Basic Comparison of Low Temperature Cooling Methods - Robert W Repke

There are various cooling methods available today that can impact both equipment and product performance. This paper aims to give an overview of the pros and cons of some of the most commonly used methods, and offer the most effective use of each.

Methods and Descriptions:
Dry Ice is a solid form of carbon dioxide with a temperature of -78.5°C (-109.3°F) at atmospheric pressure. 1 pound of dry ice = 246 BTUs. It’s colourless, odourless, non-flammable and slightly acidic. If the environment in which dry ice is used or stored is above -78.5°C the dry ice will sublime, meaning it will change from a solid to a gaseous state without going through the intermediary liquid state. Because of this phase change, the rate of which is heat dependant, dry ice is considered to be an ‘expendable’ refrigerant.

Dry ice is widely used in medical, industrial and scientific applications. Most applications involve breaking blocks of dry ice into pieces and placing them in a solvent in an insulated container and producing a slurry. The cold bath can then be used to remove heat from an intended application or used as a cold bath for maintaining the temperature of a product or process placed in the bath. In order to maintain the bath or process at a specific temperature, temperature control must be added.

Because dry ice is an expendable refrigerant, it must be constantly replenished, the rate of which is determined by the heat load imposed on the dry ice. For applications other than one-time use, dry ice must be regularly reordered so as to prevent running out and shutting down the process it’s cooling. Additionally, dry ice must be stored in well insulated containers using valuable space. It must be transported from its storage site to the process site and must be broken into useable pieces to fit the process. The storage, transportation and breaking of the dry ice is costly (in addition to the cost of the dry ice itself), in that an individual must be assigned to perform these tasks.

Because of the low temperature of dry ice, inherent hazards exist. Prolonged exposure to dry ice can cause severe skin damage through frostbite. Because dry ice is solid carbon dioxide, as it sublimes it becomes carbon dioxide gas which displaces oxygen containing air which can result in asphyxiation if the area in which it is used is not properly ventilated.