Overcoming the limitations of existing ADPI assessment with AHC

When considering the criteria involved in designing an air distribution system main objective are thermal comfort of the occupants, offsetting of the room heat loads, supply of fresh air and energy efficiency etc. The performance of an air distribution system within a room/zone can be rated in terms of Air Diffusion Performance Index (ADPI). For selecting and placing air terminal outlets to achieve a spatially uniform air distribution and thermal environment in the room, the Air Diffusion Performance Index (ADPI) is quite commonly used. The objective of this blog to clarify the application of ADPI, its formulation, implementation, limitations and relevance to the industry standards and use of Autonomous HVAC CFD (AHC) to overcome the limitations of existing ADPI Assessment, this enables designers to effectively use this tool without compromising their initial design.

What is ADPI?

Room dimensions
Room loads
Air distribution products type
Air distribution lay-out of air inlets and outlet
Throw distances of air registers

ADPI is defined as a percentage of the measurements taken within an occupied space where the effective draft temperature is between -1.7 ^oC to + 1.1 ^oC with the air velocity less than 0.35m/s (ASHRAE Fundamentals, 2001). An ADPI of 100% implies the measurements taken at all the sampling points within the occupied zone conform to the given criteria above, and therefore the thermal condition in that space is expected to be at an acceptable level. Most air distribution systems are designed to achieve ADPI of 80% or greater (ASHRAE Fundamentals-2001).

What it is Not?

ADPI should not be used as a basis on which to judge thermal comfort in a general sense where industry standards such as PMV and PPD are used as benchmarks. The reason for this is because ADPI does not take into consideration the humidity and radiant temperatures of the surrounding surfaces in the space considered, two other environmental factors determining thermal comfort. Furthermore, metabolic rate and clothing insulation of the occupants are two additional personal variables the PMV and PPD indexes take into account for a complete and in-depth assessment of thermal comfort.

ADPI Formulation

Determination of Effective Draft Temperature

Determination of ADPI

Conventional ADPI selection process and its limitations

Select type of diffuser.
Check Manufacturer’s recommendations to determine if the diffuser’s jet pattern will have excessive drop at the desired flow rate, using ceiling height as a parameter
Select the characteristic length (L) from the plans. This is the distance from a diffuser to a wall, or to the centerline between two diffusers, etc.
Determine the desired diffuser (isothermal) throw using the characteristic room length and the throw ratio from Table 1.
From the manufacturer’s performance table select a size with a T50 within range for the diffuser at the required cfm.
With the VAV system, this must be done for both maximum flow rate at maximum loads and at the lowest turn down flow rate when the space is occupied.

Many believe this selection is thought to result in high perceived comfort and ventilation mixing for the application. In light of the ADPI selection process above, it is not possible to evaluate the performance of a type of ventilation grille or diffuser and give it an ADPI rating based solely on its individual aerodynamic performance. As described, ADPI is intended to evaluate the performance of an air distribution system rather than the product alone. The resultant ADPI varies with room size and geometry, type of air terminal unit used, air outlet location relative to the walls and each other, and the way they are installed.

Also, above tables have some limitations such as applicable for up to 12’ ceiling heights, depends on space heating & cooling loads (designer loads vary with weather location, occupancy type, energy code / building insulation and construction, etc.), depends on manufacturer’s diffuser throw data but this can vary between manufacturers, cooling ADPI tables are different from heating ADPI tables, methods don’t consider return grille location into account, May not effectively predict ADPI values for rooms with multiple diffusers, irregularly shaped spaces, multi-level spaces Therefore, each air distribution system within a space is unique and possesses its own ADPI. The same air distribution system in a different space may have a totally different ADPI.

How Autonomous HVAC CFD overcomes the limitations of exiting ADPI assessment and improve accuracy

Standard Test Position Approach: ADPI rating is calculated as per ASHRAE Standard 113-Method of Testing for Room Air Diffusion.
Volume Weighted Approach: ADPI rating is calculated based on the effective draft temperature and air speed values in every cell (small volume within the simulation domain) in the occupied zone, rather than just a few test positions.

Enhancing ADPI Accuracy in AHC through following Key Factors -

Probe Location & Measurement: - AHC Uses the probe locations, measurement guidelines as mentioned in ASHRAE 113-Method of Testing for Room Air Diffusion which helps to get ADPI values closer to as build design.
Heat loads, design and performance of outlets: -Local sources of heat loads cause convection currents and stratification that determine the type, location, design and performance of the supply outlets. All these in turn may change/deviate the behavior of the original air distribution system at design stage. In AHC we are able to consider all these parameters while calculating ADPI values.
Room geometry, interior design & ceiling heights: - The spacing of diffusers is influenced by the interior design, spatial constraints and room geometry etc., which will affect air flow patterns in the room. In AHC we are able to capture the effects of all this on ADPI values.
Return air outlet location: - Its causes to short circuiting of supply air, which will compromise air distribution into the occupied zone in a similar fashion effective delivery of fresh air into the breathing zone of the room is compromised in an ACE evaluation. Particularly detrimental is the ceiling inlet & ceiling outlet set-up of entertainment air distribution because supply air is likely to exit out of the room through the return outlet before it gets mixed and delivered further into the room.
Different Scenario: - In VAV systems under turndown conditions, fixed diffusers with lower velocities such as the conventional four-way ceiling diffuser do not perform well and it affects the Final ADPI Value. In AHC we are able to get ADPI Value when VAV system work at maximum flow rate at maximum loads and at the lowest turn down flow rate at minimum load with VAV System

ADPI Test Case