Sizing a pre-heat coil with a vav system... 1

[Badbird2000]

Ok kids, hold on to your hats...
I am working on a 4 story science and mathmatics building for a local college. The system will consist of rooftop units with chilled water coils and pre-heat coils, chillers, boiler, pumps etc, with vav boxes serving the classrooms and offices. In trying to size the pre-heat coil, my intent is maintain a minimum discharge air from the unit around 55 degrees F and do heating in the spaces with reheat coils in the vav boxes. My question is, if you take a cold morning at design heating conditions, 9 degrees F, i am assuming (and that's not always a good thing) my boxes are going to be at the minimum setting, roughly 30% of my total cfm for cooling. So, i would not have the maximum airflow across the coil, mixed with the return air from the unit. In some cases, my o.a. is more than the minimum 30% airflow, so i would have to have 100% o.a. to maintain my ventilation rates. So, would i hvae to size the coil for 100% of my o.a. at 9 degrees at that point?

For example, RTU-2 has a maximum airflow of 9,500 cfm and an o.a. of 3950. 30% of the maximum is 2,850 but i need 3950 to maintain my ventilation. Would i assume it's going to see 3,950 of 9 degree air and size the pre-heat for that condition?

Any help? Am i making this harder than it needs to be?

Wayne

 

[Jazzerman]

No, you have valid thoughts Re: ventilation and VAV reheat systems. You should probably check out ASHRAE 62.1 (if you haven't already) which has a process to calculate the minimum outside air that a VAV air system should use. One design that I have done (custom air handler though, not packaged RTU) once I figured out what my minimum outside air is through the 62.1 vent rate procedure was to use a separate minimum OA ventilation duct and modulating damper (separate from the OA economizer damper). An air measuring station is then used in the MIN OA section and the min OA damper modulated to maintain the min OA setting.
Probably getting too specific for now.
Check out ASHRAE 62.1 ventilation rate procedure first. Ultimately it involves balancing your VAV box mins versus the AHU min oa rate. You might find that a certain few VAV boxes are causing the AHU sytem vent rate to need to be way higher, and if increase those box mins, you can reduce the AHU vent rate.
Another thought - if you have many interior zones they might be in cooling year round, rather than heating, and your system desing day heating AHU system cfm might be more than sum of heating mins...

 

[317069]

Your system configuration is not clear to me.
you have rooftop units with hydronic system, do you mean rooftop unit as we know which has a compressor(I doubt), or air handlers use hot/cold water located on the roof?
if you have an air handler, is it variable air volume or constant air volume, when you have VAV boxes you have to have VAV rooftop too, for a regular rooftop, the unit will have stages of operation to meet a certain level of variable load and air flow.

also you said " if you take a cold morning at design heating conditions, 9 degrees F, i am assuming (and that's not always a good thing) my boxes are going to be at the minimum setting, roughly 30% of my total cfm for cooling"
can you explain it a little more

 

[EnergyProfessional]

Why would you need heating coils n AHU? There is fresh air from ERV, and recirculated air from spaces - both likely above 55°F when mixed.. In unoccupied time (including optimized start in winter) there is 100% recirculated air.

All the heating coil does is create pressure drop and creates a potential for freezing when ERV fails. I think AHU heating coils are an archaic relict from the time when we used straight OA and didn't have ERVs. Size your VAV reheat coil properly (or better provide perimeter heat) and all will be fine.

If your AHU is an RTU, you should use glycol, or at some point it will freeze if you live in freezing climate.

 

[Badbird2000]

Jazzerman, yuo are correct. I need to break out the good o' ASHRAE book and crank through those numbers to determine actual minimum settings.
317609, sorry, yes, they are air handlers with chilled water cooling coils located on the roof. The supply fan is on a VFD to modulate with the vav boxes as required. As far as determining my pre-heat coil, i am not sure if i need to base the coil sizing on the full maximum CFM at the heating design temp, 9.
HerrKaLeun, that is where i was going with this the more i thought about last night while hanging Christmas lights. At my cfm with return air mixing with o.a., it should never see anything below 52 degrees. My goal was to leave the unit at 55 during the winter, and doing reheat at the vav boxes.

 

[EnergyProfessional]

From an energy point it doesn't matter if you add heat at zone of AHU level in heating-only mode. but you have better control at VAV level. Especially when you have mixed heating-cooling on the same AHU.

Over time AHU coil valves just get stuck open or something else bad happens. You need to size the VAV coils accordingly, since you now may get 50°F air as opposed to heated to 70°f or so.

 

[Drazen]

you reasoning in initial post seems completely valid for me.

the one practical problem mentioned by jazzerman is how to achieve what all vav boxes receive exact amount of minimum outdoor air. in practice it is mostly not possible, as some zones' min. air is 30% of the supply air, in others it can be 15% or 45%.

you should base your calculation on zones that have largest percent of outdoor air (in relation to supply air), and increase minimum outdoor air setting of other zones (proportionally). it's good if your maximum zone percentage is say 33% and average percentage is 30%. if your max. zone percentage is say 45%, that you would oversize min. outdoor air of all other zones and that could be large waste of energy. the other option is to increase design supply air of such zone to leave other zones properly sized, but if zone in question is large, there could be different troubles, first that somes to mind is how to set supply temperature after reheat coil - if your supply air is too much excessive, supply air temperature can fall unacceptably low or you can have unacceptable overheating.

i have never found or learned about universal solution for this. it would be best to separate zones with much larger percentage if possible. there are also many other means, all being somewhat unique.

 

[EnergyProfessional]

Don't use the "30%" rule, it is a myth. per ASHRAE you need (and can) calculate min-flow for each zone.

Jsut using an arbitrary percentage is wrong. The maximum flow is based on load (which can vary with windows etc.) min flow depends on people and OA in the AHU. There is no %-correlation. In addition you may need different min-flow for heating if you have ceiling distribution since warm air doesn't mix well coming from above (per ASHRAE if you have 15°F warmer than space)

 

[ChasBean1]

You are making this harder than it needs to be.

Go back to the old standby: Q = 1.08 * cfm * dT (no latent)

You’ll find that 3,950 cfm of outdoor air needs the same heating if it’s mixed with 70% return air or no return air.

Just look at the mixed section and blend it well – you can see quite a bit of stratification if your coil designed for 450 fpm is operating at 150 fpm for the same amount of heat input…

 

[irfansiddiqui]

yes keep it simple as chasbean1 has told, save cost

 

[tys90]

I concur with ChasBean. Also, if your design OA temp is 9F, I would most definitively put a pre-heating coil in with some sort of freeze protection (circulating pump or glycol). If a damper get stuck open and you get 100% of 9F degree air, you will freeze something if there is not some sort of protection in place.

Some others touched on the ASHRAE requirements - if they are applicable make sure you go through them thoroughly. Increasing a minimum flow in your critical zone can reduce your overall ventilation rate required.

 

[tys90]

I concur with ChasBean. Also, if your design OA temp is 9F, I would most definitively put a pre-heating coil in with some sort of freeze protection (circulating pump or glycol). If a damper get stuck open and you get 100% of 9F degree air, you will freeze something if there is not some sort of protection in place.

Some others touched on the ASHRAE requirements - if they are applicable make sure you go through them thoroughly. Increasing a minimum flow in your critical zone can reduce your overall ventilation rate required.

 

[EnergyProfessional]

Don't rely on circulation pump for freeze-protection, use a freeze-stat downstream of coil. those small circulation pumps can fail and you don't have redundancy. And even circulating water can freeze at some point.

this also applies to cooling coil if you don't use glycol.