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Basics of Psychrometrics
Practical Heat Load Calculation
Webinar
30 April 2020
Vikram Murthy
ASHRAE Mumbai Chapter
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Sessions
►Basics of Psychrometrics
►All about Heat
►Practical Heat Load (Cooling Load)
Calculation ( Using the E 20 , CLTD - Cooling Load Temperature Difference Method )
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Psychrometric basics
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Psychrometrics► A hundred and eighteen years ago, Willis
Carrier, developed a method that allows us
to visualize two of the variables -- the
combination of air temperature and humidity
that exist in a space. The tool he developed
is called the Psychrometric Chart.
► Psychrometrics, which Willis Carrier
developed, is the study of the mixture of dry
air and water , and is the scientific basis of
Air conditioning .
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Willis Carrier Willis began his first job at the Buffalo Forge Company .
Solving a Problem at the Sackett Wilhelm Lithographing Company in
Brooklyn, he formulated the Laws of Psychrometrics .
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Willis Carrier laid down the Equations of Psychrometrics in 1902 .
The Carrier Company he founded developed the Centrifugal
Chiller and the Weathermaker, that we call an Air Handling Unit or
AHU .
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Purpose Of Comfort Airconditioning
►To Cool or Heat
►To Dehumidify or Humidify (remove or add
moisture)
►To remove odours
►To remove particulate & microbial pollutants
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Definitions Of Air
► Air is a vital component of our everyday lives.
► Psychrometrics refers to the properties of moist
air.
► Dry air
► Moist air
► Moist air and atmospheric air can be considered to
mean the same
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Units
► We work in INCH POUND system of units, IP units.
(The other unit system in use is SI units)
Units of length: ft, inches.Units of area: sq.ftUnits of volume: cu.ft.
Weight: pound, lb.Moisture: grains.7000 grains = 1 lb.
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Units
► Temperature: Deg F
Ice = 32 deg F (0 deg C)► Boiling water = 212 deg F (100 deg C)
Body temp.: = 98.6 deg F ( 37 deg C)Karachi Summer temp = 99 deg F (37.1 deg C)
Heat: (sensible and latent) Btu
Specific Heat: btu/lb per deg F
Specific Heat of dry air: Btu/lb per deg F
Specific Heat of water vapour: Btu/lb per deg F
W = humidity ratio, lbs of water per pound of dry air
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Units
► Rate of heat flow: Btu/Hr
1 watt = 3.41 BTU/Hr1 kW = 3410 BTU/Hr1 H.P. = 2545 BTU/Hr1 Ton of Refrigeration = 12,000 BTU/Hr
K value: BTU/Hr/Sq.ft/Inch thickness/deg FU value: BTU/Hr/Sq.ft/deg F
Air quantity: cuft per minute, Cfm
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Psychrometry
►Air conditioning, by its very name means
treating air.
►How would Air behave when it is subjected
to cooling, heating, humidifying or de-
humidifying processes.
►A study of the properties of Air at normal
atmospheric pressure.
►Such a study is what is called Psychrometry.
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Psychrometry
►Psychrometry is the science of studying the
thermodynamic properties of moist air and
the use of these properties to analyze
conditions and processes involving moist
air.
Psychrometry (from the Greek word :
psukhros which means cold) , is the study of
moist air (which is mostly oxygen, nitrogen
and water vapor) and of the changes in its
condition.
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an energy or heat graph
►Any point on the psychrometric chart
represents air in a specific condition
containing a certain amount of heat.
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The following can be determined by
using a Psychrometric Chart :
►dry-bulb temperature
►wet-bulb temperature
►relative humidity (RH)
►humidity ratio
►specific volume
►dew point temperature
►enthalpy
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Dry Bulb Temperature
►air temperature
►indicated by a thermometer
►measured using a normal thermometer
►degrees Fahrenheit (oF)
►an indicator of heat content
►Constant dry bulb temperatures
►appear as vertical lines
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Dry Bulb Lines
►Any vertical line is a line of constant
temperature.
►condition of air represented by any point on
this line will have the temperature
corresponding to this vertical line.
►the temperature as recorded by a
thermometer which is dry.
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Dry Bulb Temperatures
Dry-bulb Temperature - The temperature of air as registered by an ordinary thermometer.
The horizontal X-axis denotes
dry bulb temperature (DBT)
scale.
Vertical lines indicate constant
dry bulb temperature.
DBT is the air temperature
measured in °C or °F and
determined by an ordinary
thermometer.
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Typical DB
Line
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Humidity Ratio / Absolute Humidity
Y-axis indicates humidity
ratio or absolute humidity,
which is the weight of the
water, contained in the air
per unit of dry air. This is
often expressed as pounds
of moisture per pound of dry
air.
Humidity ratio is found on
the vertical, y-axis with lines
of constant humidity ratio
running horizontally across
the chart.
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Humidity Ratio / Absolute Humidity
The Y axis shows the water vapor component and
is generally shown in lbs of water vapor.
Sometimes the vapor content is also shown in
grains of water vapor.
One pound of water vapor =7000 grains of water
vapor
Moisture is indicated in either Lbs of water vapor
or grains of water vapor, per pound of dry air
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Typical
Absolute
Humidity
Line
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Wet Bulb Lines
►There are number of parallel slant lines
which are called wet bulb lines.
►temperature of the air as recorded by a
thermometer with a wet wick on its bulb.
►air having a certain wet bulb temperature
will have a definite heat content although its
dry bulb temperature may be anything.
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Wet Bulb Lines
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Wet Bulb Temperatures
Wet Bulb Temperature
(WBT) is defined as the
temperature at which
water, by evaporating
into air, can bring the air
to saturation at the same
temperature
Inherent in this definition
is an assumption that no
heat is lost or gained by
the air.
Wet Bulb Line
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MEASURING THE WET BULB
TEMPERATURE
The wet-bulb thermometer is wrapped
in a cotton wick; when the wick is
completely wet, swing the
thermometer around, and the water
evaporating at the wick pulls the wet-
bulb thermometer’s temperature
down in direct proportion to the water
content of the air around it.
The drier the air, the more water
evaporates at the wick and the lower
the wet-bulb temperature gets
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MEASURING THE WET BULB
TEMPERATURE
The wet-bulb thermometer tells us
the relative humidity-the moisture
content of the air compared with
how much moisture it can hold.
When the dry- and wet-bulb
temperatures are equal it means
that the air is holding as much
moisture as it possibly can- i.e. air
is at 100% relative humidity.
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Relative Humidity Lines
►When the air contains its maximum moisture
content, we call it saturated air.
►when it contains anything less than this
maximum limit then it is not saturated air.
►We, therefore, say that such air is 50%
saturated or 60% saturated.
►the percentage saturation is "relative
humidity"
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The Condition of Air at
Point T is plotted on the
chart and its saturated
moisture content is then
checked
We find that the
saturated condition
moisture content is
indicated by Point 2
The moisture condition
at condition T is
indicated by Point 1
Relative Humidity
T 1
2
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T
1
2 The relative Humidity of
air at condition T is the
ratio of Moisture content
at saturation, to the
Moisture condition of air
at the specific condition
RH = Specific Moisture value ( Point1)
Specific Moisture value ( Point 2
Relative Humidity
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Relative Humidity, is
an expression of the
moisture content of a
given atmosphere as
a percentage of the
saturation humidity at
the same
temperature.
The RH lines are
shown on the chart
Relative Humidity
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Saturation Line
►The curved line on the extreme left-hand
side of the chart is what is called the
saturation line.
►condition of air represented by any point on
this line is said to be saturated air.
►the air is having the maximum possible
content in it. It cannot hold any further
moisture.
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The air is 100%
saturated when the
moisture content in the
air is at its maximum
possible and the
saturation line is shown
on the chart
Relative Humidity
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Dew Point►The Dew Point is the temperature at which
water vapor starts to condense out of the air.
►Move horizontally on the psychrometric chart and read the temperature where you intersect the saturation line.
►It is the moisture content which determines the dew point.
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Dew Point
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Dew Point Temperatures
When air, at a certain dry
bulb temperature and
relative humidity, is cooled
up to saturation condition,
from point R to Saturation,
it reaches its DEW POINT
CONDITION
Condensation occurs on
surfaces, which are at or
below the dew-point
temperature, and which
are in contact with the air
at condition R
R
DEW POINT
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Dew Point
► If the dew-point temperature is close to the air
temperature, the relative humidity is high.
► if the dew point is well below the air temperature,
the relative humidity is low.
► If moisture condensates on a cold bottle from the
refrigerator, the dew-point temperature of the air is
above the temperature in the refrigerator.
► The Dew Point is given by the saturation line in the
psychrometric chart.
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Enthalpy
►Wet Bulb Lines as lines of constant heat
content of air.
►Enthalpy is just another term used in place
of "heat content".
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Enthalpy
►At any temperature there is a limit to the maximum moisture holding capacity of air.
►At higher and higher atmospheric pressure, the moisture holding capacity at any given temperature becomes less and less.
►The enthalpy of moist and humid air consist of sensible heat and latent heat.
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ENTHALPY
Enthalpy (E) is the heat
energy content of moist air.
It is expressed in Btu per
pound of dry air and
represents the heat energy
due to temperature and
moisture in the air.
Lines of constant enthalpy
run diagonally downward
from left to right across the
chart ( As shown).
Enthalpy scale
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Enthalpy
►The enthalpy of moist and humid air includes the;
►enthalpy of the dry air - the sensible heat - and
►the enthalpy of the evaporated water - the latent heat
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ENTHALPY
Lines of constant enthalpy
and constant wet-bulb are the
same on this chart but values
are read off separate scales.
For calculating enthalpy at
point (R) the enthalpy is read
at point 1. The sensible heat
component can be read at
point 2, corresponding to the
enthalpy of dry air at the
same temperature. The
remainder, i.e.. 1 - 2, is the
latent heat content.
R
1
2
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Psychrometric Chart SI
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Applied Psychrometry
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Sensible Heating
► adding heat to air whereby the entire heat added goes to raise the temperature of the air.
► no change in the moisture content of the air. ► its condition will move on a horizontal line
corresponding to its constant moisture content. ► Since heat is being added during such process, its
enthalpy also rises. ► during the heating process the wet bulb
temperature of the air will also rise ► already seen, it is the wet bulb temperature lines
which are identified as constant enthalpy lines.
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Addition Of Moisture
►if moisture is somehow or the other added to the air without adding any sensible heat, the process would be represented by a vertical line corresponding to its dry bulb temperature.
►the moisture added carries with it the latent heat of vapourisation of water
►the heat content of the air also rises and hence its wet bulb temperature also rises.
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Evaporative Cooling
►Evaporative cooling is the process by which
air is simply subjected to a spray of re-
circulated water.
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Evaporative Cooling
►However, since we do not provide infinite or
adequate number of spray banks, the air
does not come out 100% saturated, or at
100% humidity.
►we must define some norm for specifying
the humidifying efficiency of the air washer.
►Wet bulb depression
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Wet bulb depression
► Is simply the difference between the actual dry
bulb temperature of the air and its wet bulb
temperature.
► The smaller this depression, the closer is its
condition to the saturation line.
► If this depression is zero, obviously the air is 100%
saturated.
► If the depression is more and more, then the
relative humidity of the air is less and less.
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Evaporative cooling
(adiabatic cooling)
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Outside Design Conditions
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Sensible Heat
►Sensible heat is dry heat causing change in
temperature but not in the moisture content.
►Btu/Hr = 1.08 x cfm x delta t
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Latent Heat
►Latent heat is the heat that when supplied
to, or removed from air, there is a change in
the moisture content of the air, but the
temperature of the air is not changed.
►
Btu/Hr = cfm x 0.68 x delta W
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Enthalpy
►Enthalpy is the thermodynamic term for the
heat content of air.
►Btu/Hr = 4.5 x cfm x delta H
► Since air can gain heat with either an increase in
temperature or moisture content, the terms
sensible heat and latent heat are used to
distinguish how air has gained heat.
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Heat Transfer
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Conduction
► Conduction of heat is the process of heat transfer in solids. ► In buildings, heat is transferred by conduction, mainly by
the walls or roof either inwards or outwards. Conduction flow rate through a wall of a given area can be described by the equation :
► QS = A * U * T
► where Q = conduction heat flow rate, in Btu/Hr A = surface area, in square feet U = Conductivity value in Btu/hr/sqft/ deg F T' = temperature difference in deg F
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Convection
► Convection is the process of transfer of heat in which molecules of cool air absorb heat from a warm surface air, rise, and carry it away.
► Convection heat flow in a building occurs mainly in the interior spaces - within a room, between a gap an air gap in the walls, or roof or within two layers of glass in a window.
► Convection and infiltration are both forms of mass flow but convection heat flow takes place mainly in the interiors while infiltration takes place between the building and the outside air.
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Convection heat
►Btu/Hr = cfm x 1.08 x delta t
►Btu/Hr = cfm x 0.68 x delta W.
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Radiation
►Radiation is the process of heat flow in electromagnetic waves from a hotter surface to empty space.
►The radiation balance "favors" the cold surface.
►This is the only method of heat transfer which does not require a medium for heat transfer
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Radiation
►Radiation heat gain in the buildings is
considered mainly through the window.
►Qr = A * Sc * Sg
►Sg = solar gain factor of window glass.
►Sc = Solar heat gain correction factor due to
shading
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Transmission
►Heat flows from a higher temperature to a lower temperature.
►heat transmission per hour:►H = A * U * T ►U is the overall heat transmission coefficient
expressed in BTU/Hr/Sq.ft/Deg F temperature difference.
►The product A x U is also called "conductance".
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Thermal Conductivity of a material, k
►is the heat transmitted through the material
expressed as BTU/Hr/Sq.ft/Inch
thickness/Deg F temperature difference.
► If k is the conductivity of the material then 1/
k is the resistance "R“ of the material of 1
sq.ft cross section and 1" thickness.
►If the thickness is "t" inches, the resistance
becomes (t)/(k) per sq.ft.
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Thermal Conductivity of a material, K
►If a barrier is made up of, say, three
materials having thermal conductivities k1,
k2 and k3, the total thermal resistance of the
barrier is:
t1/k1 + t2/k2 + t3/k3
Where t1, t2, t3 are the thickness of the
barriers.
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Thermal Resistance is ti/k1 + t2/k2 +t3/k3
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Calculating U value
►Outside air film
►Transmission thru the material layers
►Air space.
►Inside air film
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Thermal conductance of air space.
►Dead space of air as a layer
►Exceeds ¾” thickness
►No reflective insulation surfaces like
aluminum foil
►Transmits heat by radiation, convection and
conduction.
►Value of a = 1.1
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U Value
If "U" is the overall heat transmission of the barrier in
BTU/Hr/Sq.ft/deg F then, 1/U ( R ) is the overall
thermal resistance of the barrier.
1/U = 1/f1 + t1/k1 + t2/k2 + t3/k3 + 1/fo + 1/a
Therefore,
U = _____________1___________________
1/f1 + t1/k1 + t2/k2 + t3/k3 + 1/fo +1/a
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Typical U values in Btu/hr/sq ft/Deg F
► 8” brick wall with ½” cement plaster both sides 0.35
► 4” brick wall with ½” cement plaster both sides 0.44
► 4” brick wall, with ½” and 1” expanded polystyrene 0.24
► 6” RCC, with ½” plaster both sides 0.65
► 6” RCC, with ½” plaster + 1 in Mosaic Tile 0.40
► 4” RCC, ½” plaster both sides 0.71
► 4” RCC, ½” plaster both sides, 2” thermocole 0.12
► ¼” or 6 mm glass 1.13
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Conversion
► To convert U values from ip Units to SI Units :
Multiply U value in btu/hr.sft.F (IP units) by 5.678
to get U value in W/sqm.K (in SI units)
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Heat Load Calculation
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E 20 Method
► The E 20 method is a reliable method to calculate Peak Cooling Loads . If you calculate instantaneous loads using this method, then, in most cases, this calculation will be reasonably accurate to select correctly sized equipment. (The exceptions are if the Peak occurs under a different set of conditions than calculated )
The E 20 method is a method developed by Carrier.
Many more methods have been developed, including a "heat balance method", where you can calculate hourly loads, not just the instantaneous load at 4 PM. (Hourly Load Calculators like Carrier HAP or Design Builder or Smart energy software use this method that include schedules )
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Building Survey
► Collect architect's drawings for the building giving all details and dimensions of walls, floors, windows, etc. If such drawings are not available, then survey the place and get the details.
► Building orientation.
► Windows: Location, size and orientation, whether externally or internally shaded.
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Building Survey
► Partitions: To non-airconditioned spaces, to kitchens, to toilets.
► Roof construction, light roof, sheet roof, insulation, Medium roof (4" concrete), Hung ceiling (false ceiling), Ceiling ventilation, Ceiling, floor, AC above or not.
► Construction details like thickness of wall, material and layers of construction, type of windows, nature of ceiling, roof, floor below AC or not, orientation, occupancy, lighting load, appliances, etc.
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Thermal Zoning
► What you get as a drawing, remember, has the space divided as a geometrical space. You would need to map out the space as a thermal space!
What we mean by a thermal space, is that, all like areas, being fed by a single split or packaged or air handling system, and therefore are at the same temperatures, can be clubbed together, for the purposes of heat load.
Simply, if there is a row of 10 cabins, being fed by the same equipment, while geometrically there are 10 spaces, you could treat the entire 10 cabin space as a single thermal space.
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Thermal Zoning
► if there is a row of 10 cabins, being fed by the same equipment, while geometrically there are 10 spaces, you could treat the entire 10 cabin space as a single thermal space.
Of course, some zones, like the Data Centre or the dining area, would be treated as separate thermal zones, because their inside design conditions are different for the rest of the space.
Zoning is an art, developed by practice.
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Geometric Zones
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Thermal Zones
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Multi-story buildings
► You could treat a multi-storied building as one single thermal zone. In which case we call that a block load. you do just one heat load calculation, to get the block load. Of course, for purposes of air distribution, and equipment selection, you may need to do a load for each zone (say, floors). But the total of all the zonal heat loads will add up to the block load.
Typically, in a multi-storied building, there is a ground floor a top floor and many floor in between.
Since the load for the ground floor will be difference from the intermediate floors and the top floor (because of say, a basement below), it would be treated as a separate thermal zone. The top floor, similarly, would be exposed to sun, so that would be treated differently. But all intermediate floors, could be identical, and you could do a single heat load for that, and multiply that load by the number of floors.
If you add all the loads done above, ground, top, and (A x typical floor), the total would be equivalent to the block load.
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A Typical Commercial office
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Plan of HVACR Office for Calculation of Cooling Load
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Summary of Zones and Areas of Walls and Windows
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Details of Wall & Glass Areas , Occupancy , Lights & Equipment
Floor Area 5120 sq ft
Roof Exposed Area 5120 sq ft
North Glass Area 144 sq ft North Wall Area 576 sq ft
East Glass Area 192 sq ft East Wall Area 768 sq ft
South Glass Area 144 sq ft South Wall Area 576 sq ft
West Glass Area 192 sq ft West Wall Area 768 sq ft
Occupancy : 40 persons
Lights : 5124 X 0.2 W/sq ft LED = 1024.8 W
Computers : 200 X 20 = 4000 W
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The Heat Load Form
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Profile
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Room Size
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Outside Design Conditions
► Outside Design Data:
► Which Station► What is the Latitude► What is the Daily range
► Summer, Monsoon, Winter
► Given, DB temp. and WB temp. ► Find Grains from the psychrometric chart.
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Outside Design Conditions
►The data we use is the ASHRAE Weather
Data.
( ASHRAE Handbook of Fundamentals )
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“Comfort" variables
►environmental variables
►air temperature
►relative humidity
►air motion
►mean radiant temperature (av.)
"clo" value, “met” rate,
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Inside Design Conditions Summer
►75 deg F DB temperature►55% R.H. Or 60% RH
►76 deg F DB temperature►55% R.H. Or 60% RH
►74 deg F to 82 deg F ( 23 deg C to 28 deg C )
►30% to 70%
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ASHRAE Comfort Chart
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Inside Design Conditions ► As per ASHRAE, one would choose 75 deg F and
55 % RH.
Note that with each degree F decrease in inside room temperature the load would increase by 10 to 15%.
Usually, we do not go below 50 deg F supply air temperature for comfort applications.
The usual guaranteed inside conditions have a tolerance of + / - 2 deg F, and the Relative humidity has a tolerance of +/- 5% R.H.
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Outside and Inside Conditions
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Outside
Condition
Inside
Condition
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Design Conditions
HEAT LOAD ESTIMATE
At 4pm
Est SUMMER Peak
DB WB RH GR/LB
O.A. 99 74 21 88
Room 75 55 70
Diff 24 18
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Munters Psychro App
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Outside Air per person
► (ASHRAE standard 62.1 )
Outside air is provided for oxygen and for maintaining the area under slight positive pressure. In some applications, 100% outside air is required.
cfm / person plus cfm per sq ft
Deduct the amount of infiltration directly entering into the room. Add the amount of exhaust, if any, to get the net outside air to use in the heat load.
Calculation of air changes, is based on the volume of conditioned space. that means, that the height to be used should be upto the false ceiling, if there is one.
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Ventilation / Outside Air
( Fresh air ) Load
space
supplyfan
coolingcoil
outdoor air or fresh air
returnair
returnair
supplyair
exhaustair
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Lets take an example of an office with an area of 1000 sft having an
occupancy of 30 persons
The chart shows that for Office application
Cfm / sft is 0.06 and
Cfm / person is 5
Fresh air calculations
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Lets take an example of our selected office with an area of
5124 sft with an occupancy of 40 persons
The chart shows that for Office application
Cfm / person is 5 and Cfm / sft is 0.06
CFM OF FRESH AIR REQUIRED =
Cfm/ person* Number of persons +Cfm/ area sft*sft of the
space
CFM fresh air for the example = 5*40+0.06*5124 = 507 cfm
Fresh air calculations
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Sources of Cooling Load
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Outside Air
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3 Sources of Sensible Loads
► Heat flow from solar radiation (sometimes called radiation load).
► Heat flow from warmer surroundings (sometimes called the transmission load and sensible infiltration load).
► Heat flow into the space from energy consuming objects within the space (sometimes called internal loads); these objects usually include:
People lighting Office appliances Motors any other energy consuming devices
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Sensible Gains
►Solar gains
►Transmission gains
►Lights gains
►Equipment/ appliances gains
►People gains
►Outside air gains
►Infiltration gains
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Solar and Transmission Gains
► The sun's heat can get into a building in one of two
ways -- through glass and through walls and roof.
1. Solar gains through glass is absorbed
instantaneously in the room. This is in addition to
the conducted heat passed by the glass.
► 2. Transmission gains through glass, walls, floors,
ceilings and roofs.
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Sunlit Surfaces
sunrayssun
rays solar angle changes throughout the daysolar angle changes throughout the day
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Glass
►Remember, glass is responsible for BOTH:
►Solar gains.
►Transmission gains.
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O Lat 20 Deg N Solar Gain Tthrough Ordinary Glass Btu/hr/Sq ft Sash Area
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Latitude 20 Deg N Solar Gain Through Ordinary Glass
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Solar gain equation
► Solar Gain:
► Area x Solar Heat Gain x Overall Glass Factor =
Btu/Hr
► A x Sg x Sf = q
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Shading Of Glass
VenetianBlinds are popularly used toShade the Space andReduce sungain
VenetianBlinds are popularly used toShade the Space andReduce sungain
Alternate Shading ,Include tinted glass, exteriorFins / Awnings
Alternate Shading ,Include tinted glass, exteriorFins / Awnings
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Shading Factors for Solar Heat Gain Through Glass
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Effect of orientation and time
► Effect of Orientation and time on solar gain.
Glass facing East peaks in July - august at 10 am .
Glass facing the South shows the greatest load at noon, and is lower before and after noon. Also, it is maximum in December.
Glass on the West is the reverse of East. It peaks at 4 pm, and is max,. in July.
Glass on the North and any shaded glass all day gets some solar heat that is reflected by dust. Of course, this is very small as compared to direct sunlight.
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Solar Gain Factor
► Solar gain factor is 1.0 for clear single-paned glass.
► Solar gain may be reduced by using:
► Double paned glass (insulating glass) ► Vacuum or gas – filled, Argon, Krypton.► Heat absorbing glass
(Low e glass), (Low emissivity glass)► Tinted glass
Outside shading devices► Inside shading devices
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Solar Gain
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Solar Gains
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Transmission gain equations
► Area x (Equivalent) Temp. Diff. x U value = Btu/Hr
► A x ETD x U = q (For walls and roofs)
► A x Temp.Diff. x U = q (For other transmission
gains)
► walls, for roofs and correction to ETD
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Equivalent Temperature Walls in Deg F
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Equivalent Temperatures Roof in Deg F
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Corrections to Equivalent Temperature Deg F
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Why ETD value and not Temp.Diff?
►Walls and roofs have capacity to “store” thermal energy.
►Called “thermal storage.”
►Depending on the type of construction, there is usually a time lag of from two to ten hours before this heat reaches the room.
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ETD values depend on:
► latitude (based on 40 deg N) (Approximately correct for 20 deg N and 30 deg N latitude too).
Exposure, N,S,E,W,NE,NW etc.
► Weight of wall or roof, lbs/sft, (10lbs/sft to 80 lbs/sft)
► Wall colour (Correction normally not used, but formula exists in the► Carrier System Design Manual, if required to be used).
4" brick = light construction.6" brick = medium construction8" brick = heavy construction.
4" RCC = medium construction.6" RCC = heavy construction.
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Basis of ETD values
► Outside DB at 95 deg F , and room at 80 deg F.►
Daily range: 20 deg F daily range, and for 40 deg N latitude
► Based on 24 hour operation.
► Dark-coloured walls.
► Refer Correction to ETD for walls and roofs
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Transmission Gain Thru ceilings,
floors, glass, partitions. ► Note carefully, whether the area has a floor below AC or
non AC. Similarly, for the ceiling above. Ground floors could have basements, so floor below would be treated as non-AC.
Use a temperature difference of 5 deg f less than the outside DB temperature.
Sometimes, the floor below or ceiling above may be at different temperature, let's say a lower temperature, such as for Data Centres. Then, that needs to be accounted for. (and don't forget to insulate the slab to prevent sweating)!
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U Values ( 1/R ) Btu/hr/sqft
► 8” brick wall with ½” cement plaster both sides 0.35
► 4” brick wall with ½” cement plaster both sides 0.44
► 4” brick wall, with ½” and 1” expanded polystyrene 0.24
► 6” RCC, with ½” plaster both sides 0.65
► 4” RCC, ½” plaster both sides 0.71
► 4” RCC, ½” plaster both sides, 2” polystyrene 0.12
► ¼” or 6 mm glass 1.13
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Gable Roof
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Transmission Gains – Walls and
Roof
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Typical U values in Btu/hr/sq ft/Deg F
► 8” brick wall with ½” cement plaster both sides 0.35
► 4” brick wall with ½” cement plaster both sides 0.44
► 4” brick wall, with ½” and 1” expanded polystyrene 0.24
► 6” RCC, with ½” plaster both sides 0.65
► 6” RCC, with ½” plaster + 1 in Mosaic Tile 0.40
► 4” RCC, ½” plaster both sides 0.71
► 4” RCC, ½” plaster both sides, 2” thermocole 0.12
► ¼” or 6 mm glass 1.13
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Transmission Gain Thru Glass And
Partition ► Add all the areas for solar glass, and then add any
glass which is exposed to a non-airconditioned area.
For transmission gain thru glass, use the difference between the outside and the inside design conditions.
For transmission gains from partitions, use 5 degree less. Note, sometimes the partition, may be exposed to a hotter area like a kitchen or furnace, in which case, please take care.
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Transmission Gains - Other
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Internal Heat Gains
peoplepeople
equipmentequipment
appliancesappliances
lightslights
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Lights
► The heat given off by lights both incandescent and fluorescent (and CFL), is not affected by the room temperature. It depends only on the electricity consumed.
1 Kw lighting load generates 3410 BTU/Hr.
Ballast loads, copper ballast, electronic ballast.Halogen transformer issues.
Watts per sqft. Office.Showrooms.Jewellery shops.
Gain into return air plenum.
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Lights gain equation
►Lights:
►Area x watts/sqft x constant =Btu/hr
►A x (kW, W) x 3.41/3410 = q
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Return air plenum gain
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Appliances, kW or Watts
► Heat generated by computers.
(Refer the booklet for other appliances).
Usually, as per earlier IBM recommendations, this used to be 150 watts per PC, but would have now increased to 200 watts per PC.
Remember, that for UPS's and Data Centers you need to be very careful in determining the heat generated.
For UPS rooms, take 10% of the UPS rating, if it’s a digital UPS.
For server rooms, there is no set norm, but usually, a 42 U rack would have equipment generating about 4 KW per server rack, right upto 10Kw per blade server rack.
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Electric Motors, H.P.► The heat given off by electric motors, machines and
appliances is also fairly independent of the room temperature. It depends on the actual electricity used. Nameplate ratings may not reflect actual loads.
Motors many times, are over loaded or under loaded. So a usage factor may be used to account for this.
In addition, the heat from the motor going into the room, depends on the location of the motor, whether within the room or outside the room.
1 H.P. = 2545 BTU/Hr.
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Equipment / Appliances gain
equation
► (kW,W,HP) x Diversity Factor x constant = Btu/hr
► kW x D.F. x 3410 = Btu/Hr
► W x D.F. x 3.41 = Btu/Hr
► HP x D.F x 2545 = Btu/Hr
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People
►Heat generated by oxidation, called
metabolic rate.
►Carried by:
►Radiation, convection (skin & breathing)
►Evaporation of moisture from skin
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Heat Gain from People
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People gain equation
►People x Sensible gain/person = Btu/Hr
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Internal Heat
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Bypass factors
Coils will have a small bypass and this will
have to be factored into the heat load calculations
The bypass at the coil, leaves some of the heat , directly entering the
room and will add to the room heat
If the bypass is 10% , 10% of the heat from the outside air will be
added into the room directly , and 90% added to the coil load
The coil which has more rows , will have less
bypass . As the moisture load on the coil
increases ( as the fresh air load increases ) ,
we will require more rows in the coil design
and this will lead to lower bypass factors
The velocity of air through the
coil will also decide the bypass
with lower velocities offering
lower velocities
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Ventilation / Outside Air
( Fresh air ) Load
space
supplyfan
coolingcoil
outdoor air or fresh air
returnair
returnair
supplyair
exhaustair
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Bypass Factor
► Bypass factor calculation:
For 4 row coil = 0.1
6 row = (0.1) ^ 6/4
8 row = (0.1) ^ 8/4
(1-BF) is called Contact Factor.
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Bypassed outside air gain equation
►Outside air cfm x Temp.Diff. x Bypass factor
x 1.08=Btu/Hr
►OA cfm x Temp.Diff x B.F. x 1.08 = Btu/Hr
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Infiltration
Infiltration is the leakage of untreated outdoor air through porous walls, floors, roofs, poorly sealed windows, etc.
Infiltration can add a lot of moisture load into the conditioned space.
Generally, infiltration is caused by wind velocity, or stack effect, or both.
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Infiltration
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Infiltration
►Air Change Method:
►(0.2 to 0.5 air changes per hour.)
►Effective Leakage Area Method.
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Added Load due to
infiltration of Outside air
The air from outside could infiltrate into the conditioned
space through the door cracks and this brings in both
sensible and latent heat into the conditioned space
The amount of air that will leak in will depend on
a. The crack width in the door frame
b. The wind velocity outside
CRACK IN DOOR
Cfm leakage per linear foot length of door
5MPH 10MPH 15MPH
3/16 inch 4.8 10 14
1/8 inch 3 6 9
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Infiltration gain equation
►Infiltration air cfm x Temp.Diff. x 1.08=Btu/Hr
►OA cfm x Temp.Diff x 1.08 = Btu/Hr
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Bypassed outside air and infiltration
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Safeties and Room Sensible
Heat
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2 Sources of Latent Loads
► Moisture entering the space from bypassed outside air and infiltration.
► Moisture through permeation from spaces at a higher vapour pressure.
► Moisture generated within the space from moisture generating objects. These objects usually include:
occupants within the space moisture generated by cooking or warming appliances industrial or production machinery which evaporates water
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Latent Gains
►People
►Outside air
►Infiltration
►Equipment (steam)
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Latent gains equation
► People x Latent gain/person = Btu/Hr
► Outside air cfm x Grains x Bypass factor x 0.68=Btu/Hr
► Infiltration air cfm x Grains x 0.68=Btu/Hr
► Steam lb/Hr x 1080 btu/lb = Btu/Hr
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Room Latent Heat and Room Total Heat
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Outside Air Heat and Safety Margins
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Sensible Heat Factor
►Effective SHF
Effective Sensible heat factor =
►Room sensible heat / Room Total heat
► ESHF = RSH / RTH
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ESHF ( Room sensible heat factor)
ESHF
= Room sensible Heat
Room Sensible + Room Latent heat
193075 Btu/hr
183075+9616 Btu/hr
ESHF =0.88
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Apparatus Dew Point
►Depends on Inside Design Conditions
►Effective Sensible Heat Factor
►(Effective Sensible Heat Factor takes into account the effect of bypassed air).
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O
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SHF, ADP and Dehumidified cfm
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Dehumidified Air
►Temp. Rise = (1-BF) x (RoomDB – ADP)
►Dehumidified air =
►ESHF / (1.08 x Temp. Rise)
Note that the air quantity is inversely
proportional to the temperature rise.
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Dehumidified Rise and CFM
The Dehumidified Rise =
(Room Temperature – ADP)* (1-Bypass factor)
(75-54 )*( 1-0.1)= 21*0.9= 18.5 F
Dehumidified CFM
The Dehumidified CFM = Room sensible heat
Dehumidified rise *1.08
= 193075 = 9674 cfm
18.5*1.08
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Est Zeeshan HEAT LOAD ESTIMATE
Dtd 30-Apr-20 At 4pm
Job HVACR Society
Est SUMMER Peak
Add. Karachi 24.9 Deg N Latitude
Space Office DB WB RH GR/LB
Size 0.0 0.00 5120 SQFT 10.0 FT HT. 51200O.A. 99 74 21 88
Item Area Gain Factor Btu/Hr. Room 75 55 70
SOLAR GAIN - GLASS Diff 24 18
N glass 144 Sqftx 23 x 1.00 3312 VENTILATION
E glass 192 Sqftx 12 x 1.00 2304 5120 Sq ft 0.06cfm/sft 307
S glass 144 Sqftx 12 x 1.00 1728 40 people 5cfm 200
W glass 192 Sqftx 163 x 0.56 17526 0 Doorsx 0cfm 0
SW glass 0 Sqftx 85 x 0.56 0 0 Crack 0
SE glass 0 Sqftx 12 x 0.56 0 0 Exhaust(excess) 0
NW glass 0 Sqftx 138 x 0.56 0 CFM VENTILATION 507
NE glass 0 Sqftx 12 x 0.56 0
HOR glass 0 Sqftx 0 x 0.00 0
GAIN-WALLS/ROOF
N Wall 576 Sqftx 13 x 0.35 2621
E Wall 768 Sqftx 27 x 0.35 7258
S Wall 576 Sqftx 25 x 0.35 5040
W Wall 768 Sqftx 21 x 0.35 5645
SW Wall 0Sqftx 31 x 0.35 0
SE Wall 0 Sqftx 26 x 0.35 0
NW Wall 0 Sqftx 17 x 0.35 0
NE Wall 0 Sqftx 19 x 0.35 0
R-sun 0 Sqftx 0 x 0.35 0
Roof Sun R-sh. 5120 Sqftx 44 x 0.12 27034
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TRANS. GAIN S.H.F.AND ADP
All glass 672 Sqftx 24 x 1.13 18225 193075 RSH
Partition 0 Sqftx 19 x 0.45 0 219723 RTH 0.88 SHF
Ceiling 0 Sqftx 15 x 0.12 0 INDICATED ADP
. 53oF
Floor 5120 Sqftx 24 x 0.40 49152 SELECTED ADP 54oF
INFIL.& OUTSIDE AIR
DEHUMIDIFIED AIR
Inf. cfmx 24 oF x 1.08 0 0.88 21 = 18.5
O.A. 507 24 oFx 0.12 BFx 1.08 1578 193075 RSH
INTERNAL HEAT
1.08 18.5 = 9674 CFM
Occ. 40 People x 245 9800
Equipment Load 200Watts x 20 3.40 13600
Lights 0.30 Watts x 5120 3.4 5238 CHECK FIGURES
Appl. ( PC's) 2 230 3.40 1564 TR
18.31
Sub
total171623 CFM 9674
HEAT FAN SAFETY ADP DEG
F54.0
GAIN% 0.0 HP% 7.5 FACT 5.0 21453 CFMTON 528.3 ROOM SENSIBLE
HEAT193075 CFM/SFT 1.9
Inf. 0 cfmx 18 Gr/lb 1.00 x 0.68 0 400 CFM/TON 24.18 O.A. 507 cfmx 18 Gr/lbx 0.12 BFx 0.68 745 SQ FT /TR 279.6
Occ. 40 People 205 8200
Steam 0.00 lb/hr x 1080 0
Sub
total8945
LEAK 0 SAFET
Y0 0
LOSS% 0.0 FACT
%7.5 671
ROOM LATENT HEAT 9616
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ROOM TOTAL HEAT
202691
S.H. 507 cfmx 24 oFx 0.88 x 1.08 11569
L.H. 507 cfmx 18 G/lb 0.88 x 0.68 5463
OUTSIDE AIR HEAT 17032
Grand Total Heat Sub-Total
219723
HEAT H.P. CHW
GAIN%
0.0 PUMP% 0.0 PPg% 0.0 0
TONS 18.31 GRAND TOTAL HEAT 219723
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PSYCHROMETRY INVOLVED
Mixed air
Coil
ADP
RSH
Coil
BypassRoom
RLH
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Grand Total Heat
GRAND TOTAL HEAT IS
EFFECTIVE SENSIBLE HEAT 193075 Btu/hr +
EFFECTIVE LATENT HEAT 9616 Btu/hr +
OUTSIDE AIR HEAT 17032 Btu/hr +
GRAND TOTAL HEAT = 219723 Btu/hr
= 18.31 TR
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AIRCONDITIONING EQUIPMENT
REQUIREMENT BASED ON HEAT LOAD
GRAND TOTAL HEAT = 18.31 TR
DEHUMIDIFIED CFM THROUGH COIL = 15989 CFM
The equipment selected should have
18.31 TR capacity and 9674 cfm air flow through
the coil
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Check Figures
CHECK FIGURES
TONS 18.31
CFM 9674
ADP 54.0
CFM/TR 528.3
CFM /SQ FT 1.9
400 CFM/TON 24.18
SQ FT/TR 279.6
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Munters Psychro App
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Load Calculation Check
►Did you consider window shading?
►Did you consider zoning?
►Did you consider infiltration?
►Did you consider insulating the roof?
►Did you consider toilet exhaust?
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References► Carrier Handbook of Air conditioning System Design
https://www.scribd.com/doc/142002487/Carrier-Handbook-of-Air-Conditioning-System-Design-Part-1
►ASHRAE Fundamentals
► Consulting Engineer , Rajeev Kakkar
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