BICSI South Pacific 2011 - PUE Psychrometrics...
Transcript of BICSI South Pacific 2011 - PUE Psychrometrics...
Psychrometrics 101
Tony deFrancescoBE Mech (Hons)Technical DirectorP.U.E. Pty Ltd
1. Psychrometric basicsi. Dry & wet bulb temperatureii. Psychrometric chartiii. The heating & cooling process
2. The data centre environmenti. Data centre exampleii. What can alter the moisture leveliii. Why is precise control important
3. ASHRAE recommended guidelines4. Effect of moisture on free-cooling
i. Free-cooling according to ASHRAE indoor limitsii. How to increase free-cooling hours
5. Summary
Topics
Psychrometric basics
Psychrometrics
Psychrometrics or psychrometry are terms usedto describe the study of gas-vapour mixtures, inparticular the mixture of water vapour and air
Dry bulbtemperature
(Tdb)
Wet bulbtemperature
(Twb)
• Wet bulb temperature isan indication of theamount of moisture inthe air
• Wet bulb temperature isthe lowest temperaturethat can be reached bythe evaporation of wateronly
• Dry bulb temperature isthe temperature that isusually thought of asthe “air temperature”
• Dry bulb temperatureprovides no indicationof the amount ofmoisture in the air
Dry & wet bulb temperature
Wet bulbtemperature
Dry bulbtemperature
Moisturecontent
Relativehumidity
Psychrometric chart
Heating airCooling air
Addingmoisture
Drying air
Saturation line(i.e. maximummoisture content)
Psychrometric chart
1kg of air
15g moisture
20°C
100%RH
1kg of air
15g moisture
32°C
50%RH
+12°C
Note: No change in the amountof moisture, but a change in
relative humidity!!
Heating example
ΔTdb = 12°C
Heating example
1kg of air
20g moisture
25°C
100%RH
1kg of air
15g moisture
20°C
100%RH
-5°C
5g water
Note: No change in relative humidityand no change in the total amount ofmoisture, but a change in its state!!
Cooling example
ΔW = 5g
ΔTdb = 5°C
NOTE: When air is at 100%RH,any cooling will result inprecipitation (i.e. rain)
Cooling example
The data centre environment
13°C @ 90%RH
22°C @ 50%RH35°C @ 23%RH
Temperature & relative humidity changes throughout theroom, but the amount of moisture in the air does not change!
8.4g/kg
8.4 g/kg8.4 g/kg
Data centre example
W = 8.4g
Data centre example
In increase in the air’s moisture can occur due to:
1. Introduction of moist outside air (typically summer)
2. People
3. Humidification
A decrease in the air’s moisture can occur due to:
1. Introduction of dry outside air (typically winter)
2. Cooling the air
What changes the room’s moisture?
If the supply air cools from 13°C@50%RH to <11.5°C,water would be present in the data centre!!
13°C @ 90%RH
22°C @ 50%RH
11.5°C @ 100%RH!!!!
20.5°C @ 55%RH
Why is precise control important?
ASHRAE recommendedguidelines
Allowable Operating Environment
Revised 2008 RecommendedOperating Environment Limits
Existing 2004 RecommendedOperating Environment Limits
Class 1 & 2 Operating Environments
NOTE: The Recommended range is a statement on reliability,the Allowable range is not.
ASHRAE environmental guidelines
Effect of moisture onfree-cooling
Outside air inwhen temperature
and moisturelevels within limits
20°C
32°C
Free-cooling: Direct
Upper limit: 8.7g/kg
Lower limit: 5.6g/kg
20°C
Free-cooling: ASHRAE guidelines
Moisture must be:
> 5.6 g/kg
< 8.7g/kg
20°C
32°C
Free-cooling: ASHRAE guidelines
BRISBANEBRISBANE
NO FC
SYDNEYSYDNEY
NO FC
CANBERRACANBERRA
NO FC
Free-cooling: ASHRAE guidelines
MELBOURNEMELBOURNE
NO FC
Indirect FCIndirect free-cooling a function of:
• Ambient Temperature
Direct free-cooling a function of:
• Ambient Temperature
• Ambient Moisture (%RH)
• Ambient Air Quality
Direct FCHow to increase free-cooling hours
Sydney temperature levels basedon a 20°C S/A and 32°C R/A
Direct FC Indirect FC
FC54.1%
MIX45.7%
DX0.3%
FC49.4%
MIX49.6%
DX1.0%
Ambient temperature
Sydney moisture levels with limits shown according to ASHRAE EnvironmentalGuidelines (2008) for 20°C room setpoint temperature
FC30%
FC100%
Direct FC Indirect FC
Ambient moisture
Sydney temperature levels based on a 20°C S/Aand 32°C R/A and moisture levels according toASHRAE Environmental Guidelines (2008)
Direct FC Indirect FC
Ambient temperature & moisture
Summary1. Relative humidity is not an absolute measure of the moisture in the air.
It is an indication of the level of saturation of the air.
2. A change in relative humidity does not necessarily mean a change inthe air’s absolute moisture content.
3. The amount of moisture in the air is best measured by determining theair’s moisture content from a psychrometric chart.
4. In most of Australia’s cities, the use of outside air to directly cool adata centre is heavily restricted due to the very high and low levels ofmoisture typically present
5. The amount of free-cooling hours in a year can be greatly increasedby introducing a heat exchanger to separate the indoor and outdoor airstreams.
Thank you