Once the air enters through the floor diffusers, it spreads across the floor forming a reservoir of fresh cool air. Any sources of heat (e.g. people computers etc.) will generate a thermal plume rising upward entraining the surrounding air, as shown in the Figure. The fresh cool air at floor level will flow to replace that which is warmed and lifted into the convective plumes. By this means the fresh air continuously replaces the air as it is used and contaminated air is lifted to high level for removal from the space.
The energy conservation aspects
UFAD is a more energy efficient system than the ‘mixing flow’ ceiling based distribution system, due to the following reasons:
Air is provided at low flow rates, as buoyancy is utilized to distribute air through the space. Low flow rates result in the reduction of energy usage of the fans used to distribute air through the under floor ducts.
- The supply air is at a relatively high air temperature (i.e. 19°C), as the air is supplied closer to the source of heat gains. Relatively high air temperature means lower chilled water temperatures, resulting in an effective reduction of the heat load / tonnage of the chillers installed.
- Since new air is kept separate from stale air, the breathed air quality is substantially uncontaminated.
Most commonly asked questions
- The increase in cost for UFAD is mainly due to the requirement of false flooring or the forming of ducts (in concrete) as a part of the floor slab.
- There is a reduction in cost when a UFAD system is used due to the reduction in the cost of the chillers and fans.
- Therefore, the effective cost difference will vary depending on the size of the system.
- Saying that if operational cost is taken into consideration any increase in cost of the UFAD will have an RoI of less than a year.
Will the ducts get dirty sooner, due to the diffusers being on the floor?
This is probably the biggest misconception people have about UFAD. The answer to it is contrary to what anyone would think. The ducts do not get dirtier as the ducts are positively pressurized. This in simple terms means that the dirt would be blown out instead of sucked into the ducts.
Will the flexibility of the interior planning be restricted due to the diffusers on the floor?
This is probably the biggest disadvantage of UFAD, unlike the ceiling distribution system where the furniture can be placed directly under the diffusers. In UFAD, furniture cannot be directly placed on the diffusers. Therefore, reducing flexibility of interior planning in a speculative office space. However, if UFAD is considered during the initial architectural planning, the area & dimensions of the floor plate can be designed to get a relatively higher flexibility in planning.
A case study
At Clancy Global we got the opportunity to take advantage of this fact and install an under-floor cooling (air distribution) system at the marketing office of a project in Mumbai for RNA Builders.
There are parts of the office with sloping roof and therefore the height of the office space is above 3.5m. The reduction in the heat load / tonnage of this office was approx. 15% compared to the design for a ceiling distribution system.Firoj Kumar Jena Executive Associate-MEP, Clancy Global Consulting Engineers. Akhil Kiran Ganatra Principal Advisor, Clancy Global