Carrier 5H - Manual (Page 28)

Three common methods of gas desuperheating
and liquid cooling for direct stage systems are
illustrated in Fig. 15. In the open type systems,
the refrigerant liquid is cooled down to the satura­
tion
temperature corresponding to the interme­
diate pressure. In the closed type systems, good
intercooler
design usually results in the refrig­
erant liquid being cooled down to 10 to 20 degrees
above
the saturation temperature corresponding to
the intermediate pressure.
Oil Separators and Lubrication - In Freon cas­
cade type systems, where evaporators and suction
lines are properly designed for oil return to the
compressor, oil separators are not usually used.
In direct staged systems, however, oil may
tend to accumulate in one of the stages and thus
result in lack of lubrication in the other machine.
By the use of oil transfer lines, equalization of
the oil level between crankcases can be achieved
by manual operation at periodic intervals. Auto­
matic control of proper oil return to both com­
pressors is effected by the use of a high stage
discharge line oil separator, returning oil to the
high stage machine, and a high side float, connected
to the high stage machine crankcase, which contin­
ually drains excess oil from this crankcase down
to the next lower stage compressor (Fig. 15).
For booster application, the factory oil charge
should be drained and replaced with a suitable
viscosity oil for the low temperature application.
Control Pressurestat for Booster Application -
The standard dual pressure switch furnished with
the
5F,H compressors is not suitable for use
with booster application. It must be replaced with
one of the appropriate low temperature dual pres­
sar estats from Table 21.
Table 21 - Control Pressurestats for Low
Stage Application
REFRIGERANT 12 REFRIGERANT 22
CARRIER PART NO HK04HN-068 HK04HN-164
Switch Action - High Open on press, rise Open on press ri se
■ Low Open on press fall Open on press fall
Range - High 20" Vac. to 65 psig 30" Vac to 110 psig
- Low 30" Vac to 20 psig 30" Vac to 25 psig
Differential - High 8 to 30 psi adjust. 12 to 30 psi adjust
- Low 5 to 15 psi adjust. 9 to 30 psi adjust.
Max Pressure - High 200 psig 300 psig
- Low 120 psig 300 psig
Discharge Valve Springs - When 5Hcompressors
are used for booster applications where the dis­
charge
pressure is below 10 psig, the standard
discharge valve springs furnished with the machine
should be replaced with an equal number of lighter
weight springs. Part Number 5H41-1801.
No change in discharge valve springs is rec­
ommended for the 5F compressors.
SECTION
PAGE
DATE 5F,H-1XA
28
10-66 Printed
Water-Cooled Heads - The standard 5F,H com­
pressors are not equipped with water-cooled heads
but
they are available on special order. Water
cooling of the heads is not generally necessary in
R-12
and R-22 booster applications. For these
applications with R-22 involving high compression
ratios,
5 or above, 5F,H booster compressors
should be equipped with water-cooled heads.
Motor Selection Data - In staged refrigeration
systems, the high stage compressor starts first
and runs until the low stage pressure has been
reduced to a predetermined level before the low
stage machine starts. With direct staged arrange­
ments,
the high stage machine draws gas from
the evaporator thru the low stage machine bypass
during this initial period. The size of the selected
motor must be related to the maximum condition
at which the booster compressor can operate.
The compressor may run under heavy loads
during periods of high suction pressure, espe­
cially on starting when the system is warm. To
handle these situations the motor must be sized
larger than the actual balanced operation brake
horsepower indicates, or special attention must
be paid to the operation of the system when starting
initially.
If the system is to operate only at a fixed low
temperature, it is generally possible to avoid
oversizing of motors to any great extent, provid­
ing careful operation is followed when the system
is first put in operation.
For those cases where definite pulldown pe­
riods are involved, the motors must be sized to
handle the highest brake horsepower occurring
during pulldown.
On applications requiring reduction in tem­
perature from ambient conditions to some ex­
tremely low temperature, the compression system
will be operated at high suction pressures for
considerable periods of time. General practice
here is to drive the high stage compressor with
a motor that will carry the compressor at the
highest
expected evaporator temperature. This
is generally the "air conditioning" rating of the
unit. For intermediate or low stage compressors,
it is generally sufficient to size the motor to take
care of double the balance load indicated horse­
power plus the friction horsepower.
It is also necessary to consider compressor
starting torque requirements when selecting the
motor for a booster compressor. The starting
torque of a motor only large enough to provide the
required normal operating bhp for booster appli­
cations, may not be large enough to start the com­
pressor.
The recommended minimum motor sizes
shown in Table 22 have been selected to assure
adequate
starting torque. The actual motor size
selected is usually larger; however, depending
on the maximum bhp conditions under which the
compressor will run during pulldown or other
abnormal operating periods.
. S U P E R S E D E S
SECTION 5F,H-1X
PAGES 1-42
inU.S.A. DATE 11-63