Carrier 5H - Manual (Page 29)

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