1. Nate Lee ID:W846188
Understanding Comfort_HVAC
Comfort is a subjective term, however; comfort is not only personal but intra personal.
HVAC systems are designed to aid in comfort levels anywhere there are people or maintenance
of specific parameters is needed. Setting up and maintain these comfort levels or parameters fall
into the job of HVAC design. How well the system is set up determines the level of comfort that
will be attained.
There are many advantages of improved air quality. Because weather can sometime be
unbearable, HVAC systems maintain a uniform distribution of air in both warm summer months
and also in cold winter months. HVAC systems can also control the humidity in the air. Air
borne dust and allergies can be filtered and stagnant odor laden air can be reduced drastically
with proper HVAC. One of the most important functions of an HAVC system is to introduce
tempered fresh air to the indoor community.
The basic purpose of HVAC design is to promote a comfortable climate indoor
throughout the year. Different levels of comfort are need in different circumstances. HVAC can
be localized to bring about comfort levels in any situation. Comfort and be found when the mean
radian temperature and the dry bulb index are equal. If air speeds are from 20 to 40 feet per
minute, this too aids in median comfort level. The relative humidity should be below 20 to 25
percent to lessen the effect of static electricity formation and not to exceed 60 percent humidity.
At 60 percent and above humidity condensation may form and mildew could be a detrimental
problem.
As stated before comfort is a subjective term, therefore variances of comfort level do
occur. “Sedoc”, a lightly clothed imaginary “sedentary occupant” has been developed to model
median comfort levels. Surveys show most people prefer 75 degrees F in the summer and 70
degrees F in the winter. However the median comfort level could fall to 80 to 65 (respectively)
as indoor temperatures. What makes us comfortable or uncomfortable? The dissipation of body
heat, whether cooling or heating, plays a key role in our comfort statues.

2. There are four ways the body dissipate or loose heat:
Convection
Radiation
Evaporation
Conduction
Convection is the circulation of liquids or gasses caused by temperature differences.
Body heat can be lost when the air temperature is less than skin surface temperature. This is
good in the summer time however on windy cold days it can be uncomfortable. Increased air
motion will increase convection heat loss. In a hurricane, and tornado, warm air rises and is
replaced with cool air, this constant movement is a convective air current.
Radiation is the heat transferred by electromagnetic waves, from a warmer surface to a
cooler surface. On the body surface, radiate heat disperses to cooler surrounding but also radian
heat can be received, depending on the temperature difference. The windows of a building can
be a source of heat loss. The reason for this heat loss is radiant heat transfer. Radiant heat is an
in-line sight heat. In-line sight heat, i.e. sunlight, can only be felt in line with the source. An
example is the shade of a tree blocks the radiant heat because the heat is no longer in-line of you
and the sun.
Evaporation is a change of state from liquid to vapor. Evaporation aids in body heat loss.
In humans, evaporation absorbs sensible heat, from the skin and changes it to latent heat of water
vapor. In other words, body heat and sweat loss create water vapor. Air motion increases the
effectiveness of the evaporating cooling effect.
Conduction is heat transferred when two items at different temperatures meet. A good
example of this would be a bare hand touching a cold window. Only a small amount of body
heat is lost through conduction. Most body heat is lost between convection radiation and
evaporation. But with all this being said, how do we find comfort in the whole wide world of
heat loss? The answer is Psychrometrics.
Psychrometrics is the study of the thermodynamic properties of air and water vapor
mixtures under varying temperatures and pressures. Psychrometrics charts can predict
condensation problems and also calculate the HVAC capacity needed for an ideal comfortable
place. There are seven characteristics and or variables that are noted on the chart. Of these
seven only two need be known for the other five. The seven characteristics/variables are: Dry

3. Bulb Temp. (DB), Humidity, Relative Humidity (RH), Dew Point (DP), Wet Bulb Temp. (WB),
Heat Content – Enthalpy and Density. The only exception to not finding all the values is when
the dew point and moisture content are the same.
Understanding the terms and drawings of a Psychrometric Chart is pivotal to designing
the correct HVAC system. The DB is the temperature of a house thermometer. Humidity is the
amount of moisture, water in the air. The DP is the temperature at which the air is saturated with
water content. The DP is directly proportional to the dry bulb temperature. The WB is a
measurement made from the cooling that results from evaporation. On the Psychrometric chart
the WB and DP can be interchanged. Heat content – Enthalpy, is measured in BTU (British
Thermal Units) and is defined as the amount of heat required to raise one pound of water by one
degree F. Enthalpy can and will come in many forms. There is specific heat, sensible heat,
latent heat and body heat.
Because we are discussing HVAC systems, let’s focus on body temperature control.
HVAC systems help maintain an average body temperature. Everyone releases some amount of
body heat therefore control is needed in acclimated environments. A formula is devised to
calculate the storage or loss of body heat. M+/-CD+/-CO+/-R-E=HS
M =metabolic heat production
CD= conductive heat gain or loss
CO= evaporative heat gain or loss
R =evaporated heat loss
HS = heat storage
In the world of thermo dynamics there are rules. Heat flows from an area or object of
high temp. to an area or object of a lower temp. More energy is needed to keep a higher temp at
a higher temp. For every heat transfer it involves losses in the form of wasted heat. Friction as
well as large temperature differences increases heat loss. HVAC systems help control comfort
levels and also aid in stabilization of environments.