Comparison between flooded evaporator and D-X, the performance of flooded evaporator and D-X is such that in direct expansion evaporator (D-X), the refrigerant flows inside the pipes and the chilled water flows inside the shell, while in the flooded evaporator, the refrigerant evaporates in the shell and Water circulates in the pipes. This flow pattern in the flooded evaporator causes the energy efficiency ratio to be higher because in this case the temperature difference of the boiling refrigerant inside the shell and the cold water coming out of the chiller tubes is about 3°F. On the other hand, in a DX evaporator, the temperature difference between the evaporating refrigerant and the cold water exiting the shell is between 8°F and 10°F. Obviously, with the same compressor and the same outlet temperature for cold water, the evaporation temperature is higher in the flooded system compared to the DX system.
In the DX system, the superheated suction gas temperature is usually kept constant at 10°F to 15°F to prevent liquid backflow into the compressor. In the flooded system, the refrigerant inside the shell is boiled, evaporated and exits from the top of the evaporator with about 3°F to 5°F super heat. This decrease in the temperature of super hot suction gas increases the capacity efficiency of the compressor. In addition, in the design of DX systems, in order to ensure the return of oil to the compressor both in full load and partial load mode, the flow speed in the pipe is considered higher and this will increase the pressure drop in the evaporator. Unlike the DX system, in a flooded evaporator, the pressure drop on the shell side is very low, as a result, the excess capacity that must be considered to compensate for the pressure drop in the evaporator is much less in the flooded design, and in general, the compressor that works in a flooded system is able to provide capacity is greater than the DX system. In the Flooded evaporator, there is the assurance that all the tubes of the evaporator are completely immersed in the boiling refrigerant, as a result, the full-load efficiency will be excellent and even better in the partial-load mode, because in the mode partial-load also uses the entire heat transfer levels of the evaporator tubes.
But in the DX evaporator, due to the fact that the speed of the gas refrigerant in the pipes must be maintained at a suitable level for the proper return of the oil, some of the pipes are closed in part-load conditions. As a result, the entire heat transfer surface that is available in full-load mode is not used, and compared to the flooded evaporator in similar partial-load conditions, it has lower efficiency. In a DX evaporator, the flow of water inside the shell is established diagonally on the tubes and is guided by vertical baffles. This flow has a higher pressure drop compared to the flow of water inside the tube of a flooded evaporator. As a result, flooded chillers need smaller water pumps with less energy consumption. In general, the DX evaporator is used for low and medium capacities where the efficiency of the chiller device is not so important and the main issue is the low initial cost, while with the increase in energy cost and the need for optimal use of energy in order to prevent the phenomenon global warming, chillers with flooded evaporators will become more common not only in high capacities but also in low and medium capacities. In the DX evaporator, clogging occurs inside the shell and around the pipes and is difficult to fix, but in the Flooded evaporator, where water flows inside the pipes and the clogging occurs inside the pipes, they can be easily cleaned with a brush inside the pipes. Also, if The need for acid washing in the Flooded system uses less acid than DX. In Flooded systems, an external oil separator is used after the compressor to separate and collect the oil leaving the compressor from the refrigerant gas, then it is returned to the compressor by the pressure difference of the cycle. This causes more purity of the refrigerant in the condenser and evaporator and As a result, it leads to an increase in the efficiency of the converter. Another point that is very important is that even if the entire cycle refrigerant enters the Flooded evaporator, it cannot fill the volume of the shell, so there is no possibility of liquid entering the compressor, but in the DX system, if the operation of the expansion valve is disturbed or it fails, there is a possibility of refrigerant liquid entering. There is a compressor.
In flooded systems, if the expansion valve is damaged, bypass can be used temporarily, but in DX systems, it is not possible to use bypass for the expansion valve, because if bypass is used in the DX system, the possibility of liquid entering the compressor is almost certain. . The volume of the refrigerant liquid stored in the shell of the Flooded evaporator makes it happen quickly and immediately when the chiller starts, but there is no reserve volume in the DX evaporator, and it takes some time for the device to start producing cooling, this is in partial load mode. The fact that the device frequently goes into standby mode significantly reduces the energy consumption in the Flooded system. Also, the volume of the refrigerant liquid stored in the Flooded evaporator shell makes the chill water temperature fluctuate much less in these chillers during load fluctuations.