basics of psychrometrics practical heat load calculation · combination of air temperature and...

Basics of Psychrometrics

Practical Heat Load Calculation

Webinar

30 April 2020

Vikram Murthy

ASHRAE Mumbai Chapter

Sessions

►Basics of Psychrometrics

►All about Heat

►Practical Heat Load (Cooling Load)

Calculation ( Using the E 20 , CLTD - Cooling Load Temperature Difference Method )

Psychrometric basics

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 .

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 .

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 .

Purpose Of Comfort Airconditioning

►To Cool or Heat

►To Dehumidify or Humidify (remove or add

moisture)

►To remove odours

►To remove particulate & microbial pollutants

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

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.

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

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

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.

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.

an energy or heat graph

►Any point on the psychrometric chart

represents air in a specific condition

containing a certain amount of heat.

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

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

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.

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.

Typical DB

Line

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.

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

Typical

Absolute

Humidity

Line

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.

Wet Bulb Lines

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

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

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.

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"

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

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

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

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.

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

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.

Dew Point

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

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.

Enthalpy

►Wet Bulb Lines as lines of constant heat

content of air.

►Enthalpy is just another term used in place

of "heat content".

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.

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

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

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

Psychrometric Chart SI

Applied Psychrometry

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.

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.

Evaporative Cooling

►Evaporative cooling is the process by which

air is simply subjected to a spray of re-

circulated water.

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

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.

Evaporative cooling

(adiabatic cooling)

Outside Design Conditions

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

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

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.

Heat Transfer

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

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.

Convection heat

►Btu/Hr = cfm x 1.08 x delta t

►Btu/Hr = cfm x 0.68 x delta W.

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

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

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".

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.

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.

Thermal Resistance is ti/k1 + t2/k2 +t3/k3

Calculating U value

►Outside air film

►Transmission thru the material layers

►Air space.

►Inside air film

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

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

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” 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

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)

Heat Load Calculation

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 )

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.

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.

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.

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.

Geometric Zones

Thermal Zones

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.

A Typical Commercial office

Plan of HVACR Office for Calculation of Cooling Load

Summary of Zones and Areas of Walls and Windows

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

The Heat Load Form

Profile

Room Size


► 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.


►The data we use is the ASHRAE Weather

Data.

( ASHRAE Handbook of Fundamentals )

“Comfort" variables

►environmental variables

►air temperature

►relative humidity

►air motion

►mean radiant temperature (av.)

"clo" value, “met” rate,

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%

ASHRAE Comfort Chart

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.

Outside and Inside Conditions

Outside

Condition

Inside

Condition

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

Munters Psychro App

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.

Ventilation / Outside Air

( Fresh air ) Load

space

supplyfan

coolingcoil

outdoor air or fresh air

returnair

returnair

supplyair

exhaustair

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

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

Sources of Cooling Load

Outside Air

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

Sensible Gains

►Solar gains

►Transmission gains

►Lights gains

►Equipment/ appliances gains

►People gains

►Outside air gains

►Infiltration gains

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.

Sunlit Surfaces

sunrayssun

rays solar angle changes throughout the daysolar angle changes throughout the day

Glass

►Remember, glass is responsible for BOTH:

►Solar gains.

►Transmission gains.

O Lat 20 Deg N Solar Gain Tthrough Ordinary Glass Btu/hr/Sq ft Sash Area

Latitude 20 Deg N Solar Gain Through Ordinary Glass

Solar gain equation

► Solar Gain:

► Area x Solar Heat Gain x Overall Glass Factor =

Btu/Hr

► A x Sg x Sf = q

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

Shading Factors for Solar Heat Gain Through Glass

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.

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

Solar Gain

Solar Gains

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

Equivalent Temperature Walls in Deg F

Equivalent Temperatures Roof in Deg F

Corrections to Equivalent Temperature Deg F

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.

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.

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

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)!

U Values ( 1/R ) Btu/hr/sqft






► 4” RCC, ½” plaster both sides, 2” polystyrene 0.12

► ¼” or 6 mm glass 1.13

Gable Roof

Transmission Gains – Walls and

Roof

Typical U values in Btu/hr/sq ft/Deg F





► 6” RCC, with ½” plaster + 1 in Mosaic Tile 0.40


► 4” RCC, ½” plaster both sides, 2” thermocole 0.12

► ¼” or 6 mm glass 1.13

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.

Transmission Gains - Other

Internal Heat Gains

peoplepeople

equipmentequipment

appliancesappliances

lightslights

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.

Lights gain equation

►Lights:

►Area x watts/sqft x constant =Btu/hr

►A x (kW, W) x 3.41/3410 = q

Return air plenum gain

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.

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.

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

People

►Heat generated by oxidation, called

metabolic rate.

►Carried by:

►Radiation, convection (skin & breathing)

►Evaporation of moisture from skin

Heat Gain from People

People gain equation

►People x Sensible gain/person = Btu/Hr

Internal Heat

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

Ventilation / Outside Air

( Fresh air ) Load

space

supplyfan

coolingcoil

outdoor air or fresh air

returnair

returnair

supplyair

exhaustair

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.

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

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.

Infiltration

Infiltration

►Air Change Method:

►(0.2 to 0.5 air changes per hour.)

►Effective Leakage Area Method.

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

Infiltration gain equation

►Infiltration air cfm x Temp.Diff. x 1.08=Btu/Hr

►OA cfm x Temp.Diff x 1.08 = Btu/Hr

Bypassed outside air and infiltration

Safeties and Room Sensible

Heat

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

Latent Gains

►People

►Outside air

►Infiltration

►Equipment (steam)

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

Room Latent Heat and Room Total Heat

Outside Air Heat and Safety Margins

Sensible Heat Factor

►Effective SHF

Effective Sensible heat factor =

►Room sensible heat / Room Total heat

► ESHF = RSH / RTH

ESHF ( Room sensible heat factor)

ESHF

= Room sensible Heat

Room Sensible + Room Latent heat

193075 Btu/hr

183075+9616 Btu/hr

ESHF =0.88

Apparatus Dew Point

►Depends on Inside Design Conditions

►Effective Sensible Heat Factor

►(Effective Sensible Heat Factor takes into account the effect of bypassed air).

SHF, ADP and Dehumidified cfm

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.

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

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

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

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

PSYCHROMETRY INVOLVED

Mixed air

Coil

ADP

RSH

Coil

BypassRoom

RLH

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

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

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

Munters Psychro App

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?

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

Thank you

Vikram Murthy

ASHRAE Mumbai Chapter

[email protected]

basics of psychrometrics practical heat load calculation · combination of air temperature and...

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