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Rapport 2414173 - October 2007 __________________________________________________________________ where Popt is expressed in bars and Texit is the R744 temperature in °C at exit of the gas cooler. Thus, a gas cooler exit temperature of 45°C requires a high pressure of about 125 bars. This equation is valid for temperatures between about 38 and 53°C. 2.4.Transcritical cycle vs. Traditional cycle Table 2 summarizes the main design differences between the subcritical (traditional) cycle and the R744 transcritical cycle. As mentioned in the previous sections, the main differences lie in the pressure levels (both during operation and at standstill) and the methods implemented to ensure the control of high pressure and refrigerant flow rate. Table 2 : Main differences between transcritical R744 cycle and subcritical cycle Cycle parameters Subcritical Transcritical CO2 High pressure cooling – heat rejection device Condenser – Vapor condenses at constant temperature Gas cooler – CO2 undergoes large temperature change Discharge pressure HFCs : from 10 to 40 bars from 90 to 130 bars Suction pressure HFCs : from 2 to 9 bars from 25 to 50 bars Refrigerant discharge temperature Usually less than 95°C Up to 140°C Expansion device controls By superheat set point or fixed flow expansion device Usually used to control high pressure of CO2 High pressure controls (excluding safety shut-down controls) Not controlled - Pressure is set by condensation temperature – usually 40 bars max Required - Up to 130 bars Refrigerant state @ standstill Partly liquid and partly vapor Gas (supercritical) above 31°C ambient; vapor –liquid mixture below 31°C. Can become solid upon cooling below P<6 bars ! System pressure @ standstill (T ambient >31°C) Refrigerant vapor pressure at ambient air temperature At least 74 bars – can be higher, depending on charge and temperature. 3. R744 HEAT PUMPS TECHNOLOGY In Section 2, the basics of the transcritical cycle were examined. In this section, some R744 applications (both for cooling and/or heating) will be surveyed, with a focus on the __________________________________________________________________________________________ SHERPHA PROJECT - Transcritical R744 Heat Pumps – Technician's Manual 17/63PDF Image | Transcritical R744 (CO2) Heat Pumps 2017
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