Thursday, April 12, 2012

Hospital Operating Air Systems and ASHRAE 170

There have been a number of recent calls regarding the application of the Krueger Sterilflo air curtain system and the requirements of ASHRAE 170, the Hospital Ventilation Standard. When we noted to the committee developing 170, the chair insisted that 170 did not prohibit an air curtain system, but does not endorse them either. He mentioned that there was (at that time) no ASHRAE peer reviewed paper on air curtain systems. Gerry Cook and I immediately prepared one, and it was presented at an ASHRAE seminar. The paper has been turned into a Krueger White paper which can be found at http://www.krueger-hvac.com/lit/pdf/white_aircurtainsystems.pdf. Nonetheless, the standard was published without mention of the air curtain type of hospital operating room air delivery system.

In short, the ASHRAE Standard wants to see a series of “non aspirating” laminar panels over the patient with the rest of the required airflow delivered vertically, but not necessarily from a non-aspirating diffuser. There are two realities that one must resolve in attempting to meet this standard’s requirement. The first is that no laminar panel system is truly “non aspirating”. The committee actually witnessed this fact in a manufacturer’s lab during one of the ASHRAE meetings. Laminar panels induce some air at the perimeter of their area. The second is that this induction adds to the mass of air traveling down towards the patient’s table, increasing the table velocity. There is also the “coke bottle” effect in which the colder air tends to contract the downward air pattern, also increasing the velocity at the table. Finally, the greater the number of adjacent panels, the greater the “mass effect” which also increases the local air speed at the table top. There is a great likelihood that an all panel system will exceed the velocities specified at the operating table with the required face velocities at the panels, due to all the effects listed.

An air curtain system keeps the size of the panels as small as possible (avoiding the mass effect), and the air curtain prevents the induced air from coming outside of the sterile area surrounding the patient. Tests have been conducted measuring viable particles during actual operations (a while back, as it is unlikely one could do so today with the potential liabilities that exist these days). These tests confirmed that the air curtain system is much less likely to create a non-sterile condition than a standard laminar panel system.

Meeting ASHRAE 170 is simply a matter of managing the areas of panels and air curtains to comply with ASHRAE 170.
Authored by: Dan Int-Hout, Chief Engineer Krueger