Design Improvements for an Ice Making Machine

I used to work on such a machine. The secondary coolant was propylene glycol (antifreeze), and the machine worked like you describe. The coils were right in the tank, not outside. Eventually ice would form on the coils and slow down the process.

To improve the efficiency of this machine, I would first water cool the condenser coils. Air cooling in the tropics is not as good as water cooling, you just need a good supply of cooling water.

It may have been spilled water freezing on the coils, eliminate that by having raised lips on a cover over the tank.

Pre-cool the filling water in a coil clamped to the coil in the tank. This may melt any ice on the coil, and speed up the freezing time.

Have your ice forms tapered for easy removal of the ice. Our ice man wasted a lot of water and time trying to get the ice out of a non-tapered form.

If you have to use brine, research additives to retard freezing, antifreeze is expensive.

Have no salt in your filling water, ro it if necessary.

Have a look at how commercial fishing boats flash-freeze their catch, using brine. Sounds similar to what you are doing...

The secondary refrigerant requires two steps of heat transfer (refrigerant to brine, brine to water). This is less efficient than refrigerant directly to water.

One type of icemaker, North Star, produces flake ice (about 1.5 to 2 mm thick) efficiently, without need for rewarming the ice in order to release it. You don't say what form you need: flakes, cubes, blocks,....

The thicker the ice is, the less efficient to make it, because the heat must transfer through a greater distance of water.