TDSHS Air Change Requirements and the Basics of Psychrometrics · Air Changes and Psychrometrics...
Transcript of TDSHS Air Change Requirements and the Basics of Psychrometrics · Air Changes and Psychrometrics...
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