The refrigerant enters the condenser as superheated gas, i.e. at a temperature higher than the saturation temperature (point a in Figure 7.2). The heat rejection can be followed in a log P/h diagram. The first part of the condenser cools (desuperheats) the gas to the saturation temperature (a-b). This cooling represents 15-25% of the total heat of rejection. It is a onephase heat transfer where the temperature of the refrigerant gas decreases typically by 20-50K, depending on the system and refrigerant. When the refrigerant reaches its saturation temperature, the latent heat is rejected and a liquid film forms on the heat transfer surface. The condensing process represents the majority (70-80%) of the total heat of rejection (b-c). Finally, the fully condensed refrigerant (c) is sub-cooled a few degrees (c-d) to ensure that pure liquid enters the expansion valve (d). This is also a one-phase heat transfer operation, representing approximately 2-5% of the total heat of rejection.
Temperature profile inside the condenser
The temperature of the refrigerant decreases during the desuperheating and sub-cooling processes, but remains constant during the condensing process (see Figure 7.3). The energy rejected from the refrigerant heats the secondary medium, whose temperature thus increases.
The refrigerant pressure changes little from desuperheating to subcooling. In a similar way to evaporation, the only pressure difference between the entrance and the exit of the heat exchanger is the pressure drop. Because the flow velocity in a condenser decreases, the induced pressure drop is much lower than in an evaporator.
The temperature difference of the refrigerant between the condenser inlet and outlet is much larger than in an evaporator, due to the desuperheating. True counter-current flow in a plate heat exchanger makes it possible to utilize this temperature difference. The temperature on the secondary fluid side can be increased to approach or even exceed the condensing temperature. A temperature increase results in a smaller flow of the secondary fluid for the same heat load. This reduces the required pump capacity and the size of the liquid cooler. However, there is a "temperature pinch" that must be considered and avoided for stable operation, as discussed below.
In Figure 7.4, the inlet temperature of the secondary fluid is identical for the two cases, but the flow of curve (b) has been reduced to utilize the high gas temperature. The minimum temperature difference between the refrigerant and the secondary fluid in a counter-current condenser, the pinch, occurs at the beginning of the condensation process, as shown in Figure 7.4. The temperatures of the two media in a heat exchanger may converge but never equalize. The temperature of the leaving secondary fluid cannot therefore become more than a few degrees higher than the saturation temperature without "hitting the roof" in the tight section at the condensing temperature.
Reducing the flow of secondary fluid exaggeratedly in an attempt to approach the temperature lines results in a heat exchange approaching zero. This greatly reduces the efficiency of the heat exchanger, and may result in only partial condensation and unpredictable performance.
The effect of pressure drop
Pressure drop is created by the friction of the fluid, and is highly dependent on the fluid velocity. In a condenser, the refrigerant flow velocity is reduced as the refrigerant condenses, because the liquid phase has a much smaller specific volume than the gas phase. Hence, the major part of the pressure drop of the condenser is induced in the desuperheating operation, when the refrigerant is still a gas. Reducing the pressure results in a decrease of the saturation temperature, i.e. the level of superheating is increased. Normally, these effects are very limited. However, they are discussed further in chapter 7.6.
Counter-current vs. co-current flow
True counter-current flow is always preferred in a condenser for optimal utilization of the high desuperheating temperature. The mean temperature difference between the refrigerant and secondary fluid sides also becomes larger for counter-current flow, because there is no risk of the entering and leaving temperatures converging. However, the "temperature pinch" should still be avoided, as discussed above.
Operating in co-current flow achieves the minimum temperature difference between the leaving sub-cooled refrigerant and the leaving water, as shown in Figure 7.5. Not only do the approaching temperatures reduce the heat transfer. The already low heat transfer coefficient of the one-phase subcooling operation also results in very low heat transfer efficiency. A large extra surface is therefore required for a co-current plate heat exchanger compared with a heat exchanger operating with counter-current flow.
The sub-cooling must also be kept low for co-current condensers, because the converging temperatures greatly reduce the already low heat transfer coefficient.
Influence of inert gases
Incondensable, inert gases are normally not present in the system. However, they could be found in a system that has been unsatisfactorily evacuated before startup, from decomposed refrigerant or oil, etc. If inert gases are present in the system, they may accumulate in the condenser, resulting in phenomena that reduce the general performance. The incondensable gas may accumulate in a layer close to the heat transfer wall. This blocks direct contact of the refrigerant gas with the heat transfer surface. Instead, the refrigerant gas must diffuse through the inert gas layer. Furthermore, the partial pressure of the refrigerant gas decreases and the saturation temperature has to be lowered to compensate, resulting in a smaller temperature difference over the heat exchanger, as shown in Figure 7.6.
The condenser may act as an accumulator for inert gases because the refrigerant enters the condenser as a gas but leaves it as a liquid. Although all refrigerants except ammonia are heavier than the most common inert gases in refrigerant systems, i.e. air and carbon dioxide, the inert gases will be carried down towards the exit by the pressure drop in the channels. However, it is difficult for the gases to exit the condenser. If there is a condensate level inside the condenser, the incondensable gases accumulate in the gas/liquid interface and thus reduce the heat transfer efficiency. A venting purge should therefore be placed in the lower part of the condenser on the refrigerant side. To minimize the refrigerant loss, the purge may be cooled by cold low-pressure refrigerant gas to re-condense and collect the refrigerant.
If the pressure drop over the channels is too small, pockets of air or other incondensable gases may also form inside the condenser on the secondary fluid side. If the static pressure recovery due to the gas column inside the channels is larger than the dynamic pressure drop over the channels, there is a risk that a stable air pocket will form, as shown in Figure 7.7.
If the pressure drop is too small, all liquid will pass through the first channels, leaving the last channels without liquid. The air pocket can be emptied by means of a purge. An effective way of providing purge is by utilizing an extra connection outlet on the secondary fluid side, as can be seen in Figure 7.7. A simple mechanical float valve or equivalent ensures that only gas leaves the system.