The medium to which the heat is exchanged. Giving
heat up from air to refrigerant is an example. The best
heat exchange occurs between two liquids, such as
water to liquid refrigerant. This is because liquids
are more dense than vapors and usually have a higher
specific heat. However, this is not always practical
because heat frequently has to be exchanged
between air and vapor refrigerant. The vapor-tovapor
exchange is slower than the liquid-to-liquid
exchange, Figure 21–10.