Most experienced HVAC engineers, when questioned about what
supply air temperature they typically use for comfort air conditioning
applications, will answer 55°F almost without exception. As consistent as this answer will be
repeated, one would think that there must be some basic law of physics that
makes it so universal. In fact, the
answer is not simple at all. It is based
on a series of principles that span several interrelated topics from physics
and chemistry to the basic temperature regulation systems of the human
body. This discussion will provide a
very brief overview of the parameters that conspire to make that 55°F supply
air answer appropriate for comfort air conditioning.
The first element of the answer has to do with what
constitutes human comfort (after all, we do call it comfort air
conditioning). But before we can address
human comfort, we need to talk about the sea of atmospheric air that we all
live in. Atmospheric air is composed of
a mixture of several gases, principally oxygen and nitrogen, but also
containing a very small amount of water vapor.
Water vapor is always present in atmospheric air, and though its
relative weight in the atmosphere averages less than 1% in temperate climates,
it is nevertheless one of the most important factors in human comfort. Its impact on human activities is in fact
altogether disproportionate to its relative weight.
The human body, at the most fundamental level, is a heat
machine with a complex set of mechanisms to control core body temperature. The food we take in is used in chemical
processes to provide the energy for our body to function. In turn, this heat must be rejected to our
environment (usually atmospheric air) to maintain what we refer to as “comfort”
– not enough heat rejected, we feel warm; too much heat rejected, we feel
cold. The heat rejection that takes
place from our bodies occurs by three principal means:
·
Convection – heat transfer based on the
temperature of the air around us. The
higher the air temperature, the less heat transfer that occurs tending to make
us feel hot; the colder the air temperature, the more heat transfer that occurs
making us feel cold.
· Radiation – heat transfer based on the
difference in temperature between our skin temperature and the temperature of
surfaces around us; we lose heat sitting next to a cold window surface in the
winter even though the air temperature around us may be warm.
·
Evaporation – heat transfer based on the
relative humidity or the amount of water vapor in the air. Water leaving the surface of the human body
(sweat) evaporates and cools the body.
The higher the relative humidity of the air, the lower the evaporation
that takes place making us feel warmer; the lower the relative humidity, the
more the evaporation that takes place making us feel colder.
In the years before about 1950 when our buildings were
typically heated only, making people comfortable relied principally on
convection and radiation – maintaining the air temperature was the only
parameter that was needed for control in the winter. If there were large windows, we installed
warm radiators below them to (1) warm the air and (2) offset the radiation heat
loss from the window. Since the air in
the winter was very dry (low relative humidity), evaporation heat transfer
occurred at a relatively constant rate as needed and convection based on air
temperature could be controlled to make us comfortable. In the summertime, everyone inside buildings
knew they were going to be uncomfortable since the air temperature was
generally high (reduced convection heat transfer), the building surfaces were
warm (less or no radiation heat transfer), and the air relative humidity was
generally high (reduced evaporation heat transfer).
When air conditioning became common after the early 1950’s,
the picture changed dramatically.
Suddenly the relative humidity became an equally important parameter in
comfort. In humid climates like St.
Louis, summer outdoor relative humidity is high and the evaporation component
of body cooling is appreciably reduced.
Exhaustive tests were performed in controlled settings to determine what
combination of space temperature and relative humidity in indoor environments
made people comfortable. The result was
that 75°F, 50% relative humidity became the target that seemed to please the
maximum number of test subjects. Thus
75°F, 50% relative humidity became the indoor air standard for summer air
conditioning. To maintain the indoor air
temperature at 75°F in the summer, colder air needed to be introduced into
rooms to provide cooling. But the
physics of the cooling process could be satisfied by introducing supply air
into the room at any temperature below 75°F, as long as the quantity of such
air could be controlled (much more 70°F air would be required than 50°F air for
the same amount of room cooling). But
the evaporative side of the heat transfer equation turned out to be a bit more
complicated.
In the summertime, the relative humidity in air conditioned
spaces increases principally due to the evaporation of water vapor from the
skin surfaces of the people in the spaces.
The process of controlling indoor humidity occurs in the same basic
process as that of temperature control described above – drier air (below the
75°F, 50% relative humidity) is introduced into the spaces to limit the
relative humidity rise to 50%. Again,
the physics of the drying process could be satisfied by introducing supply air
into the room at any dryness below 75°F, 50% relative humidity, as long as the
quantity of such air could be controlled (much more 75°F, 49% relative humidity
air would be required than 75°F, 30% relative humidity air for the same amount
of room air drying). Though our
technology provides many options to heat, cool, and add water vapor to air,
only one simple option is available to remove water vapor from air. And that option is only available because of
a unique property of the air/water vapor atmosphere we live in – as air is
cooled, it can physically hold less water vapor.
If we pass air over a cooling coil that is at a colder
temperature than the air entering it, cooling of the air occurs. If the air is humid as on a rainy spring morning,
the air is cooled but because the cooler air can hold less moisture, water
vapor begins to condense out of the airstream and onto the coil as liquid
water. If the coil is very efficient,
the air leaving the coil is at nearly 100% relative humidity (a condition
referred to as saturation). The air
leaving the coil now becomes both cooler and drier. It turns out that if you pass 75°F, 50%
relative humidity air across the coil, at a coil temperature of a little below
56°F, water begins to condense on the coil and air cooling and drying begins to
take place. At a coil leaving air
temperature of about 55°F, the drying effect of this air leaving the coil is
sufficient to reduce the water vapor increase from the building occupants for
typical people densities in our modern buildings. And because of the property of air/water
vapor described above, the 55°F air is at nearly 100% relative humidity and a
single specification of 55°F supply air temperature satisfies both the cooling
and the drying of the air necessary for comfort air conditioning. Thus, all of these parameters together make
the simple answer- deliver supply air at 55°F.
As year-round air conditioning systems were then developed
over the ensuing years, most of these systems were based on delivering 55°F
supply air to satisfy the air cooling and drying required during summertime
conditions.
8760 Engineering would like to thank
Jerry Williams for this blog post, and stay tuned for several more insightful entries from our very own senior engineer!
To end this blog post we present and energy conservation measure that almost always exist and is never found unless an engineer employs data logging. This is the case of the broken heating valve, which will heat the air up in excess of 100°F before it is cooled back down to 55°F.
ECM: Replace Cracked / Failed Heating Valve
kWh Saved per CFM: 86
Article Author: Jerry Williams
jwilliams@8760engineering.com
Blog Post: Ryan Corrigan
rcorrigan@8760engineering.com
