Page 472 - Mechanical Engineers' Handbook (Volume 4)

10 Refrigeration System Specifications 461

Closed-cycle operation involves both liquid and gas phases. System designs must take
into account liquid-flow problems in addition to gas-flow requirements and must provide for
effective separation of the liquid and gas phases in different parts of the system. These factors
require careful design of all components and influence the arrangement or elevation of certain
components in the cycle.
Liquid pressures must be high enough to feed liquid to the evaporators at all times,
especially when evaporators are elevated or remotely located. In some cases, a pump must
be used to suit the process requirements. The possibility of operation with reduced pressures
caused by colder condensing temperatures than the specified design conditions must also be
considered. Depending on the types of liquid valves and relative elevation of various parts
of the system, it may be necessary to maintain condensing pressures above some minimum
level, even if doing so increases the compression power.
Provision must be made to handle any refrigerant liquid that can drain to low spots in
the system upon loss of operating pressure during shutdown. It must not be allowed to return
as liquid to the compressor upon startup.
The operating charge in various system components fluctuates depending on the load.
For example, the operating charge in an air-cooled condenser is quite high at full load, but
is low, that is, essentially dry, at light load. A storage volume such as a liquid receiver must
be provided at some point in the system to accommodate this variation. If the liquid controls
permit the evaporator to act as the variable storage, the level may become too high, resulting
in liquid carry over to the compressor.
Abnormally high process temperatures may occur either during startup or process upsets.
Provision must be made for this possibility, for it can cause damaging thermal stresses on
refrigeration components and excessive boiling rates in evaporators, forcing liquid to carry
over and damage the compressor.
Factory-designed-and-built packages, which provide cooling as a service or utility, can
require several thousand kilowatts of power to operate, but in most cases, they require no
more installation than connection of power, utilities, and process lines. As a result, there is
a single source of responsibility for all aspects of the refrigeration cycle involving the transfer
and handling of both saturated liquids and saturated vapors throughout the cycle, oil return,
and other design requirements. These packages are custom engineered, including selection
of components, piping, controls, base designs, torsional and critical speed analysis, and
individual chemical process requirements. Large packages are designed in sections for ship-
ment, but are readily interconnected in the field.
As a general rule, field-erected refrigeration systems should be close-coupled to mini-
mize problems of oil return and refrigerant condensation in suction lines. Where process
loads are remotely located, pumped recirculation or brine systems are recommended. Piping
and controls should be reviewed with suppliers to assure satisfactory operation under all
conditions.

10 REFRIGERATION SYSTEM SPECIFICATIONS
To minimize costly and time-consuming alterations owing to unexpected requirements, the
refrigeration specialist who is to do the final design must have as much information as
possible before the design is started. Usually, it is best to provide more information than
thought necessary , and it is always wise to note where information may be sketchy, missing,
or uncertain. Carefully spelling out the allowable margins in the most critical process vari-
ables and pointing out portions of the refrigeration cycle that are of least concern is always
helpful to the designer. A checklist of minimum information (Table 7) needed by a refrig-
eration specialist to design a cooling system for a particular application may be helpful.