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Maximum Air Exchange Rate per Hour

Maximum Air Exchange Rate per Hour

Maximum Air Exchange Rate per Hour

I have a very small lab which is 12' by 18' with 8' ceiling.  My client would like to install some fume hoods inside there.  Based on ASHRAE requirement, 10 air exchange per hour minimum is required in the lab area.  If that is the case, the minimum supply and exhaust CFM is 288CFM.  If I install two fume hoods there, the air flow will be around 3000CFM and the air exchange ratio will be more than 100!!!  Theroically it should be ok but I think it will be too windy inside the lab.  My question is: what is the maximum nominal air exchange rate in such a small area?

RE: Maximum Air Exchange Rate per Hour

I think you will be stuck providing the required make-up air for the fume hoods.  In order that you minimize turbulence you would likely require a number of laminar flow diffusers supplying air to the space.  (Similar to EH Price RFD diffusers for labs)

RE: Maximum Air Exchange Rate per Hour

Thanks walkes,

I agree with you I need a ceiling mounted diffusers to distrib the air flow.  My question is:  Based on a common pratrice, what is the max air exchange rate per hour without having any problem?

RE: Maximum Air Exchange Rate per Hour

Depends, I've seen air change rates as high as 60. The key point, as walkes mentioned, is that the diffusers be selected to introduce the air at a low velocity.

For reference, 3000 cfm supplied low into a 12x18 room and exhausted high results in a rough velocity of 14 ft/min through the space(3000 ft3/min / 216 ft2 = 14 ft/min pretending for a moment that the room is a duct).

14 ft/min is not a perceivable airflow, 50 ft/min is just noticeable.

Think about where the air is introduced, and where it is exhausted. Velocity will be highest off the diffuser, so control it there.

RE: Maximum Air Exchange Rate per Hour

1500 cfm per fumehood seems to be a bit high. You should maintain a velocity of 0.5m/s across the sash opening. If the fume hood width is 4', with a sash opening of not more than 1' (ideal), the make up air will be about 4*1*100 = 400cfm. This gives you about 27ACPH with two fume hoods. I used Kewaunee (check the spelling) fumehoods that consume about 300cfm/fumehood, with excellent performance.

RE: Maximum Air Exchange Rate per Hour

See if you can install a push-pull system for the fume hood(s).  Such system exist in restaurant kitchens over fryolators, grills and ovens.

RE: Maximum Air Exchange Rate per Hour

You've asked a good question but without providing enough information to answer it completely. You also need to evaluate the system you offer for complete code and ANSI standard compliance--which sadly many are not completely up to date on.

1. 1500 cfm--6' FH's have approx 12 sq-ft opening x 100 fpm = 1200 cfm. 8' FH's have approx 16.5 sq-ft x 100 = 1650 cfm. So 1500 cfm is most likely a 7' FH sized for 100 fpm.

2. OSHA requires you provide 4-12 air changes of OA. The ASHRAE ACH you mentioned is a guideline and has no legal standing--so it should be treated as a benchmark. Reducing excessive outside air in our labs is a priority of Labs21.

3. Designing for a reduced sash opening as suggested above should be avoided. ANSI/ASHRAE-110 requires the FH's be tested to abuse and that is widely known to be full sash open. Another way to read that is you, the engineer, have no clue or control over how the FH will be operated. So the best way to minimize your risk is to test the FH at full open AND at whatever sash height your Owner uses.

4. If you read ACGIH guidelines governing FH's you see that FH's should idelaly be operated horizontally. Vertical operation is intrinsically less safe than horizontal and as such we should do everything we can to avoid vertical operation. Vertical operation has only achieved prominence because none of the Lab HVAC control companies can accurately track horizontal sash movement.

5. testing--you, as the design professional, are required by ANSI/AIHA-Z9.5-2003-2.4.2 to specify the acceptable spillage rates from every FH AND verify that level of containment is achieved through rigorous testing.  

6. Exhaust airflow--there is a discernable move in the lab world towards what the EPA calls high performance (low flow-high containment) FH's. I have used 2 that operate, and have tested beautifully, at full open face velocities of 50 fpm or less. Be careful of vendors selling low flow, which means less exhaust/make-up but places limits on sash position. Such reduced sash opening designs are ergonomically problematic and should be avoided.

My recommendation might seem off the beaten path but something we are using on a daily basis. Using 2-6' high performance FH's sized for 50 fpm uses 600 cfm per FH. I'd go a step further and provide combination sashes but with a lock on the vertical sash so the FH can only be used vertically for loading. This reduces the effective opening to 50% x 12 sq-ft = 6 sq-ft x 50 fpm = 300 cfm. The ANSI/NFPA45-A.6.4.2 minimum exhaust for that FH is 25 cfm/sq-ft of work surface or 300 cfm. None of use ever design for the minimum so my design would be 2 x 350 cfm = 700 cfm. Us Ve=12x18x8 = 1728 ft-3 = 24.3 ACH.

One of the big problems we see in our lab renovation-retrofit business is excessive HVAC, excessive controls complexity, and excessive reheat. On average you'll find your cooling load is probably in the 1 cfm/sq-ft or 4-6 ACH. Meaning you need to size your reheat coil for 700 x 1.085 x (20-(4/24.3)(20)) = 12,700 bth.

The final question is airflow and of course the concern is what impact it will have on FH containment/performance. ACGIH recommends down and cross drafts be limited to <50% the FH face velocity (or less). Using 50 fpm FH's means we need max 25 fpm drafts so laminar flow devices are ideal. I've found the old supply perf's have the least impact on even the largest FH population and of course slots should be avoided at all cost.

When it comes to FH's don't believe anything any FH manufacturers say--none of their factory tests are worth a darn. Field test every single FH shooting for a spillage rate lower than the 0.1 ppm AVERAGE spillage recommended by ASHRAE. The tougher you make your spec (such as lowering the mannequin height to the height of the average lab worker 5'1, using a SF6 flow rate, add hot plates to simulate thermal loads, etc)--tougher the test the better you and your customer are protected.

RE: Maximum Air Exchange Rate per Hour

Looks like a very good application for chilled beams.
M&I of Toronto makes them, there is a good article on chilled beam technology in the last ASHRAE journal.
Essentially, the system works as a 100% OA supplying induction units.
You will need a flat plate heat exchanger in your chilled water system to avoid condensation.

Very popular in Europe. Reduces ductwork, airflow, and fan energy substantially.

RE: Maximum Air Exchange Rate per Hour

Atlas,  I looked thru that article as it applies directly to some labs I am working on (too late tho).  What shocks me is the total cooling loads they describe as "typical" -- 10 - 20 W/sf.  This is 2-3x what I have documented for a large electronics / optics lab.  Anyone got any experience in this regards?

RE: Maximum Air Exchange Rate per Hour

the ideal application for beams is where you have a low hood exhaust (say a min ACH of 4) and a thermal load of say 8 ACH. The induced air provides the added cooling affect without adding to the size of the AHU's. I have used beams on several good sized projects and generally I like them alot. You do have to be concerned over condensation on the beams--so we used heat pipe and designed for 70% RH supply. You also need to know that beams need constant volume. Even a reduction as low as 10% causes the induced airflow rates to plummet.

So 8-10 ACH worth of cooling effect using 4 ACH of primary airflow.

RE: Maximum Air Exchange Rate per Hour

Daly1 most labs we have designed use vertical sash hoods. The operator opens it fully only when loading apparatus in the hood. Then when in use, the sash is lovered below the operator's face but at a height such that he can reach in and do the experiments. Thus some labs require hoods exhaust designed at 60 FPM when full open or 100 FPM at 24" sash height whichever is maximum. Also a local hood alarm is provided for each hood to flash & sound an alarm when the sash is raised above the 24" height. The 24" sash height is maked on the hoods. Hoods are the bypass type.

Some labs use VAV non-bypass hoods. The sash is measured and the hood exhaust airflow is controlled at 100 FPM at the current hood face area. Minimum exhaust is about 150 CFM per hood. A room general exhaust is also provided to keep the room negative. Controls sum up the room supply air (VAV responding to room temperature requirement but with minimum at about 40% of maximum or equal the hood minimum CFM minus the design room transfer air in CFM.

I don't see application for chilled beams in labs. I thought they are usually for use in underflow air distribution to supplement the sensible cooling (and heating if desired) capacity at perimeter zones. They can be passive ot active (with air supply).

RE: Maximum Air Exchange Rate per Hour

Ditto Lilliput's input.  I recently did a 100,000 sf lab space and went back thru our calcs of room cooling airflow requirements vs airflow req'd for pressure balance/fume hoods.  Only 2 or 3 rooms were cooling-load governed; however, as I noted above, the cooling loads (based on both calcs and historical data at this site) are 7.5 W/sf, not the 20-30 quoted in that article.  They casting metal in these labs??

It also seems like the supply air velocity in the active chilled beam systems would need to be higher to create the desired effect.

RE: Maximum Air Exchange Rate per Hour

Lilliput1--regardless of how the FH industry sells fume hoods they are not designed to be operated safely, vertically. In your example you describe the major problem with working vertically. When you lower the sash below the operators face--then how do workers see their hands when they are working 6" deep in the FH? This forces workers to make one of two choice--lower the sash until it doesn't block their line of site (which make it tougher to work the requried 6" deep) OR raise the sash until its' out of their line of site. Neither is an acceptable choice but its' generated by designing for reduced sash openings. As engineers we're not involved in ergonimic studies but this clearly one where we need to be involved and avoid these improperly thought out designs.

In terms of your design using 60-fpm full open or 100 fpm at 24", are you doing testing at both positions? You the design professional are required to specify the acceptable spillage and prove that any operating position meets the low spillage level you specify. If you specify a traditional 100-fpm fume hood then I guarantee a moderately challenging 110 evaluation will show it spilling like a pig (or higher than the arbitrary 0.1 ppm spillage 110 recommends) at the full open sash position. So when workers defeat the sash stop and raise it full open (as they do everywhere) then your safeties have been set aside and they will be exposed to whatever they are working on. Your liability doesn't end when the safeties are bypassed because of the requirement to test at full open. Some of the largest pharmaceuticals in the world are going horizontal only--and this is something we all should study.

In terms of the alarms this is another area where we engineers just simply must do a better job of evaluating the products. Virtually all fume hood alarms work with RTD or hot wire sensors capable of discerning between 0.1 to 0.05" WC. When you convert 100 fpm to Velocity pressure you find 100 fpm = 0.000623" WC. By AIHA-Z9.5-2003 we are required to annunciate a change in face velocity of +/-20% or 20 fpm = 0.000125" WC. Now we know that virtually none of our fume hood alarms are capable of actually providing the level of annunciation required. Secondly, using Vp=0.05 and solving for fpm=4005 x sq-rt (vp), fpm @ 0.05" = 896 fpm. So we need a change in face velocity of nearly 900 fpm for these alarms to get the change in pressure needed to annunciate.

We saw the same problem with Lab HVAC controls and that led to our firm abandoning them completely. There are two types of Lab controls used--Open loop or closed loop. The closed loop controllers acutally measure airflow in Vp, calcuate exhaust/supply CFM, and can modulate any damper to maintain a specific delta CFM. They fell out of favor over thermal and dirty air fouling the airflow sensors causing error. Open loop controls don't measure airflow but stoke dampers to a preset position--as an exmaple they track vertical sash height and modulate the exhaust damper to a position commensurate with 100 fpm at that sash height. The problems are many in that 100 fpm does not equal safety, vertical is less safe than horizontal and these systems have no way of tracking the preferred horizontal movement, and they cost far more than the limited benefits they purport to offer. If you don't measure airflow precisely then how can these systems purport to provide a very precise CFM -delta into a space. The answer is they cannot and do not.

In terms of the chilled beams ROSSABQ is right on. The 7.5W/sq-ft or 26 BTH/ft is right in line with the the 20-35 BTU/sq-ft we normally find (and far below the 40-60 btu/sq-ft many engineers default to). Using 30 btu/sq-ft and the 700 cfm example I used above this gives us a delta T of 8.5F, meaning this space would be in reheat 24/7 and we could not drop supply any lower without violating NFPA45 min FH exhaust. So beams would not be a good application for this room w/2-6' hoods. However, if we have 1-4' horizontal FH then we could provide a higher temp supply, the chilled beams can add to the cooling effect of the space, and the supply/exhaust can be hard balanced. The control sequence simply becomes a wall thermostat controlling a solenoid valve on the beams to augment room sensible cooling.

So we drive the 100% OA as low as possible. In rooms with high FH polulations beams are of no value. On the projects we've found beams (or any point of use heating/cooling devices) can be used in as much as 50-60% of the spaces. The end result is less 100% OA, more spaces with recirculated air (which is a basic tenant of Desinging Green Labs-Labs21), and less expensive and complex controls.

sorry for the epistolary.

RE: Maximum Air Exchange Rate per Hour

Daly1 - I am sure fume hood operators would be safer if the sash cover their face. They should reposition their work or lower the sash to see what their hand is doing. Which pharmaceutical company wants to use horizontal sash vs vertical?

I never diD trust actual velocity reading at the sash. Instead the prefered method is to measure the sash height & calculate the proper CFM for the calculated sash opening. Fouling of air measuring station and accuracy is a problem. That is why venturi type flow control valves (Phoenix) are prefered although they do not directly measure airflow but are factory calibrated. Venturi type valves do not require a minimum straight duct run for accuracy.

Chilled beams are not suppose to handle latent loads. So I doubt their use in labs since you would probably have to cool down your supply air further and reheat to meet room humidity limit requirements.

RE: Maximum Air Exchange Rate per Hour

Abbott has long eschewed vertical, Pfizer presented a paper at Labs21 touting (amongst other findings)the benefits of horizontal, added to literally hundreds of end users following this same approach. Do a goggle search and I think you'll be amazed at how far that movement has gone and how little the MEP world has been exposed to it.

I understand our industry has gravitated towards the open loop controller connected to the vertical sash cables. For all their touted benefits these designs cannot overcome one HUGE problem--even if they control the velocity at precisely at 100-fpm (which they do not regardless of their claims), they can never make the fume hoods work as well as they need to work.

This is definitively stated by ANSI/AIHA-Z9.5-2003-3.3.1 "face velocity shall be adequate to provide containment. Face velocity is not a measure of safety".
By using VAV systems you are ignoring this simple statement and basing your entire chemical hygiene plan on the contrary that face velocity =safety. Face velocity is a lie sold by the fume hood manufacturers for decades and Lab VAV is based entirely on this lie. Basing any of your designs or your firms liability on the lies told by casework manufacturers has in the past proven to be (how to put this nicely) a poor business decision.

Our firm goes to extensive means (CFD modeling, extensive testing, etc) to eliminate the Lab Controls from our projects. While  many Owners want nothing but a Phoenix style system there are many that are fed up with the complexity, cost, maintenance, etc, etc and are looking for MEP firms that offer a different solution.

Last thought--do yourself a favor and look into what the EPA is doing. They've decided to become relevant and now have a rating system for CV, low flow, high containment fume hoods. All these hoods are reduced flow, constant volume, and 2 of the approved manufacturers are constant mass flow--regardless of sash position. With them all we need do is provide constant static pressure in the exhaust manifold and then hard or proportionally balance the exhaust from each hood. We teach our architects to specify combination windows--and the issue of sash position goes away and our liability is virtually zero.

Anyway, no insult intended but if the only basis of worker safety you consider is an 18" sash height = safety--then you're several years behind what many Lab designers would consider cutting edge.

good luck

RE: Maximum Air Exchange Rate per Hour

100 ach is no problem as i have engineered systems n the past ..... use anemostat hcr diffusers mounted perpendicular to the hoods

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