FEATURE |
INCORPORATING COLD CHAIN |
|||||
temperature of 32 ° C and a gas cooler |
|||||
approach of 6 ° C, the gas cooler outlet |
|||||
temperature would be 38 ° C.” |
|||||
To handle these loads, the system |
|||||
needed seven medium-temperature and |
|||||
five low-temperature compressors of a |
|||||
specific model. The total installed power |
|||||
for these compressors in this transcritical |
|||||
setup came to 1 009kW. This breaks down |
|||||
to 924kW for the medium-temperature |
|||||
compressors and 85kW for the lowtemperature |
|||||
ones, not counting auxiliary |
|||||
power for things like gas cooler fans. The |
|||||
|
Image supplied by SCM Ref Africa.
CO2 Rack 4.
He then introduced the concept of a typical transcritical two-stage CO2 system, briefly describing its main components like lowand medium-temperature compressors feeding evaporators and a gas cooler. He explained that in a transcritical cycle, heat is rejected at temperatures significantly above the critical point, and the gas cooler operates with an approach typically 6 – 10 ° C above the ambient dry bulb temperature. This contrasted sharply with the lower condensing temperatures achievable
|
with wet bulb approaches in evaporative
systems, setting the stage to explore the
potential benefits of operating CO2 systems
subcritically when conditions allow.
COMPARING TRANSCRITICAL AND SUBCRITICAL CO2 SYSTEM PERFORMANCE
“ To explain how a typical transcritical
CO2 system works, let ' s trace the cycle:
heat is removed in the evaporator. On
low-temperature compressors, we
|
have desuperheating, and on mediumtemperature compressors, there ' s further compression. The crucial step is heat rejection in the gas cooler, where we don ' t cross into the liquid phase – that ' s what defines transcritical operation. After that, it ' s expansion, and the cycle repeats.” He offered a comparative example.“ We set up a baseline using a transcritical twostage CO2 system designed for specific refrigeration loads: 990kW at-10 ° C and 450kW at-35 ° C. With an ambient dry bulb |
system required a heat rejection capacity of 1 530kW from the gas cooler.
The comparison scenario involved replacing the transcritical gas cooler with a hybrid condenser specifically designed for CO2 ' s higher operating pressures. This hybrid unit incorporated a dry coil at the top, acting as a desuperheater and handling approximately 30 % of the heat rejection to reduce pressure, followed by an evaporative section below. The key benefit of this hybrid, evaporative-assisted design is the ability to achieve condensation subcritically when ambient wet bulb conditions are favourable. In this example, with a 22 ° C wet bulb temperature, the system could comfortably condense at 28 ° C – significantly lower than the transcritical gas cooler outlet temperature.
Operating subcritically at this lower temperature allowed for substantial
|