# A Review of Small-Scale Vapor Compression Refrigeration Technologies
## 3 Small-Scale Vapor Compression Refrigeration (VCR) Systems
### Components
#### 3.4.4 Evaporators  
The evaporator is the cold-heat exchanger where the refrigerant removes the heating load. At the expansion device outlet, the refrigerant is in a liquid-vapor mixture, which evaporates at low temperature and pressure as it flows through the evaporator, promoting heat transfer between the area to be cooled and the refrigerant fluid. In small-scale refrigerators, this is undoubtedly the device for which more designs have been proposed and evaluated, as described next. [PERSON] et al. tested three evaporator types (spiral-tube, tube-strip, and tube-in-tube) to select the better option in terms of heat transfer performance for the miniature system they  
Figure 22: Finned-tube condenser used by [PERSON] [54]. were investigating [61]. Cooling power and weight were 300 W and 0.16 kg, 260 W and 0.12 kg, and 200 W and 0.22 kg, respectively. A schematic of the spiral-tube evaporator shown in Figure 23, it was assembled into the miniature refrigeration system because it was made of copper and therefore was easier to weld. It is worth mentioning that this device fits in the palm of a hand. The use of microchannel heat exchangers as evaporators has also been quite common, again due to their high rates of heat transfer. In some works, the effect of design characteristics has been studied. [PERSON] and [PERSON] (2018) analytically compared six microchannel designs used as evaporators in a small-scale refrigeration system for electronics cooling with different overall size and hydraulic diameter. The authors found that the cooling power decreases with the increase in hydraulic diameter, due to the decrease in overall heat transfer. In a more recent study [46], the authors indicated that microchannel heat sink application to miniature refrigeration systems for electronics cooling has received little attention in the literature, along with determination of the heat source surface temperature. They tested a miniature vapor compression system in which the evaporator was a microchannel heat sink with 106 channels with rectangular cross-section shown in Figure 24a. Channel dimensions were 150 \(\upmu\)m wide, 450 \(\upmu\)m high, and 20 mm long, with 150 \(\upmu\)m fin thickness. The microchannel heat sink was made of nickel-plated copper to prevent corrosion by the refrigerant. Figure 23: Illustrative representation of the miniature spiral-tube type evaporator. Figure 24: Miniature microchannel evaporators used by (**a**) [PERSON] et al. [PERSON] et al. (2018) and (**b**) [PERSON] et al. Continuing with the use of microchannels as evaporators, [PERSON] et al. [50] presented an even smaller microchannel cold plate evaporator made of copper (similar to the scheme shown Figure 24b). It is interesting to note that they used two evaporators connected in series, an idea that was later adopted for evaluation of a series and parallel arrangement of two evaporators [98]. The results indicate that better COP was obtained for the series system. A singular microchannel plate-fin evaporator was developed by [PERSON] et al. [63] to minimize the size and weight of the cooling system. The evaporator was made of dozens of layers of stainless-steel plates and the microchannels on the plates are formed by photochemical etching process. The etched stainless-steel sheets are then laminated together and welded by vacuum diffusion bonding process. Figure 25 represents a similar scheme to the prototype of this device. Another type of evaporator widely used at the small-scale level are the cold plates, as shown in Figure 26, similar to the one presented by [PERSON] and [PERSON] [14]. This is a small cold plate mounted on top of a heat spreader that simulates the power generation of a chip through two 100 W electrical heaters tightly fitted into the heat spreader. Thus, the heat load of the cold plate can be varied by adjusting the cartridge heaters. The dimensions of the cold plate were 75 mm length, 9 mm height, and 75 mm width.