Realize that water in 'vapor' form is hard to remove. The hot compressed air coming directly from the compressor pump contains water vapor and until that vapor cools and condenses its fruitless to try to remove it.
The best scenario is to add an 'aftercooler', which is plumbed into the system between the pump discharge and the receiver (tank) This will cool the discharge air down enough to change the vapor into 'water droplets' which can then make their way to the bottom of the receiver to be then removed by either opening a manual drain valve or installing one of many different style automatic drain devices, in either case, you can plumb the drain outside or into a container if running outside is not feasible.
The automatic condensate drains run the gambit in cost, the timed system are probably the least costly, next would be the units that drain after each compression cycle and the most costly are the 'Zero Loss' systems which utilize a float and when it's reservoir is full, a solenoid opens and the stored condensate is forced out via tank pressure.
I forgot to mention that the relative humidity in your area has the greatest influence on the amount of condensate your compressor produces. If you lived in the deserts of Arizona, it would be minimal compared to high humidity areas like Alabama or the like.
