Capacity versus thermal capability in cooling towers

Do cooling towers ever work properly?
Mostly never, even though engineers often over specify the design capacity and/or the entering wet-bulb temperature, in the hope that the cooling towers will perform without trouble.

You pay for 100% but do you get 100%?
What can you do to be sure that the towers you purchased perform according the specifications. We analyze poor performance at what consequences and the solutions we have to solve.

Capacity versus capability

Capacity:
The design capacity, calculated from the heat exchange processes, given in the following forumla:
 HWT : Hot water entering temperature
 CWT : Cold water leaving temperature
 WBT : Design entering wet bulb temperature
 FLOW: The assigned water flow for transferring the heat load.

 HWT - CWT = range
 CWT - WBT = approach

With these data, the cooling tower manufacturer calculates the size of the cooling tower required, with his typical references for heat transfer. If the tower is under designed it might be understood that the tower does not reject the full heat load. This is a false conception. The tower always transfers the heat load given, but, it will not reach the CWT at the given WBT. The cold water temperature will not reach the design parameter, but the heat load will still be rejected to the ambient air.

For example: If a tower is designed for a range of 5 degrees and an approach of 5 degrees. It needs a heat exchange surface of 100%. If the heat exchange surface is only 75%, then the approach will not reach 5 degrees, but perhaps 7 degrees. The CWT will approximately be 40% higher then the design criteria requested.

Capability:
This implicates the tower's ability to handle the heat load to its original design parameters.
The capability is the percentage of the design water flow to handle the applied heat load. In practical terms: with how much water over the tower do we have the designed range and approach? It is the quantity of water, under 100% heat load on the tower, given for a test conducted under the test codes.

Thermal performance assessment:
If a mechanical device is under designed for its duty, you will have immediate feedback if the device is not reaching it's intended performance. For a cooling tower it may be suspected that the performance is not what it should be, but proving a capacity shortage is more difficult. Checking a tower on its design conditions will be a long wait till the operating temperatures, flow and wet bulb temperature do match the design parameters. Some testing methods do allow performance tests at conditions that vary from the design conditions.
To test a tower on accurately, one will have to do a little more then to wave a sling thermometer in the vicinity of the tower, take the flow rate from a pump curve and take the temperatures with an average household thermometer. High accuracy testing equipment is required to get close to 100% accuracy in the test results.

An example, the table below gives a set of design conditions for a cooling tower and two sets of test conditions; Test A and Test B.

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Test A is the set of figures from a careful run test. Test B however, gives results where a mistake of 4% has been made in the water flow calculations, and errors of 0.1 °C have been made in the temperature readings for the hot and cold water and for the wet bulb reading. These errors may not seem to be significant, their impact however on the test result is. Results for the tests give a capability mark for Test A of 86.0% and for Test B 96.5%, more than a ten percent difference.

Test B results would generally be judged a pass, for it has a capability of 95%. When one considers the price tag for a tower like this of about $US30,000, the real shortfall of 14% in capability in the example above is worth some $US4,000. Consider the cost difference on that tower when it delivers only 70% or less in capability! It is a cause of concern that despite the existence of a number of different thermal test standards for cooling towers, we still see this situation quite regularly.

Effects of an undersized cooling tower:
A cooling tower can under perform for various reasons:

  • Wrong selection of the type of tower
  • Poor manufacturing and installation
  • Poor maintenance
  • Optimistic load ratings

These can have the following effects:

  1. Initial cost:
    All things being equal, a cooling tower that is providing only 80% capability can be expected to be around 20% cheaper in its original cost compared to a 100% tower. In a competitive situation when bidding for a project, a small price difference can win or lose a job.
  2. Running costs:
    Towers that are undersized will produce higher water temperatures than specified, and thus contribute to a less efficient operation of the total plant. A less efficient plant equates to a longer running time, causing higher energy costs.
  3. Critical plant operations:
    For critical conditions such as computer rooms, air conditioning temperatures that are higher than design can cause problems with computer operational malfunctions. In industrial applications, loss of production, or a downgraded product can be the result of high water temperature.
  4. Plant life:
    Add to the above the reduced life that can be expected from a plant because of the additional stresses caused by higher operational temperatures, and the benefit of the lower initial cost of an undersized cooling tower quickly diminishes.

How to protect your company from undersized cooling towers:
Which protective measures could you make to ensure that the installation performs satisfactorily? Most buyers of cooling towers will try to avoid buying a malfunctioning tower using the criteria below.

  • Doing business with manufacturers or representatives who state that they guarantee the tower's capacity.
  • Acquire cooling towers from a member of the Cooling Tower Institute(CTI) or buy cooling towers that carry their logo sticker or printed logo on technical bulletins.
  • Acquire a cooling tower of which the manufacturer claims the tower is tested in accordance with the Japanese Standard JIS B 8609.
  • Acquire a cooling tower of which the manufacturer claims the tower is tested in accordance with the Chinese Standard GB7190.1
  • Insist on a test on site carried out by the manufacturer.

Unfortunately, even if all of the above conditions apply, the tower's capability cannot be guaranteed. Please consider the following:

  • If a manufacturer quotes a particular sized cooling tower to a set of design conditions, it could be assumed that the manufacturer is guaranteeing that the tower will perform to those conditions. Unfortunately this is not always the case and some manufacturers are over-rating their towers as much as 40%.
  • A number of manufacturers are member of the Cooling Tower Institute, a worldwide organization headquartered in the USA. More than any other organization, the CTI has played a part in elevating the status of cooling towers, by promoting the truthful rating of them.
    Being a member of the CTI allows the member to use the CTI logo with member printed beneath on letterheads and promotional information. The use of the CTI logo in such a way does not provide any guarantee of performance of the manufacturer's range of products, it just proves they are a member.
  • The Japanese Industrial Standard, JIS B 8609 - Performance Tests of Mechanical Draft Cooling Tower covers thermal performance, sound, drift and electrical power.
    Some cooling tower manufacturers proclaim that their towers are tested in accordance with JIS B 8609. This standard states that it is for use with cooling towers of capacity of 233 kW or less at standard design conditions. This is an approximate water flow rate of 11 l/s, which is a very small machine. There is nothing to prevent the principles outlined in the standard from being used for larger towers.
    JIS B 8609 is a code for laboratory practice and is not suitable for field-testing cooling towers. In JIS B 8609, once rated the performance of the tower, there is no requirement, compulsory or voluntary, to use the actual capacity found from testing in the published rating for that tower.
  • As with the Japanese standard discussed above, the Chinese standard, GB 7190.1 covers thermal performance requirements and other matters pertaining to cooling towers. Different to the Japanese standard is that this Chinese standard also covers a form of certification of the cooling tower's thermal performance.
    However, this standard has a number of problems with its approach to thermal performance testing. Some problems are:
    The test tolerance of 10% is too high, to much for a mission critical cooling tower.
    The required instrument precision is low, making the end result dubious. As discussed and shown before , seemingly small measurement tolerances can lead to large errors in the final calculated capability of the cooling tower.
  • To overcome the problems caused by undersized cooling towers, some consultants demand a site test on the cooling tower to verify its capability. As it might not be in the best interest of the manufacturer to be very critical on the performance of it's own products, manufacturers tests might not be the most reliable.
    Most manufacturers would do a test using minimal equipment of dubious quality. In such circumstances, errors are common, and usually favor the manufacturer. Referring to the table above, small differences in the measured quantities can lead to large difference in the end result. Errors in the (handling of) measurement devices are not the only problem, there are other tricks of the trade that can be used to bias a test result in the favor of the tower manufacturer. So, even specifying a site test with all the best intentions will not necessarily ensure that the end result is a tower delivering the performance required.

If it is critical that a tower provides the performance that is specified (and shouldn't all installations be considered as such?), there are two ways to ensure receipt of a right sized tower.

These are:

  • Specify a test to a competent standard such as the CTI Test Code ATC-I05, with the test to be carried out by an independent testing authority that possesses the necessary skills and high quality instrumentation.
  • Purchase a cooling tower that has certified thermal performance. At this point of time, the only certification scheme of merit in existence is the one conducted by the CTI.

Independent cooling tower thermal performance testing
A well conducted site test true to the test codes involves the measurement of quantities as water flow rate; hot and cold water temperatures; entering air wet bulb temperatures; fan motor electrical characteristics and wind speed and direction. These quantities are specified to be within certain limits of the design conditions.
Because water cooling towers rely on both heat and mass transfer to effect the cooling function, they are rated at steady state conditions. While it is impossible to obtain a steady state condition on site, the test codes stipulate the maximum rate of change that the various measured quantities can vary over the test period.
Instrumentation is of paramount importance and, of course, should be calibrated to an acceptable standard. Logging of test measurements should preferably be automatic, particularly for larger towers where a large number of instruments are used in the gathering of information.

It is essential to the interests of all parties, that confidence exists in the ability of the tester to conduct the test and interpret the accumulated data. To ensure confidence, the CTI has a scheme where it licenses testing agencies to conduct thermal performance testing under their auspices. Such a license is only granted after rigorous examination by CTI of the applicant. The aim of the examination is to assure quality in the conduct of the test, the reporting of results and the complete independence of the licensee without the possibility of any conflict of interest.

We do not promote that every cooling tower needs to be tested. However, for larger towers and for towers on critical installations, the test price can be justified when compared with the initial cost of the tower and the impact on the life cycle costs of the overall plant that an undersized tower can have.
Should a test prove that the tower is deficient in its capability by more than 5% then, as part of contractual obligations, the tower manufacturer should be required to upgrade the tower and retest at their own expense. Should the tower still be deficient, then further upgrading and re-testing is to be carried out. It is not surprising that where such specifications are invoked, the chance of an undersized cooling tower being installed considerably diminishes.

Conclusion
Cooling towers have a poor reputation. It is generally believed that they do not deliver the performance they are rated at. To overcome the possibility of a low performing tower, design conditions are specified which are higher than required. This "solution" may be contributing to the perpetuation of the original problem.
It is possible to demand and obtain cooling towers that have 100% capability, either by specifying a site test conducted by an independent and experienced testing agency, or by the purchase of a tower with certified thermal ratings.
In the competitive process of bidding, the various products offered should be compared on size of the tower, the wet surface and the motor power ratings instead of only a towers price. This already expels in an early stage those manufacturers who, willingly or not, offer underrated towers in order to win the project. One could also invite a professional independent institute to support the purchase process.
Nonetheless, one should receive what one paid for.

A special word to:
To this brief explanation about capacity versus capability Mr. Terry Watt in 2005 contributed a lot of insights. Now retired, he is known as a critical follower of the cooling tower manufacturing industry and in his professional life he tested thousands of towers as CTI certified tester. "To keep the industry honest" is his lifetime and professional slogan.
Many of my cooling tower friends contributed to this publication by sharing their experiences in the cooling tower industry.

WACON International PTE Ltd.
Henk Janssen
February 2009