Carrier 5H - Manual (Page 26)

SPEED (RPM)
Fig. 16 - Multiplying Factors - Nonstandard
Speeds
Fig. 17 - Booster Compressor Selection
Safety Factors
Safety Factors - The use of capacity safety fac­
tors in selecting booster compressors must
necessarily be a matter of judgment on the part
of the engineer making the selection.
Factors which have a bearing on satisfactory
compressor selections are: the accuracy of the
load estimate, the amount of safety factor included
in the total load, knowledge of the degree of impor-
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10-66 Printed
tance of meeting the specified capacity at the given
condition, the temperature level of operation and
the magnitude of the refrigeration load. All of the
factors must be recognized when considering the
use
of a capacity safety factor in selecting a
booster compressor.
Figure 17 presents reasonable safety factors
for use in the selection of booster compressors.
These can be employed when it is not desired to
e stablish a factor based on the selector's judgment.
When a capacity safety factor is used, the
compressor is selected at its maximum speed
to handle the design load plus the amount of safety
factor.
Whether or not the added capacity offered by
the safety factor is incorporated at once is a mat­
ter of judgment. If it is, then the compressor will
be operated at maximum speed right at the start
and any excess capacity achieved will be reflected
in faster pulldowns or lower temperatures. It is
also, however, good practice to drive the machine
at the speed which will provide slightly more rated
capacity than is required by the design load. The
additional
speed-up available will then constitute
reserve capacity in the event that it is needed.
Motors should be sized to run the compressor at
maximum
speed to forestall any motor changes,
should this maximum compressor speed be re­
quired in the future.
Determining Intermediate Pressure- in the appli­
cation of commercial compressors to staged sys­
tems, it is found that the lowest total bhp per ton
and
most economical equipment selection results
when using approximately equal compression
ratios for each stage. However, it is also econom­
ical to juggle assigned compression ratios slightly
to fit available sizes of machines.
The use of Fig. 18 will allow direct determina­
tion of the proper intermediate pressure which
will result in equal compression ratios per stage
for
direct two-stage system. The information in
Fig. 18 is given in terms of saturated temperature
instead of pressures, for easier use with the com­
pressor ratings.
The existence of a second appreciable load, at
some higher suction pressure level, will very
often dictate the most convenient intermediate
pressure.
Gas Desuperheating - The operation of a direct
staged system requires cooling of the gas between
stages.
Otherwise, the highly superheated dis­
charge gas from the low stage machine would be
taken directly into the suction of the higher stage
compressor
and further compression would result
in excessive heating of this compressor.
Liquid Cooling - It is also necessary to employ
liquid cooling between stages and increase the
refrigeration effect of the liquid delivered to the
evaporator
to realize the rated capacity of the
booster compressor. The amount of refrigeration
expended in cooling the liquid between stages is
accomplished much more economically at the level
of the high stage compressor suction than at the
level of the low stage suction.
S U P E R S E D E S
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5F,H-1X
PAGES 1-42
inU.S.A. DATE 11-63