In the late 1970's the energy crisis reached a peak. In the years since the oil crisis, many industries who utilize electric power, such as refinery and petrochemical have started to study for the energy saving. The field of cooling tower was not excepting. To minimize investment cost and realize improved cooling tower operational economy (with minimal loss to production) the designer should evaluate the effect of cooling tower size and performance on plant output. To do this, the impact of the design point on tower size and the effect that the many possible modes of tower operation have on cold water temperature must be more completely understood.

With today's trend towards minimum energy use firmly established, it is necessary for cooling tower users to continuously reassess both the operational requirements and the design point parameters given to potential equipment suppliers. This is quite essential to insure the most economical tower operation over the life of the system.

1) Cooling Tower Thermal Design Strategy

Several parameters are used to develop the tower design. These are water flow rate, cooling range, and wet-bulb temperature. Of these only the wet-bulb temperature is not process related and therefore affords us some flexibility. Determining the impact the design wet-bulb temperature has on the size and power requirements of a cooling tower is critical to optimizing the cooling tower economics. For this reason, an example illustrating this effect will be discussed further.

In the majority of applications, the design duty of an evaporative cooling tower is based upon an acceptable/required cold water return temperature, when operating under constant heat load, and local wet-bulb temperature conditions. If investment and operational costs were not a concern, the ideal design wet-bulb temperature would be equal to, or higher than, the highest local wet-bulb temperature recorded. In this way, the returned water temperature would never be higher than the acceptable/required cold water temperature.

Unfortunately, this design methodology can lead to large, power intensive cooling towers which are rather expensive. Instead, the design wet-bulb temperature chosen is usually a compromise between operational requirements and capital cost. In order to reduce the tower's size, or power requirements, and therefore the cost of the tower, the specifier normally chooses a wet-bulb temperature which is not expected to be exceeded for more than a small percentage of the time in any given year. in effect, the actual wet-bulb temperature used for design then, is lower than the ideal wet-bulb temperature. The acceptable/required cold water temperature returned by the tower will be achieved at this new lower design wet-bulb temperature.

A value of 2.5 or even 5% of the total yearly operating hours is normally the amount of time acceptable/required cold water temperature is permitted to be exceeded. Actually, the value used should depend upon the process, and the effect cold water temperature have on the process. in some cases, values of 1-1/2 or even 1% are used.

Below figure is a typical wet-bulb temperature cumulative frequency diagram. it has the classic "5" shape and with it we can determine what percent of the year the cooling tower can be expected to experience a particular wet-bulb temperature. For this case, it can be seen that the design wet-bulb temperature would be exceeded 0.5 percent of the year, i.e., about 58 hours. For such a tower the plan area would be about 4,608 square feet (this size, of course, is dependent upon the manufacturer's design, fill and fill air velocity). In this example, we will assume that the fill, fill air velocity, water flow rate and cooling ranges are kept constant.

The percentage of time the ideal wet-bulb temperature can be exceeded. As a general rule, as the percentage of time exceeding the ideal wet-bulb temperature is increased, the plan area of the tower decrease. Thus the total evaluated cost (i.e., initial investment plus operational costs) to the user is decreased. However, it must be pointed out that since the tower's cooling capacity is reduced, the plant's output is also correspondingly reduced. If taken to the extreme this will have severe production impact. Conversely, as the number of hours is reduced, the tower size and power requirements increases. Obviously, so does the investment and operational costs, but the yearly production of the plant also increases. (This is the basis for an optimization process, since as the percentage of time increases, the average yearly production of the plant will decrease.)

Inlet (hot) water temperature: 113^oF (45^oC)
Outlet (cold) water temperature: 89.6^o (32^oC)
Wet-bulb temperature: 82.4^o (27^oC)
Water flow rate: 63,842 GPM (14,500 M³/hr)
Tower Size: 42' x 42' x 6 cells

Clearly, then, when choosing the design point the specifier must be cognizant of the effect the cooling tower design point has on plant operation. The design point should be chosen such that it yields a cooling tower which provides the minimum cooling capacity which does not produce a severe adverse effect upon plant performance. Thus, the careful consideration of the design parameters is essential as a basis for cooling tower operating economy and operating efficiency.

2) Component Design

Till now the economy was discussed for only the thermal design side of cooling tower. Let me discuss on energy evaluation for mechanical counter flow type of cooling tower further. Fans, cylinders, drift eliminators, fill and water/air distribution systems are the components most subjected to the punishment of the cooling tower environment and are therefore the most commonly replaced. Since a cooling tower is an air and water management device, those components which are subject to the maximum wear and tear fortunately also afford the greatest potential for improvement by applying state of the art technology. For the fans and cylinders were previously discussed. So, these shall not be described in this section.

(1) Fill: Counter flow tower design and operating experience has been accumulated for over 60 years. The earliest and most common designs until recently utilized splash type fills. The advent of high power evaluations, beginning in the mid-1970's, led to a predominance of film type counter flow designs using relatively low cost PVC materials. The new film type designs provide energy savings both in fan power and pump head through the high surface areas per cubic feet of fill. This "surface density" coupled with its low cost gives film type counter flow fill an overall effectiveness such that it has become an industry standard.

The component most likely to provide improvement in tower performance is the fill packing. In some cases, merely replacing the filling “in kind” can be the most cost-effective approach. If the fill is in poor condition, bringing the tower back to its original performance has an immediate, beneficial impact on the operation of the plant. In most cases, however, the owner should look at the high performance splash and film fills that are available today. High performance plastic fills, when properly installed, offer the opportunity of improving the performance of tower while at the same time providing a material that is generally impervious to rot and chemical attack.

At typical cooling tower thermal duties, the capacity of a given tower can usually be improved several percent by replacing wood lath fills with modern, high performance splash designs. Care must be exercised when replacing fills. Any new fill will likely have a pressure drop characteristic different from the original equipment. The system designer and tower owner must be aware of this and analyze the impact a pressure drop change may have on the system as a whole. High density splash fill is very efficient thermally; but at constant horsepower, the air rate through the tower reduces due to an increase in static pressure. Normally the increased thermal efficiency more than compensates for the reduction in air rate, but the fan system must be checked to assure that it is capable of moving the required air volume through the fill section without stalling. A fan operating in a stall condition is noisy, will not operate efficiency, and will not move the required amount of air through the fill.

Conversely, some low pressure drop splash fills, such as those that operate parallel to airflow, must be investigated to assure that drift problems do not occur. Due to the splash bar orientation with respect to the tower air stream, less turbulence is generated, reducing the thermal efficiency of the packing. This effect needs to be overcome by increased airflow through the tower. Fortunately, the reduction in turbulence and in the projected area of the splash bar reduces the operating pressure drop. At constant horsepower, the resulting increase in airflow normally compensates for the reduction in efficiency. The additional airflow required, however, may increase the tower drift rate. The designer must be aware of this and assure that upgrading the fill performance does not generate an objectionable drift rate.

PVC films are much more thermally efficient than splash fills due to the tremendous water surface that they expose to the air stream. As with the high density film fills impose high pressure losses on the air moving system. Additionally, film fills must be provided with very precise water distribution to assure full thermal performance. Unlike splash fills, which tend to allow water redistribution and are somewhat forgiving to marginal distribution systems, film fill does not allow redistribution and is intolerant of poor initial distribution. It is obvious that circulating water cannot migrate across or through film fill sheets, and research shows very little tendency for water to migrate parallel to the fill sheets. Note that the small stream of water applied at the top of the pack has expanded only slightly by the time it reaches the bottom of the fill. The practical implication of this is that unless the circulating water is uniformly distributed over the entire fill plan area, actual performance levels may be much less than anticipated by the owner or fill maker.

While film fill is extremely efficient thermally, extreme care must be taken in its application. Film fill is much more sensitive to fouling than splash fill, and for this reason, is normally not applied in systems where heat exchanger leaks, suspended solids or biological conditions in the circulating water indicate fouling potential. The purchaser and fill manufacturer must communicate early in the fill selection process to confirm water conditions and clogging potentials.

(2) Drift Eliminators: The design o drift eliminators has undergone tremendous improvement in the last decade. Concern about the discharge of cooling tower drift has prompted new eliminator designs that now routinely achieve elimination rates several orders of magnitude lower than those available only 10 years ago. Some new eliminator configurations accomplish this improvement while actually reducing eliminator pressure losses - affording the owner an additional opportunity to achieve operating horsepower savings.

The basic concept of eliminator design is rather simple. A cooling tower drift eliminator is a low pressure, momentum filter. Components are arranged to force the air leaving the fill section to make a series of directional changes. Water droplets, which cannot negotiate these turns, impinge on the surface of the eliminator, from which they are collected and drained back into the wet side of the tower. The designer's goal is to provide the maximum drift elimination at reasonable cost and minimum pressure loss.

The state of the art in eliminator design is the modern cellular configuration. Cellular eliminators are typically constructed of PVC sheets vacuum formed into very precise, compound shapes, with an integral honeycomb strength. The compound shape allows significant improvements in drift eliminations and the use of cellular structure appreciably reduces the pressure losses through the eliminator when compared to either the wood lath or wave form eliminators. The net free are of well-designed, modern cellular eliminators is in excess of 95%.

As with all other tower components, the design of the drift eliminator has an impact on the rest of the system. Very subtle changes in the drift eliminator can have a significant impact on the fan system. For instance, the last "pass" in the drift eliminator must direct the air upward toward the fan. If this step is not taken or is taken improperly, increased fan plenum losses will occur which could reduce tower performance by as much as 10%.

(3) 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, spray chamber height, 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. While small diameter, high pressure nozzles or nozzles with internal turbulators simplify the distribution function, they also greatly increase the risk of fouling which increases the owner's maintenance costs. Minimum clearance inside an industrial counter flow nozzle should be at least 1-1/4" to avoid the risk of plugging.
• The nozzle must be capable of proving 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.
  

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