Modern Engineering Thermodynamics

578 CHAPTER 14: Vapor and Gas Refrigeration Cycles
Steam line
Steam nozzle
To condenser
Boost ejector
Makeup water
Water return
Evaporator
Sprays
Water level Chilled water
control
Circulating pump
Cooled area
FIGURE 14.35
Steam jet refrigeration. Steam flowing through the steam nozzle and booster ejector produces low pressure in the evaporator. This
causes the water to evaporate and cool (or “chill”) the water in the evaporator.
14.16.4 Radiation Cooling
The Egyptians were known to use radiation heat loss to a black night sky (which typically has a black-body
temperature of about 150 R) from a shallow pan of water, causing it to freeze when the surrounding air temperature
is far above freezing.
14.16.5 Reduced Pressure Refrigeration
In 1755, William Cullen discovered that exposing water to a vacuum caused it to boil at a lower temperature.
This could then be used to cool a surrounding fluid. Various versions of this technique evolved over the years,
including a fascinating steam jet ejector cooling system developed by the French engineer Maurice Leblanc in
1909. Leblanc utilized the Bernoulli effect to evacuate a container of water by attaching it to the throat of a nozzle
passing high-velocity steam. The water in the container was thus cooled. This is the technology that was initially
used to air condition the Radio City Music Hall in New York City in the 1930s (see Figure 14.35).
14.16.6 Thermoelectric Refrigeration
Thermoelectricity is the direct or spontaneous conversion of heat into electrical energy. In 1834, Jean Charles
Peltier (1785–1845) observed that, when an electric current was passed through two different metallic conductors
connected in a loop, one of the two junctions between the conductors cooled while the other warmed.
When the direction of the current was reversed, the effect was reversed, with the first junction warming and the
second cooling. Thermoelectric cooling remained a laboratory curiosity until after 1950, when semiconductors
were developed. Semiconductor materials were formulated to produce much more efficient thermoelectric cooling
than pure metals. They were used effectively in the U.S. space program in the 1960s and have a growing
market today.
14.16.7 Vortex Tube
The technique for separating a flowing fluid into hot and cold outlet flows using a spiral vortex chamber was
developed by Georges Ranque (1898–1973) in 1931. Vortex tubes have a limited application in spot cooling in
manufacturing operations, electronic cabinet cooling, and body suit cooling. They have no moving parts, and
are very effective in manufacturing facilities, since they require only a source of compressed air for their operation
(see the Section 9.10 at the end of Chapter 9 for more information).
14.17 FUTURE REFRIGERATION NEEDS
The most pressing need in modern refrigeration, air conditioning, and heat pump technology is improvement in
energy efficiency (COP). It is estimated that, in the United States, over 20% of all household electricity used is
consumed by refrigerators and about 20% of all the electricity generated in the United States is used for lighting.
By the year 2030, it is estimated that the world’s energy needs will top 25 × 10 12 W (25 TW) per year. Technological
developments are needed to make household appliances and industrial operations more efficient, develop
heat pumps that operate in colder climates, devise new and more efficient air conditioning cycles and associated
technologies, and decrease the overall use of energy in illumination. It was mentioned earlier that refrigeration
technology has never had the power and glamour associated with the power-generating technologies, but it will
be very important in leading the way to energy conservation in the future.