NORTHERN ILLINOIS UNIVERSITY -
Department of Mechanical EngineeringLab:
Measurements of Flow Rate and Specific Heat of AirObjective:
To determine flow-rate and the specific heat of airApparatus:
P.A. Hilton’s Air-conditioning unit model A573 [Specifications].
Theory:
We know from the First Law of Thermodynamics, that heat (energy) rate ‘Q’ supplied to a system (air) while flowing through a control volume (duct) will increase the system enthalpy: Q = m(hout - hin) = mCP(Tout - Tin), where ‘m’ is the air mass flow rate, ‘CP’ is the air specific heat at constant pressure, and (Tout - Tin) is the air temperature difference after (at outlet) and before (at inlet) of the heating section. In this experiment, the air flows through the apparatus, heated by electrical heaters with known powers (specified by the manufacturer). We will measure the temperature of the flowing air before (Tin) and after the heater(s) (Tout). The air mass flow rate is measured by a custom-made orifice flowmeter at the end of the duct. An inclined differential manometer is used to measure the pressure difference across the orifice (see Text, Section 9.3, p.399). Using the corresponding formula (see also Text, Section 10.5, p.445) we calculate the mass flow rate of air through the duct:
In the above equation C2 is the over-all flowmeter coefficient obtained by calibration (and supplied by the manufacturer), Once the temperature difference, heating and mass flow rates are known, the specific heat is easily calculated:
CP = Q/[m (Tout - Tin)]
We will assume the air is an ideal gas and use the ideal gas equation and standard reference tables for property evaluation (rair » 1.17 kg/m3), except for the specific heat which will be used as a reference against which our measured values will be compared.
Procedure:
1. Connect the apparatus to the power outlet.
2. Check, and if necessary adjust, the horizontal level of the manometer.
3. Zero the manometer and/or note down its initial reading (mm of Water) for correction.
4. Switch ON the apparatus by turning the main yellow switch clockwise by 90 degrees.
5. Set the fan control knob at low setting.
6. Wait 5-10 minutes or so for the system to stabilize and note down the thermometer readings at points A, B and C. Since the heaters are OFF, the thermometers should indicate the same air temperature except for the measurement errors. Record the readings and use any consistent discrepancy for corresponding corrections later.
7. Turn the fan control knob at about 25% of the scale. For higher settings the manometer reading may go out of range and its liquid may spill out.
8. Switch ON the Air Pre-Heaters (1000 W each) and wait for about 10 minutes or until the system comes to a steady state.
9. Note down the manometer and the thermometer readings.
NOTE:
The manometer scale is graduated in mmH2O for a manometric fluid of S=0.784 specific gravity. However, since we use water instead (S=1), the manometer reading, the difference between the final and initial readings, has to be divided by 0.784 correction factor. The result is D h (mmH2O).Observations:
Reference Cross-Section | Temperature | FIRST | SECOND | THIRD |
(A) Fan Inlet | TA (° C) |
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(B) After Pre-Heater | TB (° C) |
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(C) At Orifice | TC (° C) |
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Heating Power | Q = S P (W) |
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Manometer Reading | D h (mmH2O) |
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Specification of PA Hilton’s Air-conditioning Unit
Air throughput:
0.13 m3/sec (max)Pre-heater: Extended fin electric heating elements 2 X 1.0 KW @ 220 V
Cooler: Direct expansion, extended fin coil. Cooling rate 2 KW approx.
Re-heater: Extended fin electric heating elements 2 X 0.5 KW @ 220 V
Fan: Centrifugal (Variable speed)
Power input: approx. 120 W @ 240V 50 Hz
Boiler: Electrically heated and working at atmospheric pressure. Fitted with water level gauge and float level controller.
Heaters: 1 X 1.0 KW and 2 X 2.0 KW @ 220 V
Refrigerator: Hermetic unit with air-cooled condenser. Refrigerant: R-12
Compressor speed: 2700 to 3000 RPM @ 220 V, 50 Hz according to load; 3300 to 3600 RPM @ 110 V 60 Hz.
Swept volume: 25.95 cm3 per revolution.
Air flow measurement: Orifice plate with inclined tube manometer
Temperature measurement: 4 pairs Wet and Dry bulb glass thermometers 300 mm long, selected to agree to within 0.2° C of each other at normal operating conditions.
Refrigerant Circuit: 3 X 300 mm Glass thermometers.
Safety: Refrigerator hi-pressure cut out (with manual reset)
All moving parts are enclosed.
All electrical components are individually fused.
To ensure heaters are not switched on without air flowing, the fan speed regulator is preset to give an adequate airflow as soon as the three-phase mains switch is closed.
NOTE:
Your previous Lab assignment and Lab report are due before the demonstration of the next Lab. It is the best for you if you do your lab experiments right after the demonstration while TA is still in the Lab. Also, you have to perform the uncertainty analysis for every experimental lab and include it in your lab report.