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The HVAC Process: Changing the
Properties of Air
Alexander Delli Paoli, Jr.
ABSTRACT
This discussion addresses the physical properties of air
and water vapor mixtures. The components of the heating, ventilation, and air conditioning (HVAC) process that
alter the moisture content of air to achieve desired levels
in the facility or manufacturing process being served are
described. Topics discussed can be thought of as the “wet
side” of HVAC. The wet side includes all aspects of removing and adding moisture to air along with removing water
condensed out of the airstream from the air handler.
This paper continues a previous discussion describing
HVAC systems (1). This and future discussions will provide a working knowledge of the HVAC process. A future
paper will address the “dry side” of HVAC, which includes
filters, fans, ductwork, controls, and safety components
in the system.
INTRODUCTION
Psychrometrics is the study of air and water vapor mixtures. It is important to understand this topic and how
those properties influence and affect the environment
being controlled. A psychrometric chart is a graphic representation of all the possible conditions of air/water vapor
mixtures (see Figure 1). Most “psych” charts are limited
to certain areas of the total mixture spectrum. Practical
limitations result in the creation of unique psych charts
for sea level and differing elevations as well as charts portraying low, normal, and high temperature ranges. It is
evident when a chart is not of the proper temperature
range. It is unfortunately too easy to use a chart of the
wrong elevation. The consequences of this error can result
in HVAC equipment components that are not suited for
the application. The moisture content levels portrayed by
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the process cycle on a psychrometric chart translate to
desired humidity ranges in the controlled environment.
Low moisture content in air is desirable when the concern is control of microbial or mold growth. However,
low moisture content is not favorable if static electricity is
a concern. Low humidity can also accelerate evaporation
of ambient moisture, which can be good or bad for different process environments. Moisture content can also
be an important factor for personnel in an environment.
High moisture content in air also has positives and
negatives, although there are usually more negatives with
high moisture content: people feel “muggy,” microbes
and molds flourish, and paper products become soft and
perform inadequately in high moisture environments.
Positive aspects of high moisture levels include the absence
of static electricity and lower evaporation rates of ambient
moisture. An optimally higher moisture level also minimizes drying of mucous membranes and could promote
a healthier environment.
THE PROPERTIES OF AIR
AND WATER VAPOR MIXTURES
The following discussion is expressed in English units.
International (SI) units could also be utilized. Familiarity with the topic is achieved regardless of the units used.
This section refers to Figures 1-3 to define the significant
parameters presented on the psychrometric chart.
Dry-Bulb Temperature
Dry-bulb temperature is located on the abscissa of the
chart (Figure 1, DB, Point 1). This is the temperature
that indicates the addition and subtraction of heat in
air. There is no connection to moisture levels when
ABOUT THE AUTHOR
Alexander Delli Paoli Jr., P.E., is managing director of Engineered Strategic Visions, Inc., Libertyville, IL,
USA. Engineered Strategic Visions is an engineering consulting firm specializing in project planning and
management, manufacturing support, asset and energy management plus several other areas of expertise.
Alex may be reached at [email protected] and at 1.847.204.9957.
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P e e r - R e v i e w e d : H VA C
is an important factor toward getting
an accurate reading. Impurities in
the water change the evaporation
rate and the evaporation temperature, delivering erroneous results.
Figure 1: Reading the psychrometric chart.
Dew-Point Temperature
The dew-point temperature parameter (Figure 1, DP, Point 3) is located
on the ordinate scale at the right side
of the chart. Dew-point temperature
is exactly as the name suggests. It
is the temperature at which air
becomes saturated with moisture
at a specific dry-bulb temperature
and begins to create dew. Dew is
the result of moisture condensing
to water and coming out of the air/
moisture mixture. If this condensation is sufficiently rapid, fog forms.
Dew point always occurs along the
edge of the wet-bulb temperature
line (Figure 1, Point 2) at the left
side of the chart.
presenting dry-bulb temperature. The “real-feel” or
“wind-chill” temperature reported by network weather
people is dependent on wind intensity. A stronger wind
will strip the human skin’s boundary layer, allowing
more heat removal from skin and clothing. Moisture
can also play a part, but is usually a small factor if skin
is substantially covered by dry clothing.
Wet-Bulb Temperature
The wet-bulb temperature scale (Figure 1, WB, Point
2) is located on the curved part of the chart running
from lower left to upper right. The terms “wet bulb”
and “dry bulb” have historical origins based on the way
the readings were taken with the technology available
at the time. Mercury-in-glass thermometers were the
instruments of greatest accuracy. Two thermometers
were mounted on a sling to be manually spun together
by a person. The bulb of one of the thermometers was
covered with a fitted sock. The sock was wetted with the
purest water available, hence the description wet-bulb
temperature. The sling assembly was then spun to cause
evaporative cooling on the wetted sock, cooling the
covered thermometer bulb, and thus, indicating wetbulb temperature. Once both wet-bulb and dry-bulb
temperatures were known, the state of the air/moisture
mixture was known. Using the purest water available
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Specific Volume
The specific volume parameter is represented diagonally
across the chart (Figure 1, Point 4) running diagonally
from upper left to lower right. Specific volume is doubly
significant because the reciprocal of the number is the
density of the air/moisture content. It is amusing to
note that while a pound of dry air is equal to a pound
of wet air, a cubic foot of wet air is lighter than a cubic
foot of dry air.
Enthalpy
This property of the air/moisture mixture (Figure 1,
Point 5) runs roughly parallel to the wet-bulb temperature scale. Enthalpy is a measure of the heat content of
the mixture. It is used to determine equipment capacities for processing the HVAC cycle.
Relative Humidity
Relative humidity is the most popular way of expressing
the moisture content of air. Relative humidity (Figure
1, RH, Point 6) is plotted on the chart running diagonally from lower left to upper right. RH is calculated
as a ratio of the partial pressure of the water vapor in
the current sample of the air/water vapor mixture to
the partial pressure of water vapor at saturation in the
current sample.
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A l e xan d e r D e ll i Paol i , J r .
Absolute Humidity
and Vapor Pressure
Figure 2: HVAC process cycle—warm climate.
Absolute humidity is not shown on the
chart. This parameter is used to determine the actual amount of water that
must be added or removed for sizing
of HVAC processing equipment. The
scale is typically located on a second
ordinate scale on the right side of the
chart. It is the ratio of the actual mass
of water vapor in a sample to the mass
of dry air in that same sample.
Vapor pressure and other measures
specific to the HVAC industry are also
available on the chart, but these are
of lesser value to understanding the
HVAC process cycle.
CASE STUDIES
The following case studies describe representative HVAC process cycles at different extremes (see Figures 2 and 3).
Case Study One—Warm
Climate
Case study one describes a facility in a warm and humid
climate (see Figure 2). The key operating parameters are
as follows:
• Desired process operating conditions:
• Dry bulb temperature: 72ºF
• Relative humidity: 50% RH
• Ambient (outside) conditions:
• Dry bulb temperature: 100ºF
• Wet bulb temperature: 80°F
• Relative humidity: 45% RH.
Note that the ambient relative humidity starts to lose
relevance for most people at higher temperatures, so the
wet-bulb temperature is provided. The wet-bulb temperature or absolute humidity readings are important for design
engineers.
The HVAC process cycle on the psychrometric chart
will start at the desired operating point (Figure 2, Point
1) in the facility. This is the point representing 72∘ F/50%
RH. Air from the facility is returned via ductwork to the
air handler (Figure 4) where it is mixed with outside air
at the conditions of Point 2. The two airstreams mix to
conditions at Point 3. The total air then passes on to
the cooling coils where conditions change from Point
3 to Point 4.
At Point 4 within the cooling coil, the air/moisture mixture reaches its dew-point temperature. Moisture begins
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condensing and continues until the air reaches the temperature at Point 5. The air then leaves the cooling coil
and is heated by the air handler or by heating coils in the
ductwork to the temperature at Point 6.
This additional heating is necessary because the air
was cooled below the temperature needed to maintain
facility equilibrium. This sub-cooling was necessary to
achieve the desired moisture content offered at Point 5.
The air then moves on into the controlled facility at Point
1 where it acquires heat being generated within the facility
to maintain the desired environmental conditions. This
completes the HVAC process cycle.
Case Study Two—Cool Climate
This case study describes a facility in a cooler climate (see
Figure 3) operating at the same process operating conditions as the facility in case study one. Comparison of
Figure 2 to Figure 3 offers a clear graphic indication of the
extremes to which the HVAC process equipment must
perform to maintain the desired facility control conditions.
The examples portray potential seasonal variations to the
HVAC process cycle of a facility located in the Northern
Hemisphere.
Key operating parameters are as follows:
• Desired process operating conditions:
• Dry bulb temperature: 72 ∘F
• Relative humidity: 50% RH
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Figure 3: HVAC process cycle—cool climate.
dry for process conditions. The air then
moves on into the controlled facility at
Point 1 to maintain the desired environmental conditions. This completes the
HVAC process cycle.
COILS FOR
COOLING AND HEATING
• Outside air taken in: 50% of total volume being
circulated by the air handler
• Ambient (outside) conditions:
• Dry bulb temperature: 34∘F
• Relative humidity: 80% RH.
The HVAC process cycle on the chart will again start
at the desired operating point (Figure 3, Point 1) in the
facility. This is the point representing 72∘F /50% RH. Air
from the facility is returned via ductwork to the air handler
(see Figure 4) where it is eventually mixes with air coming from the outside. Notice in this example the mixing
occurs only after the outside air at the conditions of Point
2 is heated through the preheat coil in the air handler to a
safe temperature at Point 3 that will not expose the other
components in the air handler to a freezing hazard.
The two airstreams of equal volume then mix to conditions at Point 4. The total air has not been sub-cooled in
this example, but it is cooler than is necessary to maintain
the facility environmental conditions. The total air passes
through a heating coil in the air handler where it is heated
to a temperature at Point 5 appropriate to still remove
the heat being generated in the controlled environment.
Also note that the absolute humidity is increasing
between Point 5 and Point 1. This suggests that humidification is occuring to prevent the air from becoming too
78 Journal of Validation Technology [SPRING 2012]
This discussion explores the characteristics of coils used in HVAC systems.
The name is somewhat of a misnomer;
it suggests copper tubing on a bootlegger’s distillation unit. That is most likely
the origin of the term, but the coils used
today must fit efficiently in rectangular
air handlers and rectangular ductwork.
For this reason, most coils are themselves put in a frame of a rectangular
geometry. There is usually copper pipe
inside the frame, typically with aluminum heat transfer plates called fins
attached to the pipe (finned tubing).
Row after row of copper is run back and
forth across the airflow surface called
the coil face. There are enough rows to
cover the entire air stream in the air handler or ductwork.
Even that is not enough to ensure complete treatment
of the airstream passing through the coil. Several other
important design details must be addressed for maximum coil efficiency.
Face Velocity
The velocity entering a coil is important for three primary
reasons: heat transfer, pressure drop, and carry-over.
Heat transfer. Heat transfer is best in a velocity range
specified by the manufacturer of the coil. If the velocity
is too high, air does not spend residence time in the coil
to pick-up or give-up enough heat to be effective. If the
velocity is too slow, the boundary layer on the fins of
the coil get thicker and less turbulent causing less heat
transfer.
Pressure drop. Pressure drop through a coil is
important to the total static pressure needed to drive
air through the HVAC system. This is usually a decision made during the system design phase. The energy
consumption, noise, and possibly lower heat transfer
efficiency of the coil will endure through the life of the
facility.
Carry-over. Carry-over refers to the moisture being
condensed on cooling coils after the air goes below
dew-point temperature and is giving up the excess
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Figure 4: Schematic of the HVAC system.
water in the saturated air. If the velocity of the air is
too great, the water droplets will be “carried-over”
back into the airstream and downstream of the cooling coil. This usually means the water lands outside
of the coil drain pan and impinges at another point in
the air handler. This can cause floods as water accumulates, or higher humidity than desired if the water
re-evaporates back into the airstream. Rare instances
of fog entering the controlled facility have also been
observed. Flooding in unexpected places is the most
common occurrence.
Number of Rows
The entire face of a coil is covered by finned tubing.
This is not enough by itself because a percentage of the
air bypasses the fins and is not treated. There is also
not enough contact time with one row of heat transfer
surface. Additional rows are needed to assure the air
has enough residence time in the coil to transfer heat or
condense moisture. Typical heating and preheat coils
range from one to six rows of depth (coil depth). Cooling coils typically range from four to ten rows deep with
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occasional needs for cooling coils of up to 16 rows of
depth. The selection is the resolution of many design
constraints by the design engineer.
Fins per Length of Pipe
Fins per length of pipe, like face velocity, is an important design consideration for the same reasons: heat
transfer, pressure drop, and carry-over. Heat transfer
is enhanced if fins are spaced close enough together
to keep air from getting in between them without
being treated. Fins too close together can lead to
excessive pressure drop in the airstream. Close fins
can be bridged by water droplets condensing in the
coil, effectively blocking the unit. This blocking could
cause water carry-over and decreased airflow to the
controlled environment: another set of constraints for
the design engineer and coil manufacturer.
Drain Pans
Drain pans should be placed under all water-bearing
components in air handlers and ductwork. It is a small
price to pay at installation to ensure a material failure
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during the life of a facility does not cause a major flood
in sensitive areas below the coils. Most facility personnel think of cooling coil pans as mandatory. Placing
drain pans under other coils is a good practice if their
rupture could cause undue hardship or business interruption. Cooling coils endure condensation from seasonal moisture in the northern climates. In warmer and
more tropical climates, the cooling coils could be wet all
four seasons. In either case, it is important to perform
periodic cleaning and sanitization of coils and drains to
minimize the possibility of microbial contamination.
Traps
A trap is a down-and-up loop in a drain to keep air from
passing through the pipe. Traps are present in all sanitary sewer systems to prevent offensive sewer gases from
migrating up through use point such as sinks in bathrooms
and kitchens. In the case of a cooling coil trap, the trap is
connected to the drain pan. The difference in this case is
the fact that the air handler is under negative pressure at
this location. This can cause two undesirable issues: air
contamination within the air handler from an uncontrolled environment and flooding of the air handler by
condensate overflow.
Within the air handler (Figure 4) the fan section is
pulling air through all the upstream components. This
creates an increasing negative air pressure as the air is
drawn across each component. By the time air has past
the cooling coil, it has been drawn through return ductwork, prefilters, heating coils, and the cooling coil.
This negative pressure can amount to multiple inches of
water column. To prevent the trap from being “blown”,
it must have an elevation difference outlet-to-inlet of
typically six inches or more so the negative pressure
can pull the water in the trap up toward the drain pan
discharge without allowing the water in the bottom
of the trap to pass air. Once the trap is blown-out, the
negative pressure causes high velocity air to pass through
the trap entraining air and conveying it back into the
cooling coil drain pan. The drain eventually fills up and
overflows inside the air handler. The leaks that ensue
are disruptive to most facilities.
Seasonal changes can cause trouble even after the trap
has been primed (filled with liquid) initially. During a
season when dehumidification is required, condensation flowing out the drain keeps the trap primed. Once
a dryer climate occurs, cooling coil condensation stops
and slow evaporation of the liquid in the trap begins.
When the trap can no longer support the column of
water created in it by the negative pressure inside the
air handler, the trap blows-out and uncontrolled air
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starts flowing. This can continue throughout the dry
season until condensation begins again, presenting the
likelihood of flooding. This could be an undesirable
annual event unless preventive measures are in place.
Mechanical devices to prevent undesirable airflow exist,
but they should be inspected periodically much like the
rest of the HVAC system. It is just as simple to make an
inspection of the traps part of the overall system preventive maintenance work orders and not introduce more
contraptions on the trap. Traps can be filled with a fluid
of low evaporation rate such as glycerin during the dry
seasons to minimize the threat of a missed inspection.
This fluid will be purged from the system when condensate begins flowing again.
DEHUMIDIFICATION SOURCES
Moisture is usually removed from air in one or more of
three ways: cooling coils, desiccants, or compressed air.
Cooling Coils
Cooling coils are the most prevalent and cost effective
method of dehumidification within typical temperature
and humidity ranges found in controlled environments.
Because temperature and relative humidity have an inverse
relationship, it is difficult to quote exact numbers. An
example of an operating point is 70∘F at 50% relative
humidity (RH). Cooling coils are limited to the freezing
point of water because the condensation forming cannot
be allowed to freeze in the coil and block the airflow.
The fluid in the coil could be circulated colder than the
freezing point of water with the use of antifreeze, but any
temperature below 34∘F is risking the freeze hazard.
Using refrigerant directly in the cooling coil (direct
expansion) is common on many smaller HVAC systems.
This eliminates the risk of the fluid inside the coil freezing, but the concern for the condensate on the outer coil
surfaces still exists. Control of the temperature leaving a
direct expansion cooling coil is also less precise across the
operating range of the coil.
Desiccant Dehumidification
Desiccant dehumidification is used when significantly
lower humidity is needed in a facility. The size of the HVAC
system is typically limited to serve just the facility requiring
the lower humidity. Facilities using this type of system are
likely to contain hygroscopic powders or other materials
sensitive to moisture pick up.
The dehumidification method can use either solid or
liquid desiccant materials. Dry desiccants are usually in
the shape of a porous wheel. The wheel is rotated between
two ducts, each covering half of the wheel. The first half
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of the rotation is in a duct performing the moisture
removal for the air serving the controlled environment.
The moisture-laden side of the wheel then rotates into
the air stream of the second duct where the moisture is
removed by warm air (wheel regeneration). The warm
air then moves on for heat recovery or is exhausted to
atmosphere. The wheel continues around again into the
controlled air stream. The liquid desiccant system is similar in operation except that the liquid is exposed to the
controlled environment airstream in a chamber to pick
up moisture. The desiccant liquid is then circulated back
to the dehumidification equipment where it is warmed to
drive out the moisture. The liquid is then ready for reuse
in the circulation loop.
Compressed Air
Air can be compressed to its saturation point and below
to achieve low humidity in very small applications. The
compressed air is usually cooled to further enhance moisture removal. When the air is later expanded the amount
of moisture in the air can be low. While this is an effective
way to remove moisture, it is not cost effective or energy
efficient on a large scale. This method of dehumidification should not be ruled-out for small-scale, temporary,
experimental, or emergency operations. The capacity of a
facility’s central compressed air system should be investigated before proceeding with this type of dehumidification
system (i.e., tied into the source of compressed air).
HUMIDIFICATION SOURCES
Moisture is usually added to air in one or more of
three ways: evaporation, steam humidifiers, or water
humidifiers.
Evaporation
Evaporation from people and processes operating within
a controlled environment contribute to the total moisture
in a controlled environment. This can be considered an
advantage in dryer seasons and part of the overall cooling
load during the seasons of higher humidity. While evaporation is a contributing source in an operating facility, it is
not considered a controllable parameter. Steam or water
can be infused into air environment in a controllable,
metered fashion.
Steam Humidifiers
Steam humidifiers disperse vapor in air handlers, ductwork, or in the controlled facility directly. Steam is already
a vapor, so it poses a minor risk of wetting or flooding
the inside of air handlers and ductwork. The steam is
distributed to a piping grid typically located across the
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downstream face of the cooling coil in the air handler.
In ductwork, steam humidifiers also consist of a piping
grid distributed across the area of the duct. It is good
practice to locate a humidity sensor downstream of any
internal humidifier to detect saturated air. Condensation
and eventual flooding would be the likely outcome of
a failure of the primary humidification controls. This
condition could occur with a control valve malfunction
when the steam is not throttled or turned off adequately.
The downstream humidity sensor could be designed to
override the normal control sensor and generate an alarm
to alert maintenance personnel.
A safety concern with steam humidifiers is the hot
pipes of the distribution grid. It is important that maintenance personnel are trained to be aware of this potential
burn hazard
Water Humidifiers
Water humidifiers also disperse a mist through a distribution grid, which is evaporated as it is mixed with passing
dry air in an air stream. The evaporation of this mist causes
cooling of the airstream. Misting humidifiers can also be
installed in the controlled facility directly, but they may
cause cool spots in rooms due to the evaporative cooling
effect of the water droplets. This type of humidifier requires
greater attention to location in air handlers and ductwork.
If the mist impinges on objects within the equipment,
the liquid can build up and cause flooding. This could
happen with the air never reaching its saturation point. A
high humidity sensor and alarm are also recommended
for the same reasons as with steam humidifiers.
SUMMARY
This paper initiated a more complete discussion of
design constraints and details concerning the removal
of moisture (wet side) from the air stream in a controlled
environment. Topics addressed included psychrometrics
and associated parameters including dry-bulb temperature, wet-bulb temperature, dew-point temperature, specific volume, enthalpy, and relative humidity.
Case studies described HVAC processes conducted at
different extremes. Coils for cooling and heating, and
associated design details are described. Methods of
dehumidification are discussed including cooling coils,
desiccants, or compressed air. Humidification sources
including evaporation, steam humidifiers, and water
humidifiers are described.
REFERENCE
1. Delli Paoli Jr., Alexander, “The HVAC Process,” Journal of
Validation Technology, Volume 17, #4, Autumn 2011.JVT
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