TDSHS Air Change Requirements and the Basics of Psychrometrics Presented by: Aric Murray December 06, 2013
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
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) Air Changes per Hour (ACH) = 60 minutes Air changes per minute 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? Answer: 17.78 ACH Answer: NO
2012 TDSHS Ventilation Table
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
Basics of Psychrometrics How do you determine wet bulb temperature? Sling Psychrometer
Example: What is the wet bulb temperature for a room temperature of 75 degrees and 45% relative humidity? Answer: 61 Degrees F
Example: What is the dew point temperature at the same room conditions? Answer: 52.2 degrees F
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?
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.
Answer: 74.6 degrees F
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.
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.
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.
Sensible Heat Ratio = Sensible Heat Load in the Room Total Heat Load in the Room Sensible Heat Ratio 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 coming off the cooling coil will be saturated with moisture at 90% relative humidity. Answer: 65 Degrees F Minimum
Questions?