Downdraft table -blocking off part of table surface to increase flow?

[Cringle]

TL;DR

Our lab has downdraft tables to remove noxious gases. Lab closed due to high fume levels. Solution: cover the table with perspex. ???How does this help???

Hello everyone. First post! I am struggling to understand an issue in our (histology) lab.

We use large downdraft tables which extract the noxious substances we use, mainly formaldehyde, while we process surgical samples. These tables have a steel cover with lots of holes about 5mm diameter throughout to allow the gases through but catch any tissue that might fall. The air then goes into a shared system and is filtered. The levels have been far too high and the lab was temporarily shut and the issue diagnosed as inadequate flow from the tables.

The solution, in the short-term at least, is for someone to put sheets of hard plastic covering and blocking about 3/4 of the holes. Apparently this should improve flow by 'simple physics'. This doesn't make sense to me so I have been reading but haven't found anything that could really explain this. However I am a bit out of my depth! Intuitively to me it seems that although locally the flow rate will be higher, it will be over a smaller area so the overall effect in the room will be the same (or less) which is really the important thing (five benches being used constantly).

Bernoulli's principle has been mentioned by a colleague but they didn't explain and I think they don't really know anything about it. I did look into it, but does it apply here? In any meaningful way at least. Apparently it can seem counterintuitive though. So by Darcy's law doesn't a reduction in the cross-sectional area of the table surface reduce the flow? Am I on the right track here or totally missing something?

As the staff still have concerns we will meet with management but I want to pose the questions re: the physics and be able to discuss with a semblance of understanding otherwise I think I might get fobbed off. I obviously have doubts but if it really does improve the flow I can explain to others and that would also really help to assuage concerns.

I am obviously at a very basic level here but willing to read and learn! Thanks!

 

[Baluncore]

The air then goes into a shared system and is filtered.

Does the air then get recirculated within the workspace, or is it removed from the building?

Have you checked to see if the flow is reduced because the filters have become blocked?

 

[Cringle]

The air is removed from the building.

The rest of the system is being looked into but at the moment re: the filters I don't know, although I wouldn't have thought so as the whole set up is fairly new.

My main concern for the time being is whether the perspex sheets blocking the holes might really make any difference as that is the only change that has thus far been made.

 

[Baluncore]

The perspex sheet will reduce the total flow. You can continue to use the area that is not covered, which will have a slightly improved flow. If the filters are becoming blocked, then the perspex will only make a slight difference for a short while.
Check the filters, and that the extractor fans are still running.
Use a smoke generator to trace or show the airflow around the table.

 

[jrmichler]

Things to check / ask / do:
1) Are the filters plugged? Do not be surprised if the filters are plugged, and nobody has the responsibility of checking them.
2) Is the total airflow out of the fan what it is supposed to be after checking the filters? If not, look for restrictions. Also check the static pressure across the fan, and compare to the fan curve.
3) Is the airflow through each table what it is supposed to be? If table airflow is too low, look for too many tables in use, or restrictions in the ductwork.
4) Are there drafts moving room air past the tables? External draft shields are a solution. Also stopping the drafts.
5) Can you put shielding around and over the tables to make them more like fume hoods?
6) Closing some of the holes will increase flow through the remaining holes, while the total flow will decrease. The overall effect will almost certainly be small compared to the items listed above.

 

[DaveE]

The cost of closing a lab seems small compared to hiring a good HVAC contractor to inspect the system. They may have more credibility with management than employees arguing about physics.

It seems there are ultimately two fundamental questions to address:
- Is it working as designed (clogged filters and such).
- Was it adequately designed for the current usage requirements.

Did it used to work OK?

 

[berkeman]

We use large downdraft tables which extract the noxious substances we use, mainly formaldehyde, while we process surgical samples. These tables have a steel cover with lots of holes about 5mm diameter throughout to allow the gases through but catch any tissue that might fall. The air then goes into a shared system and is filtered. The levels have been far too high and the lab was temporarily shut and the issue diagnosed as inadequate flow from the tables.

The solution, in the short-term at least, is for someone to put sheets of hard plastic covering and blocking about 3/4 of the holes. Apparently this should improve flow by 'simple physics'. This doesn't make sense to me

So the formaldahyde is released when you open a container to process the surgical samples, and when you close the container(s) they stop releasing the formaldahyde? What percent of the tabletop area is used typically? How many tables and how many lab technicans are typically involved in this work?

If it is usually only one lab tech working at a time, it may make sense to block off the unused tabletop areas, since this will focus the air evacuation in the area where the tech is releasing the formaldahyde.

 

[Lnewqban]

Welcome, @Cringle !

Please, see ways to better improve your tables, besides the advised inspection and balance of your exhaust and makeup air duct systems and blower:
https://en.wikipedia.org/wiki/Fume_hood

 

[russ_watters]

The cost of closing a lab seems small compared to hiring a good HVAC contractor to inspect the system. They may have more credibility with management than employees arguing about physics.

It seems there are ultimately two fundamental questions to address:
- Is it working as designed (clogged filters and such).
- Was it adequately designed for the current usage requirements.

Did it used to work OK?

I think you said that first bit backwards, but otherwise, yes, that's the correct approach. I've done this study many times for pharmaceuticals.

Formaldehyde is really nasty stuff (personnel exposure limit: 250ppB) that is rarely used in the US anymore as a result. A capture problem is a really big health concern and warrants shutting down the lab until it is figured out, approaching the problem from both sides as you listed:

Functional side:

1. Measure airflow and compare to design. Troubleshoot as needed (analyze fan performance, ductwork, controls, etc).
2. Use smoke (glycol or co2/water) to observe capture.
3. Use a proxy gas (if formaldehyde can't be measured directly) and measure actual capture/exposure under actual or simulated operation.

Design side:

1. Define use/exposure case (time and emission)
2. Identify the exposure limit.
3. Determine capture method and airflow. For formaldehyde you can't use dilution airflow, you must have 100% capture/containment.

Downdraft tables are pretty good (if functioning properly). But fume hoods are near perfect and you need near perfect to handle formaldehyde.

Please get a qualified local engineer to look at this, and engage your company's Environmental Health & Safety (EHS) department for the criteria/enforcement.

 

[Cringle]

Thank you all so much for all your replies and advice! So my question has been answered re: the table flow but also lots of other potential (probably more important) issues and I will look into what has been suggested. By way of answering some of your questions:

It seems there are ultimately two fundamental questions to address:
- Is it working as designed (clogged filters and such).
- Was it adequately designed for the current usage requirements.

Did it used to work OK?

I think this may be the root of the issue - the lab is fairly new and there have been issues going on pretty much since it opened. I suspect that it wasn't actually adequately designed (too many tables in too small a space - previous lab was somewhat low-tech in comparison but five tables were spread over three separate better ventilated rooms). Obviously there is a lot to look at first before a redesign might be considered!

So the formaldahyde is released when you open a container to process the surgical samples, and when you close the container(s) they stop releasing the formaldahyde?

Correct. However we often deal with very large samples and even though the containers are closed there will still be a lot of formalin around on the table and residually on / within the sample, although it can be rinsed off somewhat.

What percent of the tabletop area is used typically?

Variable but I would say around 50% of the time most of a table is being used with large specimens. When biopsies / small samples are being used this can be localised to a smaller area. If anyone is interested the RCPA has good videos of the sort of thing that we do (and they seem to have similar tables from what I can see!) e.g
https://www.rcpa.edu.au/Manuals/Mac...Gastrointestinal/Colorectal/Colorectal-tumour

How many tables and how many lab technicans are typically involved in this work?

Five tables are used constantly within working hours with two people per table.

1) Are the filters plugged? Do not be surprised if the filters are plugged, and nobody has the responsibility of checking them.
2) Is the total airflow out of the fan what it is supposed to be after checking the filters? If not, look for restrictions. Also check the static pressure across the fan, and compare to the fan curve.
3) Is the airflow through each table what it is supposed to be? If table airflow is too low, look for too many tables in use, or restrictions in the ductwork.
4) Are there drafts moving room air past the tables? External draft shields are a solution. Also stopping the drafts.
5) Can you put shielding around and over the tables to make them more like fume hoods?
6) Closing some of the holes will increase flow through the remaining holes, while the total flow will decrease. The overall effect will almost certainly be small compared to the items listed above.

1-2. Will ask, thanks.
3. It is too low. Restrictions in ductwork have been checked (there was one) but the issue continued. Too many tables in use is probably correct, but due to the high throughput of the lab eventually there will have to be a solution to get all of them working safely somehow. We could reduce use in the meantime though.
4. No drafts as this area of the lab is quite small and enclosed. Well insulated I suppose. No windows.
5. There is some shielding above the tables and they are up against the wall on the other side, but this could be an improved.
6. Understood.

Thank you all again!

 

[sophiecentaur]

The air is removed from the building.

I searched this thread for words like "inlet" and "input" and couldn't find any. Is there a low resistance path for air to enter the lab? Sorry if this is a daft question but it could account for the increase in flow (measured where?)when the outlet is partially blocked. Try the measurement with all the doors open or measure the ambient air pressure in the room.