How much current is drawn by an 18,600 BTU A/C compressor during free run?
It does mean that using the nominal load it draws 11.5 Amps. 

If it is an air conditioner the load is 18600 BTU / hours, which is 5.6kW -> the thermal energy it can draw out. 

However, when you use the motor don't drive it more than 11.5 Amps in long term.



The figure above shows a typical torque - motor speed diagram.

If you have no load, the motor speed will be higher, because only cooper, iron and friction losses determines the motor current. For higher load, the output power increase, resulting in a higher motor current (~ I_motor * V) and the motor speed decrease. 

BTW: 



So I would estimate, since it seems to be an older motor, that is draw ~2 Amps during free run. (efficency of 80%)