1
ME 410 MECHANICAL ENGINEERING SYSTEMS LABORATORY
MASS & ENERGY BALANCES IN
PSYCHROMETRIC PROCESSES
EXPERIMENT 3
1. OBJECTIVE
The objective of this experiment is to observe four basic psychrometric processes which
are heating, cooling, humidification and dehumidification in an air conditioning unit. The air
velocity, dry and wet bulb temperatures and the amount of water added/removed will be
measured to check the mass and energy balances of these processes.
2. INTRODUCTION
The function of an air conditioning equipment is to change the state of the entering air to a
desired state by controlling temperature and humidity of the specified space.
Air conditioning applications are divided into two types according to their purposes:
i) Comfort air conditioning, ii) Industrial air conditioning. The primary function of air
conditioning is to modify the state of the air for human comfort. The industrial air
conditioning meets the temperature and humidity requirements of an industrial or scientific
process.
In comfort air conditioning, it is necessary to control simultaneously the temperature,
relative humidity, cleanliness and distribution of air to meet the comfort requirements of the
occupants.
According to the comfort chart given by the American Society of Heating, Refrigeration
and Air-conditioning Engineers (ASHRAE), comfort conditions can be obtained at 20-23°C
dry bulb temperature (DBT) and (50 ± 20)% relative humidity in winter, and 24-27°C DBT
and (50 ± 20)% relative humidity in summer. In order to maintain these requirements, the
state of the air is modified in an air conditioning apparatus such that the varying summer and
winter loads are balanced.
3. THEORY
In air conditioning, the moist air (or simply the air) is taken as a mixture of dry air (a)
and water vapor (w) carried with it. At a given total air pressure and temperature, the amount
of water vapor that may be contained in the air is limited. The mixture existing at this limit is
called saturated air. Any excess water vapor in the air separates itself from the mixture as a
liquid (condensate) or solid (ice).
2
The dry bulb temperature (Tdb) is the familiar temperature that can be measured by a
thermometer or a thermocouple. On the other hand, the wet bulb temperature, Twb, is related
to the humidity level. The humidity of moist air may be stated in terms of either relative
humidity, Φ or humidity ratio, ω.
The psychrometric charts are diagrams giving the relationship between Tdb, Twb, Φ, ω
and h (enthalpy) by assuming an ambient pressure. For example, ASHRAE psychrometric
chart no. 5 is for 750 m. elevation (92.634 kPa barometric pressure) which may be used for
Ankara (see Fig. 7). Many psychrometric processes may be represented on these charts by
straight lines.
Wet bulb temperature (Twb) is the temperature measured when the bulb of a
thermometer or the junction of a thermocouple is wetted. For unsaturated moist air, it is less
than the dry bulb temperature; the difference being proportional to the relative humidity. In
practice Twb is assumed to be equal to the adiabatic saturation temperature, T*, which would
be reached if moisture is added in an adiabatic process until the air becomes saturated. Thus,
Twb ~ T*.
Relative humidity (Φ) and humidity ratio (ω) are defined as,
/ (1)
where Pw is the partial pressure of water vapor in air and Pws is the saturation pressure
of water at air temperature T. Relative humidity is a dimensionless quantity usually expressed
as percentage. The humidity ratio (also called specific humidity), ω, is defined as
/ (2)
where mw is the mass of water vapor in moist air and ma is the mass of the dry air.
Using the ideal gas relationship for dry air and water vapor, humidity ratio becomes
. . . (3)
The humidity ratio of air at a given P and T may be calculated from the above
relationships when T* is known:
(4)
where
. ,
T,
hf*
hg
hfg
Pw
cp
Noth En
Thbe
Se
Th
one wh
removed
wet bul
change
there is
or humi
H
Th
increase
dry bulb
T* are the d* is the spec
g is the speci
g* = (hg - hf)
ws* is the sat
a is the cons
ote that ‘*’ e wet bulb)
nthalpy (h)
he enthalpye calculated
ensible Hea
he sensible
here only
d from the
lb temperat
as a resul
no change
idity ratio o
umidificati
he process
es the humid
b temperatu
dry and wet
cific enthalp
ific enthalp
) at T* (kJ/k
turation pre
stant pressu
indicates p temperatur
)
y of the moias:
1.00
ating or Co
heat transf
energy i
moist air.
tures, relat
lt of heat
e in water v
f the air (se
ion or Dehu
of adding w
dity ratio, r
ure may slig
t bulb tempe
py of liquid
y of water v
kgw)
essure of wa
ure specific
properties wre T*.
ist air at any
35 1.872
ooling (Qs)
fer process
is added
The dry an
tive humidi
transfer, b
vapor conte
ee Fig. 1).
umidificati
water vapor
elative hum
ghtly chang
3
eratures (°C
water at T*
vapor at T (
ater evaluate
heat of dry
which are ev
y state can
@
23 2
is
or
nd
ity
but
ent
F
ion
r to the air
midity, wet b
ge or remain
C), respectiv
(kJ/kgw)
kJ/kgw)
ed at T* (kPa
air (1.0035
valuated at t
be read fro
501.4
Fig. 1 Sensib
is called h
bulb temper
ns unchange
vely,
a)
kJ/kga).
the adiabati
m psychrom
ble heating
humidificati
rature and th
ed. The reve
ic saturation
metric chart
(5)
and cooling
ion. Humid
he enthalpy
erse proces
n (that is
ts or can
g
dification
y, but the
s, which
decrease
moistur
cooling
refrigera
Co
Th
conditio
1-
1-
1-
1-
1-
In
Fig. 2 H
M
At
can be
balance
Fig. 4.
Th
es the humi
re at consta
the moist
ation.
ombined H
he followin
oning:
-6: Heating
-7: Heating
-8: Cooling
-9: Cooling
-9’: Cooling
n Fig. 3, pro
Humidificatio
Mass and En
t steady sta
obtained fr
s for a gen
he continuit
idity ratio i
ant temperat
air below
Heating and
ng combined
and humidi
and dehumi
and humidi
and dehum
g and dehum
cess 1-9 is a
n and dehumi
nergy Balan
ate, the follo
from the m
neral proce
ty equation
is called deh
ture by a d
its dew p
d Humidific
d sensible a
ification (co
idification (
ification (as
idification (
midification
actual wher
idification con
nces
owing relat
ass and en
ss as show
for dry air i
4
humidificat
desiccant (a
point tempe
cation, or C
and latent p
ommon in w
(with a desi
s in air wash
(common in
(theoretica
reas process
ncepts
tions
nergy
wn in
is given by
tion. It may
a drying age
erature as i
Cooling and
process, sho
winter)
ccant)
hers)
n summer)
l)
s 1-9’ is theo
Fig. 4 G
y be achieve
ent) as show
illustrated i
d Dehumid
own in Fig.
oretical (ide
Fig. 3 Combin
General cont
ed by absor
wn in Fig.
in Fig. 3 b
dification
3 may occ
eal).
ned processes
trol volume
rbing the
2 or by
by using
cur in air
s
an
Th
wh
m&
wh
Q&
No
Th
P
Re
Co
mechan
(or capi
In
which a
as well
nd that for th
he first law
here
zero dcondewater
wm⎧⎪= ⎨⎪⎩
&
g
f
h at w
h at Tw
⎧⎪= ⎨⎪⎩
Q = Rate of
ote that wat
he percentag
Percentage E
efrigeration
ooling the m
nical refriger
illary tube fo
n the labora
also runs the
as P-h and
he water vap
of thermod
during sensiensate remor vapor injec
2
water vapor
T for dehum
heat transfe
ter boils at a
ge error bet
TheError =
n Cycle
moist air w
ration cycle
for small sys
atory unit, th
e fan of the
T-s diagram
por is
dynamics yie
ible heatingoved during cted during
temperature
midification
er, (+) for h
about 96oC
tween the m
eoretical VaTheor
with or with
e which inc
stems) and a
Fig.
he compres
air cooled c
ms of a typi
5
elds
g or cooling dehumidifihumidificat
oe (96 C) fo
n
eating, (-) f
in Ankara.
measured and
alue Measuretical Valu
−
out dehumi
ludes a com
an evaporat
5 Refrigera
ssor is recip
condenser.
ical cycle. I
.
cation (-)tion (+)
or humidific
for cooling
d theoretica
ured Valueue
⋅
idification i
mpressor, a
tor.
ation cycle
procating ty
Figure 5 sh
In reality, th
cation
al values can
100%⋅
is usually a
condenser,
ype run by
hows the equ
he compress
(6
(7
(8
n be found b
(9
achieved by
an expansio
an electrica
uipment sch
sion process
6)
7)
)
by
9)
y using a
on valve
al motor
hematics
s will be
6
irreversible and there will be pressure losses through the evaporator, the condenser and the
connecting pipes. The isentropic efficiency of the compressor is defined as:
(10)
The parameters that are important include the compressor discharge temperature (T2),
cooling capacity, power input and coefficient of performance of the cycle which may be
defined as :
(11)
Because of the irreversibility of the expansion valve and also the other parts, the COP
becomes less than the ideal value of a reversible (Carnot) cycle,
(12)
Fig. 8 is the P-h diagram for the refrigerant, R-12.
4. Experimental Setup
The schematic layout of the set-up is shown in Fig. 6. The main parts of the set-up are as
follows:
i. Preheaters : Three electrical heaters to heat the air entering
ii. Boiler : To supply steam for humidifier. It is composed of a stainless steel container and three electrical heaters, which are dipped into the water
iii. Cooling Coil : To cool the air with or without dehumidification
iv. Rotating vane anemometer : To measure air flow rate in feet per minute
v. Reheaters : Two electrical heaters after the cooling coil which reheats the cooled air before delivery to the space, if required
vi. Compressor-Condenser unit : To complete the refrigeration cycle
vii. Fan : For air circulation
viii. Thermocouples and thermometers : For measuring dry and wet bulb temperatures
7
PROCEDURE :
Before the Experiment:
Check all the thermocouples and thermometers, they should show the same dry bulb and
wet bulb temperatures at all locations.
Start the boiler and wait until the thermometer shows 96°C. Then turn OFF the power to
the boiler, to be restarted for humidification.
During the Experiment:
Turn the fan ON and note down the air flow. Use heating, cooling, humidification and
dehumidification as required. Make the necessary measurements and note them down on the
enclosed Data Sheet. At least 10-15 minutes should pass to reach a steady state after any
modification on the operation is made. Measurement steps during the experiment:
• Start your alarm clock to measure the condensed water and water level change in the
boiler (5 min).
• During this 5 min. duration,
read the wet bulb and dry bulb temperature values for each state,
read the temperature values related with the refrigeration cycle,
read the pressure values related with the refrigeration cycle.
• After 5 min measure the amount of the collected condensed water and boiler level.
• Measure the air velocity.
After the Experiment:
(1) Plot the process lines on psychrometric chart.
(2) Estimate the Twb at section 4, based on state 5 and the processes between states 4 and 5 (Hint: use the psychrometric chart).
(3) Find h, Φ and ω from chart and from equations (1) to (5). Compare the results.
(4) Make necessary calculations for am& , wm& and Q& at each section. Compare the theoretical energy and mass changes with measured ones.
(5) Draw the refrigeration (R-12) cycle on the P-h diagram provided (Fig. 8) and estimate power input to the compressor, ( , refrigerant flow rate ( rm& ), isentropic efficiency ( ) and the COP.
8
RESULTS AND DISCUSSIONS
Questions for Further Discussion:
i. Why using the sea level psychrometric chart for Ankara is incorrect? Estimate
the error in humidity ratio and enthalpy at some selected moist air states.
ii. Estimate the heat lost or gained from the duct surfaces. Will the omission of this
cause significant errors in the energy balances? (Usur = 1.7 W/(m2.°C)) (ONLY
FOR THE LONG REPORT)
Section Preheater Evaporator Reheater Total
Lateral Area (m2) 0.72 0.6 2.28 6
iii. Comment on taking electrical heaters consumption as constant. Estimate the
variation in electrical energy supplied to these heaters if the resistance is known
within ± 20%, and voltage varies within ± 5%. (ONLY FOR LONG REPORT)
9
Fig. 6 Schematic drawing of the experimental set-up
Fan
Mixer
Compressor condenser unit Liquid
Receiver
Feed Water
Condensate
Inlet
Evaporator Mixer
Rotating vane anemometer
Boiler
Reheaters (3.6 kW)
Steam Injection
PreHeaters (2.88 kW)
Drier
Discharge
T.E.V
3 4
5
2
Mixer
d
1
1.44 kW 2.5 kW 1.44 kW
10
Date:
AIR FLOW (ft/min)92 kPa
mV (11th T.C)
Preheaters: 0.72 kW each (x3) Duct Area: 0.0875 m²Reheaters: 0.72 kW each (x2) Boiler Cross Section: 0.3 m x 0.4 m
TC No. (mV) TC No. (mV) TC No. (mV) TC No. (mV)1 & 2 1 2 9 14
3 3 4 10 155 7 8 12 16
13 18Tavg
kWkW
minmmml
1 Psi =6.895 kPa1 ft/min T (˚C) 23.46xT(mV) + 2.35
0.00508 m/s
ME 410 EXPERIMENT 3Mass & Energy Balances in Psychrometric Processes
DATA SHEET
Conversion Factors:
Lab Group :
PressureTemperature
AMBIENT
TC Readings at Section 4 (Dry-bulb)SectionTemperature
Dry Bulb Wet Bulb
ENERGY VALUESEnergy Input at PreheatersEnergy Input at Reheaters
REFRIGERATION CYCLEHigh side pressure of compressorLow side pressure of compressorTemperature at condenser inletTemperature at evaporator outlet
Psi (Red Gage)Psi (Blue Gage)
mV (20th TC)mV (21st TC)
Amount of condensate
WATER MEASUREMENTSMeasurement TimeChange in boiler level
11
ME 410 EXPERIMENT 3 OUTLINE FOR RESULTS
Table-1 Enthalpy (h), humidity ratio (ω) and relative humidity (Φ) values for each section
Section
Tdb Twb From Chart From Equations Deviations ( % )
(oC ) (oC ) h (kJ/kg) ω (gr/kg) Φ (%) h (kJ/kg) ω (gr/kg) Φ (%) h ω Φ
1 & 2 3 4
5
Table-2 Results of energy and mass balance calculations
States Process Measured Values Theoretical Values % Deviations
mQ& (kW) wm& (kg/s) mQ& (kW) wm& (kg/s) mQ& wm&
2 & 3 Preheating+ Humidification
3 & 4 Cooling+ Dehumidification
4 & 5 Reheating
: ……………………….. kg/s
: ……………………….. kg/s
: ……………………….. kW
: ……………………….. COP : ………………………..
12
Fig. 7 ASHRAE Psychrometric Chart
13
Fig. 8 P-H Chart
Top Related