When the size of a freeze dryer is to be determined several questions have to be addressed, for example:

• 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.

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.

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 deplete the pressure level and possibly damage the vacuum pump, eventually polluting the laboratory environment.

In the table we provide the freezing points/ collapsing temperatures of some commonly used solvents and known products, these are given for guidance purposes and to assist in deciding the best condenser temperature for your application.If the condenser temperature is close to the freezing point of the sample – freeze drying cycle times are extended and the product could even start to melt, giving very poor recovery rates. This phenomenon is known as a “collapse” of the sample. The driving force in freeze drying is expressed as ΔP or ΔT corresponding to the temperature/pressure differential between the product and the condenser – the larger this difference the faster the freeze drying process.

In conclusion, 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.

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\The CoolSafe family of freeze dryers comprises of a large range of different size condensers and with a choice of either –55 °C, –95 °C, –100 °C or –110 °C temperatures. The sizes offered are from 4 litres up to 15 liters for bench-top models and 80 liters on Superior floor standing models. All the condensers are heavily insulated to save energy and give increased performance characteristics.

The seamless, gasket-free construction ensures long life and maintenance-free usage, together with easy cleaning and fast de-icing.

The compact bench-top design saves valuable space but if required can be mounted on castors or trolleys for ease of re-location anywhere in the laboratory. The high cooling capacities on all models are measured over a 24 hour period and give fast, efficient and reproducible freeze drying performance

A vacuum valve between the condenser and the vacuum pump allows for separate cool down of the condenser and warm up of the vacuum pump giving an efficient start-up sequence. The vacuum valve will only open if the condenser temperature is low enough to capture vapours – otherwise it remains closed, thereby protecting the pump and the sample and not allowing premature operation.

PRO microprocessor control via display and key-pad. Temperature and pressure display and measurement to 0.001 mbar. Offers full programming capability in Automatic mode or basic operation in Manual mode.Controls temperature of chambers with electrically heated shelves, auto-start and stop, pressure regulation, de-ice and time. Including alarms for overload, malfunctions of compressors and sensors.

PC Option is the ideal choice when documentation is required – allows the monitoring of the parameters for pressure, condenser temperature and heated shelves and product sensors (if included). Full programming facility – to programmed complete freeze drying cycles for different sample types and to control and monitor the complete process automatically. Data Logging – data can be presented in Excel spread sheet format or as graphical curves with data point display and tabulation.

Vacuum regulation - Vacuum regulation and control available only on PRO versions.

This is often a wise option to select, especially when requiring a low temperature condenser with versatility, multi-purpose applicability and resistant to heavy use with the most aggressive liquids and solvents.