TDSHS Air

Change

Requirements and

the Basics of

Psychrometrics

Presented by: Aric Murray

March 31, 2014

Air Changes and Psychrometrics

Agenda

1. What are the air change requirements

and how do you calculate them?

2. Why are air change requirements

important?

3. The Basics of reading a

Psychrometric Chart

4. Practical example of using a

Psychrometric Chart to determine

Operating Room temperature and

Relative Humidity

Air Changes and Psychrometrics

Calculating Air Changes per Hour

Air changes per minute =

Room Volume in Cubic Feet (length x width x height)

Air Delivered to Room (Cubic Feet per Minute or CFM)

Answer: 17.78 ACH Answer: NO

For example: 30 foot wide by 25 foot long operating room with a

9 foot ceiling height is receiving 2000 CFM of air to the room.

How many air changes per hour is this OR receiving?

Does this room meet the TDSHS requirement for an OR?

Air Changes per Hour (ACH) =

60 minutes

Air changes per minute

Air Changes and Psychrometrics

2012 TDSHS Ventilation Table

Air Changes and Psychrometrics

2012 TDSHS Ventilation Table

Example: How much exhaust airflow do I need to meet the minimum

requirements for an isolation room that is 12 feet wide by 25 feet long

with a 9 foot ceiling? Answer: 540 CFM

Air Changes and Psychrometrics

Basics of PsychrometricsBasics of Psychrometrics

How do you determine wet bulb temperature?

Sling Psychrometer

Air Changes and Psychrometrics

Example: What is the wet bulb temperature for a room

temperature of 75 degrees and 45% relative humidity?

Answer:

61 Degrees F

Air Changes and Psychrometrics

Example: What is the dew point temperature at the same

room conditions?

Answer:

52.2 degrees F

Air Changes and Psychrometrics

Why is this important ????

How could this be useful ????

Consider the following real world example:

An air handling unit brings in outside air through an

uninsulated duct through a mechanical room that is kept at

68 degrees F. During the summer months this outside air

duct sweats on the inside bringing moisture into the air

handler.

Why does this happen?

What could we do to fix the problem?

Air Changes and Psychrometrics

What is the dew point of the outside air

travelling through the uninsulated duct?

Use the design summer outside air

conditions for Houston at 96 Degrees F

dry bulb and 80 degrees F wet bulb.

Air Changes and Psychrometrics

Answer:

74.6 degrees F

Air Changes and Psychrometrics

Why is this happening?

The cold metal of the uninsulated

outside air duct (68 degrees F) is below

the dewpoint of the outside air (74.6

degrees F) causing the moisture in the

outside air to condense on the cold

surface.

Air Changes and Psychrometrics

What can we do to fix the problem?

1. We could insulate the outside air duct

which will keep the metal surface warmer

than the room temperature of 68 degrees and

above the dewpoint of the outside air. This

option will cost $$$

2. Raise the mechanical room temperature to

78 degrees to make sure the room and the

uninsulated duct sheetmetal stay above the

dewpoint of the outside air, 74.6 degrees F.

This will solve the problem and save $$$ in

operating costs.

Air Changes and Psychrometrics

Let’s look at another real world example:

What is the coldest temperature an Operating Room can

achieve when it receives supply air at 52 degrees F while

still keeping the Operating Room relative humidity below

the code required 60% RH?

Assume the 52 degree air is coming from a standard cooling

coil and not a desiccant wheel.

What do we need to know?

We need to know the sensible heat ratio of the Operating

Room.

Air Changes and Psychrometrics

Sensible Heat Ratio

Sensible Heat Ratio =

Sensible Heat Load in the Room

Total Heat Load in the Room

For an Operating Room the sensible heat ratio

can be assumed to be around 0.86, which means

86% of the heat produced in the room is sensible heat

and 14% is latent heat.

What does this mean Psychrometrically?

Air Changes and Psychrometrics

Air coming off the cooling coil will be saturated with

moisture at 90% relative humidity.

Answer:

65 Degrees F

Minimum

Air Changes and Psychrometrics

Questions?

Air Changes and Psychrometrics