PHASE CHANGE MATERIALS AS SECONDARY COOLANTS
A pumpable ice slurry can be used instead of chilled water or brine. The composition of these fluids is quite simple, consisting of water with ice crystals mixed with another fluid such as glycol, ethanol, ammonia, NaCl. Only a fraction of the water is transformed into small ice crystals (around 1 mm). This ensures a uniform solution while enhancing cold transfer. Ice slurry can take advantage of the high latent heat of freezing whilst retaining the ability to pump the fluid to the coolers. Optimization is directed towards finding the least viscous secondary refrigerant, from within the available range, that has the highest energy efficiency.
The ice crystals can be produced by scraped surface generators that may be quite costly. Alternative generation methods are being investigated. Various methods, products and applications are given by Paul (1997, 2002).
The properties of carbon dioxide make it very suitable for circulation as a volatile secondary coolant. Liquid carbon dioxide at approximately — 10°C and 25-26 bar pressure can be pumped to coolers where a fraction evaporates. The two-phase mixture returns to a reservoir where it is separated. A conventional refrigeration circuit maintains the pressure at an appropriate saturation temperature.
Liquid carbon dioxide has a very low viscosity; and because of its high latent heat, the volume to be circulated is very small compared to any water-based solution. Carbon dioxide exhibits a high boiling heat transfer coefficient that favours the design of the coolers. This type of system is starting to be applied in supermarkets for medium temperature refrigeration. It also has significant potential in air conditioning applications (Pearson, 2004).
Posted in Refrigeration and Air Conditioning