psych rome trics
DESCRIPTION
hvacTRANSCRIPT
MCAST BTEC National Diploma in Building Services Engineering
Building Services Science
Psychrometrics
Joseph Gatt
Psychrometrics April 2010
Joseph Gatt Page 2 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
Psychrometrics April 2010
Joseph Gatt Page 3 of 16
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
Psychrometrics April 2010
Joseph Gatt Page 4 of 16
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
Psychrometrics April 2010
Joseph Gatt Page 5 of 16
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
Psychrometrics April 2010
Joseph Gatt Page 6 of 16
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%.
Building Services Handbook; Page 251
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.
Psychrometrics April 2010
Joseph Gatt Page 8 of 16
Plant SizingPlant SizingPlant SizingPlant Sizing
Building Services Handbook; Page 253
Psychrometrics April 2010
Joseph Gatt Page 9 of 16
Building Services Handbook; Page 254
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.
Psychrometrics April 2010
Joseph Gatt Page 11 of 16
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
Psychrometrics April 2010
Joseph Gatt Page 12 of 16
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
Psychrometrics April 2010
Joseph Gatt Page 13 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
Psychrometrics April 2010
Joseph Gatt Page 14 of 16
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
Psychrometrics April 2010
Joseph Gatt Page 15 of 16
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