Laminar Flow Diffusers

[williamsGA]

Please read the below email, it is in response to a questions regarding the face velocity for laminar flow diffusers in operating rooms. My understanding is that a laminar flow diffuser does not entrain any room air as it exits the diffuser. Without additional volume how does the air speed up after it has left the diffuser face as the email below suggests. Any ideas will be helpful.

Thanks,

In reference to our conversation from a couple days ago, I wanted to try to clarify some of the confusion that I find often occurs when reading this spec. Reading from the 2006 AIA "Guidelines for Design and Construction of Health Care Facilities" Page 120, Appendix A10.2.2.4(3)(a):
d. A face velocity of around 25 to 35 fpm is sufficient from the non-aspirating diffuser array....
and page 121, Section 10.2.2.4:
(a) In new construction and major renovation work, air supply shall be from non-aspirating ceiling diffusers with a face velocity in the range of 25 to 35 fpm...

This seems to contradict the accepted values for average air velocities of 40-70 fpm at table height. However, these AIA guidelines above refer to the velocity of the air leaving each perforated hole in the face of the diffuser (our LFD), after which time the air recombines and gains speed as it begins to fall. This is how the LFD can meet these guidelines even though published performance data lists the average velocity of this air to be anywhere from 60-175 fpm at just 1 foot below the diffuser face.

 

[limeycad]

From my experience the air when exiting the perforated face of the diffuser combines to give a measurable velocity around 1 foot from the face. As the air travels downwards it will start to peel off at the outer edges of the diffuser canopy (hopefully forcing any airborne particles away from the table in the direction of the extract path). This peel off will cause a reduction in the total volume (and hence velocity) by the time the air reaches the table.

If you imagine the airflow to resemble an inverted funnel, with the spout being the cross section of the canopy & the rim of the funnel being the cross section that the airflow has spread to at table height,then you can envisage the way the air velocity will reduce by the time it arrive at the table.

Rob