tc fundamentals 2011 psychrometrics

7/28/2019 Tc Fundamentals 2011 Psychrometrics

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Scott Laurila ~ Senior Application Engineer

Greenheck Fan Corporation

Schofield, Wisconsin

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gen a

Properties of Air The Ps chrometric

Chart

ApplicationsMixed Air

Cooling

Dehumidification

Energy Recovery

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ourse ec ves

At the completion of this session, you should be able to use aPsychrometric Chart to:

ent y t e propert es o a r at a g ven state con t on

Illustrate the performance characteristics of basic heating

Demonstrate how the use of Energy Recovery devices can

reduce heating and cooling loads in building designs

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syc rometrcs

Psyc rometr cs s t e sc ence ea ng w t t e p ys ca

laws or air water mixtures

Important in the design of HVAC systems as numerous air

properties can be found over a broad range of conditions

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ropertes o

Dry-Bulb Temperature

Wet-Bulb Temperature

Dew-Point Temperature

Relative Humidity

Humidity Ratio

Enthalpy

If any two of the above properties are known, the

properties

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Dry-bulb temperature is the

temperature read from astandard thermometer

heat energy at the given

condition

-

from a thermometer with the

bulb covered by a wet wick

"Sling Psychrometer"

Difference between dry-bulb

-

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dryness of air


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Dew-point temperature is thetemperature at which

condenses on objects

When the dry-bulb, wet-bulb,and dew-point temperatures

are equal, the air issaturated

saturated

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ropertes o

Relative humidity is the measure of how much moisture the airis holding versus how much moisture the air can hold at a

-

Expressed as a percentage

-

moisture the air can hold increases

Relative Humidity = moun o mo s ure e a r s o ngAmount of moisture the aircan hold

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Humidity Ratio is a measure of the weight of water in a givenamount of air

so re erre to as pec c um ty

Can be expressed ingrains

,

Humidity Ratio = oun s o mo s urePounds ofdry air

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e syc rome rc ar

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aturaton urve

Saturation Curve

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ry- u emperature

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e - u emperature

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ew- o n empera ure

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e a ve um y

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H mi it R ti

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ens e an a en nergy

Sensible Energy is the heat that causes changes in the air's drybulb temperature

atent nergy s t e eat assoc ate w t p ase c ange. t s

representative of changes in the air's moisture content with no

change to the dry-bulb temperature

Enthalpy is the total energy in a given amount of air at its

present conditions

Enthalpy (h) = Sensible Energy + Latent Energy

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nt a py

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syc rometrc art econstructe

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Example:

93F DB, 65F WB

Find: n a py

Dew-Point = 50.9F

Relative Humidity = 24.0%

Wet-BulbDew-Point

Hum. Ratio = 66.8 grains/lb dry air

Enthalpy = 32.9 Btu/lb

Humidity Ratio

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Dry-Bulb


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r ens y orrec ons

Do NOT forget about elevation! Above 2,000 feet elevation, the air density is reduced by

approxma e y . per every , ee

A change in air density also changes the physical and-

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syc rometrc rocesses

Air ConditioningMixed Air

Cooling Coils

Reheat

Energy Recovery

Sensible Devices

Total Enthalpy Devices

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Evaporative

Cooling

Steam

Humidification

Example Processes

160

140

80

- -

Latent: Up & down

DesiccantCoolin &

70120

100

DehumidificationDehumidification

40

50

80

60

40Heat

3020

20 30 40 50 60 70 80 90 100 110

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Dry Bulb Temp.


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Use the ASHRAE indoor

n oor es gn on tonses gn con t ons to e p w tprocess calculations

75F, 50% RH

Winter Indoor Design

72F, 35% RH

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xe r on ons Step 1: Identify outdoor condition, indoor condition, and

ventilation rate (% outside air)

Step 2: Draw a straight line between the outdoor and

Step 3: Draw a straight line between points 1 and 2

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xe r xampeOutdoor Air: 95F DB/75F WB

Room Air: 75F DB, 50% RH

Ventilation: 25% Outdoor Air

95F x 0.25 = 23.75F75F x 0.75 = 56.25F

Mixture = 80F

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Condenseroo ng rocess xampe

Hi h Pressure

CompressorExpansion Valve

vapora or

Low Pressure

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oo ng o rocessesCapacity (Btu/hr) = 4.5 x SCFM x h

oo ng on = , tu r

Capacity (Tons) = 4.5 x SCFM x h / 12,000

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Exercise 2: A customer wants to cool 4,000 SCFM of

oo ng xerc se

outside air from 93F DB / 65 WB to 50F saturated air

Find the following parameters for 50F saturated air:

Property Value Units

Relative Humidity 100 %

Dew Point 50 F

What is the chan e in enthal from 93DB/65WB to

Humidity Ratio 64.8 grains / lb dry ai

Enthalpy 22 Btu / lb dry air

50DB/50WB? h = 10.9 Btu / lb dry air

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e erm ne e coo ng onnage requ re n s process.

Capacity = 16.4 Tons


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Introducing cold, saturated air directly into an occupied

u w

space may be too cool for occupant comfort

Reheat can be used to bring the air to a space-neutral

Reheat Methods

Electric Heat

Hot Water Coil

Hot Gas Reheat

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Condenseroo ng rocess xampe

Hi h Pressure

CompressorExpansion Valve

vapora or

Low Pressure

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ot as e ea

ReheatCoil

Air Conditions After Reheat Coil:

65F DBT/56F WBT (50% RH)

Air Conditions After Evaporator:50F DBT / 50F WBT

Evaporator

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Supply Air


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nergy ecovery

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y nergy ecovery

Economic Benefits

Reduced Initial Costs

e uce pera ng os s

ASHRAE Standards & Guidelines

ASHRAE 62.1 2010 Ventilation for Acce table indoorair quality

ASHRAE 90.1 2010 Energy Standard for Buildings

. Performance Green Buildings

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ASHRAE Standard 62Ventilation for Acceptable Indoor Air Quality

minimize the potential for adverse health effects. Typical ventilation rates:

15 CFM per person for Classrooms

17 CFM per person for Offices

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Steven Taylor Taylor Engineering


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. -

Exhaust Air Energy Recovery (6.3.6)

Supply air is greater than 5000 CFM

70% or more of supply is outdoor air (3500 CFM)

Energy recovery system shall have a total effectiveness

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ASHRAE 90.1-2010

Table 6.5.6.1 Energy Recovery Requirement (IP)

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Standard for Design of High-Performance Green Buildingswww.ashrae.org/greenstandard

Table 7.4.3.8 Energy Recovery Requirement (IP)

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nergy ecovery as c erms

(no moisture) Also referred to as heat recover

Total Energy Recovery transfers both sensible and latent

energy

Effectiveness refers to the efficiency of the energy

recovery device (expressed as a percentage)

Total effectiveness

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ens e eat ecovery Sensible Heat Recover

Aluminum fixed plate Run-around coils

Heat-pipe heat

exchangers

ens e w ee s no

desiccant)

Transfers sensible ener

only (no moisture transfer)

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ens e a e

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Sensible Ener recover

180

moves the outdoor air dry

bulb temperature laterally

70

140

120

temperature

50

60

100

80

60

HeatExchangers

30

40 40

20

Room Air

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Dry Bulb Temp.

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ota nergy ecovery Total Ener Recover

Enthalpy wheels Enthalpy plates (or cores)

Transfers sensible energy

through the media

rans ers a en energy

through a desiccant (wheel) or

molecular transfer (core)

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nergy ecovery

47Summer Operation w/ Rotary Wheel


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Total energy recovery

160

140

80

shifts OA point towards

RA point

70120

100Outdoor AirOutdoor Air

-

40

50

80

60

Room Air

30

20

20 30 40 50 60 70 80 90 100 110

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Dry Bulb Temp.


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ar - oa on tons

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E R d C li


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Ener Recover and CoolinPotential

Energy Savings

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Winter O erationWinter O erationSupply Air

Dry Bulb 54F

Humidity 31 grains/lb.

Room Air

(to be exhausted)Dry Bulb 72F

Humidity 41

Outdoor Air

Dry Bulb 0F

Humidity 2

grains/lb.

.

Exhaust Air

Dry Bulb 18F

Humidity 12

grains/lb.

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Wint r r ti nSensible vs. Total ERV

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Possibility of condensation forming whenever a warm, moist

air stream comes in contact with a cold surface

Always consider frost control methods in when winter design

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Winter Exhaust Air ProcessSensible vs. Total ERV

Total

Sensible

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Question?

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evew

Psychrometric Charts are useful tool forunderstandin HVAC rocesses

Easily find properties of air across numerous

conditionsAllows us to plot, predict, and calculate the

heating and cooling capacities

An understanding of psychrometrics is thefoundation of energy recovery

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THANK YOU!

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tc fundamentals 2011 psychrometrics

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