Redundancy Airflow Ducting

[mech5]

I have a scenario where the client wants 50% redundancy using 2 constant volume units. This means I have 2 units each sized for 75% of the TOTAL load. My question is what airflow do I use for sizing the duct? There will be a common duct and I will use a horizontal mounted back draft damper on the connections to the units. If I size it for the total redundant load airflow, 150%, I will be over-cooling the space??? If I size it for only 1 unit operating at once, what happens when the other unit kicks on?? My duct will be too small to ahndle the airflow.??? I guess I just need to know how these units will operate.
Any help would be appreciated.
Thanks in advance.

 

[Drazen]

you may need to check velocities, there is always range of recommended velocities, so try to put your duct in a way that design velocity falls near mid-range of recommendation, while top flow velocity lays somewhere near maximum. if you cannot achieve that, than look at maximum possible velocity only.

lower value velocity is not so crucial for sanitary air applications.

 

[EnergyProfessional]

Assuming each unit has 75%, there will be times of both running. so your duct needs to handle both flows.

why not use some VAV setup? It sure also would be more comfortable f you have fluctuating loads. Your scenario gives you a 2-stage cooling and if you have over 75% load, it will cycle the 2nd unit on/off with over/undershooting the temp. The same will happen under 75% load.

depending on your actual failure scenario and needs you also could use one airhandler with 2 fans, and a DX system with 2 independent refrigerant circuits. If it is all variable speed (fan and compressor), you get big savings and still have pretty good redundancy and save on duct size.

Maybe tell us more about the application and we can come up with better ideas. How the load is over time will be important in decision-making.

 

[mech5]

The load is from VFD's and should remain constant 24/7.

 

[MintJulep]

What is the point?

If one unit fails the equipment overheats since you have only 75% of the required capacity.

 

[mech5]

Ok, well the client said they usually have 50-60 redundancy with thier HVAC equipment. How should I take that? Put in 2 systems at 75%? Three at 50% total load each? If this is what you heard from a client what how would you approach it?...

Btw, if one system goes down and one is satisfying 75% of the load, wouldn't the system only lose it's ability to cool down to the required setpoint (75F)? Instead, woulnd't it only cool down to let's say 80F ???

Thanks

 

[MintJulep]

If this is what you heard from a client what how would you approach it?...

I would ask the client to clarify what they really want before I spent a lot of time going down a potentially wrong path.

wouldn't the system only lose it's ability to cool down to the required setpoint (75F)? Instead, woulnd't it only cool down to let's say 80F ???

Heat out = heat in --> constant temperature

Heat out < heat in --> constantly increasing temperature

Depending on things that you haven't told us about the conditioned space the equipment may or may not be the only source of "heat in" and the cooling system may or may not be the only source of "heat out".

 

[CarolinaPE]

Under your scenario you would have to size the ductwork to accomodate airflow from both units. With neither sized large enough to handle the load, both are anticipated to be on-line at peak load. Some good questions have been raised as to the overall strategy to have 50% redundancy required by the client. Is the client a mechanical engineer or maybe a (micro) project manager? I have had architects insist a building be conditioned a certain way (geothermal heat pumps for example) until they see the price tag. This may be a case where the client is best served the let the mechanical engineer come up with the mechanical engineering strategy.

 

[IRstuff]

You do need to ask for clarification; the term "redundancy" can be interpreted in a couple of different ways, either to mean a backup in case of failure, or to mean excess capacity for design margin or surge capacity.

TTFN
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[SAK9]

Not a great solution to have the units sized for 75%.Most of the time you will have two oversized units trying to cool the space.Either go for 2 or 3X50% or 2&100%

 

[EnergyProfessional]

I think you should hire an engineer who has experience with such applications and its actual redundancy requirements.
The client likely doesn't know what he actually needs. Redundancy could mean the 2 systems with independent generators with independent fuel sources...

Regarding load you should find out what permanent and what temporary operating temperature those units require. I haven't come across VFD that need cooling that couldn't be met by just ventilating with OA.
If you really need refrigerated cooling, you should know what temperature is needed for permanent operation (this may be 80°F), and what the VFD can stand short term (this could be 120°F). you also need to gage how quickly a broken unit can be fixed and how long an outage of one would be the case.

for the cooling equipment you need the temperatures. Because your 10-ton equipment has 10 tons capacity as specific temperatures. If it is warmer inside, the capacity increases, so that way the temperature will stabilize at a higher temp if one unit fails.

If you end up actually needing the 100%, SAK9 advice to use 2x100% or 3x50% may be the best.

The load likely varies. first your VFD has different loads (whatever it is controlling) so it dissipates different heat amounts. Second, the heat that goes to surrounding spaces also varies as those spaces change. third, you won't be able to find a system that actually provides 100.00000% of your load. All fans, DX systems come in specific nominal sizes that unlikely are exactly the size you need.

 

[mech5]

I know I stated earlier that the load will remain constant, but after clarification, the load can vary to some degree. I figured one unit sized at 75% the total load should operate the lion's share of the time throughout the year. I'm sure if I specify units that utilize 2 stage cooling, it will help the situation.

 

[IRstuff]

There still seems to be missing requirements. If you are really sure that this "redundancy" requirement is a reliability thing and not a design margin thing, then the correct answer should be 3 units at 50% each. Then, if any single unit fails, you still have full design capacity.

TTFN
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[urgross]

Two at 100% in split casing is what I have used for BSL and AAALAC.

 

[DRWeig]

Ok, well the client said they usually have 50-60 redundancy with thier HVAC equipment. How should I take that? Put in 2 systems at 75%? Three at 50% total load each? If this is what you heard from a client what how would you approach it?...

To sum up the above, here's what I would do.

Question to client:

"You have asked for 50 percent redundancy. There are different ways to interpret that and I need an interpretation so that I give you what you want. Best would be if you could describe what you have done before in other facilities.

Or, I'll give you some examples from which to choose:

1) You need reliability to always meet your load. Say that load is 100 tons. 50 percent of that is 50 tons. The reliability solution would be 3 each 50-ton units, so that you can meet the 100 ton load with one unit out of service. We call this N+1, you have one unit considered as spare or standby that can be brought online to restore full capacity in the event of failure or maintenance downtime on one of the other units.

2) You need room for expected growth in your load but you don't want to pay for an idle unit for standby. For the 100 ton case, that means that you expect to have more cooling load added in the future, like an addition to your building or a whole bunch more computers than you have now. We call this spare capacity. In this case, two 75-ton units or a single 150-ton unit will give you the 50 unused percent you need for load growth.

3) You need your 100 ton load satisfied in normal operation, but you would be happy with reduced capacity during maintenance or failure of one unit. Your 50 percent redundancy might mean two 75-ton units = 150 tons. When one unit fails, you can meet 75 percent of the load and cover it by unplugging all of your coffee makers, snack machines, and microwave ovens. We call this 'kind of silly."

I really need you to clarify for me. Are you looking for one of those three solutions, or something different?"







Best to you,

Goober Dave

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[mech5]

Thanks everyone for the input. I like your response DRWeig. This really helps out.
Thanks again!