ACs: Back to the Future

A friend walked in the other day, sat down at the table, a little harried, and loosened his tie. “How wonderful to take in a breath of fresh air,” he said looking out the window at the coconut frond that has ben raring to wend its way into my grubby, but airy little office. He added, “Sitting in my office that is fully air conditioned, makes me yearn for this clean lungful.”
So what went wrong over thirty years of modern engineering and technology that has celebrated the duct-based AC system? So why is every office space today going in for these huge, expensive ducting systems and chiller plants and energy–guzzling AC systems?
What we call air conditioning today used to be known as Plenum ventilation. We came across a startling set of directions in a classic 1943 text of engineering written by a Majumdar. Among many things that are explained with lucidity, are the first tenets of air circulation. Ventilation, the book reminds you, is necessary to remove foul air both for health and comfort.
About 3,000 cft of fresh air per hour per person is required in enclosed spaces—be it offices, or homes. The CO2 level should not exceed half a per cent. As more people inhabit the room, oxygen content reduces, and the CO2 level rises. And organic matter and odour increases. The human breath increases humidity, and the temperature rises with the heat that human bodies generate.
Natural ventilation is the obvious solution, but it brings with it light and noise, apart from the constraints on creation of openings—windows, skylights— specially in large multi-stack buildings. Typically, you should look at door, ventilator and window areas that are 20 to 25 per cent of the floor area.
In the Plenum or propulsion system, fresh air is driven by blowers. When you need to artificially ventilate, you have to shut out the doors and windows.
How do you achieve control over the incoming air? Air is drawn into the building by a fan, and is discharged through ductwork at suitable positions; the outlets draw the foul air through ducts controlled by an extraction fan of smaller power than the inlet fan. The design ensures that a slight pressure is created thanks to the larger rate of incoming air—so any leakage has to be outwards.
Then comes the need to address humidity—the water vapour content in air. Hot air when drawn in, has to be dehumidified. A chamber to dehumidify serves just this purpose.
The air coming in is cooled by a chilled water distribution network that is centrally ducted. This cooling is from an independent boiler, fed by an ammonia refrigerating plant with gilled tubes which have a good radiating surface. The humming noise of the fans is avoided with special care taken in the placing of the fans with anti–vibration bush-mounted panels, and motors that are efficient.
The ductwork in itself can be of light GI sheeting [circular or box section], or formed in brickwork or concrete.
Essentially, what is air circulation and conditioning about? Cleanliness, humidity and temperature are the key factors. In the present–day AC systems, the trouble is that the air ducted into your room is recirculated air. The fresh air that is infused into the system is less than 10 per cent for an entire day.
Drier the air, quicker the evaporation. Heat gets oppressive with high temperature and humidity. Higher air movement and cooler air will bring comfort, apart from the fact that such air management will keep the room free of dust.
Typically, this type of conditioner draws on fresh air, and is not dependent on recirculated air. That is a big advantage since the quality of air is assured. The key components of such conditioners are the inlet for fresh air with a controlling valve, air filters, a water sprayer, the extraction fan, eliminator plates for removal of excess moisture or humidity, and a conditioned water duct. The filters remove dust and soot from air. These filters are of glass–wool and coated with a viscous non–drying oil. These filters can be cleaned with a vacuum cleaner—once a year is fine.
The saving on energy that these conditioners offer is tremendous. For cooling a room space of 2000 cft [a typical space of 200 sq. feet with height of 10 feet], between the conventional AC system and these propulsion systems, there is a saving of 50 per cent per hour of use. With cost of electricity at about Rs 5 per unit, this can mean a big saving. If the average annual use of the conditioners is reckoned at 2,000 hours—we make this annual estimate since the use of these conditioners, we believe in good conscience, will be confined to only the hotter times of the year, and so will not be uniform through the year—, the saving per annum can be up to Rs 10,000 on power bills alone.
We read the occasional depressing report on air quality in our city or town. Essential to human life is air, water and food. And in securing these, we take many things for granted. The last two decades of a consumerist market economy have lulled us into believing we can continue to get these things for ever, just the way our grandfathers and their fathers did.
Returning to the first principles for each of these resources, and the quality of what we need to imbibe, will go a long way in enhancing quality of life and in making us resource–conscious. After all, there is a legacy you have to leave behind for your own children, if not to the rest of the world.
If you seek to look at these directions, write to us. We can help you.