By Marius La Grange, general manager at Thermocoil
In a previous issue of the RACA Journal, some information was provided in terms of CO2 systems and heat exchange that can be referenced in conjunction with this content. This article is a continuation addressing further aspects of these systems.
CO2 gas-coolers
Gas coolers (GC) reject heat energy from a CO2 (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.
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Function
A trans-critical R744 multi compressor (multiplex/booster rack) would use a specific electronic controller. The progression of 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.
Figure 1 – System diagram – Gas cooler pressure control. Image source: https://core.ac.uk/download/pdf/42131256.pdf – page 16
Construction
A CO2 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 & tube surface area absorbs heat energy discharging it back into the ambient surroundings. Similar to an air-cooled condenser, a lower ambient air temperature offers a greater ΔT between the ambient and the system set point aimed for during operation. The operating temperatures can be relatively high in most cases but comparable to some HFC applications.
Figure 2 – 304SS return bends welded into tubes. Image credit: Thermocoil
Most commonly a GC is manufactured using thin-walled Grade 304SS to withstand the required safe operating pressures, but suitable copper tubing is also used in some cases. Processing the 304SS tubing requiring specific skills and it is also more time consuming, but it is more popular with manufactures.
One of the main limiting factors being the operating pressures of trans-critical system components, like compressors for example, as the safe operating pressures for compressors increase the required GC operating pressures also increase.
Adiabatic pre-cooling walls installed in the airstream prior to the gas cooler face area can reduce the ambient temperature airflow the gas cooler receives. This is primarily an option in areas with extremely high ambient conditions making a trans-critical system viable where it would otherwise not be suitable. In other cases, adiabatic pre-cooling walls are used to reduce the ambient air onto the gas cooler face area to mainly reduce operational energy consumed.
When making a gas cooler selection the following information is crucial towards making the calculations accurately. The more accurate the input parameters you can provide the more likely the selected design will serve the system needs during very warm ambient conditions.
Site Installation
The GC is installed outside within the ambient air, similar to traditional condensers and some literature refers to GC as a condenser. This is not technically incorrect since the GC will condense the R774 discharge gas during cooler ambient conditions.
Figure 3 – Trans-critical system diagram. Image credit: https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/Transcritical_CO2_Supermarket_Refrigeration_Systems.pdf
Condensed high-pressure liquid will drain from the GC outlet into the liquid receiver (flash tanks) in sub-critical operation with the HPV valve open. Installation of the GC should ideally allow condensed liquid to drain from the GC with the GC outlet positioned above the flash tank inlet. This would also allow any compressor oil that might end up in the GC to constantly drain from the GC during operation.
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This is not essential and many installations have the GC outlet below the flash tank inlet due to site limitations, but it does increase the system charge in such cases. If possible, plan to have the GC one level above the plantroom, where the flash tank is positioned, and route the drainpipe in such a way as to allow the condensed R744 to drain out the GC. Shopping malls, for example, have strict rules regarding pipe routing so start planning early and avoid p-traps for the condensed liquid to accumulate on the way to the liquid receiver.
In most cases the ambient air would also cool down the further the GC is from the ground level, potentially reducing energy usage. Having the GC installed as high as possible pays back long term. Your installation should also reduce the likelihood of warm discharge air from the GC being drawn back into the GC inlet. Should the air onto the GC be warmer that the ambient you might have warm air recirculating, so this is something to check during commissioning.
Adiabatic pre-cooling walls installed in the airstream prior to the gas cooler face area can reduce the ambient temperature airflow the gas cooler receives. This is an option in areas with extremely high ambient conditions making a trans-critical system viable where it would otherwise not be suitable.
Adiabatic pre-cooling walls are used to reduce the dry bulb air temperature onto the gas cooler face area. The energy savings are undeniable, but the regular maintenance required by adiabatic pre-cooling systems seems to be a deterrent and such applications remain limited. The moderate water consumption during high ambient conditions also being a limiting factor to some sites.
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