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Energy Saving and New Fluid Cooling Technologies Applied in Spray-Filled Cooling Towers with Dynamic Cooling.

by V.B.Ivanov, Engineer, OOO ENERGOKHRAN, Smolensk, Russia; E.A.Svidersky, Patent Agent № 020, Minsk, Belorussia

A cooling tower is a heat exchanging apparatus used to remove waste heat from various process sources to the atmosphere via evaporation of water. Typically, less than 1.5 percent of water is evaporated.

Most of CIS cooling towers are more than 30, even 50, years old. Almost all of the plants are obsolete and outdated. Moreover, the old-time cooling towers often sacrificed cooling efficiency for the sake of saving money spent to build a plant.

In processes where cooled water is used to deliver end products, eg., chemical and petrochemical industries, fertilizer manufacturing, dairy industry, an incorrect cooling method or a poorly designed cooling tower can reduce the product yield by 1.5 to 2 times, let alone quality degradation. This becomes quite a problem in warm seasons as due to the low temperature gradient the cooling tower is rendered less efficient, for the cooler the water, the better quality end product is delivered.

There is a wrong assumption that any cooling tower would be good enough or would at least meet the specific process demand. While designing and building a cooling system, one shouldn’t be tempted to make it fast, cheap and identical to what everybody has. It is a known fact that operating expenses over the life of a cooling system (typically 15 to 25 years) are many times more than capital expenses to build one. Here are some key criteria that should guide the selection of a cooling tower upgrade:

1. Reasoned selection of a cooling tower type.

2. Correlation of a cooling tower’s specific heat load and process requirements.

3. Ease of maintenance.

4. Energy saving capabilities.

5. Fire safety.

6. Frost-free winter operation.

See water cooler applications in SNiP 2.04.02-84 Table 39 below:

Table 39

Cooler Water Cooler Applications
Specific Heat Load, thousand kcal/ /(m2/h) Water Temperature Differential, С0 Cooled Water and Ambient Air Temperature Difference by Wet-Bulb Thermometer, С0
Fan-induced draft cooling towers 80-100 and more 3-20 4-5
Natural draft cooling towers 60-100 5-15 8-10
Spray ponds 5-20 5-10 10-12
Cooling ponds 0,2-0,4 5-10 6-8
Dry cooling towers - 5-10 20-35
Open and spray cooling towers 7-15 5-10 10-12

In fact, we are talking here about a choice between fill and no-fill (ejection, spray-filled) cooling towers. Spray-filled cooling towers are made more attractive through their low cost and power saving parameters. However, ejection plants feature less cooling capacity than induced draft film-filled coolers. This can be explained by that intensive cooling takes place along the spray length of about 1.5 m from the nozzle. Farther on, the so called “droplet steam flashing” effect occurs, whereby partial pressures of water on the droplet surface and in the ambient air become equalized. Small-size, 0.5 to 1 mm, droplets (2 to 3 mm being the optimal size for cooling towers) in the spray and fast flashing thereof result in instantaneous increase of concentration of water vapor inside the cooling tower. Non-flashed small-size droplets create yet another problem of droplet entrainment. There are, however, high pressure nozzles which provide for an optimal fractional composition of a droplet in the spray (see Patent №2144439 “Centrifugal Jet Nozzle”). These nozzles tend to improve the recycle water quality and reduce droplet entrainment down to 1.5 percent (see invention description to Patent №2144439). According to “Application Conditions for Ejection Cooling Towers” by V.S.Ponomarenko, Doctor of Technical Science and Yu.I.Arefiev, Ph.D. Technical Science (Russian Federal Research Center for Water Supply and Hydraulic Engineering), the spray nozzle-created surface area of heat and mass exchange is greater than that made by the fill but ejected air volume is not sufficient for efficient cooling. Production plants that don’t require maximum cooling capacity or have excessive cooling areas would better avail of no-fan, spray-filedl (ejection) cooling towers described in Eurasian Patent №6902. Coolers of this type would seem most suitable for rehabilitation of cooling towers. The cooling capacity of these plants can be assessed by water cooling rate Кт which equals the actual/maximum temperature differential ratio at the apparent cooling limit T:

Кт = т1- т2 / т1 – Т

where т1 is heated water temperature at cooling tower inlet, Сo. КТ is a value close to thermal efficiency. КТ = 1 for an ideal cooling process (т2 = Т) and КТ < 1 in real life (т2 > Т). For some cooling towers КТ rate can reach 0.53.

Let us now look at classical cooling towers, the filled fan-induced draft plants. These units use the following well-known principle: water flows into the unit via the water distribution system from top down to the fill whereas fans force air upwards through the fill. The heat and mass exchange takes place within the fill. The fill is used to increase the heat and mass exchange surface area, thus being the key method to enhance capacity (spraying density) and water cooling intensity.

The water cooling rate in film-filled cooling towers can reach as high as 0.7. However, a film fill may be called such only if its plastic surface is wettable. As we know, PVC which is used as the fill material is hydrophobic and a film of water cannot be produced on top of it. Fills made of Pentaplast (Germany) film are expensive, short-lived and unstable under low temperatures.

VODEX engineers say: “Film fills have their disadvantages such as:

  • low mechanical strength of the film under water jets vibration (no more than four to five years service life unless specifically protected);
  • water requires chemical treatment (due to hardness salts deposits on and biofouling of the film surface, higher air drag of the fill, unavoidable clogging of the fill).

Most of marketable fills are made of polyethylene. This is an unwettable plastic material, so all polyethylene fills should be referred to as drip fills. In terms of cooling intensity, this type of cooling towers is no better than no-fan and spray-filled cooling towers described in Eurasian Patent № 6902.

“A few words should be said about finned tube fills. Finned tubes were widely used for land reclamation purposes. Once the land reclamation problem was gone, the finned tube manufacturer in Tatarstan started looking for new applications for their products, and cooling towers came just handy. Aggressive marketing has promoted finned tubes packing sales in Russia and CIS. However, this packing has one and only advantage of high mechanical strength. In terms of cooling capacity it is two times less efficient than film fill. Finned tube modules are assembled at the manufacturer’s location which results in high shipping costs. Another disadvantage is low fire safety performance of the finned tubes packing. As compared to other types of cooling tower fills, finned tubes have higher unit value”, say VODEX engineers.

As can be seen from the above, filled induced draft cooling towers have many design drawbacks such as rather low efficiency, short life, aerodynamic and hydrodynamic deterioration of polyethylene fills owing to biofouling and clogging of the fill material, maintenance complications such as lengthy shutdowns for repair. Fill material destruction and biofouling products tend to get entrapped in heat exchangers and pumps which leads to failure of process equipment and related costs. Fires are not uncommon during preventive off-schedule maintenance operations.

Failures also occur due to freezing of the cooling tower fill or walls. Actually, this cooling technology itself is costly and offers no way of energy saving. Well, this may sound as a long list of bugs but it is unbiased. If all these problems are solved, would it be possible to cool water just as efficiently as with the film fill and for as long time as with the polyethylene fill? Would it be possible to maintain these cooling towers easier and safer? Would it be possible to get rid of emergency shutdowns resulting from clogging of pumps and heat exchangers? Would it be possible to render cooling towers non-freezing and noncombustible?

The history of engineering tells us that new technologies in most cases are but simple solutions applied at a new level. Looking at pros and contras of ejection cooling towers, we have noted that the heat and mass exchange surface area created by centrifugal jet nozzles is much greater than the fill’s heat and mass exchange surface area but their air supply is insufficient.

In early 90-s, AKRON company (Veliky Novgorod, Russia) launched the first spray-filled fan-induced draft cooling tower with a throughput of 6,000 m3/h. Its cooling performance was identical to or in some instances even higher than that of film-filled coolers, displaying the KT of 0.68. Later, four more cooling towers were converted similarly. The type and location of nozzles and drip basins were modified to suit new demand. This effort found its embodiment in Russian Patent №2228501 “Cooling of Fluids in Cooling Towers”. Further on, Eurasian Patent № 7724 “Fan-Induced Draft Cooling Towers” was also granted.

Here is some specification data for the patented cooling tower:

  • Cooling intensity is 2 to 4 0C higher than the wet-bulb air temperature. In real numbers, the water temperature difference between inlet and outlet is 9 to 14 0C, subject to ambient air humidity.
  • Ease of maintenance: nozzles are installed at air intakes in two rows, 1.0 and 1.7 m high, and provide unobstructed access. One cell can be shut down any time for 5 to 7 minutes for a nozzle cleanup or replacement. Water from the shutdown cell will be directed to other cells for cooling. The increased heat and mass exchange surface area allows to increase cooling tower capacity by 25-30 percent above design load.
  • Energy saving: pumps are run at light duty as water does not need to be lifted to the height of the water distribution system (7 to 11 m, subject to the cooling tower type). This layout helps save up to 30 percent of electric power.
  • In winter, up to 70 percent of water enters the tower via winterized channels and the remaining water is supplied to the nozzles under low pressure which eliminates the risk of frosting. Fans are not required.
  • Zero fire risk as there is no plastic fill material.

Today, dozens of spray-filled fan-induced draft cooling towers operate in CIS and Russia, mainly in fertilizer manufacturing, heavy organic synthesis, metallurgy, machine building industries.

As we have already noted before, the dynamic volume of sprayed water in the cooling tower is capable of intensive cooling provided that heat is forced out of the heat and mass exchange area by a stream of air produced by fans. This also applies to natural draft cooling towers. A spray-filled cooling tower will cool water to a temperature 6 to 8 0C above the wet-bulb air temperature which allows up to 3 percent savings on fuel at heat power stations.

Please visit http://gradirnia.ru for cooling tower drawings, reports and our patents.