Carrier 5H User Manual
Page 29
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It is good practice to select motors with allow
ance for 10% voltage reduction unless there is a
certainty that this cannot occur.
Compressor
Starting
Torque
-
The
required
compressor
starting
torque
is
dependent
on
the
discharge pressure as well as the pressure dif
ferential
occurring
during
start-up.
The
max
imum expected torque required during the starting
period
for
5F,H
compressors,
used
as
boosters,
is shown in Table 22 at two saturated discharge
temperatures.
Selection Procedure
- The selection of a 5F,H
booster
compressor
requires
that
the
load,
sat
urated
suction
temperature,
saturated
discharge
temperature,
type
of
system
and
refrigerant
be
known.
After
the
saturated
intermediate
temperature
is determined from Fig. 18, the booster rating on
pages 22 thru 24 can be entered and the compres
sor selected. The low stage load is then multiplied
by the "R” factor from Table 18, 19 or 20 to ob
tain
the
high
stage
compressor
load.
With
this
information,
the
single
stage
ratings
(Application
Ratings 5F,H-1XR) can be entered and the high
stage compressor selected.
SELECTION EXAMPLE:
Given:
Refrigeration load
= 5 . 7 tons
Saturated Suction Temperature = -60 F
Saturated Condensing Temperature = 80 F
Open Type Intercooler
Refrigerant 22
Find: Compressor size and motor size.
Solution:
1.
Figure 18 on page 27 indicates an optimum
saturated
intermediate
temperature
of
-1
F.
Allow for 1 F drop from the booster compres
sor to the intercooler and from the intercooler
to the high stage compressor.
Booster Saturated Suction
Temperature
= - 60 F
Booster Saturated Discharge
Temperature
= OF
2.
At - 60 F suction and 0 F discharge, the 5H60
booster compressor has a capacity of 6.8 tons
with 12.1 bhp input at 1750 rpm.
The safety factor at 1750 rpm.
0^x 100
- 100 or 20%
This is satisfactory from Fig. 17 and^a 5H60
compressor is selected.
3. Indicated hp = bhp - fhp
where bhp is given in the compressor ratings,
fhp is given in Table 22.
Indicated hp = 12.1 - 3.07 = 9.03
Recommended minimum hp
= (2 X ihp) + fhp
= (2 X 9.03) + 3.07 = 21.13
Tentatively select a 25 hp motor. Assume that
the low stage will never start against a sat
urated discharge higher than 30 F. At 30 F
discharge Table 22 indicates a starting torque
of 54 Ib-ft. Therefore, a normal starting torque
25 hp motor is selected.
4.
With - 60 F suction and OF discharge, Table 19
indicates an "R” value of 1.303. Therefore, the
high stage load is:
1.303 X 6.8 = 8.86 tons (actual load)
5.
Allowing a 1 F drop from the intercooler,
the
high
stage
saturation
suction
temperature
is - 2 F.
Allowing a 2 F drop between the compressor
and
condenser,
the
high
stage
saturated
dis
charge temperature = 80 + 2 = 82 F.
6.
Referring
to
the
5F,H
single
stage
ratings
(5F,H-1XR) the 5F60 at 1450 rpm has a capac
ity of 9.10 tons at - 2 F suction and 82 F dis
charge. The 5F60 is selected and requires 13.7
bhp at 1450 rpm.
7.
Assume that the maximum load during pull
down occurs at 50 F suction and 90 F discharge.
For
this
condition,
the
rating
tables
indicate
17.6 bhp, thus a 20-hp motor is selected.
Table 22 - Booster Compressor Starting Data
COMPR
SIZE
UNLOADING
DURING
STARTING
MAX COMPR STARTING TORQUB^LB-FT
RECOMMENDED MIN MOTOR SIZE HP
FRICTION
HP*
(FHP)
Refrigerant 12
Refrigerant 22
Refrigerant 12
Refrigerant 22
Saturated Discharge Temperature (F)
High
Torque
Normal
T orque
High
Torque
Normal
Torque
10 F
30 F
10 F
30 F
5F20
None
9
13
15
21
_
2
3
3
67
5F30
None
10
15
16
24
3
3
5
5
.91
5F40
75%
8
12
13
19
3
3
5
5
1 15
5F60
66-2/3%
10
15
16
24
3
5
5
7-1/2
1.64
5H40
75%
19
28
30
45
5
7-1/2
7-1/2
10
2.25
5H46
75%
25
36
39
59
7-1/2
10
10
15
2.79
5H60
66-2/3%
23
34
37
54
7-1/2
10
10
15
3 07
5H80
75%
26
38
41
60
10
15
15
20
3 82
5H120
66-2/3%
41
60
65
94
15
20
20
30
5 25
Based on 1750 rpm with 5FH compressors Will vary directly with rpm at other speeds
S U P E R S E D E S
SECTION
5F,H-1X
PAGES
1-42
DATE
11-63
SECTION
PAGE
DATE
5F,H-1XA
29
10-66