How cold should aircon be set on public transport?

[Bletchleyite]

On a hot day the ceiling will be warm so that's a waste of energy cooling it and warming it back up again.

 

[Annetts key]

Moving air over a ‘hot body’ (hot meaning a higher temperature than ambient and body here meaning object) will remove more heat than if there was no forced air flow. This happens at all air temperatures that humans can survive in.

As far as people are concerned, the effect is intensified if the moving air is cooler than the ambient air temperature.

 

[Domh245]

I don't understand.
Currently the flow of cooler air leaves the unit attached to the underneath of the roof of the carriage at an angle directed to passengers.
Presumably this angle of flow is based on the angle of the vents in the unit.
The correct angle should be to point the cooler air towards the ceiling, reduce its speed and then create a slower flow down.
I think the problem isn't necessarily the temperature per se, it's the speed of the cooling air as it hits you. A bit like if you are outside. It may be, say, 16c but 16c in a gale "feels colder" than 15c in still calm.

I think we were talking at cross purposes! When you said it should "point up" I took that to mean coming at floor level rather than aiming it at the ceiling from just-below-the ceiling (on the basis that I've seen floor based HVAC, but never stuff pointed at upwards at a ceiling). I do agree though that the speed of the air is very much a factor in how pleasant/unpleasant a system feels, at least in part. Many systems will try to control the air temperature to the oft-referenced 21°C, which means that if the "room" temperature is higher, it'll supply at a cooler temperature to try and bring the room down. If you're sat directly under the air outlet from the system, getting blasted with comparatively fast, chilled air will be rather unpleasant for a longer period of time and have you reaching for a jacket!

As you note, the air from the HVAC unit enters the carriage through various types of ceiling grille, having been ducted from the (usually) centrally located system* - from simple cut outs, through slatted grilles to perforated panels. Better design of these diffusing elements (or looking to building HVAC and the increasingly extensive use of swirl diffusers!) to give a more suitable air flow and pattern is the solution, rather than designing a system which bounces air off the ceiling! The design and commissioning of ventilation systems is something of an art, and something that is very easy to get 'not right'

*by extension, this means that the ends of the vehicle will likely have slower air than the middle, if you want to try and avoid this chilled effect, at least on units where the HVAC unit is centrally positioned on the length of the vehicle which is almost everything with the exception of pre-privatisation stuff and the -stars.

 

[Annetts key]

Ahh, we’ve proceeded to system design.

On the subject of control, there are a number of different techniques:

• Simple on/off fully manual control (typically only used in cheap portable units),
• Traditional thermostat control - if the thermostat senses that the air temperature is hotter than the set point, it commands the air conditioning unit to run at maximum cooling. Otherwise the air conditioning unit just recirculates the air without actually cooling the air. Some systems have a duty cycle controller to prevent the compressor (which is the bit that actually performs the cooling) from running continuously,
• As above, but the system has temperature sensor in the output airflow, so that the airflow can be automatically mixed between recirculated air/‘fresh’ air from outside/and cooled air before being output from the unit.
• Various more sophisticated temperature sensor and control systems, often using proportional temperature sensors and fuzzy logic, so that both the room/cabin air temperature and the output air temperature are converted to a value. A microcontroller then uses these values and tries to adjust the output temperature of the air smoothly so that it changes gradually when near the set temperature rather than the on/off as used with a traditional thermostat. But if there is a larger difference (such as when switched on), it will run at maximum cooling to try to quickly approach the set temperature.

The air flow control and distribution can also be basic/simple or more complex.

 

[Lost property]

My longest flight is about 12hrs to South East Asia.



No, aeroplane aircon is far more powerful, have you seen the size of the fans? (It's engine bleed air on most aircraft, which is why you get a gobful of fumes on engine startup)

As another poster correctly states, the last thing you want on a start up is the engine bleed valve operating for the very reason described...sticking bleed valves can cause all sorts of problems when they happen...hopefully, usually, percussive engineering with a hide face hammer rectifies the problem. You will have noticed the ACM in the pack suddenly goes quiet prior to engine start and resumes once the engines are stable.

The aroma you mention comes from the APU which is sucking in air to be ducted to the air cond packs which are also getting ambient air from the ram air ducts ..ram air obviously gets mixed in flight as the name suggests.. but on the ground, both sources can provide that delicate infusion of burnt Avtur and a passing toilet cart.....if you start getting "fumes " in flight, and this can be a very contentious issue I assure you, then you do have a problem.

Having sat in various carriages like everybody else, I've often wondered how the air cond is controlled..manually selected ON and thereafter fully automatic or fully automatic when power is running and also, what the re-circ rate is in a carriage compared to an aircraft.