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).

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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 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 Tdb, 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.

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 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

concepts

Fig. 3 Combined Processes

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

= 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:

(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

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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

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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 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 , and 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 ( ), isentropic efficiency(h c) and

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 emission of this

cause significant errors in energy balances? (Usur = 1.7 W/m2 ° C) (ONLY FOR

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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SCHEMATIC DRAWING OF EXPERIMENTAL SETUP

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.

34

5

2

Mixer

d

1

1.44 kW 2.5 kW 1.44 kW

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ME 410 EXPERIMENT 3OUTLINE FOR RESULTS

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

Section

Tdb Twb From Chart From EquationsDeviations ( %

)

( oC ) ( oC ) 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

States ProcessMeasured Values Theoretical Values % Deviations

(kW) (kg/s) (kW) (kg/s)

2 & 3Preheating+

Humidification

3 & 4Cooling+

Dehumidification

4 & 5 Reheating

: ……………………….. kg/s

: ……………………….. kg/s

: ……………………….. kW

: ………………………..

COP : ………………………..

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