In transport refrigeration systems, such as refrigerated trucks, truck-trailers or refrigerated containers, during periods when the compressor is operating to compress the refrigerant in the system, substantial pressures can build up on the discharge (i.e. high pressure side) of the system. The vapor compression circuit is therefore designed to safely contain these pressures. It is recognized, however, that situations may arise where the pressures will tend to exceed what is considered a safe level. Accordingly, it is necessary to provide design features which will relieve these pressures before they become excessive.
In accordance with one established protocol, three levels of safety are provided on the high pressure side of the refrigeration system. The three levels are applied sequentially and in a prioritized fashion as follows. The first level is implemented in software and is based on pressure transducer readings. That is, when a predetermined pressure level is sensed, action is taken to limit the refrigerant flow, shut off the compressor or the system, or temporarily shut off the system and restart it after the pressure drops within a tolerance range.
A second level is implemented by way of a mechanical pressure switch which responds to sensed pressures to shut the system off or temporarily shutting the system off and restart it after a period of time.
A third level is implemented by way of a mechanical relief device which responsively opens to at least partially allow the refrigerant to be released to the atmosphere in the event that prescribed pressure levels are exceeded.
Recently concerns have arisen about the environmental effects of the release of commonly used refrigerants to the atmosphere by way of leakage and the like. One approach to addressing this problem is the use of a more benign refrigerant, CO2, in place of the traditional refrigerants such as Freon. With such a refrigerant, however, it is necessary to operate at substantially higher pressures, and therefore compressors have been designed specifically for the compression of CO2. With these higher pressures in the circuit, it is even more important to continuously sense these pressures and when they become excessive, provide relief in a safe manner. For that purpose, the three level protocol as described above has been found to be satisfactory to control the operating pressures on the high pressure side during operation of the system.
With the use of CO2 as the refrigerant, the applicants have recognized that, in addition to the occurrence of excessive pressures during periods of operation, system pressures may also become excessive during periods of shipping and storing. That is, when a charged system at rest is exposed to excessive ambient temperature conditions, such as may occur in a warehouse in the summertime or when a system is exposed to direct sunlight at midday, the pressures are likely to rise to undesirable levels. Under these conditions, the three level safety protocol will be useful in relieving pressure on the high pressure side, but, unlike systems with conventional refrigerants in these situations, a CO2 system will be susceptible to excessive pressure conditions on the low pressure side as well.
What is needed is therefore a method and apparatus for relieving pressures on the low pressure side of a CO2 system which is exposed to high ambient temperature conditions during shut down.