Experimental study of throttling of carbon dioxide refrigerant to atmospheric pressure

The pressure in carbon dioxide (CO2) refrigeration system is very high, and it is necessary to install a safety valve in the refrigeration system so as to release some amounts of CO2 refrigerant through the valve as well as its downstream line into the atmosphere to eliminate overpressure. But solid CO2 might be observed in the safety valve or its downstream line and disturb the expected function of the safety valve. As the releasing flow through the safety valve is very complex and difficult to be studied, a needle valve has been used to substitute the safety valve and to control the openings in the present paper. An experimental setup to simulate the refrigerant flow through the valve and its downstream line has been developed in order to reveal the influence of the upstream vapour quality and the opening of the throttle valve on the freezing and blockage of CO2 in the safety valve and its downstream line under the upstream pressure of 5.7-6.1 MPa. The solid CO2 will block the valve itself when the mass flow rate becomes less than 0.007 kg s−1; and it will block the downstream line when the mass flow rate is larger than 0.03 kg s−1. The blocking and de-blocking in the downstream line has shown periodic characteristics. The period increases with the upstream vapor quality, which varies from 0.7 to 16 s. And it is recommended to avoid oversize of the downstream line of the safety valve and to install a device to eliminate static electricity in the downstream line.