-
Lesson18
air
conditioning system
Ships travel the
world and are therefore subject to various
climatic conditons .the crew of the ship
must
be
provided
with
reasonable
conditions
in
wihich
to
work
regardless
of
the
ture
alone
is
not
sufficent
measure
of
condition
acceptable
to
the
human
body.
Relative
humidity iin conjunction with
temperature more truly determines the environment
for
human
comfort .Relative
humidity
,expressed
as
a
percentage,
is
the
ratio
of
the
water
vapour
pressure in the air tested, to the
saturated vapour pressure of air at the same
temperature. The fact
that less water
can be absorbed as air is cooled and more can be
absored when it is heated is the
major
consideration
in
air
conditioning
system
design.
Other
factors
are
the
nearness
of
heat
sources,
exposure
to
sunlight,
sources
of
cold
and
the
insulation
provided
around
the
space(Fig.18-1).
An
air
conditioning
system
aims
to
provide
a
comfortable
working
environment
regardless
of
outside conditions. Satisfactory air
treatment must involve a relatively
air
is circulated and returned. However, some air is
there is a requirement for renewal.
Public rooms and accommodation will operate with a
reduced
percentage of air renewal since
the conditionig cost of 100% renewal would be
considerable.
Air
conditioning systems fall into two main classes:
individual unit system, in which each room
contains its own small refrigeration
plant and fan and air cooler; and central systems,
where larger
refrigeration machinery
unit are installed and their out put distributed
about the ship by a variety
of means.
Self-contained
units
are
noisier
than
central
systems,
require
more
maintenance
and
have
been
found to have a relatively short life
(about 7 years).
The single
duct system only allows for adjustment of
temperature in each room by the occupant
manually controlling the air volume
admitted. It is thus less flexible than any of the
other systems,
which allow individual
temperature control, at least of sections of the
ship if not individual rooms.
With
ducted
systems,
the
modern
tendency
is
to
use
velocity
in
the
air
ducts
with
fans
generating up to 2550 mbar(250 mm
H?
O) pressure compared to
generating
up
to
520
mbar(50mm
H?
O). This
tendency
helps
installation
as
the
size
of
ducts
is
reduced
and prefabricated standard ducts can be used, but
it incurs the heavier running costs of
more powerful fans. Air terminals lined
with sound insulation material are necessary to
reduce the
noise passing into the room
with high velocity systems.
In
a
typical
marine
pattern
self-contained
unit,
air
circulation
is
usually
effected
by
means
of
a
centrifugal
fan, for quiet running, and a direct expansion
cooler served by a hermetic compressor.
Water
cooled
condensers
are
used.
As
these
contain
small
water
passages,
choking
develops
rapidly with direct sea water
circulation and a better method is to circulate
with fresh water, itself
cooled in a
sea water/fresh water heat exchanger.
Control is on/off by a thermostat
sensing the temperature of air returning to the
unit.
The cooling coil of
the central unit may be of the direct expansion,
brine or chilled water cooled
type.
When cooling is by direct
expansion, a separate steam heater coil is fitted
in the unit for winter
brine
or
water
coolers,
a
central
heater
is
used
so
that
the
same
coil
serves
for
summer
or
winter.
Thermostatic
control
is
provided
sensing
air
delivery
temperature
itself,
the
temperature of the room, or the return
air temperture.
All
types
of
thermostats
are
found
in
air
conditioning
systems,
direct
acting,
pneumatic
and
electrical.
In
themselves,
they
are
all
satisfactory
instruments,
but
the
results
they
achieve
are
dependent
on
the
correct
sitting
of
their
sensing
elements.
Even
the
site
for
a
direct
acting
thermostst
to
control
one
single
berth
cabin
must
be
chosen
with
care-
if
it
is
masked
behind
curtains, or too far
away from the air inlet control will be too
sluggish.
The correct
location for a thermostat to control a block of
cabins is more difficult to find . One can
pick on a
Another
possibilility is to site the thermostat in the
alleyway of the block of cabin . This position
may be affected more by an open door or
draguht in the alleyway tthan by the temperature
of the
cabins . Yet another
possiblility is to site the thermostat in the
recirculation grill is to close to an
outside
door
,
this
position
too
can
be
affected
by
outside
air
temperature
when
the
door
is
open ,rather than by cabin temperature.
General operation of the
air conditoning installation
The
first
enssential
in
operating
the
air
cooling
appliances
through
out
the
ship
is
to
hhave
all
thermostats correctly
set and correctly functioning . In extreme weather
conditions , either hot or
coold ,
control of the plant usually present few
diffculties . The capacity of many installations
is
such
that
under
tropical
conditions
nearly
all
control
valves
move
to
the
full
open
position.
Although automated
control has been lost ,internal conditions are by
and large acceptable.
Control diffculties arise in
intermediate weather conditions when there is a
call for only a small
amout of cooling.
The worst case is when part of the ship,say
inboard cabins against the engine
room
,
require
cooling
and
other
parts
,say
exposed
upper
cabins
,
require
warming .For
this
intermediate
condition,thermostats
must
be
correctly
set
by
trial
and
error.
It
is
found
that
a
uniform setting of say
21
°
C throughout the ship is
not satisfactory ,but slight variations of a
few degrees up or down are needed to
suit particular regions of the ship.
Unfortunately,
these variations in
thermostat setting are not always the same for the
cooling and heating
condition and
frequent resetting may be needed for a ship
repeatedly passing from cold to
warm
weather.
The control
problem is eased if the chilled brine (or water)
of systems using chilled liquid
circulation
is
held
at
about
13°
C
in
the
intermediate
weather
conditions
and
lowered
progressively to about 5°
C
as tropical weather conditions are approached.
When air cooling
is in use it is good
practice to keep all portholes, windows and doors
shut. On passenger