During normal operation of a Stability Chamber the cooler's heat-exchanger coils (the cold source) are slowly encased in a film of ice. This is caused by the air borne moisture in the ambient air sublimating on contact with the cold heat exchanger coils of the cooler and forming ice. This ice build-up is slow, progressive and inexorable. It can; if left unchecked, transform the heat exchanger coils into a useless block of solid ice.
The ice build-up is heavily dependent on ambient conditions. If the heat exchanger was positioned or used air drawn from a climatically controlled area (low humidity), this ice build-up would be totally different from an installation that was expose to near saturated air conditions. To allow for these ambient atmospheric variations, the defrost cycle is usually controlled by time. So during the commissioning study stages, the requisite time interval between defrost cycles must be established for all stability chambers used for climatic controlled storage. cabinets.
The most common design methodology for defrosting the heat exchanger coils is simply to incorporate in the design an electric hot air blower unit mounted adjacent to the freezer coils and allow the impingement of this hot air to melt off the excess ice. The trouble with this is you are melting off the ice from the outside in; this can be a long and slow process, during which it is extremely difficult to stop this hot air from percolating into the cabinet interior and substantially raising its internal temperature.
This was the reason our cabinets were failing.
During defrost cycles the top two shelves in the cabinets were hopelessly out of specification (actual – 14ᵒC required -25ᵒC). The answer was to redesign the defrost methodology. During the defrost cycle we circulated the hot gas coming from the compressors through the heat exchange. This sudden flow of hot gas quickly broke the bond between metal parts and the encumbering ice. This bond being broken the ice dropped down in to the splash tray below.
This allowed the defrost cycle to be reset to much shorter intervals. This in turn kept the heat exchanger relatively clear from heavy icing conditions and so significantly enhanced it's thermal efficient while enabling the defrost cycle duration to be reset from seventeen minutes (pre-redesign) to less than six minutes (post redesign). Since there was now no defrost hot air being circulated the impact of this new defrost methodology on the cabinet internal temperature was found to be insignificant. The electrical power consumption for each modified cabinet dropped by 31%.