Environmental Engineering Reference
In-Depth Information
efficiency is not the only factor than needs consideration. If safety is a concern and com-
pressed air can provide a safer alternative than other systems, then safety needs to take priority
over efficiency.
Other actions that lead to energy savings are sizing the compressors to need, using efficient
motors, conducting preventive maintenance, recovering heat, turning air off when not needed,
eliminating inappropriate compressed air use, and fixing leaks (DOE, 2003).
Air leaks are common in processing plants. They can be heard but cannot seen, and in
contrast with water or oil leaks, the result of air leaks disappears without leaving a trace. For
some reason, air leaks are not associated with waste of energy and money, but they are.
Air leaks are normally small, but because they take place continuously, the cumulative
effect at the end is significant in terms of money and wasted energy. A 1-mm hole in a line or
equipment pressurized at 100 psi leaks around 108 L/min and a 2-mm hole at the same pressure
leaks 1,000 L/min measured at standard pressure and temperature (calculated according to
table presented in Compressed Air Tip Sheet #3, August 2004, U.S. Department of Energy).
More tips on improving the efficiency of compressed air systems can be found in the DOE
publication “Improving Compressed Air System Efficiency: A Sourcebook for Industry”.
Refrigeration systems
Refrigeration and freezing are two the most energy-intensive operations in food processing.
Typical refrigeration systems work according to the vapor compression cycle, which contains
four different components: a compressor, a condenser, an expansion valve, and an evaporator.
In addition, the vapor compression cycle needs a working fluid, or refrigerant, that absorbs
the heat from the space needs to be cooled, and rejects the heat somewhere else. The most
common refrigerant in the food industry is ammonia (R-717) because of its high latent heat
of evaporation, relative low cost, and the fact that it is not an ozone-depleting substance.
Hydrofluorocarbons (HFCs) are popular refrigerants in self-contained units (e.g., water
chillers), in applications where ammonia can present a hazard, in air-conditioning, and in
mobile refrigeration units, such as in trucks and trains.
In a vapor compression cycle, the refrigerant enters the compressor as a vapor and is
compressed to a higher pressure that rises its temperature as well. The hot high-pressure vapor
then flows through the condenser where it is cooled and condensed into a liquid. The condenser
is where heat is rejected directly to the atmosphere or indirectly when another cooling fluid is
used. The refrigerant, now in liquid state, circulates through an expansion valve, where the
pressure is dropped and the refrigerant delivered to the evaporator. In the evaporator, the
refrigerant boils as a result of a reduced pressure and the absorption of heat from the compart-
ment, or the substance, to be cooled. The low-pressure vapor is then fed back to the compres-
sor where the cycle starts over.
The global efficiency of refrigeration equipment can be increased by taking actions at the
four stages of a vapor compression cycle as well as by avoiding heat gains in cold rooms and
pipes. Table 11.6 contains a list of suggestions that can improve the energy efficiency of indus-
trial refrigeration systems. Some of the ideas can be retrofitted to existing systems and others
are more appropriate for new ones.
Selection of efficient compressors is probably more suitable for new installations, but even-
tually existing systems can be retrofitted. There are three different compressor technologies in
gas compression refrigeration with different efficiencies:
Reciprocating
uses around 0.27 to 0.29 kWe/kWr (kilowatt of electricity/kilowatt of
refrigeration generated), which corresponds to a coefficient of performance (COP)
approximately of 3.5.
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