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...problem is unique for C[O.sub.2] refrigeration systems and has never been studied before. In order to fill in the blank, the blockage characteristics of the different types of safety valves were experimentally studied by constructing glass tubes with different geometries similar to the flow channel in safety valves and building them into an experimental rig. And the effect of other parameters on the formation of solid C[O.sub.2] was studied by theoretical analysis of the pressures along the release path based on much simplified model. These studies showed that severe blockage will occur in the downstream line and endanger the protected system after the valve house is partly blocked by solid C[O.sub.2]. To avoid the blockage of solid C[O.sub.2] in safety valves, a simple structure of the safety valve, high fluid velocity, and long downstream pipe are preferred. But this preference will cause high pressure in the downstream line and might further lead to blockage in the downstream line. So the downstream line should be able to tolerate high pressure, and measures such as heating or blowing should be simultaneously taken to prevent the blockage in the downstream line.
INTRODUCTION
Since Lorentzen (1993a, 1993b) has proposed the reuse of C[O.sub.2] as a refrigerant, C[O.sub.2] is receiving increasing interest for various refrigeration and heat pump applications. Kim et al. (2004) reviewed over 200 documents in the open literature from all over the world on C[O.sub.2] refrigeration systems. Among these refrigeration systems, the transcritical C[O.sub.2] automotive air conditioners/heat pumps and subcritical C[O.sub.2] cascade food processing/conservation systems are two of the most promising, where C[O.sub.2] is under much higher pressures (2.8-12 MPa or 406-1740 lbf x [in..sup.-2]) than conventional refrigerants.
Safety valves are used to protect the system from an abnormal overpressure probably caused by power failure or malfunction of the compressor and throttling part. If an abnormal overpressure occurs and the safety valve cannot achieve timely release of some refrigerant to reduce the system pressure, the safety operation of the entire system will be influenced to an unacceptable level. More importantly, the damage of the high pressure will be much more severe than that of a lower pressure. Thus, the safety valve is very important for a C[O.sub.2] refrigeration system.
But the normal function of the safety valve will be destroyed by the blockage of its release path in C[O.sub.2] systems. The reason is that the C[O.sub.2] triple-point pressure of 0.52 MPa (75 lbf x [in..sup.-2]) lies between the system pressure and the atmospheric pressure of 0.101325 MPa (15 lbf x [in..sup.2]). Solid C[O.sub.2] may be formed and block the safety valve when C[O.sub.2] expands through the safety valve from the system pressure to the atmospheric pressure.
This problem is unique for C[O.sub.2] safety valves and has never been studied before. Only Krings (1997) noted it in his experiments, but he has not studied it in detail. To study it is difficult because safety valves are normally not transparent and made of metal due to the high working pressure. So the conditions for the occurrence of this problem and its characteristics are unknown so far. This work will focus on them.
DESCRIPTION OF THE FLOW THROUGH C[O.sub.2] SAFETY VALVE
The safety valve normally does not release C[O.sub.2] directly into the atmosphere, partly because of the low temperature (195 K) of the formed, solid C[O.sub.2] and partly because such installations will cause other freezing problems, i.e., the steam in the ambient air will be condensed and solidified in the safety valve. So a downstream pipe is required for the path of release. Figure 1 is a schematic view of the release path, including the safety valve and the downstream line. C[O.sub.2] flows into the inlet pipe of the safety valve and raises the disc when an overpressure occurs. Then the fluid flows through the gap between the seat and disc, inducing larger pressure drop and producing a mixture of vapor-solid C[O.sub.2] or vapor-liquid C[O.sub.2]. This mixture expands into the valve house, inducing another larger pressure drop, and finally flows through the outlet pipe into the downstream line.
Solid C[O.sub.2] will be formed when the pressure is dropped down to the triple-point pressure. So whether the solid C[O.sub.2] is formed in the safety valve or in the downstream line depends on the pressure drop at the outlet of the safety valve and in the downstream line. The pressure drop is determined by the mass flow rate and the geometry of the outlet of the safety valve and the downstream line. The mass flow rate is decided by the upstream thermodynamic properties and the geometry of the disc and seat. In all, the upstream thermodynamic properties and the geometry of the release path decide the formation of solid C[O.sub.2] and will further decide the location of the blockage.
Safety valves under such a high working pressure are usually made of metal, and the flow in it is vapor-liquid-solid three-phase flow. Therefore, the blockage in it can neither be directly observed nor numerically calculated. Thus, the influence of...
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