If the lubricating oil is insoluble in and heavier than the refrigerant, oil drainage can be installed before the pump. The preheating in the beginning of the evaporator is then increased, which may lead to the requirement of a larger evaporator, because much more heat transfer area is needed to preheat liquid instead of producing gas. a larger height difference between receiver and evaporator, increases the sub-cooling of the refrigerant. Forced-flow systems often have a larger circulation number than thermosiphon systems due to the higher mass flow created by the pump. The higher cost of a pump can still be more economical than elevating the roof of the installation room. If the installation site does not offer the minimum necessary height difference between the receiver and evaporator to allow density circulation, a forced-flow system may be preferable over a thermosiphon. The forced-flow compression cycleĪ forced-flow flooded system is identical to a thermosiphon system, except that a pump is installed before the evaporator to serve as a driving force for the refrigerant (see Figure 6.45, no. The spontaneous vaporization in the receiver ensures that no liquid enters the compressor. Please note that the expansion valve needs no regulating action, because the flooded evaporator is selfregulating. When refrigerant is evaporated inside the BPHE, the lower density of the vapor allows more liquid refrigerant to flow inside the evaporator. The force driving the refrigerant through the evaporator depends on the density difference between gaseous and liquid refrigerant. 4) in the same way as in the basic compression cycle. The gas is compressed in the compressor (no. The gas then enters the compressor, while the remaining liquid is re-circulated through the evaporator. After the evaporator, the two-phase refrigerant mixture again enters the receiver, where liquid and gas are separated.
The refrigerant at the evaporator inlet is slightly sub-cooled due to the pressure increase from the receiver to the evaporator. 2, Figure 6.44) and evaporates due to the heat transferred from the secondary side. The refrigerant from the receiver enters the evaporator (no. The refrigerant leaving the bottom of the receiver is 100% liquid. 1, Figure 6.44) to separate the twophase mixture after the expansion valve (no. evaporator, compressor, condenser and expansion valve, the flooded system needs a receiver (no. In addition to the basic equipment in a direct expansion refrigeration circuit, i.e. Forced-flow evaporators use a pump or an ejector as the driving force, while the density difference between liquid and gaseous refrigerant drives thermosiphon systems. The leaving refrigerant flow is a two-phase mixture with typically 50-80% gas.įlooded evaporators, which are sometimes called wet evaporators, are divided into forced-flow evaporators and thermosiphon evaporators. Unlike in a direct expansion (DX) evaporator, the refrigerant is not fully evaporated and superheated at the flooded evaporator outlet.
The receiver acts as a separator of gaseous and liquid refrigerant after the expansion valve and ensures a feed of 100% liquid refrigerant to the evaporator. An important difference between a flooded evaporator and a direct expansion (DX) evaporator is that the flooded evaporator operates in conjunction with a low-pressure receiver.