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Table 23 — Booster “R” Factors
Air-cooled (R-12 and R-22) Water-cooled (R-22 only)
Determining Intermediate Pressure —
In applica-
tion of commercial compressors to staged systems, the lowest
total bhp per ton and most economical equipment selection
results when using approximately equal compression ratios for
each stage. It is also economical to juggle assigned compres-
sion ratios to fit available sizes of machines.
The use of Fig. 21 (page 32), will allow direct determination
of proper intermediate pressure that will result in equal com-
pression ratios per stage for a direct 2-stage system. Informa-
tion in Fig. 21 is given in terms of saturated temperature
instead of pressures, for easier use with compressor ratings.
Existence of a second appreciable load, at some higher
suction pressure level, will often dictate the most convenient
intermediate pressure.
Gas Desuperheating —
Operation of a direct staged
system requires cooling of the gas between stages; otherwise,
highly superheated discharge gas from low-stage machine
would be taken directly into the suction of higher stage com-
pressor 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 refrigeration effect of
liquid delivered to evaporator to realize rated capacity of boost-
er compressor. Amount of refrigeration expended in cooling
liquid between stages is accomplished more economically at
the level of high-stage compressor suction than at the level of
low-stage suction.
Three common methods of gas desuperheating and liquid
cooling for direct stage systems are illustrated in Fig. 18. In
open-type systems, refrigerant liquid is cooled down to the
saturation temperature corresponding to intermediate pressure.
In closed-type systems, good intercooler design usually results
in refrigerant liquid being cooled down to 10 to 20 degrees
above saturation temperature corresponding to intermediate
pressure.
Oil Separators and Lubrication —
In cascade-type
systems, where evaporators and suction lines are properly
designed for oil return to the compressor, oil separators are
usually not used.
In direct stage systems, however, oil may tend to accumu-
late in one of the stages and thus result in lack of lubrication in
other machine. By use of oil transfer lines, equalization of oil
level between crankcases can be achieved by manual operation
at periodic intervals. Automatic control of proper oil return to
both compressors is effected by use of a high stage discharge
line oil separator, returning oil to high stage machine, and a
high side float, connected to high stage machine crankcase,
which continually drains excess oil from this crankcase down
to the next lower stage compressor (Fig. 18).
For booster application, factory oil charge should be drained
and replaced with a suitable viscosity oil for low temperature
application.
SUCT
TEMP
(F)
DISCHARGE TEMPERATURE (F)
–50 –40 –30 –20 –100 102030
R-12
–80 — 1.230 1.276 1.328 1.377 1.429 1.470 ——
–70 — 1.186 1.230 1.280 1.330 1.380 1.421 1.458 1.489
–60 ——1.183 1.233 1.284 1.334 1.375 1.410 1.441
–50 ———1.189 1.238 1.287 1.328 1.363 1.397
–40 ————1.190 1.240 1.280 1.318 1.350
–30 —————1.291 1.234 1.270 1.307
R-22
–100
1.261 1.310 1.360 1.410 1.453 ————
1.221 1.271 1.319 1.371 1.414 — — — —
–90
1.214 1.263 1.313 1.361 1.407 1.448 ———
1.175 1.221 1.270 1.321 1.368 1.408 — — —
–80
1.170 1.218 1.269 1.315 1.360 1.400 1.434 ——
1.129 1.172 1.221 1.271 1.319 1.359 1.394 — —
–70
— 1.172 1.221 1.269 1.313 1.351 1.388 1.424 —
— 1.125 1.173 1.221 1.270 1.311 1.348 1.382 —
–60
——1.178 1.220 1.267 1.303 1.340 1.377 1.406
— — 1.125 1.172 1.221 1.263 1.300 1.337 1.367
–50
———1.175 1.219 1.256 1.291 1.329 1.360
— — — 1.123 1.173 1.217 1.252 1.289 1.319
–40
————1.171 1.209 1.245 1.281 1.311
— — — — 1.126 1.169 1.205 1.241 1.261
–30
—————1.160 1.199 1.233 1.265
— — — — — 1.121 1.159 1.196 1.227
NOTE: For R-502, “R” Factor = 1 +
[0.212 x low stage bhp]
low stage capacity (tons)
50
40
30
20
15
10
9
8
7
6
5
4
3
2
1
0.5
5
10
15
20
30
40 50 60
70
SAFETY FACTOR %
TONS OF REFRIGERATION
SATURATED SUCTION TEMPERATURE F-100
-90
-80
-60
-40
Fig. 19 — Booster Compressor Selection
Safety Factors
MULTIPLYING FACTORS
FOR OTHER SPEEDS
CAPACITY
BHP
100
90
80
70
60
50
1000
1200
1400 1600 1800
SPEED (RPM)
% RATING AT 1750 RPM
Fig. 20 — Multiplying Factors —
Nonstandard Speeds