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Engineering Considerations
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Units must have an adequate supply of fresh air to the air intake(s). When units are located adjacent to building walls or in enclosures, care must be taken to ensure that the warm, saturated discharge air is not deflected off surrounding walls or enclosures and drawn back to the air intakes(s).

CAUTION: Each unit should be located and positioned to prevent the introduction of the warm discharge air and the associated drift, which may contain chemical or biological contaminants including Legionella, into the ventilation systems of the building on which the unit is located or those of adjacent buildings.

For detailed recommendations on layout, refer to the Layout Guidelines or contact your local BAC Representative.

Piping and Valves

Piping must be sized and installed in accordance with good piping practice. All piping should be supported by pipe hangers or other supports, not by the unit.

Some installations may require flow balancing valves (supplied by others) at the coil inlets to balance the flow to individual coils and cells. External shutoff valves on the closed circuit loop (supplied by others) may also be required if the system design necessitates the isolation of individual cells.

Although equalizing lines can be used to balance water levels between multi-cell closed circuit cooling towers, the spray water for each cell must be treated separately, and a separate make-up must be provided for each cell. Note that a common remote sump for multi-cell installations can simplify make-up and water treatment.

Capacity Control

Variable frequency drives offer the most precise control of leaving fluid temperature or condensing pressure and the lowest operating cost. VFDs provide compliance with the part load power consumption and speed control requirements in current energy codes, such as ASHRAE 90.1 and California Title 24. In addition, soft-starts, stops and smooth accelerations prolong the life of the mechanical system. Sound is also reduced by minimizing start-up noise and running the tower at the lowest speed necessary to meet the system demand.

VFD reliability has improved and first costs have come down over the years. This, combined with the system benefits noted above, makes VFDs the most preferred method for controlling evaporative cooling equipment. Fan cycling and two speed motors are used less frequently as a result. Note that units with VFDs require the use of inverter duty motors, designed per NEMA Standard MG 1, Section IV, part 31. This standard recognizes the increased stresses placed on motors by these drive systems. The use of a non-inverter duty motor in these applications may void the motor warranty.

WARNING: When the fan speed is to be changed from the factory-set speed, including through the use of a variable speed control device, steps must be taken to avoid operating at or near fan speeds that cause a resonance with the unit or its supporting structure. At start-up, the variable frequency drive should be cycled slowly between the minimum allowable setting (6 Hz for belt drive or 15 Hz for on gear drive) and full speed and any speeds that cause a noticeable resonance in the unit should be “locked out” by the variable speed drive.

Fan cycling is the simplest method of capacity control. However, there are drawbacks to fan cycling that limit its application. These drawbacks include:

  • Hard starts and stops for the fan and motor which stresses the mechanical drive system
  • Sudden sound level increases or decreases due to the starting and stopping of the motor
  • Difficulty maintaining control of the design set point (temperature or pressure) as the fan cycles on and off

Therefore, if capacity control is required at off-design conditions, BAC recommends using VFDs.

Vibration Cutout Switches

Vibration cutout switches are recommended on all installations. Vibration cutout switches are designed to interrupt power to the fan motor and/or provide an alarm to the operator in the event of excessive vibration. BAC offers both electronic and mechanical vibration cutout switches on all closed circuit cooling tower models.

Water Treatment

As water evaporates in an evaporative cooling unit, the dissolved solids originally present in the water remain in the system. The concentration of these dissolved solids increases rapidly and can cause scale and corrosion. In addition, airborne impurities and biological contaminants, including Legionella, may be introduced into the circulating water. To control all potential contaminants, a water treatment program must be employed. In many cases, a simple bleed-off may be adequate for control of scale and corrosion. Note: Bleed lines are to be provided and installed by others. However, biological contamination, including Legionella, can be controlled only through the use of biocides. Such treatment should be initiated at system startup, after periods of equipment shutdown, and continued regularly thereafter. Accordingly, it is highly recommended a biocide treatment be initiated when the unit is first filled with water and continued regularly thereafter. For more information, consult the Operation and Maintenance Manual.

When a water treatment program is employed, it must be compatible with construction materials. The pH of the circulating water must be maintained between 6.5 and 9.0. Units having galvanized steel construction and a circulating water pH of 8.3 or higher will require periodic passivation of the galvanized steel to prevent the accumulation of white, waxy, nonprotective zinc corrosion called white rust. Batch feeding of chemicals into the unit is not recommended. If units are constructed with optional corrosion resistant materials, acid treatment may be considered; however, the water quality must be maintained within the guidelines set forth in the Operation and Maintenance Manual. For specific recommendations on water treatment, contact your local water treatment expert.

Note: Unless a common remote sump is utilized, each cell of a multi-cell installation must be treated as a separate entity, even if the cold water basins are flumed together or equalized. For specific recommendations on water treatment, contact a competent water treatment supplier.

Sound Levels

Sound rating data is available for all BAC Closed Circuit Cooling Towers. When calculating the sound levels generated by a unit, the designer must take into account the effects of the geometry of the tower as well as the distance and direction from the unit to noise-sensitive areas. Low sound fans and intake and discharge sound attenuation can be supplied on certain models to provide reduced sound characteristics. Variable frequency drives can also be used to reduce sound during periods of non-peak thermal loads. For detailed low sound selections, please consult your local BAC Representative.

Protection Against Basin Water Freezing

When a unit is shut down in freezing weather, the basin water must be protected by draining to an indoor auxiliary remote sump tank or by providing supplementary heat to the cold water basin. Supplementary heat can be provided by electric immersion heaters or in some cases, hot water or steam coils, or steam injectors. All exposed water piping, make-up lines, and spray pumps (if applicable) that do not drain at shutdown should be traced with electric heater tape and insulated.

When dry operation is planned for low ambient conditions, centrifugal fan units should be supplied with oversized fan motors to prevent motor overload when the spray water is not operating. Dry operation with standard fan motors is acceptable for axial fan units. For remote sump applications, the spray water pump must be selected for the required flow at a total head which includes the vertical lift, pipe friction (in supply and suction lines) plus the required pressure at the inlet header of the water distribution system (2.0 psig for PF2 models). A valve should always be installed in the discharge line from the pump to permit adjusting flow to the unit requirement. Inlet water pressure should be measured by a pressure gauge installed in the water supply riser at the spray water inlet, and adjusted to the specified inlet pressure.


Adequate precautions, appropriate for the installation and location of these products, should be taken to safeguard the public from possible injury and the equipment and the premises from damage. Operation, maintenance and repair of this equipment should be undertaken only by personnel qualified to do so. Proper care, procedures and tools must be used in handling, lifting, installing, operating, maintaining, and repairing this equipment to prevent personal injury and/or property damage.

Fluid Compatibility

The fluid to be cooled must be compatible with the coil material (standard serpentine and cleanable header coils are carbon steel, hot-dip galvanized on the outside only). Fluids not compatible with coil materials can lead to corrosion and tube failure. Certain fluids may require occasional pressure cleaning or mechanical cleaning of the inside of coil tubes. In such cases the coil must be designed to provide this capability (Optional Coil Configurations).

Open/Closed System

The standard galvanized steel serpentine and cleanable header serpentine coils are carbon steel, hot-dip galvanized on the outside only, and are intended for application on closed, pressurized systems which are not open to the atmosphere. Stainless steel coils or cleanable coil units (with tubes hot-dip galvanized inside and out) are available to cool corrosive fluids or water and ethylene/propylene glycol solutions in systems open to the atmosphere (Optional Coil Configurations).

Protection Against Coil Freezing

At below freezing ambient conditions, the closed circuit cooling tower can experience heat loss even without the recirculating spray water pump and fans in operation. Without a heat load on the circulating fluid, coil freezing can occur even at full flow. Protective means are readily available to avoid potential freeze problems. Where the system will permit, the best protection against coil freeze-up is the use of an industrially inhibited anti-freeze solution. When this is not possible, maintain a heat load on the circulating fluid so that the temperature of the fluid leaving the coil will not be below 45ºF (7.2ºC).

If the process load is extremely light, or if the process is periodically shut off entirely, then an auxiliary heat load must be applied to the circulating fluid when below freezing ambient temperatures exist to prevent damage to the coil. Refer to the Heat Loss Data table for the auxiliary heat load requirement. The amount of auxiliary heat necessary to prevent coil freezing can be further reduced by the use of a positive closure damper hood and insulation. Draining the coil is not recommended as a normal method of freeze protection. However, draining is acceptable as an emergency method of freeze protection. Frequent draining can promote corrosion inside the coil tubes. If the coil is not protected by an industrially inhibited anti-freeze solution, an automatic drain valve and air vent is recommended to drain the coil if flow stops or fluid temperature drops below 45°F (7.2ºC) when the ambient temperature is below freezing. Note that cold water basin heaters will not provide freeze protection for the coil.

Code Requirement

Standard coils are ASME B31.5 compliant and are provided with a Canadian Registration Number (CRN) when required. State or local codes, or certain applications may require the use of pressure vessels designed, fabricated, tested and “U” stamped in accordance with the ASME Boiler and Pressure Vessel Code, Section VIII, Division I. In such cases, the optional ASME “U” Stamp coil must be provided.


Please refer to the Limitation of Warranties applicable to and in effect at the time of the sale/purchase of these products.

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