Rarefaction of a Gas

It is an adiabatic expansion, so the temperature will drop.

Any gas, when allowed to expand freely (ie. into an unconstrained space) will cool the remaining gas in the tank.

Pressure times volume equals (gas constant) times Temperature times Moles of Gas, so if you drop the pressure, keep the volume the same, all else stays the same except temperature must go down linearly with pressure.

I think 473K is above the critical point of NH3, so as the pressure drops from 20bar, the temp will fall although it probably won't follow the simple adiabatic expansion formula being initially too close to critical temp (from memory about 400K).

If you have good insulation, by the time you reach 1 bar, temp will have dropped significantly. It is a standard refrigerant, so a set of refrigerant tables or TS diagram will tell you what the temp will be by that time.

The same tables will tell you the temp when you have dropped it to a low pressure from your 1 bar left in the tank.

If you are simply trying to empty the tank for maintenance, you could try using dry nitrogen to purge the gas out once you get to atmospheric pressure. Compressing and cooling the resultant mixture later will allow you to reclaim the NH3 when it liquefies.