Chapter 6REFRIGERATION AND AIR CONDITIONING
specific system or unit can be found in the
manufacturers technical manual or on the ships
A refrigeration system should not be charged
if there are leaks or if there is reason to believe
that there is a leak in the system. The leaks must
be found and corrected. A system should be
checked for leaks immediately followingor
duringthe process of charging.
A refrigeration system must have an adequate
charge of refrigerant at all times; otherwise its
efficiency and capacity will be impaired.
PURGING THE SYSTEM
To determine if there are noncondensable
gases in the system, close the liquid line stop
valve. By-pass all evaporator pressure regulator
valves and allow the system to pump down one
or more times. Stop the compressor. By-pass the
water regulating valve and circulate cooling water
through the condenser. When discharge pressure
convert the pressure to
temperature and from this subtract the
temperature of the injection or overboard. (They
both should be equalized.) A variation of over 5 °F
will indicate that air and noncondensable gases
are present in the system. Crack open the purge
valve for 2-3 seconds at 2 to 3 minute intervals
until the temperature is within 5 °F.
Where a liquid line strainer is installed, it
should be cleaned at the same intervals as the suc-
tion strainer. If a liquid line strainer becomes
clogged to the extent that it needs cleaning, a loss
of refrigeration effect will take place. The
tubing on the outlet side of the strainer will be
much colder than the tubing on the inlet side.
To clean the liquid line strainer, secure the
receiver outlet valve and wait a few minutes to
allow any liquid in the strainer to flow to the cool-
ing coils. Close the strainer outlet valve and very
carefully loosen the cap which is bolted to the
strainer body. (Use all appropriate safety gear.)
When all of the pressure is bled out of the strainer,
remove the cap and lift out the strainer screen.
Clean the strainer screen with a small brush,
using an approved solvent. Reassemble the spring
and screen in the strainer body. Replace the
strainer cap loosely. Purge the air out of the
strainer, by blowing refrigerant through it, then
tighten the cap. After assembly is complete, test
the unit for leaks.
CLEANING OIL FILTERS
Compressors arranged for forced feed lubrica-
tion are provided with lubricating oil strainers in
the suction line of the lube oil pump and an oil
filter installed in the pump discharge line. A
gradual decrease in lubricating oil pressure in-
dicates that the units need cleaning. Cleaning is
accomplished in much the same manner as
described for cleaning suction strainers.
When cleaning is necessary, the lubricating oil
in the crankcase should be drained from the com-
pressor and a new charge of oil, equal to the
amount drained, should be added before re-
starting the unit. When the compressor is put back
into operation, the lube oil pressure must be
adjusted to the proper setting by adjustment of
the oil pressure regulator.
MAINTAINING COOLING COILS
Cooling coils should be inspected regularly and
cleaned as required. The cooling coils should be
defrosted as often as necessary to maintain the
effectiveness of the cooling surface. Excessive
build up of frost on the cooling coils will result
in reduced capacity of the plant, low compressor
suction pressure, and a tendency for the com-
pressor to short-cycle. The maximum time inter-
val between defrosting depends on such factors
as refrigerant evaporating temperature, condition
of door gaskets, moisture content of supplies
placed in boxes, how frequently the doors are
opened and atmospheric humidity.
Cooling coils should be defrosted before the
frost thickness reaches three-sixteenths of an inch.
When defrosting, do not scrape or break off the
frost, as this may cause damage to the coils.
EVACUATING AND DEHYDRATING
The major cause of system failures is moisture
(H2O) which is brought in through air leaks.