I work in a medical building where we have 2’x2′ ceiling tiles and the duct work for the heating and air is in the space between the ceiling tiles and roof, the diffuser in the ceiling is also 2′ x 2 ‘, I am unable to tell how large the ductwork is, but is is of a pretty good size and delivers adequate air. There are no air filters anywhere in the system. In one room we are required to reduce the air particulate count and one idea is to place an air filter, nothing special just a medium efficiency one, under the diffuser in 2 of the 4 outputs that are in the ceiling. I can see no easy way to install one in the return. A concern was brought up that this extra resistance from the 2 filters would produce extra burden on the fan motor and cause it to burn out faster. I have no information on any of the specifics regarding the heat/cooling/ fan unit itself. Any input on this idea would be appreciated.
Thank you,
Tony
Replies
I think that any competant sheet metal shop would be able to fabricate a 'proper' filter housing that would be more effective and attractive than fixing it to a diffuser. With the tile ceiling, it could be located for access any place upstream of the diffusers (not in the return, put it in the supply).
I don't think there is a concern about over-burdening the motor, as long as you keep the filter reasonably clean. It is almost always assumed that there is filtration of some sort.
Scott.
Edited 4/1/2009 10:46 am by Scott
DO NOT put the the filter(s) on the supply diffusers! Doing so will degrade the performance of the system dramatically, since the supply diffusers are designed and located to throw the air with enough velocity, and in a pattern, that mixes, or entrains, room air and provides comfort throughout the room. Well-designed supplies will entrain 10-20 times more air than the actual flow from the diffuser.
That's the reason you never see filters on supplies, but always on the returns.
Also, since there are other branch ducts serving other spaces than the one you want to fix, putting airflow restrictions (like filters) on the room will tend to make the air look for an easier path, i.e., the other spaces that might have less resistance. In other words, it will change the dynamic balance of the system airflow.
As for the effect on the system performance, filters, even clean ones, can increase the resistance, or static pressure, significantly enough to make you think you need to install larger equipment. (Low air flow can make, for example, a 5-ton cooling system perform like a 4-ton system, or even less.) The only way to know the effect is to take static pressure readings on the system. This should be done by a truly knowledgeable HVAC tech. He might suggest either enlarging the existing return grilles or increasing their number, so that the return filter grilles do not unduly increase resistance. He will know a way to install filters in the return grilles--it's done all the time.
I hate to put out a downer for you on this, but I wonder if the bigger problem is the fact that the above-ceiling space is being used as a plenum, or common return for the system. If that space is a major source of dust, filters on the return grilles will not help much. Probably, there is already a filter on the main return at the air-handling unit itself, but the system is picking up dust, etc. as the air moves thru the system, and that dust is being deposited in the above-ceiling space, not only above the room you're working on, but throughout the building.
filters on the returns will do squat if they are bringing in out side air.
If there are rooftop units the filters will be in the units. If it is a single air handler there should be filter racks in front of the fan. I find it hard to believe there are no filters anywhere.
Everyone,
Thank you for the input, I will look for other options and consult with the company contracted to do our air work. Again, thanks for the rapid replies.
Just to clarify, I have not been on the roof yet to look around, there may be filters up there, I was refering to no filters in the building that are easily accessable, because we only need to reduce particulate in this one small area of the room that is blowing air directly into an area that is used for sterile IV compounding. The air inside that area is cleaned with a hepa filter. I have several options to consider to resolve this problem, the filter just seemed to be the cheapest and easiest...as usual, cheapest and easiest may not be the "right" way to go. Thanks.
Tony
Edited 4/1/2009 1:44 pm ET by docrx
Tony,
I would recommend you find a local HVAC engineer that know the equipment and codes that apply. In my experience with critical medical spaces, relative pressurization in the "clean" spaces must be maintained. This usually a State Health Department requirement (at least it is in Illinois and Wisconsin). Seemingly simple system modifications may not only have unknown and unintended consequences, violations and/ fines might result as well. The system does have filters in it somewhere.
Edited 4/1/2009 2:24 pm by Tim
I have installed some operating rooms. The HVAC system needs to be engineered. Usually all returns have individual filters to prevent as much as possible anything from leaving a room and entering the system. There are usually special filters at the air handler and special filters at the room supply registers. There may be some additional disinfection devices placed in different locations. These types of systems can be installed by a reasonably competent HVAC contractor but he can't design it.
I have engineered operating room HVAC systems in IL. I would expect the code requirements might differ slightly from state to state, but not significantly.
Basic filtration ( MERV 7 pleats) in the unit, HEPA filtration in the supply downstream from the fan, and laminar diffusers for terminals. Alternately, the laminar diffuser will contain the final (HEPA) filtration. Filtered returns do nothing in a critical enviroment. At 20 ACH with 20 to 25% OA, the greatest source of contamination is the fresh air and the machinery that conditions it. Furthermore, the OR must positve (positvely pressurized, that is) to the clean utility, which is positive to the soiled utility and the recovery space(s). All have specific turnover and final filtration requirements.
I was told that the filtered returns help prevent buildup of PM in the return duct system that could allow bacteria ect. to bread there . I am not an engineer but the engineer that designed the system my HVAC guy installed wanted them and that was his reasoning. My earlier point was that the system needs to be designed by a competent licensed engineer. Not left up to the HVAC installer to speculate at what he needs .
I'm in full agreement with you that an engineer or an experienced HVAC designer should be involved.
There are different design approaches to almost all systems. Engineers seldom agree on a single one. I am opposed to the approach of filtering returns for many reasons, but that is not the point of this discussion. We can cover that if you or any one wishes to do so.
In commercial buildings, the filter is often on the return air side. If you have spaces where a salesman "designed" the system, this could be dicey as the gap under the door, or a louver through the door, or nothing at all is the return air path.
Unless a proper HVAC mechanical engineer was involved (and was told of specific room requirements) it's best to assume that any new room filtration requirements will need the involvment of a compentent, local, engineer to look at the existing duct work, air handlers, and the rest to address how to meet vrious air quality requirements.
At least, that is my experience; others' differ.
I have NEVER seen the filter on the return air side of a system and I've seen a LOT of systems in my 28 year careeer ... of all sizes up to 150,000 cfm. Filtering return air is stupid. Sorry to be so blunt, but it is so ridiculous it warrants no other approach. Filter the mixed air maybe, but not the return air ... never.
I think you might have got mixed up.
I think you might have got mixed up.
Nope, done in small commercial work all over Texas. First thing I check before starting any TI work. Original paper filters are usually right there in the r/a grille.
Only time they are not in the r/a is on systems where the ceiling is being used as a r/a plenum--which has become as rare as hen teeth for all of the complications for other systems that entails.
Now, HEPA installs for medical clinics are different, and you have to ride herd on the MEs giving them very clear boundaries defining those areas (or, they'll forget to tell their drafters, who never know any better).
About half-and-half on residentail systems here in majority-cooling Texas, too. Mostly predictable based on whether the return is ducted or not (some systems just have a giant ceiling register for the r/a). Ducted systems, the filter is right on the intake side of the air handler, putting it in the very hhot attic, where the filter is almost never changed.Occupational hazard of my occupation not being around (sorry Bubba)
Go figure ... it does little good on the RA side since it is only filtering part of the air. Doesn't make sense to me. Only place I've seen it is residential where you have no ventilation air typically.
THE IMPACT OF FILTER PLACEMENT IN AIR-CONDITIONING SYSTEMS ON THE BEHAVIOR OF INDOOR CONTAMINANT CONCENTRATION
https://eprints.kfupm.edu.sa/1559/1/P145.pdf
...thought it might contribute to your discussion.
RB
Interesting. I skimmed it ... shocked at the conclusions (tongue in cheek). Nothing like deep studies and analysis to confirm the obvious. Who would have thought that there was much question to generate such a study. But the point is well taken and it's a good reminder ... study the source of the contamination when making a choice in filter replacement. Seems obvious, but most of the time, we don't sit ourselves down and consciously think ... 'where is the source(s) of the contaminants.
it does little good on the RA side since it is only filtering part of the air. Doesn't make sense to me.
Well, most of the air handler techs I know want the filter the first thing on the intake side of the blower unit, to keep "crud" off the blades. They also want many filter changes, but fail to understand not everyone has ladders and is acclimatized to entering 135-145º attics and plena (or plenuae for the more-traditional Vulgate sorts).
Now, in the ME side, there is a giant debate on whether you must separately filter ERV/HRV intakes in commercial work (residential being a separate argument here where the greatest deltaT is May to October).
In the "real world" there's acres, id not square miles" of TI space which still has no designed plan for IAQ air intake, either--until the spaces turn over, it's hard to get them changed.
But, I know I'm biased, I've been the nimrod on the step stool tip-toing to reach the paired paper filters out of the r/a grille one too many times.Occupational hazard of my occupation not being around (sorry Bubba)
I haven't very frequently dealt with AHUs in e.g. ceiling RA plenums ... even in TI applications. Usually rooftop or mech rooms. Most TI spaces have at least a AHU picked, installed, and/or located. TI sharing a vent duct ... I would hope/think that would have its own filter located for ease of maintenance.
As the article the other poster pointed out ... we DO need to assess contaminant source in filter placement. I guess I've simply always assumed there is some degree of air born particulates in the ventilation air to filter. On the surface, I really can't think of a situation where I wouldn't want to filter the ventilation and RA both.
You say not everyone has a ladder to change the filter ... nor do they have the bucks that would be required to clean e.g. an evaporator coil because they were too lazy to crawl up there and change the filter. That's akin to ... I don't want to change my car oil 'cause it's too awkward to crawl under to take the pan plug out ... but I think it's OK to rebuild the engine when it fails.
You may frequently have observed filters in RA only. I'm assuming you don't suggest that as OK for the reasons you suggest (ease of maintenance). As a GENERAL statement, I contend that RA only filters is a really dumb thing to have or to suggest. The fact that You've found this a lot in your area, doesn't make it OK.
You say not everyone has a ladder to change the filter
More of an observation on human behaviour. I used to try and insist upon a nice vertical AHU in the conditioned space for a number of reasons. Easier access to filters for one. But, even when convenient, it still does not get done.
But, that's alos true of a number of 'best' practices, too. Like buildings where the only fresh air intake was from the opening and closing of the egress doors <eyeroll>. Or where the restroom fans had been disonnected ("they're too noisy").
Now, since you are making the fine point, none of the systems I've seen with filters at the r-a-g have any mixed air at all. An absurd practice in the PNW, I'll admit. But, this is Texas, where low-rise space is very common; plena with temps of 100 and above are common. Spaces where -30º deltaT to the outside only occurs for a few weeks of the year. But, where +30ºd/T is the case for months (and not at all helped with 50-60ºd/T to attic/plenum spaces--a windmill I still try to tilt at).
Oh well.Occupational hazard of my occupation not being around (sorry Bubba)
All over the Pacific NW, filters are installed on the return side. If there is make-up or fresh air, those header into a small plenum just upstream of the filter
Uh ... that isn't what I refer to as the return air side ... that is the mixed air side. Return air is air prior to the where outside air and return air are mixed. Once return air is combined with outside air (i.e. ventilation), it is called the mixed air (MA) stream. May seem like symantics to you, but when you design and deal w/ HVAC systems and controls, there is a BIG difference. I've worked HVAC all across the NW and that is typically what I find ... filter the mixed air prior to the coil(s)/fan.
Alright, you're the HVAC guy, and I'm just the dumbass superintendent. I would bet a lot of the people in this thread are making the same mistake as me, mistaking "mixed-air" for "return-air".
You are NOT a dumbass ... mistakenass maybe. :)
It's a common mistake even among many field technicians who may often speak w/ less ... how do I say it ... specific ness ... now I'm a dumbass. That is why I was being a little more specific ... to catch who might have been thinking one thing and meaning something slightly different.
In the HVAC world there is a big difference ... regardless of what you say or how you mean what you say. I've always used the rule of assumptions about what you mean to say ... you know ... assume? Just trying to keep things clarified. It can be tough in this media of communication w/ people you don't know from Jack.
The one poster seemed to be pretty specific ... place the filter just behind the return air grille in the ceiling ... assuming it is a ducted return.
pretty specific ... place the filter just behind the return air grille in the ceiling ... assuming it is a ducted return
Which is never going to pass State or hepa certifcation for medical facilities in my experience. That's why you (or OP, to be precise) needs some ME input of fixing the problem they have.
Which can be tough sledding, some docs make terrible landlords. <sigh>Occupational hazard of my occupation not being around (sorry Bubba)
you say you have no filters and then say you have HEPA filters ... what do you have? If you need filters only in one room ... you have issues. You either give that room a separate ventilation unit or you filter the entire system/building. If you need VERY GOOD filtration (which it sounds you might), you need to put good filters on the air handling unit, but maybe the unit isn't designed for it. Better filters mean more air resistance.
BTW - more filtration, more resistance to air flow, ... energy and fan/motor effort goes DOWN, not up ... it won't hurt your fan/motor, but by the same token, you won't get the performance you need. Put your hand in front of or behind the fan and the energy/work goes WAY down. Think about it ... put your hand over a vacuum cleaner ... it speeds up, not slows down ... because it is working less. If it worked more ... it would slow down. You can burn out a motor if it speeds up. Generally on fan systems, though you wouldn't really have issues.
Having a high filtration requirement in one room out of several on one heat/cool unit could be an issue. You need professional assistance ... preferrably an engineer ... not the contractor!! You've got filtration issues ... the contractor is going to GENERALLY have a tendency to take the path of least resistance and highest return on his effort. The engineer has the liability ... as maybe you do as well. The contractor really doesn't have any design liability ... nor real knowledge for that matter (no offense to the contractors out there).
Clewless,
I understand the confusion. The room where the radiopharmaceutical preparation occurs has to meet certain requirements, it has to be class 8, which in my world means no more than 100,000 particles per cubic foot of air. We meet that requirement with no special filters. The radiopharmaceuticals must be compounded in what we call a hood, there are various types, most are called a laminar air flow hood, basicaly a 5 sided box with a fan on top that recirculates the air within the hood through a hepa filter so that theoreticaly no outside air enters the inside of the hood without being filtered through the hepa filter. This hood sets in the middle of the room. The air inside this hood must be class 5 or better, or less than 100 particles per cubic foot. This hood has been tested and certified to meet that requirement. However, when we tested for bacterial contamination using agar plates and certain growth media, this hood did not pass, almost, but not quite. The only difference between this hood and the other 2 that did pass is the 2 air vents that are above and diagonal to the opening of the hood, which I suspected may be blowing unfiltered air into the opening of the hood, just a suspicion.
After reading the replies I realize I may have been using the wrong terminology to describe the opening that I called a " return". I was refering to the opening in the ceiling where air from the room is drawn back up into the system for recirculation. This "return" constantly collects particles on the grate that requires cleaning. I assumed particles could be taken up into the system then blown back out the vent and into the hood. That is why I wanted to filter the 2 vents in the problem area. The other 2 vents are across the room and are hopefully not an issue.
Thanks for the reply. Tony
If that special room shares the HVAC system serving other areas and you are only filtering at the return grille leaving the room, the main AHU is circulating (potentially) contaminated air from other rooms and/or outside (ventilation air). Knowing air leaving the room is clean doesn't do any good. I suspect, I'm not quite getting something right about your situation, though. You have something similar to a fume hood, except, you are trying to control the atmosphere inside the hood and protect it from conditions outside the hood. How do you get anything done in there, then? Work w/ gloves through the glass?
Seems to me you need some type of clean room to avoid contaminating your VERY CLEAN hood. Does this hood vent outside or back to the room?
Were getting closer to being on the same page. The fume hood is open on the side that we work out of, no glove box, no special glass doors, the dimentions are about 3ft tall x 3 ft wide and 2 ft deep. The air going through the hepa filter on top of the hood is recirculated. Air comes out the top where the hepa filter is and is blown down through the hood and is returned to the hepa filter through vents in the bottom of the hood. There is a slight "postitive pressure" inside the hood in the sense that a slight amount of air blows out toward the operator. Any air that is taken up into the system from outside the hood would most likely go through the hepa filter before reaching the working environment unless the air is being blown in throught the front of the hood from the vents in question where it reaches the test area before being filtered. Your point about cleaning air before it leaves the room is my point exactly, that is why I wanted to filter the air coming out of the vents in the trouble area only.
Most laminar air flow hoods blow a wall of air directly at the operator, thus virtually eliminating any outside air from entering, however we are working with radioactive liquids to make the medications with, so you wouldnt want any air blowing that stuff toward you, the hoods we use are the industry standard for what we do. An actual clean room to do this in is not technically required, only a room meeting the critera stated in my prior post.
Tony
Edited 4/7/2009 11:21 am ET by docrx
I see. A lot of stuff going on there. I've never worked w/ one of those types of hoods ... mostly just fume hoods for VOCs and e.g. microbes. A pretty unique situation. My opinion ... talk to an engineer expert ... there may be liability here.
So in addition to not wanting some of the radioactive material out, you don't want dust in, either, right (otherwise a hood at a negative pressure would be fine).
thanks for patiently clarifying your situation.
Thanks for your input. I will have someone look into it before I proceed.
Thanks
Tony