Handbook of air conditioning and refrigeration / Shan K

18.42 CHAPTER EIGHTEEN
18.8 STRATIFIED DISPLACEMENT FLOW
Two-Zone Stratified Model
Operating Characteristics
Stratified displacement flow supplies conditioned cold air (in Scandinavian countries usually 100
percent outdoor air at lower velocity) at a low-level supply outlet, as shown in Fig. 18.25. The cold
supply air, with a volume flow rate of s, in cfm (L/s), flows in a thin layer along the floor. Above
the heat and contaminant sources, heated air containing contaminants rises upward because of its
buoyancy effect. Supply air is then entrained into the upward convective flow with a volume flow
rate of conv, in cfm (L/s). When upward convective flow arrives at a height where its volume flow
rate is equal to the supply volume flow rate, that is, conv � s, this height is recognized as the stationary
level zstat, in ft (m). Above zstat, ambient air is induced into the upward convective flow until
it reaches the ceiling. Upward convective flow with induced air spreads laterally along the ceiling.
An amount nearly equal to the supply volume flow rate exhausts or returns to the fan room through
the exhaust or return inlet near the ceiling. The remaining portion containing the contaminant descends
to the stationary level to be induced into the upward convective flow and recirculated.
The stratified level divides the room vertically into two zones: an upper zone and a lower zone.
In the lower zone, only the supply air is induced into the upward convective flow, as its volume flow
rate is smaller than the supply volume flow rate conv � s. In the upper zone, the portion of air and
contaminants that is greater than the supply flow rate V˙ conv � V˙ s recirculates. For cold air supply,
the mean temperature of the upper zone is usually 1 to 2°F (0.56 to 1.1°C) higher than that of the
lower zone.
V˙
V˙
V˙
V˙ V˙
V˙
Stratified displacement flow (or displacement ventilation) was introduced to Scandinavian countries
in the early 1970s as a means of improving general ventilation in industrial applications. For comfort
systems in public and office buildings, it was adopted in the early 1980s.
FIGURE 18.25 Stratified displacement flow in a typical room.