A recent change in the design of milk powder plants in New Zealand has seen the traditional cyclone system used for fines recovery replaced with washable bag house systems. Fonterra Co-operative Group Ltd, the worlds' largest exporter of milk products, has installed or retrofitted five such designs on milk powder plants throughout New Zealand since the year 2000.

Bag houses were traditionally used to remove fines not recovered in cyclones attached to spray dryer systems, but were always thought to be a source of microbial contamination because powder could have long residence times in the bags. Therefore approximately 10 % of these cyclone fines were downgraded to stock food. Another hindrance with this traditional system was that cyclones often blocked thus interrupting dryer operation.

Washable bag houses with their ability to be cleaned-in-place (CIP) have changed the perception of bag houses being a source of microbial contamination. Consequently all fines collected by them retain the product quality of the dryer chamber powder, increasing profits. Because all fines are collected in a single stage, fewer unit operations are required. This leads to reduced building space requirements, lower capital costs and simpler plants to operate. The final advantage of the new system is that bag house collection is a gentler form of product collection when compared to cyclones. This is more suited to the high fat and high protein powders of the dairy industry.

Despite the numerous advantages of this new system, high differential pressures, significant caking and excessive bag damage due to bag movement has caused frequent interruption to the dryer operation.

This paper will review bag house design as applied to milk powder production. The input variables such as gas flow rate, dust concentration, dust bulk density and particle size distribution required for the design process will be introduced. The guidelines of Croom (1995), Intensiv Filter (1989), Loffler et al. (1988) and Turner et al. (1999) for milk powder bag houses will be presented and the major design parameters of air to cloth ratio, bag diameter and length, bag spacing, between bag velocity and vessel footprint covered. The large variation among these four guidelines will be highlighted and the suitability of each for application to the New Zealand dairy industry will be discussed based on experience with the Fonterra Co-operative Group Ltd milk powder plant washable bag houses.