able to condense within the cooling system, meaning that it has to be pressurized more than the other natural refrigerants. It can require up to 200 atmospheres to achieve adequate pressure for condensation. Refrigerant systems using carbon dioxide need to be built to withstand higher pressures. This prevents old coolant systems from being able to be retrofitted in order to use carbon dioxide. However, if carbon dioxide is used as a part of a cascade refrigeration system, it can be used at lower pressures. Using carbon dioxide in cascade refrigeration systems also means that the aforementioned benefits of availability and low price are applicable for a cascade system. There are also benefits to the increased required pressures. Increased pressures yield higher gas densities, which allow for greater refrigerating effects to be achieved. This makes it ideal for cooling dense loads such as ones found in server rooms. It also allows for carbon dioxide to perform well under cold (-30 to -50 °C) conditions, since there are very small reductions in saturation temperatures for a given pressure drop. Plate freezers and blast freezers have noted improvements in efficiency and freezing time using carbon dioxide. There are also propositions for improved thermodynamic cycles to increase the efficiency of carbon dioxide at higher temperatures. Equipment with carbon dioxide refrigerant is also not necessarily heavier, bulkier, or more dangerous than similar equipment despite its higher working pressures due to reduced refrigerant volume flow rates. When the pressure of carbon dioxide is raised above its critical point of 7.3773MPa it cannot be liquidized. Heat rejection must occur by cooling the dense gas, which creates a situation advantageous to water-heating heat pumps. These are particularly efficient with an incoming cold water supply. Ammonia as a refrigerant (R-717) Ammonia () used as a refrigerant is anhydrous ammonia, which is at least 99.5% pure ammonia. Water and oil cannot exceed 33 and 2ppm respectively. Ammonia refrigerant is stored in pressurized containers. When the pressure is released it undergoes rapid evaporation causing the temperature of the liquid to drop until it reaches its boiling point of -28 °F, which makes it useful in refrigeration systems. Ammonia has been used frequently in industrial refrigeration since it was first used in the compression process in 1872. It is used for its favorable thermodynamic properties, efficiency, and profitability. Ammonia is produced in massive quantities due to the fertilizer industry, making it relatively