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