A cooling tower is an air and water management device, which consists of fan stacks, fans, drift eliminators, fill and water/air distribution systems. For the fans and fan stacks were previously discussed. So, the explanation shall be focused to the air/water distribution systems.

1) Water Distribution: The distribution of water to the top of counter flow fill is a key aspect of assured performance. It is a function of nozzle design, nozzle installation pattern, nozzle distance, and the structural cleanliness of the spray chamber. The impact of water distribution on performance is a combination of uniformity of water distribution, air-side pressure drop through the spray chamber, and heat transfer occurring in the spray zone.

The challenge for a spray system designer is to accomplish an optimum balance of design parameters with practical considerations such as resistance to silt build-up, and the ability to pass objects from trash to Amertap balls. To provide the primary function of precise water distribution, the nozzle must be designed with other considerations in mind:

• The location of counter flow nozzles and the potential for poor quality circulating water demands that the nozzle system be designed to minimize fouling.
• The nozzle must be capable of providing uniform distribution over a wide range of flows, without significant loss in nozzle performance.
• The nozzle must be capable of efficient operation while consuming a minimum of expensive pump energy.

The nozzle arrangement, and the design of the tower structure in the spray chamber, are critical to provide uniform distribution to the top of the fill. The placement of the nozzles must accommodate the tower geometry and still provide even coverage for all parts of the plan area. In general, a criterion such as 90% of the plan area within 5% of the average gpm/square foot, and no areas varying more than 10% from the average will still require several percent conservatism in the thermal performance.

Structure in the spray chamber should be avoided, to prevent spray pattern interference and because any water hitting it tends to fall in concentrated zones on the fill. The impact depends entirely on the extent of structural blockage but can be very substantial for large elements like distribution pipes placed within the spray zone.

Spray water which hits walls or partitions may bypass the fill altogether, with direct impact on performance. Some hollow cone nozzle designs are more prone to structure and wall interference due to the requirement for large overlapping spray patterns. Providing uniform coverage to the edges of the fill requires nozzle placement near the walls to maintain the overlap pattern. As a result, a significant part of the water from the edge nozzles becomes wall water.

The influence of the spray system design on performance is dramatic. Even small changes in the layout of a good spray system, or variations on a nozzle design can have an effect on tower performance of 10% or more. For this reason, it is absolutely imperative that the performance of the fill and spray system be tested as they will be installed. Fill performance data in only valid with the exact spray system configuration used in the test.

2) Air Distribution: Three variables control the distribution of air to the fill in a counter flow configuration. The first is the air inlet geometry. The second is called the pressure ratio. The third is the fan coverage over the eliminators. Extensive aerodynamic modeling studies have been conducted to evaluate the impact of the air inlet design on distribution, and therefore on performance. It is especially important with film fill that air flow reach the entire plan area, including the region adjacent the air entrance. Any region having significantly reduced air flow will effectively allow a bypass of hot water to the cold water basin.

Studies showed that the portion of fill plan area adjacent to the air inlet plan is substantially starved from air flow. Since the air approaching the tower is coming from above the air inlet as well as horizontally, the air has a large downward component adjacent to the tower casing. When this air stream passes the air inlet plane, it is still moving downwards, and does not turn into the fill nearest the inlet. In round tower, this can become a very significant percentage of the total area. In a rectangular tower the effect is still significant, but less.

Critical to the effectiveness of any design, even with an inlet air guide, is that structural interference near the fill and air inlet be minimized. Since inlet velocity is highest in this zone, the wakes behind structural elements can shadow significant areas of fill. Structural interference in this area is meticulously avoided to maximize the effectiveness of a design. The wakes around structural elements at the air entrance also lead to growth of ice in freezing conditions, so avoidance of structure in the air entrance reduces tendencies for icing problems as well. Baffles used for the purpose of changing the direction of air flow in a uniform parallel manner, also utilized to prevent water droplets from splashing out of the tower on their descent through the structure.

The second variable, the pressure ratio, is the ratio of system pressure drop (from the air inlet to the eliminator exit plane) to velocity pressure at the average entrance velocity. The pressure ratio reflects the ratio of resistance to available entering air energy. The higher the ratio, the better entering air will be spread out before entering the fill. The lower the pressure ratio, the less uniform, and less stable the distribution of air flow becomes. The degradation of air flow uniformity is readily apparent, particularly at the inlet.

(Pressure Ratio = Static Pressure / Velocity Pressure at Air Inlet)

It should be noted that ambient winds can decrease the effective pressure ratio in relation to the square of wind speed. Added entering air velocity due to winds increases the velocity pressure as the square of wind velocity. A safety margin is necessary to prevent moderate (10 mph = 4.47 m/s) winds from degrading air distribution. The chosen practice is not to apply towers below a pressure ratio of 5, which is of importance particularly for highly evaluated cases.

The tendency for optimized selections is toward selections with low pressure drop (low fan power, or draft requirement) and high entrance velocity (low pump head). The pressure ratio limitation is a frequent limiting factor in optimization, so that a manufacturer who is unaware of the limitation could have a better evaluated bid - which is not likely to perform as the manufacturer might expect. A manufacturer who recognizes the limitation may be unable to respond in this case, while an unaware manufacturer and the user may discover a serious performance problem after the tower is in service.

Modeling and full scale tower studies have shown that fan plenum pressure drop is related to fan coverage, the third variable. Inadequate fan coverage has been shown to lead also to poor air flow distribution over the fill plane area. Fan coverage is a function of the size of the fan deck opening, the cell size, and the plenum height.

An approximate rule of thumb which has been shown to provide good air distribution and a low plenum pressure drop is as follows; If a circle is projected on the eliminator plan area at a 45 degree angle from the fan stack opening, the percentage of the eliminator are covered by the projected circle is the percent fan coverage. A fan coverage percentage of 30% or greater generally limits the plenum pressure drop to about 10% of the total system pressure drop, and provides good air distribution.

Ignoring this sort of guideline will allow a shorter plenum height, and lower cost tower, but higher plenum pressure drops and uncertain air distribution lead to lower and less predictable performance.

3) Exit Air Velocity: Low fan exit velocity have a two-fold effect on susceptibility to influence by ambient winds. First, at low exit velocity relative to ambient wind speed, the effect of wind is greatest on the velocity profile leaving the fan stack. With tall velocity recovery stacks, the effect is limited primarily to a reduction of the velocity recovery stack. Depending on the magnitude of recovery expected in relation to the total system head, this can be a significant loss. The shorter the recovery stack, or the closer ambient wind can penetrate the cylinder toward the fan itself, the greater will be the direct influence on the fan efficiency. For fans and recovery stacks as commonly applied in industrial applications, a minimum stack exit velocity is approximately 1.4 times the maximum wind speed for guaranteed tower performance (10 mph = 880 fpm). Use of any lower exit velocity requires substantial performance conservatism to compensate for wind effects.

It should be noted also, that tower performance capacity at lower exit velocities relative to the ambient wind speed becomes increasingly sensitive to the wind and inherently as unsteady as the wind speed is variable. It is entirely in the tower owner? best interest to avoid a tower configuration which will have highly variable performance in winds from this effect alone.

The second consequence of excessively low stack exit velocity is the tendency for effluent air to be caught in the ambient wind stream and entrained in the aerodynamic wake downstream of the tower. Since the tower generally has an air entrance face on the downstream side, a portion of the effluent air is recirculated back through the tower. The effluent air is, of course, at a higher wet bulb temperature, so the tower as if subject to hotter ambient temperature.