ME 410 MECHANICAL ENGINEERING SYSTEMS LABORATORY
MASS & ENERGY BALANCES IN
PSYCHROMETRIC PROCESSES
EXPERIMENT 3
1. OBJECTIVE
The object 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 water added/removed will be measured to check
the mass and energy balances of these processes.
2. INTRODUCTION
The purpose of 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 the 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 as given by ASHRAE (American Society of Heating,
Refrigeration and Air-conditioning Engineers), comfort condition can be obtained at 20-23 °
C DBT (dry bulb temperature) and (50± 20) % relative humidity in winter, and 24-27 ° C
DBT and (50± 20) % relative humidity in summer. In order to maintain these levels, the state
of air is modified at an air conditioning apparatus such that 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 in the air is limited. The mixture existing at this limit is called
saturated air. If there is any excess water in the air, it separates itself from the mixture as a
liquid (condensate) or solid (ice).
1
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 humidity level. The humidity of moist air may be stated in either relative humidity, f or
humidity ratio, w.
The psychrometric charts are diagrams giving the relationship between T db, Twb, f , w
and h by assuming an ambient pressure. For example, ASHRAE chart no. 5 is for 750 m.
elevation (92.634 kPa barometric pressure) which may be used for Ankara (see Fig. 6). 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 (w) is defined as,
(1)
where Pw = Partial pressure of water vapor in air
Pws= Saturation pressure of water at air temperature T
This is a dimensionless quantity usually expressed as percentage. The humidity ratio
(also called specific humidity), w, is defined as
(2)
where mw = mass of water vapor in moist air
ma = mass of dry air
Using the ideal gas relationship for dry air and water vapor gives
(3)
The humidity ratio of air at a given P and T may be calculated from these relationships
when T* is known:
(4)
where
2
and T, T* = Dry and wet bulb temperatures (° C),
hf* = Specific enthalpy of liquid water at T* (kJ/kgw)
hg = specific enthalpy of water vapor at T (kJ/kgw)
hfg* = (hg-hf) at T* (kJ/kgw)
Pws* = saturation pressure of water evaluated at T* (kPa)
cpa = Constant pressure specific heat of dry air (1.0035 kJ/kga)
Note that “* ” indicates properties which are evaluated at the adiabatic saturation (that is
wet bulb) temperature T*.
Enthalpy(h)
The enthalpy of the moist air at any state can be read from psychrometric charts or can
be calculated as :
(5)
Sensible Heating or Cooling (Qs)
The sensible heat transfer process is one
where only energy is added or removed from
the moist air. The dry and wet bulb
temperatures, relative humidity change as a
result of heat transfer, but there is no change in
water vapor content or humidity ratio of the
air.(See fig.1.)
Fig. 1 Sensible Heating and Cooling
Humidification or Dehumidification
The process of adding water vapor to the air is called humidification. Humidification
increases the humidity ratio, relative humidity, wet bulb temperature and the enthalpy, but the
dry bulb temperature may slightly change or remains unchanged. The reverse process, which
decreases the humidity ratio is called dehumidification. It may be achieved by absorbing the
moisture at constant temperature by a descicant (see fig.2) or by cooling the moist air below
its dew point temperature (see fig.3.) by using refrigeration.
3
Combined Heating and Humidification, or Cooling and Dehumidification
The following combined sensible and latent process, shown in figure 3 may occur in air
conditioning :
1-6 : Heating and humidification ( common in winter)
1-7 : Heating and dehumidification (with a descicant)
1-8 : Cooling and humidification (as in air washers)
1-9 : Cooling and dehumidification (common in summer)
1-9’ : Cooling and dehumidification (theoretical)
In figure 3, process 1-9 is actual whereas process 1-9’ is theoretical.
Fig. 2 Humidification and dehumidification
Fig. 3 Combined Processes
concepts
Mass and Energy Balances
At steady state, the following
relations can be obtained from the mass
and energy balances for a general
process as shown in figure 4.
Fig. 4 General Control Volume
4
The continuity equation gives :
Continuity for dry air
(6)
Continuity for water vapor
(7)
The first law of thermodynamics gives
(8)
where
zero during sensible heating or cooling

mw  condensate removed during dehumidification (-)
 water vapor injected during humidification (+)

h g at water vapor temperature (96 o C) for humidification
hw  
h f at T2 for dehumidification
Q = Rate of heat transfer, (+) for heating, (-) for cooling
Note that water boils at about 96 oC at Ankara.
The percentage error between the measured and theoretical values can be found by:
Percentage Error 
Theoretical Value  Measured Value
100%
Theoretical Value
(9)
Refrigeration Cycle
Cooling the moist air with or without dehumidification is usually achieved by using a
mechanical refrigeration cycle which includes a compressor, a condenser, an expansion
valve(or capillary tube for small systems) and an evaporator.
Fig. 5 Refrigeration Cycle
In the laboratory unit, the compressor is reciprocating type run by an electrical motor
which also runs the fan of the air cooled condenser. Figure 5 is the P-h chart for the
5
refrigerant, R-12. Figure shows the equipment schematics as well as P-h and T-s diagrams of
a typical cycle. In reality, the compression process will be 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 other parts, this COP
becomes less than the ideal value of a reversible (Carnot) cycle,
(12)
4. Experimental Setup
The schematic layout of the set-up is shown in figure 7. Main parts of the setup 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
6
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 psychrometric chart).
(3) Find h, f and w from chart and from equations (1) to (5). Compare the results.
(4) Make necessary calculations for ma , mw 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 and estimate power
input to the compressor, (Wc) refrigerant flow rate ( mr ), isentropic efficiency(h c) and
COP.
7
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 emission of this
cause significant errors in energy balances? (Usur = 1.7 W/m2 ° C) (ONLY FOR
LONG REPORT)
iii.
Section
Preheater
Evaporator
Reheater
Total
Lateral Area (m2)
0.72
0.6
2.28
6
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)
8
5
1
d
Discharge
Inlet
Rotating vane anemometer
Mixer
4
Evaporator
Mixer
3
Mixer
Steam
Injection
2
Fan
Feed Water
Reheaters
(3.6 kW)
T.E.V
.
PreHeaters
(2.88 kW)
Drier
Boiler
Condensate
Compressor
condenser unit
Liquid
Receiver
1.44 kW
SCHEMATIC DRAWING OF EXPERIMENTAL SETUP
9
2.5 kW
1.44 kW
ME 410 EXPERIMENT 3
Mass & Energy Balances in Psychrometric Processes
DATA SHEET
Lab Group :
AMBIENT
Pressure
92
Temperature
Preheaters:
Reheaters:
Section
1&2
3
5
Date:
AIR FLOW (ft/min)
kPa
mV
(11th T.C)
0.72 kW each (x3)
0.72 kW each (x2)
Duct Area:
Boiler Cross Section:
Temperature
Dry Bulb
Wet Bulb
TC No.
(mV)
TC No.
(mV)
1
2
3
4
7
8
0.0875 m²
0.3 m x 0.4 m
TC Readings at Section 4 (Dry-bulb)
TC No.
9
10
12
13
(mV)
TC No.
14
15
16
18
Tavg
(mV)
ENERGY VALUES
Energy Input at Preheaters
Energy Input at Reheaters
kW
kW
REFRIGERATION CYCLE
High side pressure of compressor
Low side pressure of compressor
Temperature at condenser inlet
Temperature at evaporator outlet
Psi (Red Gage)
Psi (Blue Gage)
mV (20th TC)
mV (21st TC)
WATER MEASUREMNTS
Measurement Time
Change in boiler level
Amount of condensate
min
mm
ml
Conversion Factors:
1 Psi =6.895 kPa
1 ft/min =0.00508 m/s
23.46xT(mV) + 2.35
T (˚C) =
10
ME 410 EXPERIMENT 3
OUTLINE FOR RESULTS
Table-1 Enthalpy (h), humidity ratio (w) and relative humidity () values for each section
Tdb
Twb
( oC )
( oC )
From Chart
Deviations ( %
)
From Equations
Section
h (kJ/kg)
w (gr/kg)
 (%)
h (kJ/kg)
w (gr/kg)
 (%)
h

w
1&2
3
4
5
Table-2 Results of energy and mass balance calculations
Measured Values
States
Process
Qm (kW)
2&3
Preheating+
Humidification
3&4
Cooling+
Dehumidification
4&5
Reheating
Theoretical Values
mw (kg/s)
mair
:
………………………..
kg/s
mref
:
………………………..
kg/s
Wc
:
………………………..
kW

:
………………………..
COP
:
………………………..
11
Qm (kW)
mw (kg/s)
% Deviations
Qm
mw