CAE 331/513 Building Science - The Built Environment...
Transcript of CAE 331/513 Building Science - The Built Environment...
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CAE 331/513 Building Science Fall 2014 Week 6: September 30, 2014 HVAC thermodynamic and psychrometric processes
Dr. Brent Stephens, Ph.D. Civil, Architectural and Environmental Engineering
Illinois Institute of Technology [email protected]
Advancing energy, environmental, and sustainability research within the built environment www.built-envi.com Twitter: @built_envi
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ASHRAE Scholarships
• 8 regional/chapter and university-specific scholarships – $3,000 to $5,000 each
• 12 undergraduate engineering scholarships – $3,000 to $10,000 each
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www.ashrae.org/scholarships
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HW 3 – Psychrometrics chart
• Due Thursday
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Revisit example from last class
Moist air exists at 22°C dry-bulb temperature with 50% RH
Find the following: (a) the humidity ratio, W (b) dew point temperature, Tdew (c) wet-bulb temperature, Twb
(d) enthalpy, h (e) specific volume, ν (f) density, ρ
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Also: (g) degree of saturation, µ
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Psychrometric equations
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φ =pwpws
ln pws =C8T+C9 +C10T +C11T
2 +C12T3 +C13 lnT
W = 0.622pwp− pw
µ =WWs
Dew point temperature:
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Psychrometric equations
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W =(2501− 2.326Twb )Ws@Twb
−1.006(T −Twb )2501+1.86T − 4.186Twb
= actual W
Wet bulb temperature (iterative solver):
v = RdaTp− pw
=RdaT (1+1.6078W )
pv ≈ 0.287042(T + 273.15)(1+1.6078W ) / p
ρ =mda +mwV
=1v1+W( )
*where T is in °C
h ≈1.006T +W (2501+1.86T )
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• d
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Relative Humidity φ ≈ 50%
Dry Bulb Temp T = 22°C
Dew Point Temp Tdew ≈ 11.7°C
Wet Bulb Temp Twb ≈ 15.5°C
Humidity Ratio W ≈ 8.2 g/kgda
(i.e., 0.0082 kg/kg)
Enthalpy h ≈ 44 kJ/kgda
Specific Volume v ≈ 0.848 m3/kgda
Density ρ ≈ 1/v ≈ 1.18 kgda/m3
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Revisit another example from two classes ago
Moist air exists at 30°C dry-bulb temperature with a 15°C dew point temperature
Find the following: (a) the humidity ratio, W (b) wet-bulb temperature, Twb
(c) enthalpy, h (d) specific volume, ν (e) relative humidity, ϕ
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Also: (f) degree of saturation, µ (g) density, ρ
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• d
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Dew Point Temp Tdew ≈ 15°C
Dry Bulb Temp T = 30°C
Wet Bulb Temp tb≈ 20°C Humidity Ratio
W ≈ 10.7 g/kgda (i.e., 0.0107)
Enthalpy h ≈ 58 kJ/kgda
Specific Volume v ≈ 0.875 m3/kgda
Relative Humidity φ ≈ 40%
Saturation W Ws ≈ 0.27 kgw/kgda
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Humidity ratio
• For a known Tdew = 15°C, we know that the actual humidity ratio in the air, W, is by definition the same as the saturation humidity ratio, Ws, at an air temperature of 15°C
W = 0.622pwp− pw @T=30°C
W@T=30°C =Ws@T=15°C = 0.622pwsp− pws @T=15°C
pws@15C = 1.7057 kPa
W@T=30°C =Ws@T=15°C = 0.6221.7057
101.325−1.7057= 0.01065
kgwkgda
Assume p = 101.325 kPa (sea level)
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Degree of saturation
• Need the saturation humidity ratio @ T = 30°C:
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µ =WWs
!
"#
$
%&@T=30°C
Ws@T=30°C = 0.622pwsp− pws @T=30°C
pws@15C = 4.2467 kPa
Ws@T=30°C = 0.6224.2467
101.325− 4.2467= 0.02720
kgwkgda
µ =WWs
=0.010650.02720
= 0.39
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Relative humidity
• From previous:
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φ =pwpws
pw@T=30°C = pws@T=15°C =1.7057kPa
pws@T=30°C = 4.2467kPa
φ =1.70574.2467
= 0.40 = 40%
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Enthalpy
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*where T is in °C
h ≈1.006T +W (2501+1.86T )
h ≈1.006(30)+ (0.01065)(2501+1.86(30)) = 57.4 kJkg
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Specific volume and density
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v ≈ 0.287042(T + 273.15)(1+1.6078W ) / p
v ≈ 0.287042(30+ 273.15)(1+1.6078(0.01065)) / (101.325)
v ≈ 0.873 m3
kgda
ρ =1v1+W( ) = 1
0.8731+0.01065( ) =1.157 kgm3
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Wet-bulb temperature
• Wet-bulb temperature is the Twb that fits this equation:
Procedure: • Guess Twb, calculate pws for that T, calculate Ws for that T
– Repeat until W calculated based on those values (and original T) in equation above is equal to actual W (0.01065 in our case)
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W =(2501− 2.326Twb )Ws@Twb
−1.006(T −Twb )2501+1.86T − 4.186Twb
= 0.01065
Ws@Twb=?= 0.622
pwsp− pws @Twb=?
where: T = 30°CTwb = ?°C
Twb = 20.1°C
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• d
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Dew Point Temp Tdew ≈ 15°C
Dry Bulb Temp T = 30°C
Wet Bulb Temp tb≈ 20°C Humidity Ratio
W ≈ 10.7 g/kgda (i.e., 0.0107)
Enthalpy h ≈ 58 kJ/kgda
Specific Volume v ≈ 0.875 m3/kgda
Relative Humidity φ ≈ 40%
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PSYCHROMETRIC PROCESSES
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Use of the psychrometric chart for processes
We can use the psychrometric chart not only to describe states of moist air, but for a number of processes that are important for building science and HVAC applications
Examples: • Sensible cooling or heating • Warming and humidification of cold, dry air • Cooling and dehumidification of warm, humid air
– Sensible + latent cooling
• Evaporative cooling • Mixing of airstreams
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Typical commercial HVAC system air handler
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Supply air
Fan
Heating coil Cooling coil Filters
Mixed RA + OA
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Typical forced air distribution system
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Common processes: • Air mixing • Heating • Cooling • Dehumidification • Humidification
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Sensible and latent heat
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• Sensible heat transfer – Increases or decreases temperature of
a substance without undergoing a phase change
• Latent heat transfer – Heat transfer required to change the
phase of a substance (e.g., heat req’d to change liquid to vapor)
Qtotal
=Qsensible
+Qlatent
Qtotal
= mair (hexit − hinlet )
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Sensible heat transfer equation
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Qsens
= mcp(Texit−T
inlet)= Vρc
p(Texit−T
inlet)
Qsens =Rate of sensible heat xfer [Btu/hr or ton or W]
m =mass rate of air flow [lbm/hr or kg/s]V = volumetric flow rate of air [ft3 /hr or cfm or m3 /s]
ρ = density of air [lbm/ft3 or kg/m3]cp = specific heat of air [Btu/(lbm-F) or J/(kg-K)]
Texit ,Tinlet = exit and inlet temperature [°F or °C]
Qsens > 0 for heating
Qsens < 0 for cooling
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Latent heat transfer equation
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Qlat= !m
whfg
Qlat = rate of latent heat transfer [Btu/hr or ton or W]
Qlat is positive for humidification processes
!mw = rate of evaporation/condensation [lbm/hr or kg/s]
hfg = enthalpy of vaporization [Btu/lbm or J/kg]
also called latent heat of vaporization (hfg = 2260 kJ/kg for water)
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Heating and humidification of cold, dry air
• Example: Heating and humidifying coils – Process: Adding moisture and heat (sensible + latent heating)
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8°C 40% RH 45°C
30% RH
Q1: What is the enthalpy change required? Q2: What is the split between sensible and latent load?
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• d
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2
1
Warming and humidification of cold, dry air
ΔW
Δhtotal
ΔT
Δhsensible = 46 - 16 = 30 kJ/kgda Δhtotal = 64 - 16 = 48 kJ/kgda
Δhsensible
SHR = 30/48 = ~0.62
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Sensible heat ratio (SHR)
• The sensible heat ratio is defined as:
• Allows for understanding sensible load relative to latent load
• Typical SHR: 0.6 to 0.9
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SHR =!qsens!qtotal
=!qsens
!qsens + !qlatent=ΔhsensΔhtotal
Here is a process with an SHR ≈ 0.4
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Cooling and dehumidification of warm, humid air
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38°C 34% RH
13°C 90% RH
• Example: Cooling coil • Removing both moisture and heat
– Sensible + latent cooling
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• d
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1
2
Cooling and dehumidification of warm, humid air
ΔW
Δh SHR ~ 0.63
The red line is actually impossible… no moisture removal The blue line shows what really happens
ΔT
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Example: Sensible cooling
• Moist air is cooled from 40°C and 30% RH to 30°C – Q1: Does the moisture condense? – Q2: What is the RH at W at the process end point? – Q3: What is the rate of sensible heat transfer if the airflow
rate is 1000 ft3/min?
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• d
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1
Sensible cooling
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• d
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1 2
Dew point is ~19°C New T = 30°C • No condensation New RH is ~53% W does not change W1 = W2 = 0.014 kgw/kgda
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• d
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1 2
h1 ≈ 76 kJ/kg h2 ≈ 66 kJ/kg Δh ≈ -10 kJ/kg SHR = 1.0
Dew point is ~19°C New T = 30°C • No condensation New RH is ~53% W does not change W1 = W2 = 0.014 kgw/kgda
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Example: Sensible + latent cooling
• Moist air is cooled from 40°C and 30% RH to 15°C – Q1: Does the moisture condense? – Q2: What is RH at W at the process end point? – Q3: What is the rate of heat transfer if the airflow rate is
1000 ft3/min?
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• d
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1
Sensible + latent cooling
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• d
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1
2 Dew point is ~19°C New T = 15°C • There is condensation New RH is 100% W decreases to ~0.010
Two part cooling process: 1. Constant humidity ratio cooling until sat. 2. Constant RH cooling w/ moisture condensation
Δh = 42 – 76 = -34 kJ/kg
Sensible + latent cooling
SHR ~ 0.75
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Real data: ASHRAE RP-1299 Energy implications of filters
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0.008
0.010
0.012
0.014
Hum
idity
ratio
, kg/
kg
0
10
20
30Te
mpe
ratu
re, C
6 PM 7 PM 8 PMHour
Temp - before coil Temp - after coilW - before coil W - after coil
AC on AC off
Temperature and humidity ratio differences across AC coils in homes
T before coil
T after coil
W before
coil W
after coil
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Real data: ASHRAE RP-1299 Energy implications of filters
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What are we looking at?
Before coil
After coil
AC on/off
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Real data: ASHRAE RP-1299 Energy implications of filters
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0
20
40
60
80
100
RH
, %
18 19 20hod
RH - before coil RH - after coil
AC on AC off
Relative humidity differences across AC coils in homes
RH before
coil
RH after coil
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• d
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1
Sensible + latent cooling
2
Δh = 35 – 52 = -17 kJ/kg SHR ~ 0.85
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Mixing of air streams
• Often in HVAC systems we mix airstreams adiabatically – Adiabatically = Without the addition or extraction of heat – e.g. outdoor air mixed with a portion of return/recirculated air
• For most parameters, the outlet conditions end up being the weighted-averages of the input conditions – Dry bulb temperature – Humidity ratio – Enthalpy – (not wet-bulb temperature or RH though)
40
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Mixing of airstreams example
• Hot, humid outdoor air is mixed with recirculated indoor air at an outdoor air fraction of about 35% – Q1: What is T, W, RH, and h at the mixed condition?
41
20°C 40% RH
40°C 30% RH
![Page 42: CAE 331/513 Building Science - The Built Environment …built-envi.com/wp-content/uploads/2014/07/cae331_513… · · 2015-03-05CAE 331/513 Building Science Fall 2014 Week 6: September](https://reader034.fdocuments.in/reader034/viewer/2022051507/5aa03cdf7f8b9a6c178dcbbb/html5/thumbnails/42.jpg)
• d
42
2
Mixing of airstreams
1
![Page 43: CAE 331/513 Building Science - The Built Environment …built-envi.com/wp-content/uploads/2014/07/cae331_513… · · 2015-03-05CAE 331/513 Building Science Fall 2014 Week 6: September](https://reader034.fdocuments.in/reader034/viewer/2022051507/5aa03cdf7f8b9a6c178dcbbb/html5/thumbnails/43.jpg)
• d
43
2
Mixing of airstreams
1
3
T3 =mda1T1 + mda2T2mda1 + mda2
W3 =mda1W1 + mda2W2
mda1 + mda2
h3 =mda1h1 + mda2h2mda1 + mda2
T3 =0.65(20)+ 0.35(40)
1.0= 27°C
W3 =0.65(6)+ 0.35(14)
1.0= 8.8 gw
kgda
h3 =0.65(34)+ 0.35(77)
1.0= 49 kJ
kgda
![Page 44: CAE 331/513 Building Science - The Built Environment …built-envi.com/wp-content/uploads/2014/07/cae331_513… · · 2015-03-05CAE 331/513 Building Science Fall 2014 Week 6: September](https://reader034.fdocuments.in/reader034/viewer/2022051507/5aa03cdf7f8b9a6c178dcbbb/html5/thumbnails/44.jpg)
Evaporative cooling example • Hot, dry outdoor air is cooled with an evaporative cooler, or “swamp
cooler” – Q1: What is RH and W of the supply air? – Q2: Why would we choose this system?
44
32°C 20% RH
17°C ?? RH
![Page 45: CAE 331/513 Building Science - The Built Environment …built-envi.com/wp-content/uploads/2014/07/cae331_513… · · 2015-03-05CAE 331/513 Building Science Fall 2014 Week 6: September](https://reader034.fdocuments.in/reader034/viewer/2022051507/5aa03cdf7f8b9a6c178dcbbb/html5/thumbnails/45.jpg)
• d
45
1
Evaporative cooling
?
![Page 46: CAE 331/513 Building Science - The Built Environment …built-envi.com/wp-content/uploads/2014/07/cae331_513… · · 2015-03-05CAE 331/513 Building Science Fall 2014 Week 6: September](https://reader034.fdocuments.in/reader034/viewer/2022051507/5aa03cdf7f8b9a6c178dcbbb/html5/thumbnails/46.jpg)
• d
46
1
Evaporative cooling
2
![Page 47: CAE 331/513 Building Science - The Built Environment …built-envi.com/wp-content/uploads/2014/07/cae331_513… · · 2015-03-05CAE 331/513 Building Science Fall 2014 Week 6: September](https://reader034.fdocuments.in/reader034/viewer/2022051507/5aa03cdf7f8b9a6c178dcbbb/html5/thumbnails/47.jpg)
• d
47
1
Evaporative cooling
2 Cooling occurs along constant wet bulb
![Page 48: CAE 331/513 Building Science - The Built Environment …built-envi.com/wp-content/uploads/2014/07/cae331_513… · · 2015-03-05CAE 331/513 Building Science Fall 2014 Week 6: September](https://reader034.fdocuments.in/reader034/viewer/2022051507/5aa03cdf7f8b9a6c178dcbbb/html5/thumbnails/48.jpg)
• d
48
1
Evaporative cooling
2
RH near 100% W = ~12.2 gw/kgda Δh = 0
![Page 49: CAE 331/513 Building Science - The Built Environment …built-envi.com/wp-content/uploads/2014/07/cae331_513… · · 2015-03-05CAE 331/513 Building Science Fall 2014 Week 6: September](https://reader034.fdocuments.in/reader034/viewer/2022051507/5aa03cdf7f8b9a6c178dcbbb/html5/thumbnails/49.jpg)
• d
49
1
Evaporative cooling
2
RH near 100% W = ~12.2 gw/kgda Δh = 0
This means no energy is required other than for fans (for air) and pumps (for water)