3 Reasons to Rebuild Your Tower

Assume fuel cost is $1 per million BTU. For 400 MW production, the added cost of the 5.5°F (3.05°C) increase in condenser water temperature is:

Alternatively, instead of in-creased fuel cost, consider lost production for the 5.5°F increase in condenser temperature:

The example shows a $204,300 per year increase in fuel costs to maintain production due to the 5°F (2.7°C) performance shortage in the cooling tower.

Tennessee Valley Authority (TVA) and the Electric Power Research Institute (EPRI) conducted a survey of wet cooling towers in the power generating industry. They found that 65% of the cooling towers surveyed were performing at only 80% capability. TVA and EPRI assert that this deficiency is costing the American electric generating industry more than $100 million per year in added power.

Thermal calculations indicated that with the present water distribution system, 24" of cellular fill would more than adequately return the 8,000 gal/min of water to the process equipment at 90°F (32°C). A change in the wet decking heat transfer surfaces alone produced startling results.

Further calculations indicated that adding another 24" (total of 48") deep pack of cellular fill, together with a new water distribution system consisting of PVC piping and ceramic square spray nozzles, would expand the tower's water cooling capability by 30%. Because the cost of this conversion is $19,500, an actual saving of $20,500 was calculated. An additional $16,300 was saved by not operating an additional 25-hp motor for the new tower.

When investigating the problem from a rebuilding engineering point of view, it was determined that if the large, inefficient, triangular wood splash bars were replaced with cellular film fill, the desired expansion results would be obtained.

Another key to making this upgrade work was replacing the old fashioned gravity water trough distribution system with a PVC piping system and installing 1.25" orifice ceramic nozzles. As an extra margin, new cellular drift eliminators also were installed because they generate less static pressure than heavy, cross-section wood drift eliminator blades. Figure 1 shows the relationship between production vs. cold cooling tower process water.

A decrease of 3°F (1.1°C) results in an extra 12 tons per hour of production. The cooling tower rebuilding cost of $750,000 was more than paid back because increased production created approximately $1 million in extra product sales per year.

Not all cooling towers can be rebuilt to return significantly cooler water or save money and power. Each installation must be treated on an individual basis. Thermal, hydraulic and aerodynamic calculations of existing conditions should be studied by a competent cooling tower consultant or engineer to see where improvement can be anticipated. A testing requirement should be made part of the contract.

Thermal testing can be conducted in-house for a close estimate. However, a good operator can tell by production results if cooling tower cold water is adequate or not. For a precise evaluation, test in accordance with the CTI Acceptance Test Code ATC-105 and allow a qualified cooling tower engineer with approved instrumentation to handle it. Only a certified CTI test will provide the capability percent at which the tower is operating, compared to the 100% for which it was purchased. Due to the added cost of new concrete ba-sin, piping and wiring, rebuilding and retrofit should be considered. The expense of retrofitting and rebuilding a cooling tower to upgrade its capacity or capability has a rapid payback if the proper investigative engineering is performed.