– Dan Mizesko
It is unfortunate that enhanced tubes are not being maintained properly in the region, and thus most centrifugal chillers are consuming greater energy than they should be, leading to exponentially higher catastrophic tube failures. I would, therefore, encourage chiller owners to adopt the following recommendations:
System surveying, monitoring and troubleshooting
A chiller specialist professional should inspect the present condition of the tubes at least once annually. There are two basic non-destructive test procedures that need to be followed: Eddy Current Testing and Fiberoptic Scoping.
Eddy Current Testing:
Eddy Current Testing (ECT) has been in use for many years, and is widely employed to determine the condition of tubes by professional, independent chiller specialist service companies.
ECT has positive features, such as the ability to do a rapid internal and external tube surface and wall thickness inspection. Its sensitivity is a direct function of the calibration of the equipment and the identical nature of the calibration standards used by an ECT operator.
The primary benefit of ECT is that it examines the entire thickness of the tube wall, not just the inner visual surface.
However, the disadvantage of ECT is that it cannot determine the root cause of imperfections on the tube surface. For example, if a deposit is detected, this method cannot identify its origin. The same issue arises with a crack or a pit – it is difficult to ascertain why they have occurred.
Here, it is important to note that the services of only experienced and certified technicians should be used to conduct this type of testing in order to distinguish between pitting and deposit formations in the tubes. This is because an uncertified and inexperienced technician can misinterpret results leading to misinformation.
Fiberoptic Scoping (FS) is a modern technique of visual observation that uses a small optical device with a light source attached to a video monitor via a fiberoptic cable.
The advantage of FS is that it is a visual technique that helps see colour and can identify and distinguish between deposits, bio-fouling or other external material, such as epoxy coating, dirt or debris from the tower that have been causing heat transfer losses within the tubes.
It needs to be noted that FS cannot quantify how deep pits or cracks are. However, some units claim that through digital depth measurement capabilities it is possible to measure the thickness of the deposit or the depth of a clean depression or a pit.
As with ECT, the FS procedure should be performed by trained and experienced technicians, because:
- It is a visual technique
- The magnification properties and examination of the internals via a uni-dimensional monitor could sometimes lead to misinterpretation
- Having different angled lenses and a proper light source is important to help identify the problems, which only trained personnel can handle
Need for regular monitoring regimen
Techniques such as ECT and FS should become part of a routine chiller inspection regimen to verify how well the water treatment programme is doing its job. These NDT (Non-destructive Testing) inspection techniques should be performed at least annually unless other problems or operational conditions indicate the need for more frequent monitoring.
Monitoring of corrosion, scaling and bio-fouling
Monitoring of corrosion, scaling and bio-fouling are extremely important for the efficiency and longevity of a chiller. Here are a few of the methods of combating them:
Corrosion coupons and total dissolved metals analyses can provide the chiller specialist information about potential for corrosion.
Scaling indices and elemental mass balances that require complete water analyses can provide indicators of scaling or deposition. Deposit monitors can be used to verify that the water treatment programme is working. When scales or deposits do get formed, it is important to determine where the water treatment programme is failing or the root cause of the deposition.
Dip slides, Biological Activity Reaction Test (BART), serial dilutions, and Adenosine Tri-phosphate (ATP) technology should all be deployed and cross-checked to verify that bio-fouling is not occurring.
Additional testing using removable spool pieces and slime monitors are recommended to ensure that there are no hidden biofilms.
It is important to note that chiller service professionals cannot rely on any one test to establish if there is a biological problem. Cross-check verification with another test is always helpful to ascertain if corrosion is occurring and/or if there are any operational problems.
The above-mentioned monitoring techniques should be standard tools employed by chiller specialists and water treatment professionals and must be applied in their entirety to determine if typical water problems can pose serious issues for enhanced tubes.
It is evident that the less the particulate matter circulating, the better the system will perform and the more effective the water treatment programme.
However, because cooling towers function to some degree as air scrubbers, particulate contamination of the recirculating water always occurs. Many enhanced tube failures occur due to particulates or debris contamination. To reduce debris and its potential settling in the tubes, chiller owners should consider the use of side stream filtration, especially where enhanced tubes are in use.
Bag, cartridge, disk or sand filtration can aid in removing particulate, old corrosion by-products, microbiological components, dirt and debris, etc from the cooling water.
The probability of galvanic corrosion on tube sheets can be controlled by installing sacrificial anodes on tube sheets to prevent corrosion attack on the tube sheet or water box metal.
Alternatively, an epoxy coating, which acts as a barrier layer, could be used to limit corrosion on the tube sheet. However, while a high quality epoxy coating can be beneficial to the system, a poor coating can be just as detrimental as it generates corrosion by-products. Also, epoxy coatings require proper preparation of the tube sheet surface and precise and careful installation. Proper surface preparation guidelines, such as SSPC-5, which include achieving a clean metal surface and a minimum attachment profile should be adhered to, or else, the coating will not be adherent enough to protect the surface of the tube sheet.
Tubes should be cleaned at a scheduled frequency. Specialist rotor mechanical brushing of the tubes can remove deposits or fouling before severe problems develop. It needs to be remembered that enhanced or super-enhanced tubes require special procedures and special brushes that must be used for cleaning them. Using a straight tube brush can scratch the enhanced tube and lead to a more severe corrosion problem. It is recommended that all units containing enhanced tubes be cleaned annually at a minimum.
One of the biggest operational issues is how to handle the off-line chiller, as off-line status could result from alternating operational chillers to achieve uniform service life patterns.
In comfort cooling systems, special attention needs to be paid to units that cycle off at night. An offline chiller, even when kept full, invites fouling and corrosion.
Stagnant water is especially prone to the hazard of increased microbiological growth, settling of water-borne solids, and generation of gas bubbles that get separated from the water and get trapped in the rifling of the tubes. These conditions can lead to localised corrosion of the tubes.
Consideration should be given to lead/lag timing, recirculation with load, or draining and flushing. Typically, if a condenser is to be left full and is not operating for more than two to three days, it should be recirculated every six hours for one hour to replace stagnant water.
Some experts also recommend that if this condition is to exist for more than one week, the system should be drained, flushed and allowed to dry.
The use of enhanced tubes to increase heat transfer rates is an effective technology to reduce energy consumption and greatly benefits the chiller owner through significant operational cost reductions. However, they are efficacious only if proper evaluation, water treatment, and system maintenance programmes are scrupulously followed.
The writer is the Managing Partner of Al Shirawi US Chiller Services Int, HVAC & Energy Services. He can be contacted at: Dmizesko@aol.com