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Refrigeration Systems 295

erence and the percentage change in tonnage noted for Units are usually operated with 50—200 psig steam,
change in water-off temperature. A unit producing 100 tons although pressures down to 2 psig are possible, and 30 psig
at 50°F chilled water will also produce 115 tons when the units are giving economical performance for their specific
chilled water is used at 55°F or will produce only 70 tons situation.
when the chilled water off the unit is 40°F. In transporting this water through insulated pipes to the
The thermal efficiency of the units is approximately: process equipment, it is good practice to allow a 2°F tem-
perature rise when planning heat transfer calculations.
Operating Steam Pressure, psig Thermal Efficiency, % Cooling water must be specified at maximum expected
temperature, otherwise the unit cannot condense in hot
15 30
weather and still maintain full load.
100 60
400 80
Utilities
Operation
The utilities required for steam jet refrigeration operation
often determine the selection of these units, between man-
At the low absolute pressure of the flash chamber, the
ufacturers and between types of refrigeration. As the chilled
entering water partially evaporates and in so doing absorbs
water temperature off of the unit approaches 32°F, the cost
heat from the bulk of the water in the compartment. The
of the basic unit and its steam and cooling water require-
latent heat of steam (greater than 1,000 Btu/lb) at the evap-
ments rise rapidly.
orator pressure is removed and the water in the compart-
Figures 11-9, 11-10, 11-11, and 11-12 present estimating
ment is cooled an equivalent amount. Figure 11-8 indicates
operating utility requirements for barometric type units
the conditions for one system.
when referenced to a given chilled water temperature and
Although it is usually not desired, water may be cooled to
100 psig motivating steam. These curves allow the designer
the freezing point, and ice has been formed in units. Chill-
and operator to vary conditions to suit the relative costs of
ing water less than 40°F becomes expensive.
steam and cooling water (utility, not the chilled water) and
still maintain the tonnage from the unit. The performance
of specific units can be improved usually over the values of
the curves. Higher pressures are some advantage to a maxi-
mum of 12%. When the steam pressure to the boosters is
reduced from 100 psig to about 30—50 psig, the quantity
required will increase by a factor of 2 for 40°F chilled water
to only about 1.5 for 55°F chilled water.
Referring to the curves for 40°F water a unit initially
designed for 90°F cooling water with a steam consumption
of 30 lb of 100 psig steam per ton of refrigeration will use
10.9 gpm of the 90° water per ton of refrigeration. In the
winter when cooling water temperatures drop to a maxi-
Figure 11-7. Variation in tonnage with water temperature for steam jet mum of 75°F, the steam consumption will drop to 15.5
refrigeration systems. (Used by permission: Havermeyer, H. R. Chem. lb/hr/ton when the water rate is maintained at 10.9
Eng., Sept. 1948. ©McGraw-Hill, Inc., New York. All rights reserved.)
gpm/ton. If 45°F chilled water is needed in place of the 40°F
chilled water, and if the steam is the most expensive utility,
then 10.9 gpm/ton of cooling water (utility) will require
only 22.7 lb/hr of steam.
Make up water is about 1% of the water circulated in a
13
closed system. For water being chilled from 50° to 40°F and
used at 40°F:

1 lb water through 10°F t 10 Btu
At 40°F, the latent heat is 1,071 Btu/lb (Figure 11-3).
Lb water evaporated/lb water recirculated 10/1,071 0.0093
Figure 11-8. Operation conditions for a steam jet refrigeration sys- Approximate % make up 0.0093 (100)/1 0.93 1.0%
tem. (Used by permission: Rescorla, C. J. Chem. Eng., June 1953. Typical performance of a 150-ton unit, using 100 psig steam is as
©McGraw-Hill, Inc., New York. All rights reserved.) follows for a barometric refrigeration unit: