Technical
TRANS-CRITICAL
CO 2 SYSTEMS
By Marius La Grange , general manager at Thermocoil
In a previous issue of the RACA Journal , some information was provided in terms of CO 2 systems and heat exchange that can be referenced in conjunction with this content . This article is a continuation addressing further aspects of these systems .
CO 2
GAS-COOLERS Gas coolers ( GC ) reject heat energy from a CO 2
( R744 ) system similarly to an air-cooled condenser installed as part of an HFC refrigerant system . The cooling medium being ambient air , drawn across fin & tube type heat exchanger reducing the systems discharge gas enthalpy .
Similarly , heat rejection is adversely affected as the ambient temperatures increase at the specific installation ’ s location . A GC will function as a condenser in low ambient temperatures ( sub-critical ) but will only be able to reduce the heat energy from the compressor discharge gas during high ambient temperatures ( trans-critical ). This reduction in heat energy being at constant pressure in both sub-critical as well as trans-critical operation .
A sound understanding of a basic HCFC or HFC system is crucial towards understanding R744 systems . They are not that different and function in similar ways , albeit with distinct differences . Industry development has resulted in simpler R744 systems making it easier ( and less costly ) to install in applications to which it is ideally suited . If you have not looked at R744 systems before , it is certainly not too late with valuable industry lessons to benefit from currently .
So , what is a GC which is commonly used in R744 systems ? It is simply a special type of air-cooled condenser . A trans-critical system would be designed and built capable of trans-critical operation , but it is far more likely to operate in sub-critical mode for the greater number of hours each year .
The flow of heat energy also being counter flow in the case of a GC with the cooler outlet gas in contact with the inlet air . With the air flow passing through the rows , it absorbs heat energy from the R744 discharge gas , and the warmest air will ultimately be in contact with the warmest R744 discharge gas entering the GC .
Function A trans-critical R744 multi compressor ( multiplex / booster rack ) would use a specific electronic controller . The progression of https :// core . ac . uk / download / pdf / 42131256 . pdf - page 16
Figure 1 - System diagram – Gas cooler pressure control .
these components has simplified the required control functions for the trans-critical system . One of the most crucial functions being the GC operating pressure to achieve the optimal system coefficient of performance ( COP ).
Figure 1 refers . Two main inputs provide the controller with operational metrics . The one being a pressure transducer and the other being a temperature probe to measure the GC outlet temperature . The optimal operating pressure for the specific suction pressure the system is operating at would be calculated using default algorithms in the controller with the ICMT-valve and the GC fans used to try and achieve the optimal GC pressure .
Operating pressure in the GC could be lowered with the ICMT-valve letting the higher pressures into the flash tank / liquid receiver and or by the GC fans drawing higher volumes of air across the GC face area . EC fans are most commonly used to draw the ambient air across the GC face area and allow for a control speed from 10 % up to 100 % to aid the process of achieving the optimal GC operating pressure .
Construction A CO 2 gas cooler is in essence a fin & tube type heat exchanger rejecting heat energy from the HP side of the system and at pressures as high as 120 bar . Ambient air drawn across a fin &
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RACA Journal I April 2023 www . refrigerationandaircon . co . za