6.9 Flooded evaporators

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. 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. Unlike in a direct expansion (DX) evaporator, the refrigerant is not fully evaporated and superheated at the flooded evaporator outlet. The leaving refrigerant flow is a two-phase mixture with typically 50-80% gas.

Flooded evaporators, which are sometimes called wet evaporators, are divided into forced-flow evaporators and thermosiphon evaporators. 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 thermosiphon compression cycle

In addition to the basic equipment in a direct expansion refrigeration circuit, i.e. evaporator, compressor, condenser and expansion valve, the flooded system needs a receiver (no. 1, Figure 6.44) to separate the twophase mixture after the expansion valve (no. 5). The refrigerant leaving the bottom of the receiver is 100% liquid.

The refrigerant from the receiver enters the evaporator (no. 2, Figure 6.44) and evaporates due to the heat transferred from the secondary side. The refrigerant at the evaporator inlet is slightly sub-cooled due to the pressure increase from the receiver to the evaporator. After the evaporator, the two-phase refrigerant mixture again enters the receiver, where liquid and gas are separated. The gas then enters the compressor, while the remaining liquid is re-circulated through the evaporator. The gas is compressed in the compressor (no. 3) and condensed in the condenser (no. 4) in the same way as in the basic compression cycle. The force driving the refrigerant through the evaporator depends on the density difference between gaseous and liquid refrigerant. When refrigerant is evaporated inside the BPHE, the lower density of the vapor allows more liquid refrigerant to flow inside the evaporator. Please note that the expansion valve needs no regulating action, because the flooded evaporator is selfregulating. The spontaneous vaporization in the receiver ensures that no liquid enters the compressor.

The forced-flow compression cycle

A 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. 6).

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 higher cost of a pump can still be more economical than elevating the roof of the installation room. Forced-flow systems often have a larger circulation number than thermosiphon systems due to the higher mass flow created by the pump.

Larger static head, i.e. a larger height difference between receiver and evaporator, increases the sub-cooling of the refrigerant. 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.

If the lubricating oil is insoluble in and heavier than the refrigerant, oil drainage can be installed before the pump. Oil droplets on the heat transfer surface may decrease the heat transfer dramatically.

Co-current versus counter-current flow

For thermosiphons, it is often recommended that evaporation be operated in co-current flow, despite the mean temperature difference (MTD) being less favorable. Co-current flow ensures a large temperature difference in the beginning of the evaporator, enabling the boiling process to start as soon as possible. Because the thermosiphon evaporation process is driven by differences in density, the evaporator performance depends more on stable circulation than a large MTD. If the temperature difference between the refrigerant and secondary fluids is higher than 10K, counter-current flow may be the favored flow configuration.

Characteristics of flooded systems

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.


When considering flooded systems, the circulation number is important. Because evaporation is incomplete, the two-phase mixture must flow through the receiver-evaporator circuit more than once to achieve 100% evaporation. The degree of evaporation at the evaporator outlet is stated as xout, which is measured in kg vapor per kg total inlet mixture. The circulation number is defined as the reciprocal of the degree of evaporation:


The circulation number indicates how many times a certain liquid volume has to pass through the evaporator to be completely evaporated. A smaller circulation number indicates less pipework, a smaller receiver and a lower refrigerant charge in the flooded-flow circuit. The circulation number of a BPHE is between approx. 1.1 (xout=0.91) and 1.4 (xout=0.71), and the circulation number of an S&T unit lies between 5 (xout=0.2) and 10 (xout=0.1).

Design

The driving force of the thermosiphon process is based on natural density differences. It is therefore of vital importance that the relationship between the liquid static head and the two-phase static head (see Figure 6.48) is correct, i.e. the driving force should be larger than the restraining forces. For the two-phase mixture to return to the separator, the static head pressure must be larger than the total pressure drop through the evaporator and reconnection pipe. The driving force can be calculated as:


The restraining forces in the two-phase connection are more complicated to estimate, because the outlet flow from the evaporator is in two phases. The evaporator pressure drop is given in SSP. Table 6.2 is a receiver height converter for different refrigerants. The heights are calculated using the same formula as for the driving force.

Table 6.2 Examples of how the height between the receiver and evaporator varies with the static head pressure.

Refrigerants with a large glide are not recommended for flooded evaporators. The refrigerant is not fully vaporized in a flooded evaporator, and the composition would consequently change for a refrigerant with glide. This would make it difficult to evaluate temperature changes throughout the system.

When are Flooded Systems Used? The advantages and disadvantages of flooded systems are shown in Table 6.3. Flooded systems are economic for large refrigerant systems, due to the lower requirements on power input to the compressor. For smaller systems, the pay-back time for the larger installation cost of a flooded-flow system is often considered to be too long despite the smaller power input.

Table 6.3 Advantages and disadvantages of flooded systems (compared with DX systems).

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