Handbook of air conditioning and refrigeration / Shan K

10.20 CHAPTER TEN
During the design of a liquid overfeed cooler, the following choices should be carefully
considered:
Mechanical Pump or Gas Pump. Gas pumps generally require more energy than mechanical
pumps. According to Cole’s (1986) analysis, based on a typical plant of 275 tons (967 kW) of cooling
capacity and an evaporating temperature of 20°F (�6.7°C), the energy requirement of a gas
pumping system is about 7 percent of the total plant power, and that of mechanical pump is about
2 percent. However, the difference in energy consumption is a small percentage of the total plant
power, so in many circumstances, gas pumps are still used.
Among mechanical pump systems, centrifugal pumps are most popular. Rotary and gear pumps
may be suitable in circumstances where cavitation is critical.
Down-Feed or Up-Feed. In down-feed circuits, refrigerant is fed to the highest tube of the evaporating
coil. This arrangement improves oil return and eliminates static head. Up-feed circuits
simplify liquid distribution. The relative locations of the evaporators and receivers become less
important, and the system design and layout may be simplified.
Counterflow or Parallel Flow. Because of its lack of superheat, as in DX coils, a parallel flow
arrangement between air and refrigerant in a liquid overfeed air cooler has a small performance advantage
over the counterflow arrangement. Also, frosting occurs evenly in parallel flow, whereas
frosting is heavier in the first and second rows of the finned-tube coil in a counterflow arrangement.
Circulating Rate. Circulating rate or number n is defined as
n � (10.14)
where m˙ cir and m˙ ev � mass flow rate of refrigerant, circulated and evaporated, respectively, lb/min
[kg/(60 s)]. According to ASHRAE Handbook 1998, Refrigeration, the minimum recommended
circulating rate is as follows:
m˙ cir
m˙ ev
Liquid overfeed air coolers are widely used in low-temperature refrigeration plants for food storage
and processing. Recently, overfeed liquid coolers have also been used in ice storage systems.
10.5 CONDENSATION AND CONDENSERS
Condensation Process
Ammonia (R-717)
Down-feed (large-diameter tubes) 6 to 7
Up-feed (small-diameter tubes) 2 to 4
HCFC-22-Up-feed 3
HFC-134a 2
When saturated vapor comes into contact with a surface having a temperature below the saturation
temperature, condensation occurs on the surface. There are two types of condensation:
1. The condensed liquid, often called the condensate, wets the surface and forms a film covering
the entire surface. This type of condensation is called filmwise condensation.
2. The surface is not totally wetted by the saturated vapor, and the condensate forms liquid droplets
that fall from the surface. This type of condensation is called dropwise condensation.