Evaluation of corrosion problems in a closed air-conditioning system: a case study

Chilled water air-conditioning systems are the lifelines of cooling for large commercial, industrial and public buildings. The aim of this paper is to examine and evaluate damages sustained to a large hotel in Greece. The damage that discovered concerns corrosion and salt deposition problems of a chiller in the air-conditioning system. The hotel’s air-conditioning system is part of the facility’s electromechanical infrastructure and is comprised of three centrifugal chillers with chilling capacity equal to 500 tons each. The aforementioned chillers are supplied with water through the public water network, which firstly undergoes a softening process, performed by a softener, and covers all the hotel’s requirements. The chillers supply chilled water to seven central air-conditioning units, which in turn supply with fresh air all the hotel facilities. The above system is not supplied by water continuously, but replenishes with the needed quantity, when necessary, usually due to evaporation during the chilling cycle (closed air-conditioning system). The results of the system’s examination indicated accumulation of salts inside chiller tubes, which create a corrosive electrolytic environment, leading to the oxidation of metal and the creation of abrasion along its length.

It is often that air-conditioning installations after their completion are received by the maintenance department of the building which usually allocates craftsmen (plumbers, electricians) and has as head a Mechanical or an Electrical Engineer. The head directors of the maintenance department as rule have been informed for the operating conditions of the air-conditioning appliances. However, deprived of chemistry knowledge they cannot appreciate the actual importance of the facts. In this way they take water samples at the exit of the softener, where the hardness of water has low values and the chloride concentration is low. The water though that circulates the system after the repeated evaporations has high chloride ion concentration and water hardness higher than that of the water at the softener exit. Also since the water is continuously evaporated and lost make up water must be added to the cooling tower usually to the basin. Evaporation removes only water and not dissolved or particulate materials therefore “blowdown” is necessary so that these dissolved materials will not accumulate in the cooling system. It is obvious that in the system examined it was neglected (mostly for financial reasons) to replace the water in a regular manner. Thus by ignoring all that facts ignored that the corrosion ability of the water increases and causes intense corrosion at the metallic surfaces with which it comes in contact as it happened with the chillers tubes. The corrosion process followed by the unpleasant financial losses it could have been avoided if there was the assistance of a Chemical Engineer or a Chemist either as an employee or as a counsellor who could have known where the samples should have been taken and that with the right manner. Also having knowledge of the corrosivity of the anions he/she could have been imposed frequent water changes at the airconditioning system or he/she could have had suggested appropriate safety measures, for example corrosion inhibitors. Another proposition at that particular system is the use of a desalinator instead of a softener and proper mechanical or chemical cleaning of the chillers’ tubes as the manufacturer instructions for proper operation suggests.