Total CFM with VAV Master Slave

[edtpro]

Our company does environmental testing for medical and surgery centers and I've run across something I need some help understanding. I have an OR that according to the plant ops dept has a master/slave VAV set up with the master reporting supply of about 1100 CFM and the slave reporting about 800. Therefore, he is expecting about 1900 cfm of supply going into the room. When I use an Alnor hood on the diffusers in the room I am only reading about 1100 cfm TOTAL supply. In addition, there are 3 return grills in the room that are reading a total of about 1400 cfm. The room is negative to the corridor (about -.005" wg) as I would expect with these readings.

Another OR has a master/slave setup reporting abbout 2000 cfm each. But, I'm reading about 2300 total supply, 1800 return/exhaust and a positive .003" presure to the corridor.

Can someone help me understand the way master/slave VAVs operate? Is it just a simple matter of adding the two together? Are we looking at a sensor/software issue? Thanks for any help.

 

[Drazen]

there is so large diversity of applications and practices to give "one-fits-all" answer, but first what comes to mind is that customary master/slave means master controls supply, slave controls exhaust or vice-versa, but they are not adding to each other.

inconsistency of measured and read data can be addressed to sensors calibration or duct leakage, which can be confirmed only by adequate testing - ductwork leakage retesting, sensors calibration, measuring device recalibration, introducing another measuring device...

 

[urgross]

I've never heard of an OR room intentionally being set up to be negative relative to adjoining space. Master-slave valve or box typically uses the offset differential volume to reach a required differential static.

 

[edtpro]

Drazen - Thanks for your quick reply. Unfortunately, I have little experience with the source equipment/software side of HVAC. We only do the ventilation measurements in the rooms as one aspect of understanding IAQ and infection/exposure control. I do know our balometers are calibrated yearly. That's not to say we can't be wrong, but I handle about 100 medical centers and do thousands of readings a year. Based on consistent and expected readings at other facilities, I would assume they are fairly accurate at this facility as well. The maintenance guy didn't know much about the master/slave setup and couldn't say with confidence they were to be added together. It's just what he was guessing at.

I did mention the possibility of duct leakage as we have run into this before at another facility or two. I also mentioned the possibility of the system's sensors needing recalibration.

A follow up question: Is there a scenario you can think of where the info I have provided makes sense? That is, given his system is showing 1100 CFM(M) and 800CFM(S) and I'm reading about 1100 CFM supply and about 1400 Return, does that all add up somehow? I find it interesting that his master is reporting about the same number I am reading...or is this just coincidental?

Thanks for any more help or insight you can provide.

 

[edtpro]

urgross - Thanks for your reply. My comment, "The room is negative to the corridor (about -.005" wg) as I would expect with these readings" may have been misleading. I didn 't mean it was what the room was set up to be. I just meant that given I read more return in the room than supply, I would expect the room to measure negative to the adjacent corridor. The target differential pressure for the OR is a minimum .01" wg. The thing is, his (the maintenance guy) thought that his system is putting in a total of 1900 CFM supply certainly doesn't make any sense when we can both stand at the door and confirm negative pressure with our meter and his little piece of toilet paper being sucked into the room. I've never seen a piece of tp that needed calibration!

Frankly, I don't know much about the source side of the ventilation beyond the basics. We're not a T&B company and only read at the end. Any additional info or help you can give in plain ole non HVAC speak would be much appreciated. Thanks again!

 

[Drazen]

being focused only on initial question, i omitted what urgross reminded us -it does not make any sense that surgery room, intensive care or similar room has negative pressure differential.

unfortunately, data is in so large conflict that i believe speculation cannot help. something is wrong. if data from supply side are consistent with readings, than it is possible that exhaust side has some large malfunction, it could be faulty exhaust sensor, some problem with place of sensor installation and, without field checks, elaboration is possibly not helpful.

working in maintenance department some wisdom is always adopted over years, like the concept that simplest causes should be checked first, otherwise lot of time can be wasted. anyhow, adding up figures for supply and exhaust would not be an option.

 

[urgross]

One way that this might make sense is if this OR is dedicated for airborne isolation, and return is exhaust. I've only seen that in animal BSL applications.

If you know the air changes per hour required, and the volume of the room, correct supply flow could be verified.

If all OR's are off a single AHU, and only one OR is giving trouble, then the issue could be narrowed. Most frequent problems I've seen are with filter loading and volume dampers being moved/access panels left open during duct cleaning.

 

[ChasBean1]

In positive pressure spaces like ORs, supply should lead exhaust. So if supply reads 1100 cfm and the offset is 200 cfm, the exhaust should maintain 900 cfm. In negative pressure spaces such as BSL-3 labs, exhaust should lead supply. So if exhaust maintains 1100 cfm, supply should track by -200 cfm. An OR supply/exhaust failure scenario should leave the room positive. A BSL-3 lab supply/exhaust failure scenario should leave the room negative.

In a clean, positive pressure space if supply fails and airflow goes to zero, exhaust sees this and likewise goes to zero. If exhaust had failed in a clean space, supply would continue to deliver design airflow, keeping the space positive, although more positive than usual.

If supply fails in a BSL-3 space, you may see pressure issues and difficulty with door operation. But supply airflow going to zero would not mean that exhaust air should be interrupted. If exhaust airflow fails in a BSL-3 the failure should be indicated and supply air should track. The loss of exhaust would cause a corresponding reduction of supply, leading to a neutral space rather than ex-filtering BSL-3 lab air.

Edtpro, you said: "according to the plant ops dept has a master/slave VAV set up with the master reporting supply of about 1100 CFM and the slave reporting about 800. Therefore, he is expecting about 1900 cfm of supply going into the room." No, I'd instead expect this means 1100 cfm supply and exhaust tracks 300 cfm lower.