Apparatus Dew point (ADP)

Apparatus dew point is the effective coil surface temperature when there is dehumidification. This is the temperature to which all the supply air would be cooled if 100% of the supply air contacted the coil.

When the air is to be cooled & dehumidified from t₁ to t₂ condition, the cooling coil has to be kept at temperature(t_s) lower than dew point temperature of air .

As the air passes over coil, heat transfer takes place & it rejects heat to cooling medium & also water vapor in air starts condensing on the coil surface resulting in the dehumidification. Lets understand this on psychrometric chart.

• As we move towards left on X axis, dry bulb temp decreases results in cooling of air & DB temp rises while going towards right.
• Similarly on Y axis , going upward will add humidity & downward movement leads to dehumidification.
• So the combine effect will results in Cooling & dehumidification process as shown by the slope line.
• The slope will be define by the Sensible heat ratio or Grand Sensible heat ratio of the given space.
• Sensible heat ratio: Also called Room Sensible Heat Factor is the ratio of room sensible heat to room total heat (room sensible heat + room latent heat).

• Grand Sensible Heat Factor (GSHF): is the ratio of total sensible heat to the grand total heat including the outdoor air heat loads that the apparatus must handle.

• The Slope can be plotted as shown in figure.

• When GSHF line further crosses the saturation curve, the intersection point is the ADP or effective coil surface temperature.

It is also the temperature at a fixed flow rate at which both sensible and latent heat gains are removed (from the conditioned space) at the required rates. It is also often called as the ‘Coil Temperature’.

Significance of Effective Surface temperature & ADP

The surface temperature of the conditioning equipment varies throughout the surface of the apparatus as the air comes in contact with it.

The effective surface temperature considered to be the uniform surface temperature which would produce same leaving air condition as the non uniform surface temperature that actually occurs when the unit is in operation.

• The same can be illustrated for heating , direct expansion coil, etc. The direction, slope & position of line may change but theory is identical.
• Since Conditioning of air reduces to the basic principle of heat transfer between the heating or cooling media of the conditioning apparatus & the air thru that apparatus, there must be a common reference point. This point is the effective surface temperature of the apparatus.
• To obtain the most economical apparatus selection, the effective surface temperature is used in calculating required air quantity & quality.
• As we have seen , for application involving cooling & dehumidification, the effective surface temperature is at the point where GSHF line crosses the saturation line on psychrometric chart. This effective surface temperature is considered to be the dew point temp of the apparatus. Hence the term Apparatus dewpoint (ADP) has come into common usage for cooling & dehumidification process.
• Since Cooling & dehumidification is one of the most common application for centralized air-conditioning apparatus, the “Heat load calculation” is designed around the term ADP.

Bypass Factor

• It represents the portion of the air which is considered to pass through the conditioning apparatus completely unaltered.
• When the by-pass factor is zero, the supply air temperature is equal to coil ADP.

In other article we will discuss the Significance of Bypass Factor.