General FAQs
What
is air conditioning?
What is relative humidity?
Is there a difference between comfort
conditioning and industrial conditioning?
What is meant by a 'ton' of refrigeration?
What is direct expansion equipment?
Is is possible to vary the off coil
temperature during the cooling cycle of direct expansion
equipment?
General FAQs
What
is air conditioning?
The purpose of air
conditioning is to control the filtration, air movement,
temperature and humidity of an atmospheric environment.
Air conditioning is always associated with the cooling
and dehumidification process of air and is always
therefore identified with refrigeration equipment.
The full control
over relative humidity by the addition of moisture
by means of a humidifier and the use of a humidifier
constitutes full air conditioning, but this control
is not always exercised. However, the more
often used partial or comfort air conditioning which
uses refrigeration equipment only and is therefore
capable of cooling as well as dehumidifying is still
referred to as air conditioning.
From the foregoing
therefore the term 'ventilation' should not be confused
with air conditioning as refrigeration equipment is
not necessarily provided with ventilation equipment.
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What
is relative humidity?
The atmosphere always
contains moisture in the form of water vapor. The
maximum amount of water vapor that may be contained
in the air depends on the temperature of the air and
the higher the temperature of the air, the more water
vapor may be contained. At high temperatures and high
moisture contents extreme discomfort is experienced
as the evaporation of moisture from the body into
the atmosphere by the process of perspiration becomes
difficult.
In the air conditioning
process the moisture content of the air may be reduced
by the use of a cooling coil or added by the use of
a humidifier.
The term relative
humidity is simply a ratio between the actual moisture
content of the air compared with the moisture content
of the air required for saturation at the same temperature,
ie at 100% relative humidity (also known as saturation
point).
The air conditioning
engineer uses the psychometric chart to analyze how
the state of moist air alters as an air conditioning
process takes place.
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Is
there a difference between comfort conditioning and
industrial conditioning?
Yes, the object
of comfort conditioning as the name implies is solely
to provide a comfortable environment for the majority
of occupants. Humans are reasonably tolerant to humidity
and may be comfortable from a range of between 55%
and 20% relative humidity at normal comfort temperatures.
It is therefore common when specifying to limit the
humidity in summer and not specify a limit in winter.
Typically therefore a specification would state an
internal condition of 22°C / 50% relative humidity
being maintained at 30°C / 20°C wet bulb external
conditions in summer. In winter the specification
may typically be 21°C internal temperature at -3°C
saturated outside air temperature.
Industrial conditioning
is provided generally for a process which requires
a closely controlled atmosphere. A typical specification
may be that an internal environment is required of
say 21°C ±0.5°C and 50% relative humidity ±2.5% at
all external conditions. It will be seen therefore
that the industrial conditions for clearly defined
limits rather than comfort conditioning which is based
on statistical surveys of occupants feelings.
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What
is meant by a 'ton' of refrigeration?
Confusingly the
unit has little to do with weight, as used in common
parlance. One ton of refrigeration is the term used
to refer to 12,000 B.T.U.s/hour (British Thermal Units/Hour)
of cooling effect. Thus a chiller or condensing unit
with a cooling capacity of 60,000 B.T.U.s/hour is
said to have a capacity of 5 tons. It should be noted
that the unit B.T.U./hour is a unit of heat flow still
widely used in North America, Canada and parts of
Asia whereas Europe uses the 'watt'. One ton of refrigeration
approximates to 3.5kW of cooling.
The origin of the
term is the amount of heat absorbed by one ton of
ice when melting from solid to liquid state at 32°F
and assuming a latent heat of ice of 144 B.T.U.s/lb.
The heat absorbed is found to be 288,000 B.T.U.s over
24 hours, or 12,000 B.T.U.s/hour (in reality the latent
heat of ice is slightly less than 144 B.T.U.s/lb.)
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What
is direct expansion equipment?
'Direct expansion,'
'DX,' 'refrigeration' or 'split' units are all generic
terms used to identify the same equipment. The terms
are in fact rather loose but in any event it has become
accepted that the terms refer to two or more units,
one usually positioned externally and one or more
usually positioned internally. The units are connected
together by site installed refrigeration pipe work
which is charged with a refrigerant. The external
unit may take one of three forms:
- (a)The heat pump - which consists
of a fan, compressor, coil and reversing valve,
and rejects unwanted heat to atmosphere during the
cooling cycle and extracts heat from the atmosphere
during the heating cycle.
- (b)The condensing unit - which
is as described above but does not have a reversing
valve and therefore cools only.
- (c)The condenser which consists
of a fan and coil (as the compressor is contained
in the indoor unit); the condenser is used less
often than (a) and (b).
The indoor units consist of fan
coil units or air handling units which may be located
in the atmosphere being air conditioned or remotely
in a plant room. Some manufacturers produce 'external'
units that may be located internally and in the case
of these units ductwork is usually connected to atmosphere
to reject heat or extract heat. DX systems are in
direct contrast to hydraulic systems or chilled water
systems. With these systems cooling is achieved by
circulating chilled water with a hydraulic pump Generally
speaking with direct expansion equipment the manufacturers
match the indoor and the outdoor units and many well
not sell the units individually for fear of 'mismatching'
occurring.
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Is
is possible to vary the off coil temperature during
the cooling cycle of direct expansion equipment?
It is possible to
vary the off coil temperature using the direct expansion
equipment by the use of 'hot gas bypass'. Hot gas
bypass maintains the evaporator coiled temperature
independent of the load on the coil by taking hot
gas directly from the compress output and mixing it
with gas on the output side of the expansion valve.
This technique keeps the compressor(s) running, thus
reducing start-up surges, temperature swings, and
humidity problems.
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