Correcci´ on atmosf´ erica

For an R744 two evaporator air conditioning system based on Configuration 3 (see Figure 4-1) the accumulator is usually installed downstream the evaporator and upstream the internal heat exchanger. In this case the accumulator serves as a storage vessel for the excessive refrigerant since the mass stored in the other

components in the system strongly depends on the system operating condition. Ideally, the accumulator should separate the refrigerant phases and the oil to allow only refrigerant vapor and oil to exit the accumulator. Then, in steady state operation and assuming no pressure drop, the inlet quality equals the exit quality of the accumulator which in an idealized system (no refrigerant in oil) is one. Therefore, for a single evaporator system the exit quality of the evaporator is one which has the advantage that the all phase change energy can be used for the heat transfer. It should be noted however, that in a real application, refrigerant may be dissolved in the oil which effectively lowers the outlet quality and reduces the amount of “useable” phase change energy.

One common accumulator design is based on a J-tube design. In this case a usually cylindrical shaped vessel has an inlet where refrigerant and oil from the evaporator enters the vessel. A J-shaped-tube is mounted inside which has a hole on the bottom. This J-tube is the only exit of the accumulator. The idea for this design is that the J- tube opening is above the liquid level of the refrigerant and therefore ideally only vapor is sucked into this pipe. The hole at the bottom should ideally suck in only oil from the bottom of the accumulator. In the real application, however, the bottom hole is surrounded by mixture of liquid refrigerant and oil. Furthermore, the liquid column above the hole determines the pressure head and therefore how much liquid refrigerant and oil is passed through. The drawback is that at different operating conditions the refrigerant liquid column height is changing which means that the refrigerant exit quality of the evaporator might not necessarily be one for certain conditions.

To avoid this problem and its effects on the system performance, a prototype accumulator that did not have this configuration was used for the conducted experiments. Figure 4-10 shows the design of this accumulator.

Figure 4-10: Prototype accumulator used for experiments From Evaporators From Evaporators Sight Glass Metering Valve From Evaporators From Evaporators Metering Valve To IHX

evaporators whereas the other port serves as vapor phase outlet. An external metering valve at the bottom outlet allows controlling the outflow of liquid R744 and oil. The sight glass allows monitoring of the liquid refrigerant level. A camcorder was used to monitor the liquid level during operation.

Figure 4-11 shows an interesting phenomenon which was observed while monitoring the liquid level of the accumulator. When the system was not running a distinct phase separation boundary between liquid and vapor was visible at sub critical equilibrium. However, when the system was run and reached steady state, small bubbles were observed in the liquid R744 phase making it appear opaque. This means that an ideal phase separation was not taking place. For this reason the liquid level inside of the accumulator is reported as the apparent liquid level.

Figure 4-11: Visualization of refrigerant liquid level in accumulator; the left picture is taken at sub critical equilibrium, the right picture is taken at steady state

This prototype accumulator was installed based on Configuration 3 in an R744 two evaporator system. Due to the imperfect phase separation and to assure that enough oil was returned to the compressor, the exit quality at steady state was held at 0.95 to 0.99 for all conditions at which there was apparent liquid inside the accumulator. For a single evaporator system at steady state the inlet and exit quality of the accumulator must be equal. This still holds for a two evaporator system, but for a two evaporator system in this configuration, the inlet quality of the

accumulator is the combination of the outlet qualities of the evaporators. The constraint is that the mixing quality of the streams from the evaporators must be equal to the accumulator inlet and exit qualities. Therefore, depending on the conditions and distribution of the refrigerant mass flow rates over the evaporators it is possible to have, e.g., one evaporator with an exit quality of 0.8 and the other evaporator with superheated exit conditions and still have the accumulator inlet/exit quality of 0.97.

In conclusion for an R744 two evaporator system based on Configuration 3 even an ideal accumulator, one which has an inlet/exit quality of one at steady state, does not guarantee that the refrigerant exit qualities at both evaporators are also one. It only guarantees that the quality of the mixed stream from both evaporators has a quality of one. Therefore, to have equal refrigerant exit qualities at each evaporator it is still necessary to adjust the refrigerant mass flow rate distribution over the evaporators.