15A1. Necessity for compressors of different
capacities. The air-conditioning system and
the refrigeration system aboard a submarine
are designed as two separate and distinct systems. Each is capable of performing its task
independent of the other. However, in practice
it is desirable that these two systems be
interconnected so that the air-conditioning
compressor can serve as a standby for the
refrigeration system. This insures continuous
operation of a compressor in the refrigerating
system; otherwise, spoilage of the foodstuffs
stored in the refrigerating rooms might result.
In earlier references and explanations within
this text, the rated capacity of the refrigeration
system compressor has been given as 1/2 refrigeration ton, while the rated capacity of each
of the air-conditioning system compressors is
given as 4 refrigeration tons. This difference
in rated capacity of the two units results from
the fact that the air-conditioning system performs a greater amount of work than does the
refrigeration system.
The refrigeration system must perform
sufficient work to remove heat from the comparatively small space of the cool room and
refrigerating room, with the minor addition of
the ice tuber.- Since these two rooms are
thoroughly insulated, little or no heat enters
them from the outside. The only source of
heat inside the rooms is from the stowed
foodstuffs and from persons entering for supplies. On the other hand, the air-conditioning
system must remove heat generated throughout the vessel, that is, heat that passes into
the air of the vessel from the crew, engines,
cooking, batteries, electric light bulbs, equipment, and at times from the surrounding water
outside the hull.
These requirements determine the work
load of each system. This work load expressed
in refrigeration tons determines the capacity
of the compressor needed for each system.
15A2. Relation of capacities. The capacity
of the compressor is the amount of work,
expressed in refrigeration tons, that a compressor is capable of doing under a given set
of operating conditions. A change in the
operating conditions causes a corresponding
change in the rated capacity of the compressor.
Therefore, the relation between the capacity of
a compressor on a refrigeration system and a
compressor on an air-conditioning system is
a comparison of the operating conditions of
evaporator temperature, the speed of the compressor, and the temperature of the cooling
medium for each system, and not a comparison of the compressor or its maximum work
load under optimum conditions.
The misunderstanding of this relationship
has often given rise to the question: Is there
a difference between a ton of air-conditioning
and a ton of refrigeration? The cause of this
question is the apparent increase in the capacity in refrigeration tons developed by a
compressor on an air-conditioning unit over
the capacity developed by the same compressor
when operating on a refrigerating unit. It
would appear that there were a difference
between a refrigeration ton and an air-conditioning ton. Actually there is no difference and
the term air-conditioning ton is not in common usage. The basic rating of a refrigeration
or air-conditioning system in refrigeration tons
is a measure of the heat-removing capacity of
the system and determines the rate at which
heat is removed from a substance in a refrigerator and the rate at which heat and humidity
are removed from air. However, a compressor
rated at 2.95 refrigeration tons when operated
at -5 degrees F evaporator temperature, running at
600 rpm with the same temperature of cooling
water in the condenser in both cases, has a
rating of 8.348 refrigeration tons if it is operated at a 35 degrees F evaporator temperature (see
the following tables). Thus the rating of a
compressor may vary with the evaporator
temperature; also the rating of a compressor
109
may vary with the speed of the compressor
and the temperature of the cooling water
flowing through the condenser.
The operating temperature of the evaporator
(suction pressure) has the greatest effect on
the number of refrigeration tons that the
compressor may develop. Refer to the table
of properties of saturated Freon 12 vapor,
Section 5D1. This table illustrates why the
capacity of a compressor varies with the
evaporator temperature (suction pressure),
with constant compressor speed and condensing temperature. For example, at 0 degrees F the
density of 1 pound of Freon 12 vapor is 0.6109
and at 40 degrees F the density is 1.263. In other
words, the compressor handles approximately
twice the gas at 40 degrees F that it does at 0 degrees F.
Therefore, more refrigeration tons are developed at the higher evaporator temperature
(suction pressure). The following tables illustrate this fact and also the effect of varying
temperatures of the cooling water.
The figures given in the tables are obtained
from engineering tests and computations.
The functional reason for this variation in
capacity is that the capacity of a refrigeration
machine is influenced by the operating pressures. The capacity increases as the suction
pressure increases. The capacity decreases as
the discharge pressure increases. The power
requirement also increases under these same
pressure conditions, although, as seen in the
table for the 4 x 4 inch compressor, the relative
power per refrigeration ton decreases.
Since Freon 12 has a positive pressure
temperature relationship, it may also correctly
be said that the capacity increases as the
evaporator temperature increases. Tables such
as the previous ones are given in terms of the
evaporator temperature, rather than suction
pressure, because the temperature of the space
cooled by the evaporator is the primary consideration.
The table does not give decrease of capacity against increase of discharge pressure, but
against increase of condenser water temperature. The discharge pressure depends upon
the temperature of the condensing water, as
stated in Section 6B5. The condensing water
temperature is of primary interest because it
varies according to geographical location and
season of the year. Therefore, although increase in discharge pressure decreases the
capacity, it may be said that the capacity
decreases as the condenser water temperature
increases.
The discharge pressure also varies according
to the speed of the compressor, but the usual
practice is to operate the compressor at a
relatively constant speed.
COMPARISON OF REFRIGERATION AND AIR-CONDITIONING COMPRESSOR RATINGS
CHARACTERISTIC
REFRIGERATION
AIR-CONDITIONING
Bore
2 5/8 inches
4 inches
Stroke
2 1/2 inches
4 inches
Cylinders
2.
2.
Displacement
27.10
100.50
Rpm
600.
600.
TR at -5 degrees F evap. temp.
0.742
2.95
TR at 35 degrees F evap. temp.
2.17
8.348
Bhp at 5 degrees F evap. temp.
1.89
6.94
Bhp at 35 degrees F evap. temp.
2.42
8.95
110
VARIATION IN COMPRESSOR RATINGS IN TR AND BHP
FOR 4 X 4 INCH, 2-CYLINDER COMPRESSOR
Evaporator Temperature
Condensing Water Temperature in Degrees Fahrenheit
90 deg.
95 deg.
100 deg.
105 deg.
110 deg.
Tr
Bhp
Tr
Bhp
Tr
Bhp
Tr
Bhp
Tr
Bhp
-20 deg.
2.2
5.9
2.1
6.0
2.0
6.1
1.8
6.2
1.7
6.3
-15 deg.
2.6
6.3
2.4
6.4
2.3
6.5
2.1
6.7
2.0
6.8
-10 deg.
3.0
6.7
2.9
6.8
2.1
7.0
2.5
7.1
2.4
7.2
-5 deg.
3.5
7.0
3.4
7.2
3.2
7.3
3.0
7.5
2.8
7.7
0 deg.
4.2
7.5
4.0
7.6
3.8
7.8
3.5
8.0
3.3
8.2
5 deg.
4.8
7.8
4.6
8.0
4.4
8.2
4.1
8.4
3.9
8.6
10 deg.
5.6
8,0
5.3
8.3
5.0
8.5
4.8
8.7
4.5
8.9
15 deg.
6.3
8.3
6.0
8.6
5.7
8.8
5.4
9.1
5.1
9.3
20 deg.
7.1
8;5
6.7
8.8
6.4
9.1
6.1
9.4
5.7
9.7
25 deg.
7.9
8.6
7.5
9.0
7.2
9.3
6.8
9.7
6.5
10.0
30 deg.
8.8
8.7
8.4
9.1
8.0
9.5
7.6
9.9
7.3
10.3
35 deg.
9.8
8.8
9.4
9.2
9.0
9.6
8.6
10.1
8.2
10.5
40 deg.
10.9
8.7
10.5
9.2
10.1
9.7
9.7
10.2
9.3
10.7
45 deg.
12.2
8.7
11.7
9.2
11.3
9.8
10.8
10.3
10.4
10.9
Ratings at 650 rpm. To obtain capacity rating of any intermediate speed proportion directly to compressor rpm.
B. INTERCONNECTION OF SYSTEMS
15B1. Cross-connection of air-conditioning
and refrigeration systems. In an emergency
necessitating the securing of the refrigerating
compressor, it is possible to cross-connect at
least one of the air-conditioning compressors,
condensers, and receivers to the refrigerating
system evaporator, and maintain the desired
temperatures in the refrigeration rooms.
On some classes of vessels, either of the
air-conditioning compressors may be cross
connected to the refrigeration system; on
other classes only the No. 1 air-conditioning
compressor can be used. As the arrangement
and location of valves and lines vary on each
installation, no detailed description can be
given here. It is never necessary or desirable
to cross-connect the refrigerating compressor
to the air-conditioning evaporators.
In cross-connecting the air-conditioning
compressor to the refrigerating evaporator,
several major adjustments must be made on
all installations. Reset the low-pressure cutout
switch on the air-conditioning compressor so
that it does not stop the compressor until the
suction pressure drops down to 2 psi. Normally this cutout is set to stop the compressor
when the suction pressure on the air-conditioning evaporator reaches 32 psi. If the compressor is to operate both the air-conditioning
system and the refrigerating system, the
bypass around the suction pressure regulating
valve should be closed, the stops opened, and
111
the valve cut into the system. With the suction
regulating valve in operation, a 32-psi suction
pressure in the air-conditioning evaporator is
maintained while the compressor is operating
at the lower suction pressure necessary on the
refrigerating system. The operation of the two
systems in this manner is desirable because the
capacity of the air-conditioning compressor is
much more than is needed to maintain the refrigerating rooms at the desired temperatures.
Another point that must be checked is that
electric current is supplied to the thermostatic
control circuit on the refrigeration system;
otherwise, the solenoid valves remain closed
and no refrigerant flows through the system.
On some vessels, the thermostatic control
circuits are energized from the 110-volt lighting circuit. In this case, the main switch to
the refrigeration compressor may be pulled
and the thermostatic control circuits energized
through the refrigerating control panel. When
the main switch is pulled on the refrigerating
compressor, it interrupts the supply of current
to the thermostatic control circuits. In this
case, the following procedure should be followed:
Leave in the main switch supplying current
to the refrigerating compressor. With a
wooden pencil or other insulated material, lift
the overload relay cutout located on the bot
tom left side of the half-ton compressor control
panel, to the OFF position, making sure that
the overload relay cutout stays up. Then turn
the selector switch on the half-ton system to
either MANUAL or AUTOMATIC. This
insures a supply of current to the thermostatic controls.
15B2. Cross-connection of air-conditioning
systems. On some classes of vessels, it
is possible to operate the No. 1 compressor
and condenser on the No. 2 evaporator, and
vice versa. The air-conditioning systems on
the 300 class submarines are cross-connected
by the compressor discharge lines and the
high-pressure liquid lines only. There is no
cross-connection between the suction lines.
Because of this arrangement, in the air-conditioning system the No. 1 compressor can be
connected to the No. 2 condenser, and the
No. 2 compressor can be connected to the No.
1 condenser. But the No. 1 compressor cannot
be connected to the No. 2 evaporator, or the
No. 2 compressor to the No. 1 evaporator.
