Redundancy with common duct system? 1

[txdilbert]

What is the typical approach method we use when attempting to provide redundant floor-mounted indoor air handlers if only one duct system will fit in a space? Are they stacked and ducted together at the supply and return flanges? Do we provide duct-mounted motorized dampers to minimize the pressure drop at the common connection points?

Thanks in advance.

 

[ChrisConley]

I'm really not seeing this configuration. You have two air handling units (one is redundant) feed a single supply and single return duct? Or are there redundant ducts?

Motorized dampers to isolate the non-functioning AHU would be a start.

 

[DrRTU]

I like to use heavy duty back draft dampers in place of a motorized damper as a means of isolating a redundant unit. GDDs do not require wiring but have a larger delta P - which means higher operating costs. Motorized dampers take time to stroke and if a second unit is opened during first unit operation, the second fan may trip on OL if the fan spins backwards. I prefer gravity dampers to be in the horizontal configuration i.e. upflow air direction. This isolation configuration is common on upflow Liebert units in an N+1 or N+2 configuration in ducted phone switches. The most I have done were six on common duct design.

 

[quark]

A redundant AHU? I heard it for the first time. Can I know the logic behind redundancy at AHU level. I couldn't think of anything.

 

[RossABQ]

If you need to keep a process room ventilated, cooled, etc, without any shutdowns 24/7/365, how else would you deal with a coil failure? (or at least be able to replace the failed coil). Very common on semiconductor fabs, cleanrooms, HPM rooms, etc. Preventive maintenance, filter changes, etc. all drive a completely redundant unit.

 

[marcoh]

Recently we did a flight simulator facility which required a redundant AHU which was installed with a single SA and RA duct with motorised dampers.

The clients logic was that the facility runs 24/7, is booked out a year in advance and the additional AHU cost was insignificant to loosing a days revenue and having to reschedule their customers.

 

[AbbyNormal]

I did a telco centre majority of the systems were redundant air handlers, small ones I used gravity back draft dampers, larger ones I used motorized dampers.

was all DX, from pairs of 2000 CFM up to around pairs of 20000 CFM for the main switch

most challenging was a pair of 40 ton lieberts, each liebert had two separte blowers in it.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[quark]

I understand fully that a stand by equipment supports if the active equipment fails. My question is how often this happens with an AHU. I have been handling pharmaceutical cleanrooms (including high containment ones) for 12 years and we have fans and coils as spare, at the most. We always keep a spare set of filters.

If the recovery test is done periodically, along with other environmental performance tests, revalidation doesn't take much time.

I would prefer actuated dampers.

 

[AbbyNormal]

i would say it is common in data centres etc.

The lieberts I was mentioning were in essence, 2 20 ton units in one casing, thwo separate blowers with motors, two separate evaporators, 2 separate semi hermetics and yet being a smidge low on oil takes them out.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

alternate them daily, have the controls switch over to the idle unit if room temperature rises a couple degrees above normal and send a warning alarm.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

quark

I would suggest it is more like having an automatic back up that switches over when a system goes down, as opposed to having the repair parts on hand and having to wait for the repair to be made. It would take some time to pull a coil and replace it.

Temperature can rise pretty quick in some applications when a system goes down.



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[mauricestoker]

Typical method I have used is to have common duct manifolded between the units, with end switch and delay for automatic rollover on lag-lead, discharge static pressure sensor (preferred to paddle), and CT on control circuit if using belt driven. If provding "redundant" on the supply side, same consideration should be given to exhaust or return. "Redundant" should include control as well; if using a VFD, that means a bit more cost for bypass.
Redundant unit is mandatory for BSL-3 (BMBL) and typcial for vivarium. As on renovations, floor space is limited, I have frequently used split case units, such as by Flanders.
Before going too far, I would recommend having "redundant" defined very clearly, as even when it appears to be explicit by criteria, the interpretation will vary widely. An example which I have encountered frequently is on the power side: does redundancy include splitting feeds between MCC's? I've had a BSL-3 drop on failure of normal distribution, meaning the gennie doesn't turn on. Did not happen twice, and the point was no longer argued. Yesterday I spent 3 hours in a meeting going over this exact issue, defing "redundancy" as opposed to only having the word inserted in a contract (a computer center). End result was customer agreeing to more money than orginally expected to provide "redundant" feeders to a dedicated double end for full A-B supply, with separate UPS. As part of the compromise, the customer agreed that "redundant" fuel supply for the gennie was not required, and should improve lead time. For USP 797 compiance, "redundant" meant two sources of cooling medium, chilled water primary dedicated (a 3-TR dedicated unit) backed by house system and no DX allowed. Heating medium was "redundant" with electric (on e-power) as primary and building system as redundant. No redundant AHU or exhuast was needed, and this met the USP 797 criteria for the contract.
Redundancy is very costly, and no assumptions should be made. State exactly what you are proposing before the dollars come due.

 

[RossABQ]

Excellent points Maurice, the only one I'd add is that the PLC controls need to be included in the discussion, as there are options there too -- UPS/battery backup/ride-thru, redundant sensors with averaging, and so on.

 

[quark]

Redundant unit is mandatory for BSL-3 (BMBL)

Can I know which regulation you are referring to?

 

[mauricestoker]

The regulation is the Biosafety in Microbiological and Biomedical Laboratories. May as well include ILAR and Biosurety to the publications where "redundant" is required, but not well defined.

This one is tricky, as the criteria is to "minimize" potential release based on risk assessment. On two different jobs on the same floor of a building, redundancy requirements between BSL-3 vs. ABSL-3 have been over $1.3M. The fun stories from those jobs (and a couple more) make me always verify what is intended by "redundant". Adding in biosurety was the final difference between "difficult" and ulcerous.

Very happy to no longer be in that business. Too many unpleasant memories for a life time.