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Crankcase heaters Most compressors in the field are fitted with crankcase heaters, but some are not. The decision to use a heater or not is left up to the system designer or manufacturer. However with R744, the use of a heater may be mandatory due to the high solubility of the R744 in POE oil.
Superheat requirements When the switch from CFCs and HCFCs to HFCs was made it was necessary to switch to POE oils, in order to maintain a miscible mixture that would travel freely through the system. This is most important in the low-side of the system where the liquid is boiled off to a vapour, making it more challenging to get the oil back to the compressor.
Safety Limits
Since R744 has a higher affinity to haemoglobin in red blood cells than oxygen, R744 will displace oxygen and then be transported throughout the body. Depending on the concentration of R744 in the air, harmful side effects can occur. These can range from drowsiness at a one per cent concentration, to the possibility of death when exposed to concentrations above five per cent.
As it turned out, the miscibility of HFC refrigerant in POE oil is greater than what we experienced Since R744 has a density that is higher than air, we have to install in the past with some of the other refrigerant-oil monitoring devices at low heights (close to the floor) in mechanical rooms. combinations. In the case of R744, its solubility in POE oil is even greater. This makes it more challenging to maintain sufficient viscosity levels within the compressor. This is the main reason why a higher base oil viscosity is required when using POE with CO2. From a compressor perspective, it is essential that the manufacturer’s requirements for minimum superheats are met. For example, a minimum suction superheat of 35 to 40°F is not abnormal for an R744 subcritical compressor. In additional to return gas superheat, minimum discharge superheat values may be required. Refer to the manufacturer’s compressor operating instructions for exact values.
Temperature differences in heat exchangers Due, in part, to the high thermal conductivity of R744, the temperature difference required within the heat exchanger is generally lower. This applies to both the high and low sides of a system. Oftentimes this point is overlooked when designers and engineers are making comparisons between HFCs and R744. From an installer’s and servicing technician’s perspective, this should be remembered because it is quite possible that lower temperature differences will be experienced as system engineers work towards the highest practical efficiency.
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Water solubility
Remember the days converting from CFC refrigerants to R22? One of the discoveries made was that R22 in its vapour state had less capacity for moisture than it did in its liquid state. This means that when the moisture level was above a certain level it would separate from the refrigerant. During defrost, ice would melt and run into the cracks and crevices found in brazed joints, etc. Then, once the system returned to its refrigeration mode, the moisture would freeze and of course, expand. With R744, we have a similar dynamic. The water carrying capacity of the vapour is quite low. As a result, ice may form and create a blockage to refrigerant flow. This may be damaging to the system.