In today's extensive fields of biological research and medical innovation, the market for -80°C deep freezers is increasing. Here we consider the latest developments in design.
During the manufacture of most deep freezers, the cooling coils are wrapped around the outside of the inner body and the space between the inner body and the outer bodies is then injected with a foam mixture. As the foam expands, it compresses the air and bubbles are to be found, typically in the loops of the coils and at the corners.
Recent attempts to gain space have seen vacuum insulation panels used. These include a sealed sandwich of open cell foam or of micro cells of insulant from which the air has been evacuated. Performance is adequate at first but the differential expansion of the aluminium foil and the contents during de-frosting and re-freezing render the design susceptible to leaks in the long term. Thus the half thickness walls, which enable more sample storage, become half as effective as insulation when a leak occurs.
Some recent developments in in design include: u Insulation by foam blocks. Here, the coils are not around the outside of the inner body but are to be found within the shelves and the roof of the chamber. The space between the bodies can then be filled with pre-cut blocks of foam of homogeneous efficiency. Non-setting mastic at the junctions ensures no heat path between the panels. The closed cells of such foam ensure no water permeation. u Sample stability and protection. The quality of preserved samples is strongly related to their speed of freezing. Slow freezing allows large crystals to form within cells, frequently destroying them. Fast freezing ensures that only small crystals can form. Classical freezer design depends upon convection for removal of the heat from the samples. Movement of the air takes heat from the samples to the walls where it passes through and is removed by the cooling fluid in the coils outside the chamber.
Freezers with coils within the shelves enable the heat to be removed by conduction as the samples are sitting directly on the coils in the shelves. This greater efficiency is simply explained. Imagine a hand held a few centimetres above a pot of boiling water. The temperature difference is 100°C37°C = 63°C. The hand feels warm. Imagine that the hand is plunged into the boiling water. It is scalded. This compares heat transfer by convection with that by conduction. In a -80°C deep freezer the temperature difference for a newly introduced sample is 100°C!
The NapCOIL series of deep freezers from Napco optimises sample protection by combining block insulation with internal cooling coils. u
ENQUIRY No 37
Melissa Backus and Glenn Pickett are with Napco, Winchester, VA, USA, www.napco2.com
