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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