Cooling Tower Selection:
Cooling towers are designed and manufactured in many sizes and configurations. Recognizing and understanding the different configurations and advantages is essential for specifying the most cost effective solution for the end user. The purpose of this bulletin is to highlight the differences between crossflow and counterflow cooling towers.
We will give some guidelines for the selection of each type of cooling tower
Cooling tower overview:
The primary task of a cooling tower is to cool water by rejecting heat into the atmosphere. This heat rejection is accomplished through the natural process of evaporation that takes place when air and water is brought into direct contact in the cooling tower. The evaporation is most efficient when the maximum water surface area is exposed to the maximum flow of air in the longest possible period of time.
Cooling towers are designed in two different configurations, counterflow and crossflow. The specific configuration indicates the direction of the air flow in relation to the water flow. Cooling water distribution and air distribution are designed in a harmonious configuration and are equally important to determine a maximum efficiency in heat rejection.
[img_assist|nid=44|title=Wacon Cross Flow Cooling Tower|desc=|link=url|align=center|width=300|height=210]
[img_assist|nid=45|title=Wacon Counter Flow Cooling Tower|desc=|link=url|align=center|width=300|height=210]
Hot water distribution systems:
The overall efficiency of a cooling tower is directly related to the design of the towers water distribution and heat exchange surface. The design of the distribution system affects the pump head to bring the hot water to the tower inlet. Depending on the type of distribution system the pump head required is the static lift plus the required pressure at the distributor.
Counterflow towers use, in general, low pressure spray nozzles to divide the hot water equally over the infill surface in fine droplets. A drift eliminator section above the water distribution rejects all the water in liquid form and keeps this in the circulating system. Only water evaporated in the air leaves the tower with the saturated air. A nozzle distribution is sensitive to changes in water flow thus it affects the over all efficiency.
The air movement is vertically through the fill in counter flow to the downwards fall of the water. Counterflow towers have in general a smaller footprint than crossflow towers but require a higher pump head due to the typical distribution system with nozzles.
Crossflow towers use a distinctly different type of water distribution. The hot water is distributed to the fill by gravity through perforated basins which may be supported by metering orifices in the floor plate of the distribution basin. The water runs down the infill surface as a fine film. The turbulent air will flow in the honey comb infill structure to maximize the contact with the hot water. The hot water enters the distribution system at atmospheric pressure. The pump head required is in general the tower height as static lift. The air movement is horizontally through the infill, across the downward fall of the water.
Compared to crossflow towers, counter flow towers may require up to 0.5 bar / 7 PSI pressure at the water inlet to achieve an efficient water distribution in the tower. This leads to higher operational costs in pump energy consumption.
Air flow distribution systems:
Cooling tower performance is also related to the amount of air moving through the tower and coming into direct contact with the water.
In crossflow towers the air comes into the infill over the total infill height. Due to the fan configuration on top of the tower the air distribution is not equal over the whole inlet surface.
In counterflow towers the air passes through the air inlet louvers and bends into the infill section. The inlet louvers prevent splash water coming out of the tower and create a slightly higher air side pressure drop.
Noise is a critical item in cooling towers. There are two major sources of noise in cooling towers; The movement of air and water and the mechanical noise from the fan, gear and motor.
Silencers are used to reduce the spreading of noise but more importantly is the selection of the right type of tower and the mechanical components in it when noise is a critically important.
The fan produces a typical sound character in lower frequencies. When the fan is close to an obstacle, e.g. a support for the gear it will produce a chop noise, which also has effect on the force of the blade. Our blades are designed for low noise production.
Gearboxes are typical noise producers, so one might want to go with V-belts. Those have, if installed and serviced properly, a lower impact. We also offer a wide range of gearboxes, from expensive low noise solutions to more costly nut nosier options.
Motors have a typical noise at full load.
In counterflow towers water produces a rain like noise in the spraying area as well when it is falling from the infill on to the water surface. While in crossflow towers the noise of water is nearly absent.
Service and maintenance:
Cooling towers need regular service and inspection. Both types of towers have infill and a mechanical drive system which need easy access. However, counter flow towers have a separate drift eliminator which can obstruct the accessibility of the infill.
Some crossflow towers have drift eliminators in the plenum of the tower, however normally there is enough space to access the infill.
The mechanical part of both towers is mostly easy accessible.
The easiness to clean or exchange the infill is slightly more practical in crossflow towers as these are mostly designed with loose hanging sheets while the counter flow towers have glued or welded fill packs.
Tower characteristics compared:
Crossflow cooling towers
+ Low pumping head, thus lower operational cost.
+ Accepts variations in water flow without changing the distribution system.
+ Easy maintenance access to vital parts.
+ Reduced drift (water) loss due to the absence of water droplets.
+ Lower in noise due to absence of water noise.
- Larger foot print compared to counter flow towers.
- Large air inlet surface makes icing difficult to control.
- Tendency of uneven air distribution through the infill due to the large inlet surface.
Counterflow cooling towers
+ The coldest water comes in contact with the driest air maximizing tower performance.
+ Smaller footprint compared to other tower.
+ Smaller tower height due to compact infill.
+ More efficient air/water contact due to droplet distribution.
- Noise production due to spraying and falling water.
- Direct sunlight in the tower basin might trigger algae growth.
- Water distribution system might clog due to water borne debris.
- More complicated maintenance of the water distribution system.
- Drift loss due to droplet distribution system.
- Icing of the air inlet louvers in cold periods.
Conclusions and recommendations:
The air distribution systems for counterflow and crossflow towers have advantages and disadvantages inherent to their respective designs.
Both tower systems are designed to a required cooling tower duty thus the thermal performance and cooling capability for both tower systems, if designed well, are equal.
Crossflow towers should be specified when the following characteristics are important:
- Minimal pump head.
- Minimal pumping and piping cost.
- Minimal operating cost.
- Minimal noise.
- When variations in hot water flow are expected
- When easy maintenance is a concern
Counterflow towers should be specified when the following characteristics are important:
- Minimal space (footprint)
- When icing is of an extreme concern.
- When pumping is designed for additional pressure drop.
Never replace a crossflow cooling tower with a counterflow tower or visa versa without engineering evaluation of the total cooling system and pump systems.
Henk P. Janssen