heat from air conditioning or industrial process systems. Many of the air conditioning systems currently in use only operate during the summer cooling season, but there are numerous air conditioning and process systems that require cooling year-round. In some cases, the entire cooling system is required to operate during the winter. The cooling tower is required to provide the same 85° F (30° C) or colder water to the system as it does in the summer, but it does so at lower ambient temperatures. However, there are some applications designed to use the cooling tower for “free cooling”. Free cooling is when chilled water is cooled by cooling tower water through the use of heat exchangers without the use of refrigerant compressors. Free cooling can be accomplished when ambient conditions allow the cooling tower to produce “chilled water” for the system.
When the cooling tower is providing “chilled water” to the system, there are periods of time when it must operate in subfreezing conditions. During these periods, when the tower is subjected to very cold ambient conditions, there is greater potential to produce ice in the cooling tower or elsewhere in the system. If an inappropriate cooling tower design is chosen, or if the unit is not operated or winterized properly, excessive amounts of ice can form in the unit resulting in decreased capacity, operational difficulties, and potential damage to the tower.
Cooling tower performance in free cooling applications is dependent upon both the system and cooling tower design. The control sequences applied to the cooling system must consider management of both the air and water side of the cooling tower. It is essential that the proper control sequences be applied during free cooling to ensure adequate operation of the cooling tower during low ambient conditions.
When a requirement for free cooling is specified for a project utilizing cooling towers, certain considerations must be made from the very beginning of the project design.
First, special care must be taken when laying out the cooling towers to prevent recirculation due to poor tower location and prevailing winds. If a strong prevailing wind is present, adding “wind walls” to an induced draft tower should be considered.
Second, the cooling tower should be equipped with basic options such as basin heaters, electric water level control, and vibration switches to prevent operational issues due to basin freezing if a remote sump is not possible.
Lastly, capacity control must also be carefully considered, especially if the winter cooling load is significantly less than the summer load. Shutting down individual cells of a multiple cell cooling tower or employing a low-flow header on a singe cell should be utilized. A minimum leaving water temperature of 45°F must be maintained at all times.
In a counterflow cooling tower, the fill is completely enclosed and protected from outside elements, such as wind, that can cause freezing of the fill pack in low ambient conditions. Additionally, the fill pack is supported from below to prevent sagging in the event freezing should occur due to a system imbalance. The fans, fan motors, and drive systems of Evapco counterflow cooling towers are also designed to be safely run in reverse at up to 50% normal fan speed. However, there are several items to consider when operating a counterflow cooling tower in a free cooling mode.
This engineering bulletin will examine the cooling tower design considerations for both forced and induced draft units, and proper maintenance procedures to ensure successful operation of the cooling tower during free cooling.
An examination of free cooling should begin with a review of the normal operation of the cooling tower during the summer cooling season. The schematic shown in Figure 1 details the operation of the cooling system during typical summer conditions. The chiller is operational and cooling the system chilled water. In a traditional system, the chilled water returns from the conditioned space at 55° F where it has absorbed the heat from the conditioned space. It is then cooled in the evaporator shell of the chiller before being sent back to the conditioned space at 45° F. For these conditions, the chiller and the cooling tower are operating while the heat exchanger shown is isolated from the system and is not included in the system operation.
Simultaneously, the cooling tower is absorbing the building load plus the heat of compression. The cooling tower then transfers this heat load to the atmosphere. In a typical air conditioning system, the water leaves the condenser shell of the chiller and enters the cooling tower at 95° F. The hot water is then cooled to 85° F and then sent back to the condenser shell of the chiller to continue the heat transfer process.
Free Cooling Operation
During free cooling, the chiller is not operating. The cooling tower absorbs the building heat load and rejects it to the atmosphere. During the free cooling operation, the cooling tower does not need to remove the heat of compression since the chiller is not operating. The chiller is isolated, and the water from the cooling tower and the conditioned space is bypassed to the heat exchanger. Low ambient conditions allow the cooling tower to provide “chilled water” temperatures as low as 45° F to the primary side of the heat exchanger where it absorbs the building heat load before being returned to the cooling tower. A 2° F approach can be achieved in the heat exchanger where 47° F water on the secondary side provides cooling to the building. Since the cooling loads and the requirement to remove the humidity from the building are reduced in the winter, the “free cooling” chilled water temperatures can be higher than those during summer operation.
The temperatures shown in the schematic are typical but are dependent on system loads, winter design conditions, and desired building temperatures. The design engineer is responsible for defining the system parameters which will allow you to select an appropriate cooling tower for the free cooling operation.
Free Cooling: System Design Considerations
When considering a free cooling application, it is important to properly plan the design of the cooling tower system. The following items should be considered during the design phase of a project:
- Cooling Tower Selection
- Unit Layout
- Cooling Tower Piping
- Cooling Tower Accessories
- Transition from Free Cooling to Mechanical Cooling
Cooling Tower Selection
The first item to consider when designing a cooling tower system is the primary design condition – summer conditions or winter (free cooling) conditions. This design condition will drive the unit selection. If the winter condition is driving the unit selection, a larger unit will be required than would be normally selected for summer only operation. This occurs because it is more difficult for the cooling tower to reject heat at low ambient operating conditions.
Further, although a single cell unit may meet the summer and winter design conditions, a multiple cell unit may be a better selection for winter operation. Since the water flow rate for winter operation may be less than the summer flow rate, it can be concentrated in fewer cells, which allows the flow rate per cell to remain high, thus reducing the potential for ice formation inside the tower. Multiple cell units also provide backup capacity if an operating cell requires defrosting or were to fail.
Careful consideration must be given to the proper location and layout of the cooling tower(s) on every project. Adequate unobstructed air flow must be provided for both the intake and discharge of the unit. It is imperative for cooling towers used for free cooling that the equipment layout minimizes the potential for recirculation.
During summer operation, recirculation can dramatically reduce the cooling tower capacity, however during winter operation it can result in condensate freezing on the inlet louvers, fans, fan shafts, and fan screens. The buildup of ice in these areas can adversely affect air flow to the unit or, in more severe cases, lead to failure of these components. Manufacturing and consulting companies strongly encourages the use of a vibration switch on units that are to be used for winter operation.
See Figure 3 for correct and incorrect installations for forced and induced draft units. Cooling tower performance can be affected by prevailing winds. High winds can create icing conditions on the inlet louvers and fan screens, adversely affecting air flow to the tower.
In addition, the prevailing winds in poor unit layouts can cause a downward airflow of the moisture laden discharge air, which can condense on the unit surfaces and quickly freeze. This phenomenon promotes ice formation on the inlet louvers of induced draft units and on the fans of forced draft units.
Cooling Tower Piping
When designing a cooling tower system for free cooling applications, several piping details should be considered to ensure proper winter operation of the unit. A cooling tower bypass needs to be incorporated into the system design to allow water to “bypass” the tower’s water distribution system as a means of capacity control during low load conditions. There are several ways to design the system piping to accommodate the cooling tower bypass.
It is recommended that the cooling tower bypass be installed in the condenser water piping system. A bypass installed in this manner will require a section of pipe between the condenser water supply and return leading to and from the cooling tower.
Bypassing the cooling tower water directly into the cold water basin is another method of a cooling tower bypass. In either method of bypass (in the system piping or tower sump), it is good practice to install the bypass valve below the cold water basin level to assure good head pressure on the valve.
Regardless of what type of bypass arrangement is used, it is recommended that only a FULL FLOW BYPASS be used during free cooling operation. This means that the total flow rate to the tower must either be sent to the water distribution system or bypassed.
ALTHOUGH IT MAY BE ACCEPTABLE TO USE A PARTIAL BYPASS DURING THE SUMMER, NEVER USE A PARTIAL COOLING TOWER BYPASS DURING FREE COOLING OPERATION!
Reduced flow over the tower can result in uneven water flow over the heat transfer media (fill) which can cause scaling during summer operation and ice formation during winter operation.
Another important consideration during free cooling system design is to ensure that the necessary piping and accessories are heat traced and insulated. All water inside the cooling tower drains (by gravity) to the cold water basin – no additional provisions are required within the cooling tower. However, all external piping that does not drain (makeup water lines, equalizers, and riser piping) must be heat traced and insulated to ensure that they do not freeze. System piping accessories (makeup water and control valves, water circulation pumps, and water level control packages) also require heat tracing and insulation. If any of these items are not heat traced and insulated, the ensuing ice formation in these components may result in failure causing a shutdown of the cooling tower(s).