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UK rolling stock Air conditioning - recycled air or not?

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Jozhua

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Why can't it take tunnel air? It's not poisonous.

I presume it's to do with particulates and pollution? The London Underground has significant issues with particulate emissions, so perhaps this is an issue in the Eurotunnel too. I don't know how effective the ventilation is, but it could be a challenge over 30 miles.

This being why you get a (rather unpleasant) nose full of paraffin fumes on startup, in case anyone wondered :)

Yes!

Yes, this. I’m sure the media seek out the craziest members of the science community for sound bites... This HVAC sucking up virus’s route is an extremely unlikely method of transmission, it’s much more likely you’ll touch a contaminated surface or breathe in droplets exhaled by someone within 2m.

Yeah, very true. I mean, the chances of HVAC sucking the virus all the way in and out of the system, if it gets through the filters (those on planes and better maintained buildings are quite effective in filtering out bugs), aren't the highest, if it survives that long outside of a host.
 
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allotments

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Several replies downplaying the importance of duffield's prescient question. Now 3 months later there's emerging evidence of airborne transmission via air-conditioning. People on trains need to know what the risk is and how to mitigate it.

It would be interesting to know whether air con systems on trains can be adjusted to minimise recirculation.

Passengers meanwhile now open every window in sight and relax on trains that allow this.

Perhaps air con trains should be finally be retrofitted with windows that can be opened!
 

Fincra5

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Several replies downplaying the importance of duffield's prescient question. Now 3 months later there's emerging evidence of airborne transmission via air-conditioning. People on trains need to know what the risk is and how to mitigate it.

It would be interesting to know whether air con systems on trains can be adjusted to minimise recirculation.

Passengers meanwhile now open every window in sight and relax on trains that allow this.

Perhaps air con trains should be finally be retrofitted with windows that can be opened!

Perhaps all passengers should wear masks, as instructed... that would be a better start.
 

Skie

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It would be interesting to know whether air con systems on trains can be adjusted to minimise recirculation.

Which would entirely defeat the point of the systems, or make them so inefficient you'd be better running the train with no aircon at all.

I would like to see HEPA filters mandated for all public transport. Planes already use them and have done for years, and they aren't expensive. I'd be surprised if very modern stock didnt already use them or at the very least builders didn't start thinking about offering them in new models soon.
 

Ethano92

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On a similar note sometimes usually in weather when it's not hot enough for AC/cold enough for heating, trains without openable windows feel very stuffy, I've experienced this most on Electrostars although I know they do have windows that can be opened with a key. I would've assumed air needs to be continuously circulated through some sort of ventilation system on such trains whether that be fresh or recycled air but sometimes it doesn't feel like it. Do these trains when their AC system isn't needed really just not have any sort of ventilation/fresh air?
 

Jozhua

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Which would entirely defeat the point of the systems, or make them so inefficient you'd be better running the train with no aircon at all.

I would like to see HEPA filters mandated for all public transport. Planes already use them and have done for years, and they aren't expensive. I'd be surprised if very modern stock didnt already use them or at the very least builders didn't start thinking about offering them in new models soon.
For now it would be nice to have the option of taking in larger volumes of fresh air. The temperatures are fairly reasonable too, especially over this summer period.

Totally agree on HEPA filters. Frankly, I would want TOCs to address the air issue over social distancing/surface cleaning.
 

superkev

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Several replies downplaying the importance of duffield's prescient question. Now 3 months later there's emerging evidence of airborne transmission via air-conditioning. People on trains need to know what the risk is and how to mitigate it.

It would be interesting to know whether air con systems on trains can be adjusted to minimise recirculation.

Passengers meanwhile now open every window in sight and relax on trains that allow this.

Perhaps air con trains should be finally be retrofitted with windows that can be opened!

I'm not so sure about the opening windows bit as the draught from then at any sort of speed would distribute any viruses throughout the carriage. Certainly not practical at express speeds where modern trains are a semi sealed box to minimise pressure effects when entering tunnels.
Masks and rigorous mist type cleaning a far better solution.
K
 

Fincra5

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On a similar note sometimes usually in weather when it's not hot enough for AC/cold enough for heating, trains without openable windows feel very stuffy, I've experienced this most on Electrostars although I know they do have windows that can be opened with a key. I would've assumed air needs to be continuously circulated through some sort of ventilation system on such trains whether that be fresh or recycled air but sometimes it doesn't feel like it. Do these trains when their AC system isn't needed really just not have any sort of ventilation/fresh air?

I believe 377s AC has HEPA Filters. They have 4 Hoppers Windows per coach that open with a key.

However these are only to be used if BOTH AC units fail in that coach. Or the AC units will overwork and cut out.

As has been pointed out the Units will add fresh air at various times in the cycle, filtering out the recycled air in the system.

Plus you'll get fresh air everytime the doors open at stations.
 

aleggatta

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I believe 377s AC has HEPA Filters. They have 4 Hoppers Windows per coach that open with a key.

377's were changed from paper filters to (I believe) an activated carbon style, not sure if it has full HEPA qualities, but they do get quite mucky at the end of their life cycle! There is a constant stream of supply/extract air, but I couldn't tell you how much air is recirculated, the air con units don't take fresh air in themselves, the fresh air is taken in through the side vents on the coach and ducted up, there is no variable damper control on how much air is recycled, but thats not to say there isn't a fixed damper providing a certain level of recirculation for efficiency. Unfortunately I never got a look at the duct layouts on a coach!
 

mmh

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There is of course a huge difference between a plane and a train. The train isn't a sealed tube and has doors that open, regardless of whether it's air conditioned or not.
 

Skie

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There is of course a huge difference between a plane and a train. The train isn't a sealed tube and has doors that open, regardless of whether it's air conditioned or not.

Planes do actually take in a significant volume of external air from outside, even at high altitudes.
 

py_megapixel

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I believe 377s AC has HEPA Filters. They have 4 Hoppers Windows per coach that open with a key.
However these are only to be used if BOTH AC units fail in that coach. Or the AC units will overwork and cut out.
In cases where opening windows would be preferable to AC is it not possible to isolate the AC units?
 

Fincra5

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In cases where opening windows would be preferable to AC is it not possible to isolate the AC units?

It is but only 4 Hopper Windows provide a small amount of ventilation. Not idea on a warm day. Wearing a Mask and Cleaning your Hands regularly are still the best option to isolating AC.
 

Greybeard33

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For now it would be nice to have the option of taking in larger volumes of fresh air. The temperatures are fairly reasonable too, especially over this summer period.

Totally agree on HEPA filters. Frankly, I would want TOCs to address the air issue over social distancing/surface cleaning.
Does anyone know for sure which classes of train already have HEPA filters in the HVAC system, and/or have a "fresh air mode" with no recirculation?
 

Skie

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It is but only 4 Hopper Windows provide a small amount of ventilation. Not idea on a warm day.

Anyone unfortunate enough to use Merseyrail on a hot summer day can attest to that. 10-12 windows open in a carriage all day and it will still be stifling even when the sun has been down for a few hours and the air is cooling off. Modern(ish) trains just don’t generate much airflow from open windows.
 

FQTV

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The key thing as far as transmission is concerned, as I understand it, is that the airflow direction is the first and arguably most important factor.

Aircraft are generally fitted with HEPA filters, but the key characteristic to start with is that aircraft ventilation is negative pressure, with air being sucked out at floor level and drawn in above head height, the same as in hospital high-dependency settings.

In an aircraft, or an environment where temperature is also regulated by the system, there will be an element of recirculation and therefore filtration is appropriate.

However, if air passes in any other direction than directly downwards, HEPA filters are compromised as the air passing in any other direction than vertically down could transmit infections between people long before the filters are reached. Likewise, opening the windows of a moving vehicle might have the principal effect of simply creating an air flow that means that any airborne/aerosol infection at one end of the vehicle could be transmitted swiftly to the other.

These diagrams are from a Chinese research study, showing cases in an air conditioned (non-recirculated) restaurant, published on the US Centre for Disease Control website at https://wwwnc.cdc.gov/eid/article/26/7/20-0764_article

Abstract
During January 26–February 10, 2020, an outbreak of 2019 novel coronavirus disease in an air-conditioned restaurant in Guangzhou, China, involved 3 family clusters. The airflow direction was consistent with droplet transmission. To prevent the spread of the virus in restaurants, we recommend increasing the distance between tables and improving ventilation.


20-0764-F1.jpg

Extracted conclusion from the report is that:

We conclude that in this outbreak, droplet transmission was prompted by air-conditioned ventilation. The key factor for infection was the direction of the airflow. Of note, patient B3 was afebrile and 1% of the patients in this outbreak were asymptomatic, providing a potential source of outbreaks among the public (7,8). To prevent spread of COVID-19 in restaurants, we recommend strengthening temperature-monitoring surveillance, increasing the distance between tables, and improving ventilation.
 

Greybeard33

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These diagrams are from a Chinese research study, showing cases in an air conditioned (non-recirculated) restaurant, published on the US Centre for Disease Control website at https://wwwnc.cdc.gov/eid/article/26/7/20-0764_article
On the contrary, the aircon in that restaurant did recirculate the air. From the linked report:
The air outlet and the return air inlet for the central air conditioner were located above table C
The path of the return air is shown by the dashed arrow on the diagram and provided the possible route by which the virus droplets were conveyed "upstream" from the infectious person, A1, to the diners on Table C, who were closer to the aircon unit.

The airflow pattern in this restaurant looks somewhat similar to that in a railway carriage with recirculating aircon. The concern is that, especially without a HEPA filter, the gentle recirculating airflow could distribute virus aerosol droplets from one infectious passenger to a number of others, who are much more than 1m distant.

The risk is much lower in a vehicle with open windows. The vigorous, turbulent airflow will rapidly disperse aerosol droplets, as happens in the open air. The scientific consensus appears to be that aerosol transmission is much less likely outdoors than in an enclosed, confined space, and that ventilation can offer some mitigation. As the linked report concludes:
...we recommend... improving ventilation.
 

FQTV

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On the contrary, the aircon in that restaurant did recirculate the air. From the linked report:

The path of the return air is shown by the dashed arrow on the diagram and provided the possible route by which the virus droplets were conveyed "upstream" from the infectious person, A1, to the diners on Table C, who were closer to the aircon unit.

The airflow pattern in this restaurant looks somewhat similar to that in a railway carriage with recirculating aircon. The concern is that, especially without a HEPA filter, the gentle recirculating airflow could distribute virus aerosol droplets from one infectious passenger to a number of others, who are much more than 1m distant.

The risk is much lower in a vehicle with open windows. The vigorous, turbulent airflow will rapidly disperse aerosol droplets, as happens in the open air. The scientific consensus appears to be that aerosol transmission is much less likely outdoors than in an enclosed, confined space, and that ventilation can offer some mitigation. As the linked report concludes:

You're quite right - I used the description 'non-recirculated' badly there. The distinction that I was trying to draw was between the more controlled, closed-loop, vertically downward flow systems employed in aircraft and parts of hospitals, to domestic and commercial systems that are generally less rigid in the ways that they operate.

The point in this study was that lateral unidirectional airflow was deemed to be the cause of the particular pattern of transmission. Additional ventilation would potentially have served to disrupt that direction of flow.

With regard to the turbulence of airflow, I haven’t yet found any research in this regard. In the aviation scenario, it’s considered that the most significant risk (apart from the most direct transmission) is from touching an infected surface and then the mouth nose or eyes.

Extrapolating that, any airflow which can spread contamination laterally or radially, for it to land on a surface and then be picked up, would be a risk, presumably. The vigour of the flow (up to a point) may be less consequential than its direction(s).
 
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