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 20-23C

dry bulb temperature (DBT) and (50 20)% relative humidity in winter, and 24-27C 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).

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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, or humidity ratio, .

The psychrometric charts are diagrams giving the relationship between Tdb, Twb, ,

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 () and humidity ratio () 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), , 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*

hghfgPwcp 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

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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

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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 96C. 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, and 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.

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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)

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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

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Date:

AIR FLOW (ft/min)92 kPa

mV (11th T.C)

Preheaters: 0.72 kW each (x3) Duct Area: 0.0875 mReheaters: 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

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ME 410 EXPERIMENT 3 OUTLINE FOR RESULTS

Table-1 Enthalpy (h), humidity ratio () and relative humidity () values for each section

Section

Tdb Twb From Chart From Equations Deviations ( % )

(oC ) (oC ) h (kJ/kg) (gr/kg) (%) h (kJ/kg) (gr/kg) (%) h

1 & 2 3 4

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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 : ..

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Fig. 7 ASHRAE Psychrometric Chart

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Fig. 8 P-H Chart