Air Conditioning Process in psychrometric chart

[nevinjohn]

Hi,

Most of us know that air conditioned air is generally dry (RH very less...). My question here is referring to the psychometric chart. For the air conditioning process, the air leave the coil at almost saturated condition (100% RH), where as practically (I am aware of the by pass factor) the air entering the room is not saturated.. It is well below 50-60%RH.

14452


Note that most of the air conditioners (even split units) are not equipped with a heater. So what enables this 'sensible heating process' to achieve the desired room condition (of 50-60% RH) after the air leaves from the cooling coil from the saturated condition?

Thanks!
Nevin

[B G Scott]

The air leaving the coil may well be saturated, at the dry bulb temperature it leaves the coil but as it absorbs heat and its temperature rises it will not be saturated at the desired room condition.

As the temperature rises any given quantity of dry air can absorb (suspend if you like) more water vapour.
50% RH means that a given quantity air is 50% saturated AT the TEMPERATURE at which it is measured.

[frank]

At point 3 the air leaves the air conditioner and at point 4 it has absorbed the heat from the room.
If you look at the bottom line of the chart it is temperature

[nevinjohn]

At point 3 the air leaves the air conditioner and at point 4 it has absorbed the heat from the room.
If you look at the bottom line of the chart it is temperature

This is understood for ducted systems, however when its a small split unit, is it still the same? Would the air instantaneously heat up from saturated condition (as soon as it leaves the coil) to an unsaturated one?

[frank]

Not an instantaneous process but over time

[B G Scott]

The entrainment of secondary air from the conditioned space into the primary air flow from the diffuser is where the interchange takes place, and as Frank has said it is not instantaneous.

In the case of air discharged from a supply air grille the rate of entrainment is also influenced by the velocity of the air.
The velocity however has to be kept within limits to prevent discomfort to the occupants within the conditioned space.