2017 Tech Report Nov-Dec

TRANSCRITICAL SYSTEM

Figure 3 Simple Transcritical System

point B to point C and some of the gas flashes to a liquid as it drops below the critical point. See a simple single stage system in Figure 3. It is typical to use air cooled gas coolers in the more northern and cooler regions. By using adiabatic gas coolers, the use of transcritical systems can be moved south. Because air- cooled gas coolers depend on the dry bulb temperature of the air, these systems are limited to cooler climate areas. Adiabatic gas coolers use

evaporation of water to cool the air stream before it passes over the refrigerant coils. This process is dependent on the wet bulb temperature of the air. During most normal conditions, the wet bulb temperature will be lower than the dry bulb temperature. As an example: with an outside air temperature of 95ºF dry bulb a transcritical systemwould not be practical using an air- cooled gas cooler. However, if an adiabatic gas cooler is used, with a 95ºF dry bulb, at 50% relative humidity, the wet

Figure 2. From point C to point D, the system acts like a sub- critical system. The refrigerant absorbs heat in an evaporator, boils, and the vapor is drawn to the compressor. At point D, the superheated refrigerant vapor enters the compressor or compressors. From point D to point A the CO2 vapor is compressed. At point A the gas, now above the critical point, is discharged into the gas cooler. The gas is cooled in the gas cooler from point A to point B. The cooled gas is expanded from

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