It's a Schriber Chiller and it does have a cover that you lift off to access the evaporator and glycol. There is also a gauge glass to see the level.
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Hands, drain the system and fill with fresh water, add a product called Citranox for cleaning. I try to run the cleaner for 24hrs at a elevated temperature for the best results. After its clean pull the top and wash the tank out very good and drain several times. Try to get all the tap water out of the system, then use a 50/50 mix of distilled water and propylene inhibited glycol.
Stay on top of the 50/50 mix and maintain.
BTW, I use Protek 100 in all my open loop chillers.