and thereby to initiate water circulation between the bottom water pool 28 and the sprinklers 20 which eject the water so as to form a curtain which effectively separates off the paint drops or particles which during the spraying operation have been spread out into the air after having passed through the water curtain the air escapes from the booth through the outlet duct 14 over the heat exchanger 32 back into the atmosphere when after finished spraying operation the spray pistol is put back onto or suspended on the position indicator 36 the water circulation is stopped but not earlier but after some delay such as from 5 up to 120 seconds i e after the time required for effecting thorough purification of the air in the booth from all dust particles still floating therein the control system further includes a temperature regulator 38 which is connected to a central control station 40 and over this latter determines the number of revolutions of the exchanger motor 34 and also a setting member 42 which actuates a multi way valve 44 this valve is located in a conduit circuit 46 for hot water e g from a central source of hot water the conduit circuit 46 further includes a pump 48 and a radiator 50 which is located in the duct 12 for the supply air admitted into the spray booth the conduit circuit for the hot water may comprise a shunt conduit 52 via which depending on the set position of the valve 44 a major or minor quantity of water is circulated through the radiator 50 past the central hot water source depending on the heat requirement when the spray.