Marko:
Contamination of the heat transfer surface can indeed influence the efficiency of the system, I assume by the fact you are asking you have doubts about the cleanliness of the system.
Also external factors such as ambient air temperature and wind can influence ACC performance. Warmer air will of course cool less, due to lower temperature differential and lower density. Also wind draws extra load on the fans decreasing static pressure output of the fans into the ACC.
You could consider having your bundles cleaned as this is typically good practice, I know many sights prefer the spring for this to be ready for hot summer weather, also it could be a good idea to have your fan peformance evaluated to see if your ACC is receiving sufficient air flow, or if more air flow is required to meet this new cooling capacity requirement.
Please feel free to send me a PM (desmond_DOT_dehaan@Howden_DOT_com) if you need any local references that we may be able to provide assistance for you.
