Many managers do not consider or are not aware of the role of cooling towers and the application of cold water. A cooling tower is a high-energy utilization machine as well as a mass heat transfer device. Frequently, cooling capacity is the limiting factor in high unit production.
After a cooling tower has been erected, proper operation and maintenance is required to yield full unit production. With poor maintenance, towers can lose appreciable performance. A deficient (low bid nominal) cooling tower plus improper maintenance can mean tower performance loss, which can amount to as much as 5°F (2.7°C) increase in cold water temperature (table 1). But what does a 5°F (2.7°C) temperature increase mean to an operating unit? The following three examples demonstrate what a small temperature increase can mean to a real-world application.
Example 1 – Crossflow Power Plant Cooling Tower
Consider a 600 MW steam turbine power plant, operating at 440 MW with a heat rate of 7,946 BTU/kW. An increase in condensing pressure from 3 to 3.5″ mercury absolute (115°F [46°C]) will increase heat rate by 0.7%; that is, the added heat rate is 0.0067 x 7,946 = 53 BTU/kW.
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:
440 x 1,000 x 0.068 =
2,992 kW/hrThe 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.
Example 2 – Counterflow Industrial Cooling Tower
While a 5°F (2.7°C) reduction in efficiency is considered quite high, it is not uncommon. Table 2, on a straight line proportion, indicates the loss for less than a 5°F (2.7°C) deficiency. Multiply the numbers in table 2 by a 25-year life cycle of a generating plant, and the enormous losses caused by a deficient cooling tower can be seen. However, purchasing agents still purchase a cooling tower by looking at the lowest bidder rather than the performance guaranteed by thermal testing in accordance with the Cooling Technology Institute (CTI) Acceptance Test Code ATC-10. For towers specified under that code, severe financial penalties are written into the contract for tower performance that does not meet specified design conditions.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.
How to Know if a Rebuild Will Help
A consultant can perform a thermal test before and after rebuilding. In addition to capability percentage, the consultant should provide a report outlining reasons for the tower’s deficiency as well as specifications and an estimated budget to correct the shortage. Inefficient wood splash bars were replaced at the chemical plant with 42″ of cellular fill. Three 50-hp motors were rewound for 60-hp duty. Each motor generated sufficient air to cool 4,160 gal/min of water per cell, which could then be cooled to the design requirements of water entering the tower at 102°F (39°C), cooling to 85°F (29°C) at an ambient wet bulb temperature of 77°F (25°C).
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.
1. Allied Chemical Co., The Pressure Enthalpy Diagram, Its Construction, Use and Value.
2. Burger, Robert. Cooling Tower Technology Upgrading and Rebuilding, Prentice-Hall, 3rd Edition, Chapter 6, Thermal Analysis.
3. Lefevre, Marcel. Evaluation of Cooling Tower Test Accuracy, CTI Paper 2-4-98
4. Smith, Matt. Paper, ASME, June 1993.
5. TVA-EPRI Report, Power Generating Magazine June 1988
6. Weiss, Charlie, National Engineer, March 1985