Fish Nutrition & Feed Technology

Unit 10 - Methods of feed formulation and manufacturing

10.5.4.Pelleting

A control valve introduces dry steam into a header from which, through several port-holes, steam enters the conditioning chamber in contact with dry feed. Between this valve and the steam generator or boiler are a strainer and trap to remove condensate, providing only dry steam in the mill. At the discharge end of the conditioning chamber is a gate to restrict feed from immediately leaving and allows more time for moisture to be absorbed into the feed. A chute or funnel usually guides moisture-conditioned feed into the pellet chamber where compression and extrusion occur. If the ammeter goes much above the optimum reading, this chute may be quickly raised to prevent a choke-up of feed in the die holes. From the pellet chamber, feed may be directed by means of a butterfly valve to the cooler or on the floor for inspection.

Another variable that may be introduced into the operation of a pellet mill is the rotational speed of the die. For the production of small diameter pellets (i.e., 3 mm or less) high rotation speeds are used. This results in a thinner layer of soft feed inside the die ring ahead of the rolls, and for a given volume of feed the efficiency of pelleting and pellet hardness are improved. Die speeds may be changed by replacing the pulley on the main motor shaft of the pellet mill. Speeds generally range from 130-400 rpm. Feeds-of low bulk density are formed best in dies rotating at higher speeds.