Page 331 - Applied Process Design For Chemical And Petrochemical Plants Volume III

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290 Applied Process Design for Chemical and Petrochemical Plants

In most process systems, the evaporation of the refriger- Steam Jet Refrigeration
ant is carried out in shell and tube heat exchange equip-
ment, and allowance must be made for a reasonable In steam jet refrigeration, water is the refrigerant being
temperature approach between the process fluid and the evaporated at low pressures created by the steam jets. These
evaporating refrigerant. The process fluid always leaves units have a barometric direct contact condenser, Figure
the evaporator at a higher temperature (by 3°—15°F) than 11-1, or a surface condenser as in Figure 11-2. The former
the refrigerant. system wastes or loses the steam in the water; whereas the lat-
ter allows the steam to be condensed and reused.
2. Suction of Absorbing Pressure of Refrigerant This type of system may be used to cool water on a once-
through basis where the water is wasted, or the water may be
When the process circulating coolant and required refrig- in a closed system, and the refrigeration unit used to remove
erant evaporator temperature level are established, the suc- the heat taken up by the circulation to another part of the
tion pressure to the compressor of a mechanical machine or process. The warm water enters the active compartment(s),
the absorbing pressure of an absorption system is set. Keep depending upon the refrigeration load, and flashes due to
the low pressure point of the system at atmospheric pressure the reduced pressure maintained by the steam jets. The
or above in order to avoid air in-leakage that would later water boils at this low pressure, and the vapor is drawn into
have to be purged. This is not possible for some systems; the booster jet where it is compressed for condensation in
however, the issue is important and must be recognized, as the direct contact or surface condenser. The cooled or
explosive mixtures can be formed with some refrigerants. chilled water is pumped from the compartment with a
Moisture also enters the systems with air in-leakage. pump capable of handling the boiling water at this pressure.
Figure 11-3 indicates the expected water temperature and
3. Discharge or Condensing Pressure corresponding pressure in the active or flash compartment.
In general, the number of boosters determines the opera-
In mechanical systems, the temperature of the available
tional flexibility of the unit with respect to the refrigeration
water (or coolant) to condense the refrigerant from the
load. A single booster unit operates continuously, regardless of
compressor determines the pressure level of this part of the
load. A two booster unit can operate at 50% load by shutting
system. Generally speaking, it is less expensive to operate at
off one unit; at lower load levels it uses a pressure controller on
as low a pressure level on the discharge as is consistent with
the steam actuated by the condenser pressure. Because jets are
the suction pressure and with the physical characteristics of
not usually very flexible with respect to steam consumption
the refrigerant. Sometimes the cost of the refrigerant and
and vacuum, load control may be in increments as compared
the cost of its replacement on loss dictate that the optimum
to continuous variation. If a 100-ton unit is expected to oper-
situation is not determined by the system and refrigerant’s
ate an appreciable portion of the time at 25% of load, it may
physical properties.
prove economical to install a four-booster unit and to operate
only one for this period. Auxiliary ejectors remove uncon-
4. Refrigerant Characteristics
densed water vapor and air from the main condenser.
Available refrigerants for various levels or conditions of Low-head jet units without a barometric leg use a pump to
operation may be toxic, flammable, irritating on exposure, withdraw the water from the barometric condenser. This
hydroscopic, and expensive. These characteristics cannot be pump must be carefully selected as it must operate under
ignored, as large systems contain large quantities of refrig- vacuum suction conditions.
erant, and a leak or other failure can release a potentially
serious condition into a building or process area. Advantages of Steam Jet Units
The thermodynamic properties of the refrigerant determine
the suitability for a given condition of operation, particularly Assuming water greater than 35°F is needed and that
5
when compared with the same requirements or other refriger- water and steam costs are reasonable:
ants. The quantity of refrigerant needed for a particular level of
evaporation is a function of its latent heat, except when using 1. No moving parts exist, except water pumps.
steam jet refrigeration, because the use of its chilled water 2. Refrigerant (water) is nonhazardous and has a low cost.
involves only sensible heat transfer to process fluids. 3. System pressures are low.
4. Unit can be installed outdoors, if desired, and is self
5. System Maintenance supporting.
5. Physical arrangement is flexible and can be made to fit
The maintenance requirements for operation of the dif- odd spaces.
ferent types of refrigeration systems vary somewhat and 6. Steam and cooling water requirements can be adjusted
should be evaluated along with the particular performance. to reasonable economical balance.