|
An ice storage system manufactures
its own ice and stores it until chilled water is needed by
the air conditioning equipment. An ordinary reciprocating
compressor is used with whatever condensing means is appropriate
for the installation. One type of evaporator, Figure 7-P (see below),
consists of hollow plates submerged in a water tank. The expansion
valve meters refrigerant into one end of the plates and the
suction line of the compressor is connected to the other end.
The heat of vaporization for the refrigerant is taken from
the water surrounding the plates, causing the water to freeze
on the outer surface of the plates. Controls are arranged
to stop the compressor when the ice has reached the desired
thickness. A float valve for controlling water level and a
three-way mixing valve for temperature control can be installed
in the same manner as for the ice melting system. Tanks of
evaporator plates can be connected in parallel, Figure 7-Q,
to obtain any desired ice storage capacity.
The ice storage system uses a small refrigeration
plant to meet a high peak demand of short duration by providing
storage of refrigeration. This points to the advantage of
the ice storage system which is a lower initial cost than
a larger system that must be designed for a high peak load
without storage.
In some localities a rather high demand charge
is placed on electric service. In such instances the smaller
equipment means considerable savings in power cost because
of a much lower demand charge. Compared with conventional
refrigeration systems the ice storage system has the disadvantage
lead time before occupancy. The lead time is the number of
hours of compressor operation required to freeze ice to a
thickness necessary to meet the peak load.
As an example of the capacity of an ice storage
system, the evaporator plates in the picture below are approximately
3 ft. x 9ft. and, with ice one-inch thick, would yield approximately
7.5 tons of refrigeration per plate. Such an installation
could replace a 100 ton refrigeration plant. Electric utilities
look with favor upon such installations, for an ice storage
system means a 15 or 20 horsepower compressor operating 10
to 12 hours in a day as contrasted to a 100 horsepower unit
running for two hours.
For ice systems the cost (in dollars per ton)
of secondary equipment (bunker or evaporator plates, piping,
etc.) tends to decrease as the capacity of the system increases.
Thus, for a small system of 20 tons the secondary equipment
is a relatively large percentage of the refrigeration plant;
whereas, for a 200 ton system, the secondary equipment has
a lower unit cost. This act, coupled with the development
of larger unitary equipment, means that an ice system could
sometimes be attractive for large loads. However, considerable
space is needed for ice bunkers, which may be a disadvantage.
An economic study preceding the selection of an ice refrigeration
system should include:
- Daily load in tons plotted against
clock time for each zone of the building
- Days during the week when each area
will be occupied
- Diversity factor between zones or
areas
- Initial cost of equipment
- Electric power rate schedules
- Operating costs: labor, purchased
ice, electric power, condensing water, and maintenance.
|
