and mass transfer equation. In the early stages of sublimation, the
equilibrium is simple to maintain because the dry structure offers
minimal resistance to vapor flow. However, as drying progresses
and the depth of the dry layer increases, impedance to vapor flow
will also increase and the product may warm sufficiently to melt or
collapse unless the process temperature is reduced. One consequence of reducing the energy input will be to reduce drying rate
and prolong cycle times, but this may be unavoidable if product
quality is to be preserved [25].
Cooling and Warming
the Product
The shelves fitted into the freeze-dryer to support product containers may be alternatively cooled to initially freeze the product or
maintain shelf at a constant temperature throughout the drying
cycle or warmed to provide energy for drying. Basically, two systems may be fitted:
1. An independent cooling coil is embedded in the shelf through
which cold refrigerant is supplied (this system is termed direct
expansion), and a heating element is bonded into or onto the
base of the shelf. Shelf control is maintained by alternately
operating either the heater or cooler. Direct expansion systems
are relatively inexpensive but fail to achieve temperature control much better than Æ5
C.
2. For industrial or development activities, where shelf control to
Æ1
C is necessary to meet good manufacturing practice
(GMP) requirements, a diathermic fluid, which is invariably
silicone fluid, is circulated through the shelves, and a separate
refrigerator/heat exchanger maintains the diathermic fluid at a
preset temperature.
The mechanism and the relative quantities of heat entering the
product will depend on:
1. The nature of the product, its fill depth, consistency, and so on.
2. The dimensions and geometry of the product container and
whether the container rests directly on a shelf or is supported in
a tray.
3. The freeze-dryer design.
4. Chamber vacuum conditions.
Product temperature can be maintained by either raising or
reducing shelf temperature or by alternating system pressure
which has the effect of improving or reducing heat transfer efficiency. Regardless of the precise system incorporated into the
freeze-dryer, shelf temperature conditions may be controlled manually or programmed using a PC or microprocessor control.
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Kevin R. Ward and Paul Matejtschuk
equilibrium is simple to maintain because the dry structure offers
minimal resistance to vapor flow. However, as drying progresses
and the depth of the dry layer increases, impedance to vapor flow
will also increase and the product may warm sufficiently to melt or
collapse unless the process temperature is reduced. One consequence of reducing the energy input will be to reduce drying rate
and prolong cycle times, but this may be unavoidable if product
quality is to be preserved [25].
Cooling and Warming
the Product
The shelves fitted into the freeze-dryer to support product containers may be alternatively cooled to initially freeze the product or
maintain shelf at a constant temperature throughout the drying
cycle or warmed to provide energy for drying. Basically, two systems may be fitted:
1. An independent cooling coil is embedded in the shelf through
which cold refrigerant is supplied (this system is termed direct
expansion), and a heating element is bonded into or onto the
base of the shelf. Shelf control is maintained by alternately
operating either the heater or cooler. Direct expansion systems
are relatively inexpensive but fail to achieve temperature control much better than Æ5
C.
2. For industrial or development activities, where shelf control to
Æ1
C is necessary to meet good manufacturing practice
(GMP) requirements, a diathermic fluid, which is invariably
silicone fluid, is circulated through the shelves, and a separate
refrigerator/heat exchanger maintains the diathermic fluid at a
preset temperature.
The mechanism and the relative quantities of heat entering the
product will depend on:
1. The nature of the product, its fill depth, consistency, and so on.
2. The dimensions and geometry of the product container and
whether the container rests directly on a shelf or is supported in
a tray.
3. The freeze-dryer design.
4. Chamber vacuum conditions.
Product temperature can be maintained by either raising or
reducing shelf temperature or by alternating system pressure
which has the effect of improving or reducing heat transfer efficiency. Regardless of the precise system incorporated into the
freeze-dryer, shelf temperature conditions may be controlled manually or programmed using a PC or microprocessor control.
114
Kevin R. Ward and Paul Matejtschuk