Page 434 - Mechanical Engineers' Handbook (Volume 4)

3 Refrigeration Cycles and System Overview 423

a lower pressure in the expansion valve. The expansion valve can be a passive device, such
as a capillary tube or short tube orifice, or an active device, such as a thermal expansion
valve or electronic expansion valve. At the exit of the expansion valve, the refrigerant is at
a temperature below that of the product to be cooled. As the refrigerant travels through the
evaporator, it absorbs energy and is converted from a low-quality, two-phase fluid to a su-
perheated vapor under normal operating conditions. The vapor formed must be removed by
the compressor at a sufficient rate to maintain the low pressure in the evaporator and keep
the cycle operating.
Pumped recirculation of a liquid secondary refrigerant rather than direct evaporation of
a refrigerant is often used to service remotely located or specially designed heat exchangers.
This technique is called indirect refrigeration and provides the user with wide flexibility in
applying refrigeration to complex processes and greatly simplifies operation. Secondary re-
frigerants or brines are also commonly used for simple control and operation. Direct appli-
cation of ice and brine storage tanks may be used to level off batch cooling loads and reduce
equipment size. This approach provides stored refrigeration where temperature control is
vital as a safety consideration to prevent runaway reactions or pressure buildup.
All mechanical cooling results in the production of a greater amount of heat energy than
cooling energy. In many instances, this heat energy is rejected to the environment directly
to the air in the condenser or indirectly to water where it is rejected in a cooling tower.
Under some specialized applications, it may be possible to utilize this heat energy in another
process at the refrigeration facility. This may require special modifications to the condenser.
Recovery of this waste heat at temperatures up to 65 C can be used to achieve improved
operating economy.
Historically, in the United States, capacities of mechanical refrigeration systems have
been stated in tons of refrigeration, which is a unit of measure related to the ability of an
ice plant to freeze one short ton (907 kg) of ice in a 24-hr period. A ton is equal to 3.51
kW (12,000 Btu/hr).

3 REFRIGERATION CYCLES AND SYSTEM OVERVIEW
Refrigeration can be accomplished in either closed-cycle or open-cycle systems. In a closed
cycle, the refrigerant fluid is confined within the system and recirculates through the com-
ponents (compressor, heat exchangers, and expansion valve). The system shown at the bottom
of Fig. 1 is a closed cycle. In an open cycle, the fluid used as the refrigerant passes through
the system once on its way to be used as a product or feedstock outside the refrigeration
process. An example is the cooling of natural gas to separate and condense heavier com-
ponents.
In addition to the distinction between open- and closed-cycle systems, refrigeration
processes are also described as simple cycles, compound cycles, or cascade cycles. Simple
cycles employ one set of components (compressor, condenser, evaporator, and expansion
valve) in a single refrigeration cycle as shown in Fig. 1. Compound and cascade cycles use
multiple sets of components and two or more refrigeration cycles. The cycles interact to
accomplish cooling at several temperatures or to allow a greater span between the lowest
and highest temperatures in the system than can be achieved with the simple cycle.

3.1 Closed-Cycle Operation
For a simple cycle, the lowest evaporator temperature that is practical in a closed-cycle
system (Fig. 1) is set by the pressure-ratio capability of the compressor and by the properties
of the refrigerant. Most high-speed reciprocating compressors are limited to a pressure ratio