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Freeze dryers can be complicated pieces of specialised equipment, with controlled heating, cooling and vacuum interacting with each other to provide the best drying conditions. They are used for many different types of product and there are therefore a variety of different types of system with hundreds of options to enable them to suit the requirements of different industries and applications.
To make sure you get the right system make sure you’ve considered these questions.
What solvents are involved?
Most products for freeze drying have only one solvent: water. Water is straightforward to deal with as its high freezing temperature makes it simple to process and it does not react with any of the materials in the freeze dryer. However some organic solvents are corrosive to components especially plastics. To avoid constantly replacing parts, machines can be built to resist corrosion with stainless steel replacing acrylic parts, for example the chamber door or the manifold. However not all solvents are suitable for freeze drying, either because they are too corrosive or because their freezing temperature is too low. You should discuss this with your supplier early on.
What products are being dried?
Products may be freeze dried with different goals in mind– for example, to maintain flavour or appearance, to extend shelf life, to create a particular structure, to slow down biological activity, to prevent decay. Different products can have vastly different regulatory requirements and cost tolerances: freeze-dried strawberries for a breakfast cereal will not have the same burden of regulation as an injectable vaccine, but it will be much more cost-sensitive. It’s important to understand exactly what the goal is and how success is measured to make sure that the equipment is suitable for the job without being overcomplicated.
Some of the relevant equipment variables may include the tolerance criteria for temperature control; whether the temperature of the product is directly monitored, and how; the process temperatures and condenser capacities; and what level of programming, and reporting is required. To avoid doubt or confusion, it is useful to consider writing an User Requirement Specification (URS) which clarifies all your requirements.
What is the unit or container type?
Products for freeze drying use a range of containers – slurry in bulk trays, diagnostics in 96 well plates, pharmaceuticals in vials, R&D products in flasks. Items such as waterlogged paper or timber may not require any containers, depending on their size.
The container type will influence the type of dryer required. Vials, 96 well plates and trays are best suited to freeze dryers with shelves whereas flasks are better suited to manifold dryers. Solid products like timber or fruit can be processed in either configuration.
If your product is a liquid or slurry, care needs to be taken of how the containers are filled. The greater the product depth is, the longer it will take to dry. For example, freeze drying flasks are designed to be filled only ¼ or 1/3 full, depending on your freezing method.
Containers must be prefrozen before attaching to a manifold. “Shell baths” are freezers designed specifically to freeze product in flasks in a manner that will optimise drying.
How much product do you want to freeze dry?
There are two limiting factors in a freeze dryer: the size of the drying chamber or product container, and the capacity of the condenser.
The overall condenser capacity needs to be greater than the total batch volume – a condenser with an overall capacity of 5 litres should not be used to process a batch of 5 litres. This is to allow for the irregularity of vapour flow rates and the condenser’s decreasing efficiency as the ice builds up on its surface. This is especially relevant with large volumes or very wet products, and with manifolds with a large number of flask attachments.
In freeze dryers with shelves, the dimensions of the container may limit the amount of product that can be fitted into the chamber. It’s sometimes possible to increase the batch size simply by changing container – care must be taken to sure the additional product vapour does not overload the condenser.
Where will the freeze dryer be positioned?
Both manifold systems and temperature-controllable shelf systems are available in bench-top or floor-standing configuration. Larger condenser capacities and drying chambers are only available on floor-standing systems.
Like many refrigeration based systems freeze dryers will work more efficiently in a cool environment. 20°C to 22°C range is ideal, with a relative humidity of less than 70%. The systems will operate outside this range, but may be slower to cool or may even fail to reach particularly low temperatures.
Many systems can be built in different configurations to make them fit in a different space or to make them cleanroom-friendly, but make sure you discuss these requirements before construction starts!
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