A heat pump uses a thermodynamic cycle to convert low temperature heat to a higher temperature. The thermodynamic cycle of an ideal basic heat pump is depicted in a T-s diagram in figure 1.3.
The basic heat pump cycle consists of 4 main components in a closed circuit:
a compressor, a condenser, an expansion valve and an evaporator. The fluid used in the cycle is called the working fluid or refrigerant. First, the compressor compresses the gaseous refrigerant to a higher pressure. At this higher pressure, the refrigerant condenses at the saturation temperature corresponding with the imposed pressure. In this process, heat is rejected from the condenser. After the
T
s
expansion
compression condensation
evaporation
liquid two phase gas
Figure 1.3: A basic thermodynamic heat pump cycle depicted in a T-s diagram.
(a) Manifold type (b) Tubular type
Figure 1.4: Different types of distributors.
condenser, the liquid refrigerant is expanded over an expansion valve. This lowers the pressure. Due to the adiabatic expansion, part of the liquid flashes to a gaseous state. The refrigerant is now in the two-phase region. Finally, the two-phase refrigerant evaporates at a lower saturation temperature in the evaporator by extracting heat from a source. The Coefficient of Performance (COP) is a measure for the efficiency of the cycle. The COP is the heat released by the condenser divided by the work delivered by the compressor.
The refrigerant flow entering the evaporator is in the two-phase region as seen in figure 1.3. Furthermore, a typical evaporator, which is heated by air, consists of multiple parallel tubes to increase the heat transfer area while keeping the pressure drop low. To distribute the two-phase refrigerant mixture evenly over the different parallel tubes, a distributor is used. As shown in figure 1.4, two different distributor types exist: tubular distributors and manifold distributors. This work focuses on the tubular distributor.
In reality the distribution of the two phases is often not homogeneous over
a distributor. This can have different causes: manufacturing tolerances, uneven heat load of the different tubes, fouling, frost formation, partial load operation...
Maldistribution leads to a non-uniform superheat at the outlet of the different tubes, which reduces the effectiveness of the heat exchanger. Several authors studied the effects of maldistribution in heat exchangers experimentally [8–12] and numerically [2, 13–18]. All authors agree that maldistribution leads to a reduction in capacity and the coefficient of performance (COP) of the heat pump. Depending on the boundary conditions, the reduction in capacity varied between 2% and 40%.
According to Bach et al. [10], no strong maldistribution occurs in a well-designed and well-maintained heat pump. Hence, in reality the reduction in capacity will normally never reach values of 40%.
Vist [14] observed less capacity reduction caused by maldistribution for CO2 than for R134a. Further he noticed a strong connection between the flow regime at the inlet and the extent of the maldistribution.
Mader et al. [2] did an economic study on the costs of phase maldistribution.
The authors found that maldistribution will increase the annual operating cost up to 5%.
Several counteracting technologies for mitigating the performance drop due to maldistribution exist [18]. One of the simplest solutions is to avoid the origin of maldistribution such as reducing fouling and manufacturing tolerances.
Mader et al. [2] noticed that oversizing of the evaporator can partly recover the maldistribution-induced losses. However, this increases the pressure losses and adds to the investment cost.
Figure 1.5: Configuration of the flash gas bypass method of Tuoet al. [12]
Tuo et al. [12] suggested a flash gas bypass system. After the expansion valve, the liquid phase is separated from the vapour using a flash tank or another separating device such as a vertical T-junction (see figure 1.5). The vapour bypasses the evaporator; only the liquid phase is evaporated in the evaporator.
Hence, maldistribution of the phases cannot occur. However, uneven superheat can still occur if one tube is for example fouled. Further, the size and price of the phase separator is large. Milosevic et al. observed a 55% increase of their experimental setup’s COP by applying the flash gas bypass when there is a large maldistribution.
Figure 1.6: A vapour compression cycle equiped with individual superheat control (Kimet al. [15])
Both Kim et al. [15] and Kærn et al. [17] proposed individual superheat control of the parallel evaporator passes, but with different techniques. Kærn et al. [17]
used an expensive expansion valve for each evaporator pass, while Kim et al. [15]
uses one big expensive expansion valve combined with smaller and cheaper valves for the different passes (see figure 1.6). With both systems, one can change the mass flow rate of each pass and thereby control the outlet superheat. Technically this is the best solution but also the most expensive one. Mader et al. [18] did an economic study on the individual superheat control strategy. The payback time in an average climate is about 10 years while it is 2 years more for the flash gas bypass system. Finally, evaporator and distributor designs are currently optimised experimentally for its nominal operating condition. However this optimal design can be suboptimal for part load operation. The optimisation of the system over the whole working range would make the system more robust.
To develop optimised distributor designs, a good understanding and model should be available. However, currently little is known about the distribution of two-phase refrigerant flows in distributor heads. To the author’s knowledge, only two experimental studies aimed to improve a tubular distributor head (Nakayama et al. [19] and Yoshioka et al. [20]). Both authors optimised the geometry of an existing distributor head using experimental techniques which leads to case specific models.