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Rapport 2414173 - October 2007 __________________________________________________________________ Transcritical R744 heat pumps are fundamentally different because: • there is no distinction between liquid and vapor phase before expansion; • there is no value of "subcooling" after the gas cooling process – because subcooling does not exist as such; • the relationship between high pressure and refrigerant charge in the system is more complex than for conventional refrigerants. Transcritical R744 domestic systems are normally factory-charged. The refrigerant charge has numerous effects, particularly on the value of the compressor discharge pressure. Inappropriate refrigerant charge could thus lead to improper heat pump operation and/or equipment damage. This explains why most residential heat pumps are only serviced in the manufacturer workshop or at an authorized service station. Thus, any attempt to recharge R744 heat pumps should not be made, unless it is authorized by the manufacturer. Recharging a R744 system can only by carried out if full and explicit instructions are provided by the heat pump manufacturer. Annex 1, which shows excerpts from an Ecocute Service Manual issues important warnings against attempts to recharge the unit. Systems using R744 must be thoroughly evacuated like conventional refrigeration systems. Before any servicing, make sure that all the following is available to carry out refrigerant recharging : • Instructions from the manufacturer, including required information, e.g. the total amount of refrigerant for recharge. • Components required, e.g. filter/driers for R744 and suitable for the required design operating pressures should be used. • Appropriate recharging station if specified by manufacturer. Recharging R744 systems like conventional systems is not readily possible. With HFC refrigerants and other systems it is common practice to recharge liquid via the liquid line upon completion of the evacuation i.e. when the plant is still under vacuum. This is not possible with an R744 system because dry ice would form internally at the charging point when the system pressure is less than around 6 bar, because of expansion (see Section 6.1.1). __________________________________________________________________________________________ SHERPHA PROJECT - Transcritical R744 Heat Pumps – Technician's Manual 45/63PDF Image | Transcritical R744 (CO2) Heat Pumps 2017
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