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1 ENME 485 Winter 2004 #2: HVAC Experiment

The University of Calgary

Faculty of Engineering

ENME 485 Mechanical Engineering Thermodynamics Laboratory

Experiment 2: Heating, Ventilation and Air-Conditioning (HVAC) Experiment Objectives: i) To demonstrate the principles of air conditioning (Chapter 13 of Cengel and Boles) by comparing one experimental measurement to others using theory. ii) To demonstrate the principles of a refrigeration cycle (Chapter 10 of Cengel and Boles). Introduction: The purpose of the experiments is to demonstrate the basic principles of Air Conditioning, i.e. how heat and moisture can be added to or subtracted from a moving mass of air, enabling control of environment and comfort levels. The air-conditioning unit, shown in Fig. 1, consists of two main sections. The first main section is the air system including a blower fan with speed controls, air heaters, a steam injection system, and air flow instrumentation. The second main section is a refrigeration system consisting of a compressor-condenser unit, an evaporator and water extraction tube (dehumidifier), and controls and instrumentation.

Figure 1: Air-Conditioning Unit


As seen in the illustration of the air-conditioning unit in Fig. 2, a variable speed fan (A) forces air to flow through a 254 mm (10”) square ducting. Air mass flow is measured by means of an orifice plate and inclined manometer, calibrated in mm of water, located at the exit of the ducting. The calibration equation for the orifice is given by:

D

zmν

0517.0=& (1)

where: m& is the air mass flow rate in kg/s ( )]1[ ω+=+= awa mmmm &&&& z is the orifice differential (mm H2O, readings from 0.0 mm to 12.0 mm) Dν is the specific volume of air at Section D (m3/kg)

Both heating and cooling sections are incorporated into the unit. Heat is added by electric resistance elements, two of which are located at Section B providing 1 kW each and two are located at Section F, providing 500 W each. Moisture can be added to the air flow through the injection of steam at Section C. The steam is generated using three 1.5 kW electric immersion heaters. An R12-based vapor compression refrigeration cycle extracts heat and, within certain limits, moisture from the flow at Section E. Relative humidity and temperature is measured at three points in the system, the first being upstream of the fan inlet (Section A), the second at Section D, and the third in Section G. Relative humidity and temperature measured at these three points is performed using a Siemens Model 538-894 RH/T sensor having 2% accuracy. Temperature in the refrigeration cycle is measured using three Omega type-K thermocouples. The mass flow rate of refrigerant is measured using a Rotameter in kilograms per hour, and the pressure both before and after the compression stage is measured using pressure gauges (in bars).

Figure 2: Schematic of the HVAC System


Theory: For the purpose of analysis, it is convenient to assume that the heating or cooling surfaces are external to the duct, as illustrated in Fig. 3.

Figure 3: Duct Nomenclature for Application of Energy Equation

Applying the steady flow energy equation at Sections D and G:

( )

−+−=−

2

22DG

DGaaaVVhhmWQ &&& (2)

where: am& is the air mass flow rate in kg/s

Gh is the enthalpy at section G (kJ/kg dry air)

Dh is the enthalpy at section D (kJ/kg dry air)

GV is the air velocity at section G (m/s)

DV is the air velocity at section D (m/s) Since 0=aW& and DG VV ≅ , Eq. 2 reduces to:

( )DGaa hhmQ −= && (3)

am& may be determined from the continuity equation:

D

DDa

VAmν

=& (4)

where: DA is the cross-sectional area of the duct (m2)

DV is the air velocity at section D (m/s)

Dν is the specific volume of the air at section D (m3/kg)

Air Flow

D G

hD

VD

hG

VG

Heating or Cooling


aQ& is the sum of the heat transfer at the heating or cooling surface and the external heat transfer to or from the surrounding atmosphere. Since the apparatus runs at nearly ambient temperatures, external losses or gains are very small, and close agreement is achieved between the enthalpy change rate ( )[ ]DGa hhm −& and the measured heat transfer. The measured heat transfer is determined from either heating through the direct measurement of the electrical input or from cooling by the measurement of the refrigerant mass flow rate and the enthalpy change across the evaporator. Relative and Absolute Humidity Equations (Chapter 13): Atmospheric air consists of a mixture between dry air and water vapor. The amount of water vapor present in atmospheric air can vary and is quantified by the humidity ratio (either relativeφ or absoluteω ). If we were at atmospheric pressure we could use the psychrometric chart to do calculations. Being that we are at an altitude over 3000 feet in Calgary, the atmospheric pressure is lower (usually around 89 kPa). As a result of this we are forced to use equations to compute properties such as enthalpy and specific volume of atmospheric air. The atmospheric pressure that we measure is composed of the partial pressure of dry air and the partial pressure of water vapor: va PPP += where aP is the partial pressure of dry air and vP the partial pressure of water vapor. The relative humidity (φ ) can be evaluated from

g

v

PP

=φ

where gP is the saturation pressure at the temperature from the steam tables. This also leads to:

gv PP ∗=φ The enthalpy of both the dry air and the water vapor can be computed using:

( )TTh 82.13.2501005.1 ++= ω kJ / kg dry air whereT is temperature in degrees Celsius. The specific humidity can be computed knowing the vapor pressure:

v

v

PPP−

= 622.0ω kg water / kg dry air

and the relative humidity can be computed from the saturation pressure and the specific humidity:

( ) gPPω

ωφ+

=622.0


Example Problem 13-74: An air-conditioning system operates at a total pressure of 95 kPa and consists of a heating section and a humidifier that supplies wet steam (saturated water vapor) at 100oC. Air enters the heating section at =1T 10oC and 70 percent relative humidity ( 1φ ) at a rate of 70 m3/min, and it

leaves the humidifying section at =3T 20oC and 60 percent relative humidity ( 3φ ). Determine (a) the temperature and relative humidity of air when it leaves the heating section, (b) the rate of heat transfer in the heating section, and (c) the rate at which water is added to the air in the humidifying section. Schematic:

Solution: Pressure does not equal 101.325 kPa, therefore we cannot use the psychrometric chart.

CT o101 = ; kPaPg 2276.11, = (vapor pressure from steam tables at 10oC)

kPakPaPP gv 8593.02276.170.01,11, =∗=∗=φ

1,1,1 va PPP +=

kPaPPP va 1407.948593.0951,11, =−=−= We need the specific volume of dry air in order to compute the mass flow rate of dry air:

mRTPV = ; PRT

mV

==υ

8628.0141.94

283287.0

1,

11, =

∗==

a

aa P

TRυ m3/kg

3522.18628.01666.1

1,

11, ===

aa

Vmυ

&& kg/s

Although we do not need it, we could also compute the specific volume of water vapor using:

Air Flow

1 3

10oC

70%

Heating Coils

2

70 m3/min

20oC

60%

Saturated Vapor, 100oC


989.1518593.0

2834615.0

1,

11, =

∗==

v

vv P

TRυ m3/kg

Performing an air balance between Stations 1, 2, and 3: aaaa mmmm &&&& === 3,2,1, (P.1) Performing a water balance between Stations 1 and 2: 2,1, ww mm && =

aa mm && 21 ωω =

From the air balance we can rewrite this as:

21 ωω = (P.2) This tells us that no water is added between Stations 1 and 2. Performing a water balance between Stations 2 and 3:

3,,2, winww mmm &&& =+

ainwa mmm &&& 3,2 ωω =+

( )23, ωω −= ainw mm && (P.3) We can use an equation to compute the specific humidity 1ω :

00568.0622.01,

1,1 =

−=

v

v

PPP

ω kg/kg dry air

From Eq. P.2: 00568.02 =ω We need to compute the specific humidity at Station 3 ( 3ω ):

CT o203 = ; kPaPg 339.23, = (vapor pressure from steam tables at 20oC)

kPakPaPP gv 4034.1339.260.03,33, =∗=∗=φ

00933.0622.03,

3,3 =

−=

v

v

PPP

ω kg/kg dry air

From Eq. P.3:

( ) 00494.023, =−= ωωainw mm && kg/s Applying the First Law between Stations 1 and 2 (neglecting KE and PE):

∑∑ ++=++ eeoutoutiiinin hmWQhmWQ &&&&&&


( )12 hhmQ ain −= &&

( )2222 82.13.2501005.1 TTh ++= ω

( ) 3608.2482.13.2501005.1 1111 =++= TTh ω kJ/kg dry air

Applying the First Law between Stations 2 and 3 (neglecting KE and PE):

∑∑ = eeii hmhm &&

33,22, hmhmhm ainina &&& =+ From the air balance we know that:

aaa mmm &&& == 3,2, and from the steam tables (saturated vapor at 100oC):

1.2676, =inwh kJ/kg providing us with an equation that we can use to evaluate 2h :

a

inwina

mhmhm

h&

&& ,32

−=

Evaluating 3h :

( ) 7767.4382.13.2501005.1 3333 =++= TTh ω kJ/kg dry air

With 3h we can evaluate 2h :

00.343522.1

1.2676*00494.07767.43*3522.1,32 =

−=

−=

a

inwina

mhmhm

h&

&&

( )2222 82.13.2501005.1 TTh ++= ω

49.192 =T oC Knowing 2T we can compute the relative humidity at Station 2:

CT o49.192 = ; kPaPg 2749.22, = (vapor pressure from steam tables at 19.49oC)

00568.012 ==ωω

( ) 3779.0622.0 2,2

22 =

+=

gPPω

ωφ

%8.372 =φ Heat in is evaluated from applying the First Law between Stations 1 and 2:

( )12 hhmQ ain −= &&


( )36.2400.343522.1 −=inQ&

034.13=inQ& kJ/s 0.782= kJ/min Refrigeration Cycles (Chapter 10): A standard R-12 based refrigeration cycle is used to cool the air stream during the experiment. A schematic of the refrigeration cycle is shown in Fig. 4. During the experiment, temperatures are measured at states 1, 3, and 4, pressure is measured at both the condenser (Phigh) and evaporator (Plow), and the mass flow rate of R-12 is measured at state 3. The throttling process that occurs in the expansion valve (between states 3 and 4) is a constant enthalpy process. We will use property data for R-12 in the form of a pressure-enthalpy diagram to estimate the amount of heat removed from the air stream. In performing this we need to know the enthalpy at states 4 and 1 and the mass flow rate of refrigerant. (Hints: Determine the enthalpy at state 3 in order to find the enthalpy at state 4. This is an experiment, there are errors in both pressure and temperature measurements, thus expected points may not lie exactly where expected.)

Figure 4: R-12 based Refrigeration Cycle Operational Notes: 1. It is recommended that at all times the blower fan is operated at a minimum speed (to show 4.0 mm H2O on the inclined manometer) before any other circuit is brought into operation. 2. Ensure that the water supply to the boiler is turned on before the immersion heaters are switched on. This can be checked by manually tapping the ball valve. 3. Switch on the compressor. After initial gassing in the flowmeter, the flow should become steady. Check that evaporator inlet temperature is sufficiently below evaporator outlet temperature to give adequate superheat and so to ensure that all vapor has been evaporated. 4. Under all test conditions allow time for the equipment to settle down, particularly when using steam injection. The ball valve is in fact a safety measure to prevent the water level from becoming to low and uncovering the immersion heaters.

1

3

2

1

T

s

14 4

3 2Condenser

SuperheatRegion

Vapor

Liquid

Two PhaseRegion

Phigh

Plow

Evaporator

Expansion Valve

Compressor


5. Always start testing with small flow rates of steam and increase as necessary. Excess steam is liable to condense in the ducting which may then take a considerable time to evaporate and hence upset subsequent test balances. Procedure: Experiment #0: Local Conditions, Zero Data, Blower Work 1. Measure the local atmospheric pressure and the room temperature. 2. Record data from the computer without any flow. This will be the zero data. 3. Turn the fan on to 1.0 mm of water. Allow the system to stabilize for 1 minute. 5. Collect air data (temperatures and relative humidity). 6. Turn the fan on to 2.0 mm of water. Allow the system to stabilize for 1 minute. 7. Collect air data (temperatures and relative humidity). 8. Repeat Steps 6 and 7 for 3.0, 4.0, … 11.0, 12.0 mm of water. Experiment #1: Air Stream Humidification with Refrigeration 1. Turn the Fan On and set the speed to 10.0 mm of water. 2. Turn on all three 1.5 kW steam heaters. 3. Turn on the refrigeration system. 4. Allow the system to stabilize. This will probably take around 20 minutes. 5. Collect data, including air temperatures, relative humidity, refrigerant temperatures, pressures and mass flow rate. 6. Reduce the fan speed to 1.0 mm of water, and allow the system to stabilize. 7. Collect data, including water condensation rate, air temperatures, relative humidity, refrigerant temperatures, pressures and mass flow rate.. 7. Set the Fan speed to 12.0 mm of water. 8. Turn off the steam generator heaters and the refrigeration system. Experiment #2: Air Stream Heating 1. Turn on the two 1 kW air preheaters. 2. Allow the system to run for 5 minutes. 3. Turn on the two 500 W air heaters. 4. Allow the system to stabilize. 5. Collect data including air temperatures and relative humidity. 6. Reduce the blower speed to 4.0 mm of water. 7. Allow the system to stabilize. 8. Collect data including air temperatures and relative humidity.


Useful Data: The system power inputs are as follows: Boiler, Lower 1.5 kW: 124 V; 12.9 A; 1600 W Boiler, Upper 1.5 kW: 122 V; 12.8 A; 1561.6 W Boiler, 1.5 kW: 123 V; 12.7 A; 1562.1 W 1st Pre-heater, 1 kW: 123 V; 8.8 A; 1082.4 W 2nd Pre-heater, 1 kW: 122 V; 8.8 A; 1073.6 W 1st Re-heater, 500 W: 124 V; 4.2 A; 520.8 W 2nd Re-heater, 500 W: 122 V; 4.1 A; 500.2 W Calculations: Experiment #0: Local Conditions, Zero Data, Blower Work Before starting to process the data, you will need to remove offset errors from the instruments. Compute the average temperature for all three air sensors, the average relative humidity, and the average refrigerant temperature. Determine how much you need to add or subtract from each reading so that the measured air temperatures, relative humidity, and refrigerant temperatures equal their respective averages. You will have to add/subtract this amount to all of the measurements made during the experiment. Apply the first law between Sections A and D to estimate the amount of blower work for each of the air flow rates (1.0 mm, 2.0 mm, 3.0 mm, …, 11.0 mm, and 12.0 mm). Plot blower work versus mass flow rate of air and comment. Experiment #1: Air Stream Humidification with Refrigeration 10.0 mm of water 1. Determine the mass flow rate of dry air. 2. Compute the mass flow rate of steam going into the duct. ( )fgstboiler hmQ && =

3. Compute the heat removed by the refrigeration cycle. ( )411212 hhmQ RR −= &&

4. Perform a water mass balance between Sections A and D to estimate stm& .

5. Apply the First Law between Sections A and D to estimate stm& . 6. Apply the First Law between Sections D and G to estimate the refrigerant load. 7. Compare and comment on all results. 1.0 mm of water 1. Determine the mass flow rate of dry air. 2. Compute the mass flow rate of steam going into the duct. ( )fgstboiler hmQ && =

3. Compute the heat removed by the refrigeration cycle. ( )411212 hhmQ RR −= &&

4. Perform a water mass balance between Sections A and D to estimate stm& .

5. Apply the First Law between Sections A and D to estimate stm& .

6. Perform a water mass balance between Sections D and G to estimate condensedm& . 7. Apply the First Law between Sections D and G to estimate the refrigerant load. Use the experimentally measured value of condensedm& . 8. Compare and comment on all results.


Experiment #2: Air Stream Heating 12.0 mm of water 1. Determine the mass flow rate of dry air. 2. Apply the First Law between Sections A and D to estimate heaterQ& .

3. Apply the First Law between Sections D and G to estimate heaterQ& . 4. Compare and comment on all results. 4.0 mm of water 1. Determine the mass flow rate of dry air. 2. Apply the First Law between Sections A and D to estimate heaterQ& .

3. Apply the First Law between Sections D and G to estimate heaterQ& . 4. Compare and comment on all results.

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