The evaporation process
When sub-cooled liquid refrigerant at high pressure (state (a) in Figure 6.1) expands through the expansion valve, the pressure and thus the saturation temperature decrease (b). The amount of flash gas formed after the expansion valve decreases with the level of sub-cooling and the evaporator inlet pressure. The mixture of liquid and gas from the expansion valve enters the evaporator and starts to boil, because heat is transferred from the warmer secondary fluid (b-c). The evaporating refrigerant absorbs energy from the secondary fluid, whose temperature is reduced. After full evaporation, when 100% of the refrigerant has become saturated vapor (c), the temperature of the vapor will start to increase, i.e. the vapor will become superheated. The refrigerant flow leaving the evaporator will be 100% superheated vapor (d).
The total energy absorbed by the refrigerant is often called the Total Heat of Absorption (THA). It consists of the latent energy of evaporation (b-c in Figure 6.1) plus the sensible energy of superheating (c-d in Figure 6.1). The refrigerant vapor is superheated mainly to ensure that dry gas enters the compressor. Many control systems, such as thermal expansion valves, also regulate by means of the outlet temperature, and so superheating is necessary to achieve stable control of the evaporation process.
The evaporation temperature of a pure refrigerant corresponds to a certain pressure level and remains constant unless the pressure is changed. In reality, however, the evaporation temperature is never constant through the evaporator. Inside an evaporator, the increased velocity of the liquid/gas refrigerant mixture will induce a pressure drop, which thus reduces the saturation temperature. Refrigerant mixtures consisting of refrigerants with different boiling temperatures will increase in temperature during the boiling process; the refrigerant is said to "glide".
A good evaporator is able to provide a good, stable boiling process with a small temperature difference between the refrigerant and the secondary fluid. A low temperature difference means that a higher evaporation temperature is possible, which corresponds to a higher pressure. Decreasing the pressure difference from the low-pressure side (evaporator) to the high-pressure side (condenser) will decrease the energy use in the compressor. The higher evaporation pressure will also increase the density of the refrigerant gas. For each stroke, the compressor will therefore transport more refrigerant through the system. Lower electricity consumption and higher refrigeration capacity will increase the total system efficiency (COP).
A direct expansion (DX) system is recognized by the expansion valve that lowers the pressure of the warm liquid condensate (see Figure 6.2). This creates a cold gas/liquid mixture that enters the evaporator. Normally, there is no collecting vessel after the evaporator, and thus the refrigerant must be superheated a few degrees before the compressor to avoid liquid refrigerant leaving the evaporator. DX systems require fewer components than flooded systems and are less expensive to build.
A flooded (wet) evaporator works instead with a low-pressure receiver situated after the expansion valve. The receiver separates vapor from the liquid. The receiver ensures that vapor is fed to the compressor and that 100% liquid is fed to the evaporator. Typically, not all refrigerant is boiled off after one pass through the evaporator, and the refrigerant has to be re-circulated. No superheating is therefore possible. The receiver will continuously separate the vapor from the expansion valve and the outlet of the evaporator and feed it to the compressor, while redirecting the remaining liquid to the evaporator as described in section 6.9. A flooded evaporator system is shown in Figure 6.3.