The main disadvantage to using R-744 systems is its relatively higher operating pressures. Trans-critical systems operate at as high as 1305 PSIG on the high pressure side.
The pitfalls to be cognizant about while using R744 as a refrigerant include, asphyxiation, as a leak could cause the high pressure refrigerant to expand suddenly and voluminously displacing the oxygen in the surrounding and exceeding risk concentrations. A concentration of 30,400 ppm can be achieved as quickly as 5 mins at a leak rate of 1lb/min in a 1,000 ft^3 room if the pressure of the refrigerant is as low as 214.91 PSI . A concentration of 300,000 ppm could cause instant death. It is essential therefore to ensure that the system components are rated for the high operating pressures.
Other risks include freeze burns on contact with solid or liquid CO2, pressure changes in trapped liquids due to temperature variations ( a 1 degree Celsius temperature increase can increase pressure of the trapped liquid by 145 psi).
We designed the bullhorns and the network and provided the client with an automated calculator for sizing the evaporator suction and liquid lines.
Installation of CO2 refrigeration systems require adequate care especially during the charging process.
References: IIAR R744 (CO2) Industrial Refrigeration Handbook, CO2 Transcritical Systems, Thermophysical Properties of R744, Perry's Chemical Engineering Handbook, Transcritical Systems Training Manual