ASHRAE 62.1 Outdoor Air and Ventilation 5

[Mike1833]

Currently I am in the point of design where I must determine the outside air/ ventilation requirements for a building by using the 62.1 2010 "ventilation rate procedure." I am a bit confused about the two terms "ventilation" and "outside air"that are being thrown around and would appreciate any guidance.

Standard 62.1 lists a method called "ventilation rate procedure." This method calculates the required outside air needed based on a zone's occupancy and square footage. (I thought ventilation was just replacing air in a volume regardless if it is outside air or return air)

I thought there would be another set of requirements aimed specifically towards ventilation, independent of outside air. I would think ventilation would simply be the amount of air changes per hour that your supply air (return air plus outside air) can achieve on a space. I was anticipating a table that would provide such values, but all I see is an outdoor air requirement method that I described above.

Now to me, finding the required outdoor air amount to be used in a building does not really tell me Im getting proper ventilation (air changes per hour) into my zones because the supply air rate is what really does the air changing, and I can only base this supply air on my cooling load since I have no other standard to look at here. This is the only method I see, and I am skeptical that there should be more info stating how much supply air is required to each zone to reach an acceptable air change per hour. I am new at this so I may be missing a key point. Your thoughts are appreciated. Thanks.

 

[DRWeig]

Hi Mike,

Welcome to eng-tips.

In my experience, ventilation is always outside air, which is dictated by occupancy and square footage in each space. Supply air CFM is a function of the heating and/or cooling load in the space. To my knowledge, there is no standard for supply air per square foot (or per person) that is independent of outside air. Ventilation = outside air.

Note: I'm a controls and energy-simulation guy, my experience is in making the ventilation work right and analyzing what somebody else designed to compute energy use. I'm not a designer. Hang around, there are some very experienced MEs in this forum.

Best to you,

Goober Dave

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[joeag]

I'm with DRWeig.

I'm still pretty new to this HVAC stuff myself, but as far as I know, there is no other standard that dictates supply air other than the outside air specified in ASHRAE.

I've been working on a couple projects, and quickly looking over our calculations and assumptions everything is wholesomely based on your required cooling and heating loads.

 

[urgross]

I think you would need to consider specific applications. A wet lab may take 10 air changes per hour, but the 10 ACH doesn't need to be outside air. Health care facilities have OA and total ACH well spelled out. Pharmacies and clean rooms will have ACH typically based on ISO level. Federal agencies typically have their own requirements for OA and total ACH.

While I know of no one-size fits all, airborne odor control would be a typical area where OA is not a requirement; public bathrooms would be an example. Outside air is not always required, return air under the door can be used.

 

[cdxx139]

Take another look at 62-2010. The Ventilation Rate Procedure uses Table 6-1 gives People OUTDOOR Air Rate and Area OUTDOOR Air Rate. If that amount of air doesnt meet your cooling or heating load (as stated by drweig), or ACH rate requirements (as stated by urgros), then increase the supply cfm (usually with return air), to get the supply air needed.

I suggest you re-read 62's fine print when you want to fall asleep at night.

knowledge is power

 

[Mike1833]

Thanks everyone for your help. Ugross, are these ACH requirements in 62.1? These are the kind of requirements Im speaking of. Thanks.

When taking HVAC design classes (some time ago), there were three possible constraints that determined your supply air. The first was to figure out your cooling load on the zone. This would be the minnimum supply air rate ( never should it go under the cooling load supply rate). The second constraint was occupancy (say 15 cfm of supply air per person). If this "supply air" requirement was greatest, than it was to be used for the supply air rate instead of the cooling load supply air rate.(this is generally where Im troubled because this was a requirement (for school, not in the standard) based on supply air, not outside air, and that's why I was surprised to not see it in the standard 62.1). [The third constraint was makeup air from exhaust. The supply air would have to increase if your blowing out more than whats required by the cooling load or cfm/person (this constaint is a bit off topic since exhaust is independent of this discussion)].

So in summary, Im looking for supply air constraints based on occupancy (like Urgross pointed out), but also a cfm/people supply air constraint.

Consider this scenario... I have a specific room that requires 1000CFM of outside air (found by following the "ventilation requirements"). If my air conditioning unit services 10 different rooms, each room having a different outside air requirement, how can this requirement be met for every room? The supply air rates to each room can be controlled with actuated dampers, but the outside air is just going to be a fixed percentage of the supply air regardless, right? So when a specific room requires ventilation, I would think to look for a supply air constraint.
Your thoughts are greatly appreciated!

 

[urgross]

No, the ACH requirements are not in 62.1. I don't think there is a one-size-fits all. Criteria is usually based on a specific function or occupancy. Hospitals would be a prime example. Refer to ASHRAE 170 and you will get recommended ACH for most areas of a hospital. Even with that, additional criteria goes into specific areas, such as pharmacy. Room ACH might go up to 40 for toxic compounding based on ISO level requirements. Some areas the ACH and OA are spelled out based upon the HVAC system being used. As an example, nursing homes require 6 ACH per ASHRAE and 2 OA ACH. If using a 4-pipe fan coil for envelope load, the total ACH can be reduced to 4.

The OA fraction calculationsalso change with system configuration. No overall guidance is known, you have to look at the occupancy and the proposed system.

 

[DRWeig]

This portion of your question confuses me:

The second constraint was occupancy (say 15 cfm of supply air per person). If this "supply air" requirement was greatest, than it was to be used for the supply air rate instead of the cooling load supply air rate.(this is generally where Im troubled because this was a requirement (for school, not in the standard) based on supply air, not outside air, and that's why I was surprised to not see it in the standard 62.1)

Are you saying that your space had a requirement of 15 CFM supply air per person, not 15 CFM of outside air per person? If so, what sort of space was it? I'm curious, because 15 CFM per person was the standard outside air quantity for a lot of ordinary occupancies back when all we had were tables in the standards.

What everyone is getting at is this: For special areas (see urgross's replies), there will often be set air changes that must be met. For non-special areas, such as an office or classroom, there is no standard requirement. There is only a standard for ventilation (outside air).

As for apportioning outside air amongst your zones, you'll have to control the outside air fraction of the AHU to meet the worst case zone's needs.

Take a look through this thread for some discussion of one way to vary ventilation in a VAV system:

http://eng-tips.com/viewthread.cfm?qid=326735

Best to you,

Goober Dave

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[Mike1833]

Are you saying that your space had a requirement of 15 CFM supply air per person, not 15 CFM of outside air per person? If so, what sort of space was it? I'm curious, because 15 CFM per person was the standard outside air quantity for a lot of ordinary occupancies back when all we had were tables in the standards.

Right. At least that is the way it was formatted in my class. It was a standard office space. Even when analyzing standard classrooms or offices we used a certain CFM per person of supply air to make sure it was not higher than the required load. I am going to contact my old professor to clarify this. I may have interpreted it wrong. I knew it was given as a supply air constraint but it really never did go over the cooling load requirement anyways.

If it did, perhaps I would have had to increase the supply air requirement for that zone, not to 15 cfm/person, but to a higher supply air amount that had the correct percentage of outside air of 15cfm/person.

As for apportioning outside air amongst your zones, you'll have to control the outside air fraction of the AHU to meet the worst case zone's needs.

Great point. In lieu of this topic, what if the cooling load supply requirement was lower than the amount of outside air needed (just out of curiosity since Iv'e never encountered this)? I would think I would first have to predetermine an outside air percentage, then raise the supply air requirement for that zone to a CFM that contains that outside air needed. But how can I predetermine an outside air percentage without knowledge of the new supply air requirement? It seems like this may be an iterative process.

 

[DRWeig]

If it's a constant-volume air handler, no problem. The minimum outside air for the AHU was set at commissioning time, and the supply air to the space remains constant. Thus, outdoor air is fine at all times as long as the fan is running. The cooling coil simply cycles (dx) or modulates (chw) to meet the load with the constant air flow, and if the supply air flow is greater than what is needed even with supply air temperature reset, the reheat system comes into play.

If it's a VAV air handler, it can be done in several different ways. The one in the thread I linked above is my favorite. Without demand control for ventilation, the controller needs to know the air flow to the space and the percent of osa in the air from the air handler. It will maintain air flow to meet the minimum ventilation rate, no matter the load. Air flow monitoring at each VAV box is not totally necessary to accomplish this. With a known duct pressure and VAV box damper position, the percent of maximum supply air flow to the space can be estimated.

In both cases, if the required osa cfm exceeds the supply cfm for a space, the osa flow rate is maintained and the supply air is either reheated to maintain comfort or else the supply air temperature is modulated (within limits).

You might benefit from looking into sequences of operation for AHU and VAV systems that have already been done. Examples are usually available online from public universities -- the specs for jobs to be bid are public. There might be some building automation tutorials available that cover the topic as well. What you're asking is more of a controls issue than an equipment design issue.

Again, my humble opinion only. I teach a couple of classes that cover ventilation control with building automation, so I'm used to covering it from that standpoint.

Best to you,

Goober Dave

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[Drazen]

mike, i believe there is nothing confusing about that, but you need to study basic knowledge firstly, than you will not have to waste your energy in reinventing the wheel.

ventilation rate is rate of replacement of room air with outside air. period. supply air has nothing to do with that. relation between supply air and outdoor air does affect all your design concepts, decisions on system and equipment, but you calculate fresh air and supply air requirements independently, one per 62.1 or other applicable standard, the other per thermal load calculations.


==

Drweig, i opened one post some time ago, about how to ensure minimum outdoor air in different vav setups, but did not reach "one bullet kills all" answer, which is not surprising as that is not fully covered anywhere. if you size your vav system to ensure minimum outdoor air rate at minimum supply flow situation, you are still not sure that each and every zone will receive minimum, for simple reason that not all zones have the same supply air/outdoor air ratios, and that is what system fan can deliver. if you size your system based on zone with lowest outdoor/supply air ratio, than your other zones will be over-ventilated, which poses energy efficiency questions. i believe vav concept itself never solved that problem fully, and it is only dcv concept that can make more sense in such setups, in future (until something new would be invented).

 

[Drazen]

oops, i did not realized that you linked to that very post ...

 

[urgross]

Drazen,

That sounds lika a real good argument for DOAS and delinking ventilation air from thermal loading.

 

[DRWeig]

You're on the money as usual, Drazen. A minimum outside air injection fan helps, but it's not perfect either.

I love DOAS, urgross! Apparently, though, only the military and the IRS can afford it.

Best to you,

Goober Dave

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