This question on the effects of the thickness of the ice in the heat transfer coils and why sometimes the duty/ capacity increases instead of decreasing ?
When we use the thickness of the ice we get two effects:
the ice introduces a thermal resistance that decreases the heat exchange coefficient
the ice decreases the passage area of the air, therefore having the same flow , the cross velocity increases, thus the heat exchange coefficient air side increases. This (coefficient) usually in the heat transfer coils is the binding factor.
The addition of these two effects give an increase or a decrease of the duty.
If the flow instead is determined by adding a fan, the ice presence will give an increase of the pressure drops air side, thus bringing the balance point to a lower point, so decreasing the duty.