Wich type of expansion valve use in flooded evaporators
Answers
An advantage of flooded evaporators is that the potential problem of poor refrigerant distribution in the evaporator is reduced. The refrigerant is 100% liquid, and a liquid stream is much better distributed between the channels compared with the two-phase mixture of DX systems. Thus, when selecting a flooded evaporator, a SWEP B-model should be used for flooded evaporators, in contrast to DX systems where BPHEs with refrigerant distribution device are the better choice.
The receiver separates the refrigerant vapor before feeding it to the compressor (see Figure 6.46), so there is no need for superheating the refrigerant in a flooded evaporator. A larger portion of the total heat surface area will thus be used for evaporation compared with a DX evaporator, where 10-30% of the total heat surface area may be dedicated to superheating.
Figure 6.46 compares a flooded system and a DX system. The greenline detour (b-c) to the liquid saturation line (bubble line) shows the phase separation in the flooded system receiver. Because of the pressure gain (c-d) in the pipe that connects the receiver and the flooded evaporator, the evaporator inlet liquid is sub-cooled. Please note that the (c-d) line in Figure 6.46 is exaggerated. The pressure gain in the receiver-evaporator pipe is actually approximately 5-50 kPa (0.05-0.5 bar), while the pressure lift over the compressor is roughly 12-17 bar. Because there is no need for superheating in a flooded evaporator, the evaporation temperature can be a few degrees higher than in a DX evaporator (see Figure 6.47).
Energy transfer is much more efficient through a boiling turbulent liquid film than through dry superheated vapor. As a consequence, the temperature program is "closer" for a non-superheated evaporator than for a superheated evaporator (see Figure 6.47). A "closer" temperature program means a smaller difference between the leaving secondary fluid temperature and the evaporation temperature (LWT-Tevap).
Due to the higher evaporation temperature in a flooded evaporator, the pressure lift between the evaporator and condenser sides is smaller. The advantage is that less compressor work (W) is needed. Perhaps the largest advantage of flooded evaporators is that they use all the latent energy of the refrigerant in the phase transition between liquid and gas to cool a fluid. This is shown in Figure 6.46, where line (d-e) stretches over the whole transition length, while the red line (g-h) does not utilize the entire phase transition length. In other words, the COP (coefficient of performance) is higher for a flooded system.
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