We were walking through a District Cooling plant recently; the plant was in the process of being commissioned for operation. We questioned the engineer in charge of the plant whether the condensers were filled with water and we were told, “Yes”. We further enquired how long the chillers have been sitting with water in the tubes and

were informed, “For a few weeks now”. We informed the engineer in charge that it was detrimental to the health of the chillers to allow water to sit stagnant in the tubes for long periods of time and the response I received is what inspired me to write this chapter of ‘License to Chill’. The engineer in charge told us that “It’s not a problem, the lines have been flushed and all chemicals were charged in the loop and protection is satisfactory.” This statement could not be further from the truth.

Enhanced and super-enhanced tubes are now standard in the comfort cooling segment of centrifugal chillers. The consequences of ignoring or not understanding the special needs these tubes require, will be catastrophic and lead to excessive power consumption, but more importantly, premature condenser tube failure. Special attention needs to be given to these types of tubes. I’m just going to touch on the operational considerations of enhanced and super-enhanced tubes in this segment – stagnant water is a prime consideration. However, there are other considerations that the reader should be up to speed on regarding enhanced tubes, to be sure the chillers are properly protected and operate efficiently.

One of the biggest operational issues is how to handle the off-line unit. Such off-line status could result from alternating operational units to achieve uniform service life patterns. In comfort cooling systems, special attention must be given to units that cycle off at night. An offline unit, even when kept full, invites fouling and corrosion. In stagnant water, increased microbiological growth, settling of water-borne solids and entrainment of gas bubbles that get separated from the water and are trapped in the rifling of the tubes can occur. 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 not operating for more than two to three days it should be recirculated every six hours, for one hour, to replace stagnant water. Others in the industry have recommended that if this condition is to exist for more than one week, the system should be drained, flushed and allowed to dry.

One other means to prevent deposition and corrosion circumstances under stagnant conditions is to install a small re-circulation pump on the unit. This pump should be integrated into the system so it maintains flow when the unit is off-line. A small filter housing could be installed to allow addition of biocide and/or extra corrosion inhibitors or dispersants during down times. This arrangement would provide some flow, possible filtration and a means to restore protective levels of chemical inhibitors. As the chillers and chilled water plants in the Gulf region are rather large, the installation of the re-circulation pump would be a more economical solution, as draining and taking down large condensers can be time consuming.

In closing, the facility I described at the start of this segment had the stagnant water in the tubes for weeks! Obviously, this engineer is just uninformed of the requirements of enhanced and super-enhanced tubes. Everyone wants to do a good job and excel in his or her industry and I see stagnant water as an issue in the majority of sites I visit worldwide, thus the reason for this short but extremely important article.

Dan Mizesko is the Managing Partner of Al Shirawi US Chiller Services. He can be contacted at dan@uschillerservices.com.

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