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Relative Humidity of Air Objective: To determine the relative humidity of ambient air using Sling Psychrometer. Apparatus: Sling Psychrometer comprises of two thermocouples; one dry bulb and the other wet bulb having its bulb covered with wet wick. Both the thermometers are fixed on to a wooden, frame provided with a handle (Sling) so that the frame along with thermometers could be vigorously rotated. Theory: Relative humidity is an important parameter indicating the moisture content of air as well as its capacity to absorb moisture under given conditions of temperature and pressure. It is defined as the ratio of partial pressure of water vapor present in air to the saturation pressure at the ambient temperature. It can be expressed as: Relative Humidity, ϕ = pv / pvs,dbt (1) The value of partial pressure of water vapor, pv, is determined from the dry-bulb and wet bulb temperature of air, using carrier’s equation given as: pv = pvs – [(pv - (pvs)wbt)( tdbt - twbt )] / [1547 – 1.44 twbt] (2) Where: pv is the partial pressure of water vapor in air (bar)

(pvs)wbt is the saturation vapor pressure at wet bulb temperature (bar) tdbt and twbt are dry bulb temperature and wet bulb temperature respectively (°C) (pvs)dbt is the saturation pressure of water vapor at dry bulb temperature (bar) Procedure:  Ensure that there is sufficient water in the wet bulb thermometer container so that the cloth is wet.  In order to determine wet bulb and dry bulb temperature at a given location, the frame is vigorously rotated for 2-3 minutes before the reading is noted. It should be ensured that the sling is rotated for sufficiently long time to reach steady state conditions i.e. the temperature does not change with further rotation.  Reading of dry bulb and wet bulb temperatures are taken at 4-5 different locations, say in the middle of a room, outside in the sun, outside but under the shade, outside near a wall etc.  The sets of reading are to be entered in a table as shown below: Observations: S. No. 1. 2. 3. 4. 5.

Location Middle of room Corner of room Outside-Sun Outside- shade Outside-near wall

Temperatures Dbt Wbt

Relative Humidity Calculation Chart

Fig. 1 ASHRAE Psychrometric Chart No. 1

Calculation Procedure: Using the set of values of dbt and wbt; the relative humidity can be determined either using psychrometric chart or using carrier’s equation. The procedure is described below: Using Psychrometric chart: The intersection of dbt and wbt lines determined the relative humidity, given usually in percentage. Using Carrier’s equation: As discussed in the theory, the value of relative humidity, φ; can be determined using equation labeled as (1) and (2). Results:  The value of relative humidity at different locations are entered in the last two columns of the observation table.  Compare the values at different locations and try to give the reasons for difference in values of relative humidity

Energy balance of a Refrigeration unit Objective: Production of an energy balance for the refrigeration unit. Apparatus and theory: Hilton refrigeration unit R-134a (Fig. 1.1) has an attractive glass reinforced plastic panel, housing a belt driven twin cylinder reciprocating compressor. This is driven by a trunnion mounted electric motor connected to a spring balance. By measuring the motor torque and speed; the shaft power required to drive the compressor can be determined.

Fig. 1.1. R712 Refrigeration Laboratory Unit

Refrigerant R-134a vapor is drawn into the compressor from the evaporator mounted on the front of the panel. Work is done on the gas and its pressure and temperature are raised. This hot, high pressure gas is discharged from the compressor and flows into the panel mounted water cooled condenser. A measured and controllable flow of cooling water passes through copper coil sealed inside the condenser cylinder. The hot gas de-superheats and then condenses to a liquid on the surface of the cooling coil. The condensed liquid collects at the base of the cylinder where it is subcooled and may be observed through a sight glass. This liquid then flows through a refrigerant filter/drier and a variable area flowmeter to the thermostatic expansion valve. Here it passes through a controllable orifice which allows its pressure to fall from that of condenser to that of the

evaporator. The liquid immediately starts to boil and takes in heat to accomplish this at low temperature. In order to allow and control and measurement of the heat input at the evaporator, two electric heater elements are used. These are rolled concentrically inside the copper tube carrying the low temperature liquid/vapour mixture from the expansion valve. The voltage across the heater elements may be varied from zero to that of the mains supply voltage panel. Measurement of the voltage, current and hence power is carried out by a panel mounted voltmeter and ammeter. A range-over switch allows the motor voltage and current to be measured. The sensing bulb of the thermostatic expansion valve is mounted on the exit pipe from the evaporator and this detects the degree of superheat of the gas leaving the evaporator and entering the compressor. If the superheat is low the valve will close and reduce the flow and if too high the reverse will occur. By this means stability is maintained under all conditions of operation. A panel mounted digital temperature indicator allows measurement of all relevant system temperature and a digital tachometer indicates the rotation speed of the compressor. Individual pressure gauges indicate the pressure in the condenser and evaporator. Procedure: Start the unit and run it for a few minutes at a moderate evaporator load of about 250 W and then set to the desired conditions for the test ( The evaporating temperature, t4 is set by adjustment of the evaporator heat input). The condensing temperature is set by adjusting the condenser cooling water flow until the condenser pressure is the saturation pressure at the desired condensing temperature. The unit is left running to stabilize and then a set of readings taken as given in the table below. Increase the evaporator load in step of 200-250 W and adjust the condenser cooling water flow rate to maintain the constant condensing pressure. Take four/five sets of reading for different values of evaporator load. Compressor (Two cylinder-single acting) Bore-40 mm Stroke-30 mm Swept volume-75.5 cc per revolution Clearance volume/ swept volume=0.025 Belt pulley ratio = D/d=3.17 Hence if compressor RPM Nc= 480 (at 220V) Motor rpm= 480*3.17=1522 RPM Dynamometer- Torque arm radius=0.165 m

Fig 1.2. p-h chart for R-134a

Observations: Atmospheric pressure = …….. Pilley ratio Nm/Nc = ……… Series

Test No.

Condenser pressure (abs)

Pc (KN m-2)

Evaporator pressure (abs)

Pe (KN m-2)

Compressor suction temperature

t1 (°C)

Compressor delivery temp.

t2 (°C)

Liquid leaving temp.

t3 (°C)

Evaporator inlet temp.

t4 (°C)

Water inlet temp.

t5 (°C)

Water outlet temp.

t6 (°C)

Water flow rate

mw (gm/s)

R-134a flow rate

mr (gm/s)

Evaporator volts

Vev

Evaporator amps

Iev

Motor volts

Vm

Motor amps

Im

Spring balance

F (Newton)

Compressor speed

Nc

Motor speed

Nm

1

2

3

Results and Discussion: From the p-h chart (Fig. 2.1), find h1- enthalpy of vapor at compressor suction h2- enthalpy of vapor at compressor outlet h3- enthalpy of liquid at the outlet of condenser h4- enthalpy of liquid at the evaporator inlet Calculate the energy balance for various components (refer Fig 1.2) as described below: Evaporator: Evaporator heat input Qe = Vev * Iev

4

R-134a enthalpy change rate = m(h1-h4) Compare Qe and m(h1-h4) Condensor: Heat transfer to water Qc = mw Cp (t6-t5) R-134a enthalpy change rate = mr (h3-h2) Compare Qc and mr (h3-h2) Compressure: Shaft power Ps= (torque) (rotation speed) = 0.165 F * 2ᴨNm/60 R-134a enthalpy change rate= mr(h2-h1) Heat loss by convection and radiation from compressor to surroundings Qc= Ps – mr (h2-h1) (a) Electric Motor: Electric power input Pe1= Vm Im cosφ Power factor, cosφ = True power (kW)/apparent power (VA) Heat loss by convection and radiation from motor Qmotor= Pe1-Ps (b) Complete plant: Qnet=Pnet Qc + Qe+ Qnet = Pe1 Qrad = Pe1 – (Qc + Qe) (c) Check the Qrad given by equation (c) is equal to the sumof Qrad given by eqns. (a) and (b)

Heating and Humidification (1) Object: Performance of air conditioning plant for humidification of air with steam injection. Apparatus and Theory: Hilton air conditioning unit is mounted on a mobile frame which houses the refrigeration unit and a steam generator. The refrigeration unit has a fin coil air cooled condenser (cooling approx. 1.7 kW) and a hermetic compressor having: Refrigerant: R-134a Compressor speed: 2700 to 3000rev. /min. according to load Swept volume: 21 cm3/rev Power factor typically 0.9 The steam generator is electrically heated and works at atmospheric pressure and is fitted with water level gauge and float level controller. The heaters are: one 1.0 kW and 2.0 kW nominally at 220V. The glass reinforced plastic (GRP) ducting has a clear perpex front and all the components through which the air flows may be seen. Untreated air entering the ducting passes in series through the following: I. An air measuring intake orifice with inclined tube manometers. II. A mixing zone (where it may be mixed with recirculated air) III. A preheater (extended fin electric heating element 0.5 and 1.0 kW nominally at 220 V) IV. A humidifier supplied with steam from the generator. V. A cooler/ dehumidifier with a precipitate water outlet. VI. A re-heater (extended fin electric heating element 0.5 and 1.0 kW nominally at 220 V) VII. An axial flow fan with infinitely variable speed control. VIII. An air measuring duct orifice. IX. A damper which controls the quantity of air discharged to the atmosphere (any air not discharged is recirculated and mixes with untreated air in (ii)) All controls and instrumentation are at eye level and logically arranged. The process is shown in Fig. 6.1. In case of zero recirculation.

Fig 1.1. Humidification with steam injection

mB = mA= 0.0757 (Z/VA)0.5

Kg/s

z= manometer reading at intake orifice, mm of H2O VA= specific volume corresponding to state A m3/Kg Heat energy supplied to boiler, Qs = E Is

(1)

E – Voltage, V Is – Boiler Current, A Enthalpy change rate = mB (hc-hB)

(2)

Difference between (1) and (2) in largely attributed to heat loss from steam generator. Increase of moisture = mB (WC-WB)

(3)

Theoretical evaporation at boiler = Qs/ (hm-hw)

(4)

hm – enthalpy of injected steam hw – enthalpy of feed water The discrepancy between (3) and (4) in attributed to heat losses from the steam distribution system; some of which appears in the air stream as sensible heat due to heat transfer from the steam distributer. Procedure: Turn on the water supply to the boiler and check that the water level in the gauge glass stabilizes at a depth which will cover all the gauge glass stabilizes at a depth which will cover all the heating elements (i.e., about 120 mm from the bottom of the boiler). Rotate the fan speed controller fully clockwise and close te dampers fully so that there is no recirculation of air. Switch on the electric supply at the isolator. Switch on the unit at the main switch. Check that the following are immediately operative. I. The fan II. The main warning lamp III. The voltmeter IV. The temperature indicator Close the three boiler switches until steam is seen to issue from the distributor (this takes about 5 minutes). Then the boiler output can be adjusted to the desired rate by switching up to 5 KW in 1 KW increments. When the unit has stabilized, note down the various parameters as are given in the table below. Take four to five readings for different values of steam injection.

Observations: Station

A B

Intake After mixing C After steam injection Orifice differential intake Voltage Boiler current 2.0 kW Boiler current 2.0 kW Boiler current 1.0 kW

As Dry Unit observed bulb / screen Wet bulb (sling) t1/t2 °C t4 t3/t4 °C t6

t5/t6

Reading 1

2

3

4

°C

Z

mm of H 2O

E IS

V A

IS

A

IS

A

Calculate the mass flow rate mB = mA and energy supplied to steam generator Qs. Using psychometric chart Fig. 1.2., find specific enthalpy hB and hC and specific humidity WB and WC corresponding to states B and C. Then compare equations 1 & 2 and 3 & 4 to find the discrepancies. Result and Discussion: Use distilled water to fill reservoirs for the wicks of the wet bulb sensors. Do not exceed the air temperature beyond 50 °C anywhere in the duct. If ice forms on the air side of the evaporator tubes and fins and on the expension valve, increase the air flow rate/or switch on the air preheater to avoid icing. Then allow it to run for at least five minutes, after which the mains isolator may be switched off. Plot the states B and C on the psychometric chart for different set of reading – compare the observed to calculate states C. Comment on the results obtained by you.

Fig 1.2. psychrometric chart

Precautions: Before switching off Move the damper to zero recirculation position  Switch off all boiler heater  Switch off all air heater  Switch off refrigeration unit  Set the fan to maximum speed

Vapor Absorption System Object: Study of three fluid vapor absorption system. Apparatus and Theory: An absorption refrigerator is a refrigerator that uses a heat source (e.g., solar energy, a fossil-fueled flame, waste heat from factories, or district heating systems) to provide the energy needed to drive the cooling process. The vapor absorption refrigeration system comprises of all the processes in the vapor compression refrigeration system like compression, condensation, expansion and evaporation. The refrigerant produces cooling effect in the evaporator and releases the heat to the atmosphere via the condenser.

Fig 3.1. ET 480, general view, main components and measuring points

This basic principle of an absorption refrigeration system is demonstrated in the ET 480 experimental unit taking the example of an ammonia-water solution with the ammonia acting as refrigerant. In the evaporator the liquid ammonia evaporates and withdraws heat from the environment. To keep the evaporation pressure low, the ammonia vapor in the absorber is absorbed by the water. In the next step, ammonia is permanently removed from the high concentration ammonia solution to prevent the absorption process from being halted. For this purpose, the high concentration ammonia solution is heated in a generator until the ammonia evaporates again. In the final step, the ammonia vapor is cooled in the condenser to the base level, condenses and is returned to the evaporator. The low concentration ammonia solution flows back to the absorber. To maintain the pressure differences in the system, hydrogen is used as an auxiliary gas. In process technology systems the resulting waste heat can be used for cooling. In small mobile systems, such as a camping refrigerator or minibar in a hotel, the required heat is generated electrically or by gas burner. ET 480 demonstrates the functional principle of an absorption refrigeration system with its main components: evaporator, absorber, boiler as generator with bubble pump, condenser. The boiler can alternatively be operated with gas or electrically. Another electric heater at the evaporator generates the cooling load. Temperatures in the refrigeration circuit and the heating power at the boiler and at the evaporator are recorded and displayed digitally The function of an absorption refrigerator without a mechanically powered compressor is based on two fundamental facts.  Water has the property that it can absorb large quantities of ammonia gas (NH3) when cold. This ammonia can be expelled again at a higher temperature.  Ammonia vapor can be condensed in an enclosed system under pressure and at room temperature. If it absorbs a large amount of heat, it can be condensed again at a lower temperature in the presence of an auxiliary gas. Absorption refrigeration system offer the following advantages over compression refrigeration system:  The dissipated heat produced can be used to generate cooling power (e.g. in process engineering system)  Almost silent operation  Cooling power can be generated even when no electrical current is available (e.g. propane camping refrigerators) Experimental Procedure: 1. Set the main switch to “1”. 2. Read off and record the available measured values. 3. Switch on the electric heater and start the stopwatch 4. Read off and record the measured values at regular intervals (e.g. every 5 mins) 5. Observe the change in the recorded temperature. 6. The stationary state is reached when these changes becomes small (without a counter heater, this will be approx. 60 mins after the start of the experiment).

7. Read off and record the measured values of the stationary state. 8. Start the electric counter heater and set it to around 5 W. 9. Observe the power display of the electric counter heater and adjust the heater if there are clear vibrations. 10. Read off and record the measured values at regular intervals (e.g. every 5 mins). 11. Observe the power display of the recorded temperatures. 12. The new stationary state is reached when these changes become small. 13. Read off the new stationary state. 14. Turn the electric counter heater upto 10 W. 15. Repeat step 9 to 13 16. Turn the electric counter heater up to 20 W 17. Repeat step 9 to 13 18. Switch off the electric heater and electric counter heater 19. Set the main switch to “0” Observations: The measured values shown below were taken during an experiment in which the absorption refrigeration system was switched off over night before the start of the experiment. The experiment was conducted in a closed room temperature TA of around 20.5 deg. C. Note: Different experiment result in different measured values. Tab. 5.1 shows the available measured values on reaching the stationary states, depending on the power P2 of the connected counter heater. The temperature T1 to T4 along with the power P1 of the electric heater are measured. Comments

P2 W

T1 °C

T2 °C

T3 °C

T4 °C

P1 W

ΔT °C

0

21.8

20.6

20.3

20.0

111

-0.1

0

0

144.1

-12.4

26.1

28.0

109

32.9

0

5

144.8

-5.8

27.8

30.0

102

26.3

0.05

10

144.3

10.7

29.6

30.6

99

9.8

0.10

25

144.4

19.3

29.0

29.3

100

1.2

0.25

ε -

Start of experiment

Stationary states

Tab.5.1 Demonstration experiments, electric heater, connected counter heater, measured values.

Fig 5.1. COP (ε) as a function of temperature difference (ΔT)

Results and discussion: The cooling effects originates at the evapora...