Psych Rome Trics

MCAST BTEC National Diploma in Building Services Engineering

Building Services Science

Psychrometrics

Joseph Gatt

Psychrometrics April 2010

Joseph Gatt of 16

ContentsContentsContentsContents

Task 1 - P.33.6 – Psychrometric Properties ....................................................... 3

Introduction ............................................................................................... 3

Processes .................................................................................................. 4

Air Mixing .................................................................................................. 6

Plant Sizing ................................................................................................ 8

Condensation ............................................................................................. 9

Task 2 – M.33.5 – Calculations ....................................................................... 11

A ............................................................................................................. 11

B ............................................................................................................. 12

Task 3 – D.33.5 – Calculations ....................................................................... 14

A ............................................................................................................. 14

B ............................................................................................................. 15

References................................................................................................... 16


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Task 1 Task 1 Task 1 Task 1 ---- P.33.6P.33.6P.33.6P.33.6 –––– Psychrometric PropertiesPsychrometric PropertiesPsychrometric PropertiesPsychrometric Properties

Define and describe the important psychrometric properties of air and

water vapour mixtures. Show values, from tables of psychrometric

properties and accurate values for a range of specified conditions when

given two know conditions. You should also recognise and explain the

psychrometric charts and graphical representation of psychrometric tables.

You must also identify and explain the various values that can be obtained

from a psychrometric chart.

IntroductionIntroductionIntroductionIntroduction

The characteristics of mixed air and water vapour, which is indeed the science of

moist air conditions, represent the psychrometry. For simplified presentation of

data, graphical psychrometric details are also available. A set of graphs which are

combined so that they plot the relationships between the different variables used

to specify humidity represent also the psychrometric chart.

Environmental Science in Building; Page 85


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The following measurements can be determined from the above psychrometric

chart.

• Dry bulb temperature

• Wet bulb temperature

• Dew point temperature

• Relative humidity

• Moisture content

• Vapour pressure

It should be noted that the saturation curve represents 100% relative humidity.

Moreover, the DB, WB, and DP temperatures all have the same value at this

saturated condition. A psychrometric chart is strictly valid or one value of

atmospheric pressure, which is the sea level pressure of 101.3kPa or 1 bar. This

pressure is however the common standard. Sensible and latent heat contents,

which are of particular use to building services engineers, can be of additional

information and found on some versions of the psychrometric charts.

ProcessesProcessesProcessesProcesses

Two properties of the air must be known in order to locate a representative air

condition on the psychrometric chart. The dry and wet bulb temperatures are

however the easiest coordinates to obtain. These two temperatures can be

measured by means of a sling psychrometer, also known as a whirling or sling

hygrometer. For instance, if the meter indicates 24°C DB, and 17°C WB, the

following air properties can be determined from the chart.

AIR CONDITION PROPERTY VALUE

DB Temperature 24°C

WB Temperature 17°C

Relative Humidity 50%

DP Temperature 13°C

Specific Enthalpy 48kJ/kg

Specific Volume 0.86m3/kg

Moisture Content 0.0096kg/kg

Vapour Pressure 1mB


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Air condition is based on heating, cooling, humidification and dehumidification. By

means of lines drawn on the psychrometric chart, these processes can be

represented.

• A horizontal line drawn from left to right represents the sensible heating

(DB temperature) with no change in moisture content, but a reduction in

relative humidity.

• A vertical line drawn from bottom to top represents the latent heating,

which is also the effect of steam humidification. This process keeps the DB

temperature constant, but increases the moisture content and the relative

humidity.

• A horizontal line drawn from right to left represents the sensible cooling.

This process indicated a decrease in DB temperature, constant moisture

content, and an increase in relative humidity. An incline following the WB

temperature line represents cooling by water spray humidifier, which is also

known as the adiabatic humidification. This process also indicates an

increase in relative humidity.

• A vertical line drawn from top to bottom represents the dehumidification,

where the moisture content and relative humidity decreases.

Building Services Handbook; Page 250


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The relative humidity may be reduced to an unacceptable level by sensible heating,

i.e. <30%. On the other hand, the relative humidity may be increased to an

unacceptable level, i.e. >70%.


Air MixingAir MixingAir MixingAir Mixing

Mixing of two airstreams frequently occurs when combining fresh air with re-

circulated air from within a building. By drawing a straight line between the two

conditions and calculating a point relative to the proportions of mass flow rate, the

process can be represented.


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Plant SizingPlant SizingPlant SizingPlant Sizing



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CondensationCondensationCondensationCondensation

By providing a balance between heating, ventilation, and insulation, the internal

surface condensation can be minimised. A situation where the internal surfaces are

colder than the adjacent air temperatures can be produced by inadequate,

intermitted or partial heating. From the moisture in the warmer air, this will attract

dampness to the surfaces. A high level of humidity will also be present by a low

rate of ventilation. External and internal environment conditions can be plotted on

a psychrometric chart to predict the risk of surface condensation, as illustrated

below.


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Task 2 Task 2 Task 2 Task 2 –––– M.33.5 M.33.5 M.33.5 M.33.5 –––– CalculationsCalculationsCalculationsCalculations

AAAA

A body of fresh air of DB temperature 30°C and WB temperature 27°C is

mixed with return air of DB temperature 21°C and WB temperature 16°C.

Twice the mass of return air is used as fresh air. Find the DB and WB

temperatures of the mixture, together with the relative humidity, the

moisture content and specific enthalpy.

�� 2 � 21� � �1 � 30�

3� 24°�

From the chart

Reference Value

DB Temperature 24°C

WB Temperature 20.3°C

Relative Humidity RH 70.2%

Moisture Content 0.0134kg/kg

Specific Enthalpy 58.5kJ/kg


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BBBB

A body of air is at a temperature of 24°C and RH of 50%. If the moisture

content is increased from 0.0086 – 0.0124kg/kg;

i. Find the increase in WB and DP temperatures and specific enthalpy

At 24°C DB 50% RH

WB = 16.3°C

DP = 12°C

Specific H = 46kJ/kg

At 0.0124kg/kg of moisture content

DB = 24°C

WB = 19.1°C

DP = 17.3°C

Specific H = 55kJ/kg

Hence the increase in

WB = 19.1 – 16.3 = 2.8°C

DP = 17.3 – 12 = 5.3°C

Specific H = 55 – 46 = 9kJ/kg

ii. Determine the change in RH

RH at 0.0086kg/kg of moist air = 50%

RH at 0.0124kg/kg of moist air = 70%

Hence 70 – 50 = 20% change in RH

iii. If 300m3 of air passes through the system every minute and the

density of air is 1.3kg/m3, calculate the mass of moisture added per

minute

Mass of air passing per minute

200m3 x 1.3kg/m3 = 260kg


Joseph Gatt of 16

Moisture added

0.0124 – 0.0086 = 0.0038kg/kg

Mass of moisture added per minute

260kg x 0.0038kg/kg = 0.988kg/minute


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Task 3 Task 3 Task 3 Task 3 –––– D.33.5 D.33.5 D.33.5 D.33.5 –––– CalculationsCalculationsCalculationsCalculations

AAAA

An air conditioning plant is used to cool supply air at 30°C DB and 22°C WB

to 20°C DB and 14°C WB, in factory of 1500m3 volume requiring 6 air

changes per hour. Find the chiller and reheater rating.

��

��6 � 1500

3600� 2.5��/�

Convert m3/s to kg/s by establishing the specific volume

At 30°C DB 22°C WB = 0.88m3/kg (chiller)

At 10°C DB 10°C WB = 0.81m3/kg (reheater)

2.5

0.88�2.8 !

� �"#�$$��

2.5

0.81�3.1 !

� ��#�%��

Enthalpy values for chilling and reheating

65 – 52 = 13kJ/kg (chilling)

39 – 29 = 10kJ/kg (reheating)

Chiller rating

2.8kg/s x 13kJ/kg = 36.4kW

Reheater rating

3.1kg/s x 10kJ/kg = 31kW


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BBBB

Supply air at 5°C DB and 1°C WB is required at the delivery condition of

20°C DB and 14°C WB in the same factory. By plotting the preheat and

adiabatic saturation process on the chart find the preheater rating.

Convert m3/s to kg/s

At 5°C DB 1°C WB = 0.79m3/kg

Using same Q of question A (2.5m3/s)

2.5

0.79� 3.2 !/�

Enthalpy values

28.5 – 11.5 = 17kJ/kg

Preheater rating

3.2kg/s x 17kJ/kg = 54.4kW


Joseph Gatt of 16

ReferencesReferencesReferencesReferences

• Building Services Handbook; 5th Edition; Hall & Greeno

• Environmental Science in Building; 6th Edition; Randall McMullan

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