Most substances used in drugs today are hygroscopic and any absorption of moisture during the manufacturing process will affect the final weight, quality and durability.

When hygroscopic ingredients are weighed and mixed, it is important that the products”” weight is not influenced by absorption of moisture from the surrounding air.

In powder form, moisture will affect the flow, caking, compaction and strength properties of the manufactured solid form.

Yeast, moulds and bacteria require a certain amount of moisture to support growth. Controlling relative humidity is probably one of the most important factors in maintaining a sterile environment.

A cool dry environment generally enables faster production, a better quality product and a longer shelf life. The majority of tableting processes require humidity control between 20 and 35%RH at 21 to 24°C.

Moisture Loads

In most pharmaceutical systems, the greatest moisture load is from the fresh air component which is typically between 5 and 20% of the total supply air volume.

Depending on moisture gains across the room, it may only be necessary to “deep dry” the fresh air component using a pre-cooling coil (> 10°Cdp) and a desiccant dehumidifier (< 10°Cdp).

Dehumidifier capacity is controlled by an RH sensor installed in the return air duct. This simple design is often adequate to maintain the required RH in the room.

Air conditioning systems with a low fresh air volume and rooms with excessive moisture gains from high infiltration rates (negative room pressures), frequent door usage, product load, etc, will normally require all the fresh air and a proportion of the return air to be dried.