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To determine the size of a freeze dryer needed, several questions have to be addressed:

• What is the maximum number of samples to be freeze dried at one time?

• What is the total volume of the samples?

• What is the size of container/tray you want to use for the samples?

• What is the sample volume in each tube/vial?

   – remembering that the depth of sample should not exceed its diameter, the greater the surface area the quicker it will dry.

• If “bulk drying” on trays or shallow containers, the sample depth should ideally be less than 1.5 cm.

• What size of condenser is required ?

• When all of these questions have been answered the size of the freeze dryer can be determined.

What size of condenser is required?

The size of a freeze dryer is rated in size by the capacity of the condenser to hold ice that is produced in a 24 hour drying cycle (i.e. water from the sample).

As a minimum, the condenser capacity must be adequate to handle the total amount of sample volumes in a single batch before de-icing is required. The larger the condensers surface area, the more efficiently vapour is converted to ice and consequently the thinner the ice-layer formed. This means processing time is faster, de-icing frequency is minimised and a stable low temperature maintained.

What condenser temperature is needed?

The choice of condenser temperature is important, if the temperature is not low enough a correct freeze drying process cannot take place.

The lower the condenser temperature the better! – for aqueous samples a –55 °C condenser temperature is the preferred choice – but a lower temperature condenser will provide faster freeze drying, avoid melting and give better results with the added benefit of protecting the vacuum pump and the environment.

• What is the maximum number of samples to be freeze dried at one time?
• What is the total volume of the samples?
• What is the size of container/tray you want to use for the samples?
• What is the sample volume in each tube/vial? (Remember that the depth of sample should not exceed its diameter; the greater the surface area the quicker it will dry.)

The size of a freeze dryer is rated in size by the capacity of the condenser to hold ice that is produced in a 24 hour drying cycle (i.e. water from the sample).

As a minimum, the condenser capacity must be adequate to handle the total amount of sample volumes in a single batch before de-icing is required. The larger the condensers surface area, the more efficiently vapour is converted to ice and consequently the thinner the ice-layer formed. This means processing time is faster, de-icing frequency is minimised and a stable low temperature maintained.

The choice of condenser temperature is important, if the temperature is not low enough an efficient freeze drying process cannot take place.

If the sample contains even a small amount of acids or organic solvents and the effective condenser temperature is above –90 °C, the vapours will not be trapped in the condenser and will exit into the vacuum pump. This will decrease the vacuum level, possibly damage the vacuum pump and eventually pollute the laboratory environment.