Maximum Air Exchange Rate per Hour
Maximum Air Exchange Rate per Hour
(OP)
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
RE: Maximum Air Exchange Rate per Hour
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
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
RE: Maximum Air Exchange Rate per Hour
RE: Maximum Air Exchange Rate per Hour
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
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
RE: Maximum Air Exchange Rate per Hour
So 8-10 ACH worth of cooling effect using 4 ACH of primary airflow.
RE: Maximum Air Exchange Rate per Hour
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
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
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
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
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